CLINICAL CHEMISTRY PRACTICAL MEDICINE, Summaries of Clinical chemistry

Download CLINICAL CHEMISTRY PRACTICAL MEDICINE and more Summaries Clinical chemistry in PDF only on Docsity! CLINICAL CHEMISTRY IN PRACTICAL MEDICINE BY C. P. STEWART M.Sc.(Dunelm.), Ph.D.(Edin.) Lectu", in Bioc"'",sslry, University of Edinbu,gh: Senio, BiocJumist, Royal Infirma'Y, Edinburgh AND D. M~ DUNLOP B.A.(Oxon.), M.D., F.R.C.P.E. Chrisi;soll Professor 0/ T"',aj>suttes aM Clinical Metlie;n. Uni_sily 0/ lUlinb",g" SECOND EDITION EDINBURGH E. & S. LIVINGSTONE r6 & r7 TEVIOT PLACE 1937 F neve ap 4 HI: 7 “vgs Compuieri 30 NLVS/VRI WAT 00245 Printed tn Great Britain 4s PREFACE vii multiplied, as they are certain to be, far beyond those in use to-day, there is yet no reason why they sho)J.ld divert attention from the bedside study of disease. The second edition has been thoroughly revised in the light of the authors' own and others' experience; some tests which have been superseded or have not stood the test of time have been deleted; others which have proved more valuable have been introduced. Certain sins of omission, to which reviewers of the first edition called our attention, have, it is hoped. been remedied. The arrangement of the book has been so altered as to allow the technical details to be collected into an Appendix. This has enabled the argument in the text to continue without interruption, and the new Appendix, with the additional methods we have added, includes all the side-room tests (apart from bacterio­ logical) which the student is usually required to know. The authors wish to take the opportunity of -thank­ ing Dr. Harold Scarborough for his most helpful criticism of the manuscript, for his aid in proof­ reading, and for contributing the haematological section in Appendix I. They also wish to acknowledge the courtesy of Messrs. Heffer of Cambridge, who granted permission for the adaptati.on of the diagram appearing on p. 185 of Cole's Practical Physiological Chemistry, 8th edition. C'LINI(,AL LABORATORY, ROYAL INFmMARY, EDINBUltGH, October 1937. C. P. STEWART. D. M. DUNLOP. CONTENTS CIlAP'lEIL PAGB I. INTRODUCTION 1 II. THE COLLECTION AND PRESERVATION OF SAMPLES 7 III. TIlE BASAl. METABOLIC RATE 27 IV. THE MECHANISM OF NEUTRALITY REGU- LATION 49 V. GLYCOSURIA 78 VI. ALBUMINURIA AND TESTS OF RENAL FUNCTION ·101 VII. THE EXAMINATION OF STOMACH CONTENTS 150 VIII. TESTS OF PANCREATIC FUNCTION 183 IX. TESTS OF HEPATIC FUNCTION 195 X. THE CEREBRO-SPINAL FLUID 217 XI. CHEMICAL TESTS IN PREGNANCY 237i XII. THE BLOOD CALCIUM AND PHOSPHORUS 247 XIII. THE BLOOD SEDIMENTATION RATE 263 ApPENDIX I 269 ApPENDIX II 359 ApPENDIX III 360 INDEX 361 ix CHAPTER I INTRODUCTION SIMPLE chemical tests such as thc detection of sugar and protein in urine have long been used as aids in the diagnosis of disease. The last twenty or twenty­ five years, however, have witnessed an enormous increase in our knowledge of the chemical abnor­ malities to be found in disease. This increase has occurred along with (though beginning rather later than) the growth of biochemistry, and has been greatly aided by the development of analytical methods suitable for estimating on a micro-scale many of the substances prcsent in the body fluids and excreta. As usually happens when a new department of knowledge is rapidly made available, there is difficulty at first in appreciating its real worth. The enthusiasm of its sponsors and the hostility of its critics both tend for a while to hinder formation of an unbiassed judg­ ment. In the case of biochemistry as applied to medicine that .phase is now, happily, passing, and it is possible to assess the new science at its true value, and to rank it as one of many valuable aids in the diagnosis and prognosis of disease. It is not, has never been, and almost certainly never will be, the philosopher's stone of medicine; its methods, both as regards number and accuracy, are still far from per­ feet; it is still growing so rapidly that many of its conclusions may require modification; even with 1 4 CLINICAL CHEMISTRY IN PRACTICAL l\IEDICINE heard over the apex-beat of the heart is not, by itself, evidence of mitral incompetence, so a high blood urea is not itself sufficient justification for a diagnosis of nephritis. A high blood urea may be due to cardiac failure, intestinal obstruction, dehydration from physio­ logical or pathological causes, or any acute fever; and only a consideration of other clinical factors can differentiate amongst these possible causes. Similarly, a rather high fasting blood sugar may be due merely to nervousness on the part of the patient, but if it is accompanied by polyuria, thirst, or emaciation, it becomes valuable evidence in support of a diagnosis of diabetes mellitus. Hence, per se, the chemical findings may mean little, but considered as a link in the chain of evidence they may be of great importance. It is important also to realise that though a chemical analysis giving abnormal results may support a certain diagnosis, one giving normal results does not neces­ sarily exclude it. The organs of the human body are mostly constructed on a very generous scale. Thus it seems that half, or more, of the kidney may be destroyed by disease before its function becomes markedly impaired. Even then impairment of one function does not necessarily mean impairment of all ; so that clinical signs may lead to a suspicion of nephritis, and quite rightly, before the blood urea is raised. Hence a normal value for the blood urea or non-protein nitrogen is not incompatible with the existence of chronic Bright's disease. It has further to be remem­ bered that most substances of biological importance have, even in health, a considerable range of variation on either side of the normal mean. Hence an analytical result within the normal range may actually be pathological in a particular case, although of course INTRODUCTION 5 there is usually no means of knowing whether it is so or not. Thus the normal carbon-dioxide combining power of the blood lies between 53 and 73 volumes per cent., and a result of 56 volumes per cent. must be taken as normal. Actually, however, it may represent a pathological fall from an original value of 70 volumes per cent., a fall which might be of considerable signifi­ cance if the original value of70 were known. It is rarely that such values within the normal range are known to be actually pathological, and this wide range of physio­ logical variation therefore constitutes an important limitation of the biochemical method of investigation. The value of biochemical methods of investigation is not confined to diagnosis, but extends to prognosis and the control of treatment, where, indeed, some of their chief uses are found. It sometimes happens that aftcr a diagnosis has been made, and treatment has been instituted, considerable improvement takes place, and can be demonstrated by chemical means before the clinical picture has shown much alteration. Thus in a patient with meningococcal meningitis the first sign of improvement after serum treatment may be the return of sugar in the cerebro-spinal fluid, on which a more favourable prognosis may be given. On the other hand, an apparent clinical improvemcnt may occur unCler treatment without any real ameliora­ tion in the underlying disease, and the true state of affairs may "be revealed by chemical examination. An instance of this is found in the advanced nephritic, who apparently improvcs under rcst and dictetic treatment, but for whom the prognosis remains desperately bad, as is evidenced by the blood creatinine remaining at a high level. While in some diseases chemical examinations are 6 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE an aid in the control of treatment, in others successful treatment can hardly be carried out without them. No physician, for example, would care to treat a case of diabetes mellitus without frequent reC9urse to chemical examination of the urine, if not of the blood, and no surgeon should nowadays undertake the risk of radical prostatectomy without previous investigation of the blood urea. . Most of the investigations mentioned above, and many others like them, may be successfully carried out in general practice-with, perhaps, the technical aid of the biochemist-and do not necessitate the sending of the patient to hospital. In some cases even the actual analysis can be carried out by the practitioner himself, if he has the time and inclination to do so. When the samples are sent to a biochemist for analysis they should invariably be accompanied by a short note of the clinical condition of the patient, since the biochemist's experience may then suggest some further useful test. Close co-operation between the chemist and the clinician is always necessary, for both are partners to the transaction, and a successful accomplishment of any investigation can be achieved only if each does his own part efficiently, and knows the full facts of the case. Besides being of service in the hetter-known and more usual methods of chemical investigation, such as are dealt with in this book, it sometimes happens that the biochemist is able to render assistance in the elucidation of obscure and difficult cases. No single book could deal with all such con­ tingencies, and this one pretends to no more than a treatment of the commoner and more generally useful tests, but in cases of difficulty a consultation between the clinician and the biochemist may be fruitful. TIlE COLLEC"l'ION AND PRESERVATION OF SAMPI,ES 9 may be used for separate testing, and if this is kept as low as possible-only a few cubic centimetres need be used-the error in the analysis of the remainder of the twenty-four hqur sample will be slight. Preservation of Urine for Analysis.-For hospital work, probably the most efficien t method of preserving the urine for analysis consists in the addition of a little toluene-enough to form a continuous film over the surface of the fluid-and the keeping of the samples in a refrigerator. Toluene alone is not very satis­ factory, for it does not always prevent the growth of bacteria completely. It is to ,be remembercd that the great object of preservatives is to guard against the growth of bacteria, since the organisms live at the expense of the urinary constituents, and so, if allowed to grow, may profoundly alter the composition of thc urine. Thus, in particular, sugar tends to disappear and ammonia to be produced at the expense of Urea. Hence, unless the urine is examined while it is fresh, or with adequate precautions against bacterial growth, estimations of the output of glucose, or of the ratio between ammonia and urea, may bc entirely fallacious and misleading. The choice of a suitable preservative in private practice is not so easy. Chloroform (5 C.c. to each 100 C.c. of urine) is more efficient than toluene in rctarding bacterial growth, but has the objection that it reduces copper solutions and so simulates sugar. If it be used, thereforc, the urine must be boiled to remove the chlorof'(J),m beforc tcsts are applied for sugar. Formalin, which also has been suggested as a preservative of urinc, similarly reduces coppcr solu­ tions, and as it is not removcd complctely by boiling it is thcrefore dcfinitely undesirable in cases suspectcd 10 CLINICAL CIIEl\USTRY IN PRACTICAL MEDICINE of glycosuria. Thymol, too, though efficient as regards prevention of bacterial growth, interferes with certain tests. Possibly the best means of preserving urine for chemical examination is simple acidification to a reaction at which the ordinary bacteria cannot grow. For this purpose the addition of 1 c.c. of concentrated hydrochloric acid to 100 c.c. of urine suffices. The acid usually precipitates uric acid and destroys casts (so that for microscopical examination it cannot be used). Before the urine is examined chemically it is advisable to neutralise it by thc addition of sodium hydroxide (1 c.c. of 40 per cent. NaOH per 100 c.c. of sample is a suitable amount), and the volume of alkali so added must be taken into account in the subsequent calculations. Use of acid for preserving the urine prevents any estimation of the titratable acidity or pH; but this disadvantage is far outweighed by the usefulness of the method with respect to the com­ moner tests. BLOOD Amount.-It is impossible to lay down any simple hard-and-fast rule as to the amount of blood required for chemical analysis, though in the majority of cases 2 c.c. for each separate estimation is an adequate amount. The actual amount needed for a number of the commoner estimations is given in Table I, but it must be remembered' that these are minimal and do not allow for any duplication of analyses. Wherever possible it is wise to send more than the bare amount required for a single estimation, for it is most annoy­ ing to be deprived of useful information through an accident during an analysis which cannot be repeated. Again, it is annoying to the chemist to receive, as he so THE (,OI.LECTION AND PRESERVATION OF SAMPLES 11 --- --------------------------------------- TABLE I Test Method Quantity Coagulation -- -- - - ---- ---------1------1-------- ('°2 combining power Total Sugar Lnn:nlos(' Non-proh'in nitro- gen rn'u, nitro_[(('n ( !r(>atinino l:rie a(,l<1 lnorc;anic phos- phato Chloridos ;\let,llUf'moglo bin, .'tr. Pla~ma Proteins Chole.-,;terol Total fatty acids Lipoid Phosphorus Bilirubin lett -ric' iwiox Van Slyke { -Benedict or Folin and \Vu Hagedorn and Jenwn Micro-Kjeldahl Urease Folin Benedict Fiske and Sub­ arrow Silver-nitrate methods Spectroscope Micro-Kjeldahl 'Bloor Stewart and Hendry Stewart and Hendry Van den Bergh Meulengracht Ph.-n( ,1.tc,traehlor-1 \ Rosenthal I'hthakin J Cakium ·,l.n<1 Magnesium Carhon monOXIde Kramer and Tisdall (Tannin } 4 c.c. 2 C.c. 0·2 c.c. • 2 c.c. 2 c.c. 1 c.c. 2 C.c. 5 c.c. 1 C.c. 1 c.c. 2 c.c. 5 C.c. 1 C.c. 2 c.c. 1 c.c. 7 C.c. 3 C.c. Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated Non-coagulated { Non-coagulated or coagulated f Non-coagulated l or coagulated ( 10 c.c. } first sample -l 2 c.c. Non-coagulated 8nhseq nently 5 C.c. Coagulated 3 C.c. Non-coagulated 1 1 Spectroscopo ~Van Slyke --- - - -- - _ .----------__ --L ____ _____!_ _______ _ often does, a minute amount of blood with a demand that several complex anulyses be carried out on it. It is(JIusually a simple matter to obtain sufficient 14 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE present in the diffusible state, since citrate com­ pletely upsets the distribution of the blood calcium even if ·it does not alter the total amount. The deter­ mination of sodium, of course, cannot usefully be made after addition of any sodium salt, and that of potassium is best done without addition of any anti­ coagulant, since added salts may alter the distribution of potassium between cells and plasma. For these estimations also the blood should be allowed to clot. Some tests, such as that of Van den Bergh, and the determination of the icteric index, demand the use of blood in which no haemolysis has occurred. Hence in obtaining samples, for such tests particularly, neither ether nor alcohol must be allowed to come in contact with the needle and syringe, or with the collecting vessel. Water also must be avoided, and so all the apparatus should be clean and dry. If the syringe must be washed out, sterile physiological salt solution should be used for that purpose. The older practice in carrying out the Van den Bergh test was to use blood serum, but it is now recommended that plasma be em­ ployed, since with the use of a suitable anti-coagulant such as sodium oxalate haemolysis is less likely to occur than when the blood is allowed to clot spon­ taneously. Sodium oxalate is to be preferred to the potassium salt. A second important reason for the use of plasma in the Van den Bergh reaction is the desirability of carrying out the test as soon as possible after the blood has been drawn. It is stated that if more than two hours elapse before the test is made the results may often be misleading. Preservation of Blood Samples • .....-All blood analyses should be carried out as soon as possible. Besides the Van den Bergh reaction, this is particularly true of THE COLLECTION AND PRESERVATION OF SAMPLES 15 sugar estimations. Loss of sugar from shed blood may begin within a few minutes of withdrawal, though usually the loss is inappreciable for the first hour or so. Sugar estimation is valueless in blood which has been kept overnight without special precautions, since l;ly morning half or more of the sugar may have dis­ appeared. For sugar estimation it is a usual, and good, procedure to add the blood at once, without the use even of anti-coagulants, to the protein precipitant. After this treatment the mixture may safely be kept for a few hours, and may even be sent through the post. Many preservatives have been suggested for blood, and, for example, a trace of formalin has been found to prevent loss of sugar for a considerable time, even when, added in amount far too small to interfe:.:e with the estimation. Possibly the most useful pre­ servative-which, however, cannot be used if urea: estimation is required (since fluoride is an enzyme poisoll)-is a mixture of one part of sodium fluoride to three parts of sodium or potassium oxalate, 40 mg. of the mixture being added, without other anti­ coagulant, to each 10 C.c. of blood. With blood pre­ served in this way, estimations of sugar, non-protein nitrogen, and creatinine can safely be carried out after two or three days, so that samples so protected may be sent by post to the laboratory. If urea estimation is rf:quircd, and the analysis cannot be done within a few hours of drawing the blood, a drop or two of formalin may "be used as a preservative. . The inorganic phosphate rapidly increases in shed blood owing to hydrolysis of phosphate esters, and it is doubtful if any really satisfactory method exists for preventing this action over a long period. The first step in the estimation of inorganic phosphate Hi CJ.lNICAT. CHEMISTRY IN PRACTICAL l,\IEDJCINE is the precipitation of the blood proteins by addition of an equal volume of 25 per cent. trichloracetic acid. After the addition of this reagent thc hydrolysis of phosphatc is very much slower, although it still proceeds, owing, probably, to the acid reaction of the mixture. The immediate addition of trichloracctic acid to the blood sample (using accurately meaSllrf''' amounts of each) allows thc remainder of thc analysis to be postponed for some hours. No satisfactory method is known for preserving blood required for determination of the carbon-dioxide combining power. If really accurate results are to be obtained this analysis must be carried out within an hour or two of the blood being withdrawn. As in the cases of other samples tor analysis, the chemist should always be informed what prcservati \"es and anti-coagulants have been added to the samplcs of blood he reccives. Collection of Blood Samples.-When only a small amount of blood is required, or whcn other methods are unsuitable, it may be obtained by pricking the finger or the lobe of the ear. In order to ensure a free flow of blood the part selected should be warm, and should be congested by slight constriction, or, in the case of the finger, by swinging the arm. The stab should be made with a blood gun or a small scalpel, which are much more efficient than a needle and are no more painful. The stab should be resolute and firm, since half-hearted efforts are really more uncomfortable to the patient and generally have to be repeated. After the stab has been made the blood is massaged out into a suitablc vessel. Whcn the finger is selected as the sourcc of the blood it is advisable to make the stab at the tip rather than at the base of the nail, sincc it is THE COLLECTION AND PRESERVATION OF SAMPLES 19 occur. Once the sample of blood has been procured it is immediately transferred to a test-tube and thoroughly shaken up with a knife-point of sodium oxalate, 01' left to clot according to the nature of the analysis in view. The syringe and needle should, as soon as possible, be washed out with water and then with acetone, and the syringe should bc lubricated with a little liquid paraffin. Difficulty may sometimes be experienced with very emaciated patients, whose veins are not anchored by subcutaneous fat, and thus tend to be pushed away from the point of the needle. If the vein is sufficiently steadied by tightening the skin over it with the thumb of the left hand this difficulty can be overcome. In many fat subjects, on the other hand, especially women, the vein never becomes visible, even after considerable constriction has been applied to the upper arm. It is usually palpable, however, and often much less' difficult to puncture than the freely mobile vein of a thin subject. If it is neither visible nor palpable, the arm should be immersed for a little while in hot water, which almost invariably makes the vein more prominent. If this fails, some other vein, which may be more obvious, on another part of the body, such as the dorsum of the foot, may be tried. In young children it is usually impossible to obtain blood by puncture of the veins of the arm, and the ~xtcrnal jugular vein is generally selected, with the child's head turned so as to steady the vessel over the sterno-mastoid muscle. The fact that the child usually eries during the proceeding causes the vein to be satisfactorily distended. If a large quantity of the child's blood is not required, a specimen of 2-8 c.c. may easily be collected from a small stab wound in the 20 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE heel. The ankle is grasped so as to congest the heel, and, the stab having been made with a sterile scalpel, the resulting blood is massaged into a test-tube. In infants, blood may be obtained by puncture of the superior longitudinal sinus through the anterior fontanelle. This procedure, which sounds rather formidable, is ·in reality perfectly simple and safe, provided the child's head is held very still by an assistant and the needle is not inserted farther than the depth of its bevel, since the sinus lies immediately beneath the surface of the skin. The needle should be inserted in the middle line through the posterior angle of the fontanelle. It is important to remember that many of the blood constituents are increased in amount for some time after a meal, and in estimating these substances, -therefore, it is absolutely essential to obtain the blood while the patient is fasting. Though for other analyses fasting blood is not really necessary, its use is not harmful, and for routine analysis ·it is always desirable to use fasting blood, the blood being with­ drawn in the morning before thc patient has had breakfast. FAECES For chemical examination it is desirable that the complete stool be sent to the laboratory and that the analysis be commenced as quickly as possible, since, even more than in the case of urine, the ch$!mical composition of faeces is liable to u:ndergo alteration owing to bacterial action. Probably the best preserva­ tive which will prevent the alteration in amount and distribution of the fats of the faeces is alcohol, which, if added in considerable amount, will not only kill THE COLLECTION AND PRESERVATION OF SAMPLES 21 I bacteria, but prevent the continued action of lipase. When the faeces have to be sent through the post, however, this procedure is inadmissible, and formalin should be substituted. The chemist should be informed that .preservative has been added. Samples of faeces obtained by the use of purgatives or of an enema are of no use for chemical examination, and no liquid paraffin or other oil should be given for at least three or four days prior to the collection of the sample. CEREBRO-SPINAL FLUID The cerebro-spinal fluid may be withdrawn from the lumbar theca, the cistern space, or from the lateral ventricles. Of these the safest and most usual method is withdrawal from the lumbar theca. In performing the operation of lumbar puncture, absolute asepsis is again essential, since the most disastrous results may be occasioned by an artificial infection of the meninges. The ideal lumbar puncture needle is some 7-9 cm. (3-4 in.) long, moderately pliable and certainly not brittle. The bevel must be absolutely smooth and the shaft perfectly rustless. It is provided with a handle and a stylet, and its bore should be capable of being fitted on to a "record" syringe. The needle should be sterilised by boiling rather than by ether or alcohol. Two types of needle are in general use. The Graham type, a straight needle, is the more suitable when a manometer is bcing used; thc White­ Jenselme type, providcd with aT-handle through which the cerebro-spinal fluid drips, is perhaps easier to introduce. The pQSition of the patient during the op.eration i~ exceedingly important, since, if this is faulty, an easy ~4 CLlNlCAL ClfEM1STRY IN l'RAC'l'lCAL MED1C1NE the needle should be withdrawn until its point is just under the skin, and a new attempt made. If no fluid is obtained although it is believed that the needle has reached the spinal canal, haphazard plunging of the needle in all directions should on no account be made, as this will undoubtedly cause bleed­ ing and post-operative headaches. Rotation of the bevel of the needle should be tried, after which the fluid may escape easily, since the point may have become obstructed by a nerve root; or slight con­ striction may be applied to the veins of the neck, thereby raising the intracranial venous pressure and so increasing the tension of the cerebro-spinal fluid. If these measures fail, the needle should be inserted a little farther. If this also fails, and if by means of the stylet it is ascertained that the lumen of the needle is quite clear, the needle should be withdrawn and an attempt made in the next interspace. Occasionally all efforts to withdraw cerebro-spinal fluid by lumbar puncture fail. Sueh a " dry puncture" may be due to absence of fluid in the lumbar theca, as may be eaused by an obstruction in the spinal canal; or to thick pus blocking the lumen of the needle, as occasionally happens in suppurative meningitis; but one should be very chary of ascribing one's failure in obtaining fluid to these causes, since a " dry puncture" is much more usually due to failure of the operator to puncture the dura and enter the spinal canal. In very fat persons, or in patients with some degree of spinal arthritis, it may be impossible with the patient lying in bed to get the back sufficiently arched to carry out the puncture successfully. Under such circumstances lumbar puncture is more easily per­ formed with the patient sitting on a stool, his legs TIlE COLLECTION AND l'lt'ESEltVATION OF SA1\ll'LES 25 separated, llnd his head and shoulders bent down­ wards towards his knees as much as possible. Provided the needle is not inserted to a very ex­ cessive depth, so as to pierce the aorta, and provided excessive force is not used, so as to break the needle, the dangers of lumbar puncture are very slight. A few cases of death from ccrebral hernia have bcen reported following its performance, but in these the puncture was done with the patient in a sitting .posture. The rapid removal of 15-20 C.c. of cerebro­ spinal fluid from any case suspected of suffering from a cerebellar or cerebral tumour, whether associated with papilloedema or not, is an exceedingly dangerous procedure. When a lumbar puncture is performed in such cases a simple manometer should always be attached to the needle, the intelligent use of which will obviate the dangers of lumbar puncture under such circumstances. Owing to the presencc of the manometer the fluid will not be rapidly withdrawn, and an undue lowcring of thc cerebro-spinal fluid pressure can be prevented. (Sec p. 222.) Lumbar puncture is definitely contra-indicated in erysipelas or septic skin conditions of the lumbar region, and in cases of the exanthemata it should be performed only when absolutely necessary. It is ideal to kecp the paticnt in bed for twenty-four hours after the operation, and to tell him to take things easily for a day after that. It is essential, if hcadache is to be avoided, that he should at any rate lie flat for two or three hours afterwards. Headache is thc commonest sequel to the puncture, and may indicate that too mueh fluid has bcen removed, or may be due to non-closure of the puncture-hole in the arachnoid, so that leakage of fluid into the tissues 26 CLINICAL Cn~M1S'l'1tY IN PRACTICAL MEDlCIN~ occurs for some time afterwards. This headache is usually satisfactorily treated by kceping the patient at rest with the foot of the bed raised, giving him plenty of watcr to drink and prescribing aspirin. Occasionally an intramuscular injection of pituitrin relieves it. Pain in the extremities is a less common sequel, and is probably due to injury of one of the filaments of the cauda cquina. It is of a very temporary character, and yields to rest in bed and some mild sedative. Cerebro-spinal fluid may also be withdrawn through thc occipito-atlantoid ligament from the cistern space. This procedure may be called for in order to procure fluid in spinal-subarachnoid block, and may occasion­ ally be useful in the diagnosis of tuberculous menin­ gitis, since the tubercle bacilli are more easily demon­ strated in thc cistern than ·in the lumbar fluid. Owing to the proximity of the vital centres the procedure is not free from risk in the hands of an inexperienced operator. The technique should not, therefore, be learned from a book, but from repeated trials on the cadaver. Puncture of the lateral ventricle, which is usually performed for therapeutic purposes in hydrocephalus, is a much more formidable operation, and calls for great care and precision. It should not, therefore, be undertaken without skilled surgical aid. THE BASAl. METABOLIC RATE 29 however, the R.Q. for an average protein is found to be 0·80. The actual R.Q. found in the course of metabolism is due to the oxidation of a mixture of the three types of foodstuff; but it is possible, knowing the urinary nitrogen excretion, from which the amount of protein metabolised may be obtained, to calculate the relative amounts of fat, carbohydrate, and protein oxidised, with any given R.Q. In practice it is found that the rate of protein metabolism under basal conditions varies so slightly that, under these conditions, it need not be separately determined. For clinical purposes it is sufficiently accurate to assume it to have the normal value. The amount of heat evolved during the combustion of each of the foodstuffs has been determined experi­ mentally, and from the ·data so obtained the amount of heat evolved per litre of oxygen-the "calorific value of oxygen "-has been calculated. Thus experi­ ment shows that 1 g. of carbohydrate in the form of glycogen or starch gives 4·18 cals. when it is burnt to carbon dioxide and water. The equation for the combustion allows it to be calculat~d that 1 g. of either of these substances uses 0 ·828 litres of oxygen during the combustion, so that for each litre of oxygen used in the combustion of carbohydrate 4·18 , or 5'047 0'828 cals., must be evolved. In other words, the calorific value of oxygen used in the combustion of carbo­ hydrate is 5·047 cals. per litre. Similarly it appears that the calorific value of a litre of oxygen used in the combustion of fat is 4·686 cals., and of protein, 4'463 cals. Using these figures, and the knowledge of the relati ve amounts of the different foodstuffs oxidised 30 CLINICAL CIIEMISTRY IN PRACTICAL MEDICINE at a given R.Q., we are able to calculate the calorific value of oxygen for all possible values of the R.Q. This has been done in Table II. It need hardly be said that experimental work has amply shown that the heat production in metabolism calculated in this way from the R.Q and oxygen consumption agrees with that found by direct measurement in the calorimeter. TABLE II THE RELATION BETWEEN THE RESPIRATORY QUOTIENT AND THE CALORIFIC VALUE OF OXYGEN The figures given here are calculated on the assumption that only fat and carbohydrate are being oxidised. Actually, of course, protein also is being burned, but the error introduced by this simplification of the calculations is not considerable. ---- - -- Reapiratory GalB. per litre RllBpiratory Gals. per litre Quotient oj Oxygen wed. Quotient oj Oxygen wed. 0·70 4·686 0-86 4-875 0·71 4·690 0-87 4-887 0·72 4·702 0·88 4-900 0·73 4-714 0-89 4·912 0-74 4·727 0·90 4·024 0·75 4·739 0·91 4-936 0·76 4·752 0·92 4-948 0-77 4·764 0·93 4·960 0·78 4·776 0-94 4-973 0-79 4-789 0·95 4·985 0-80 4-801 0·96 4·997 0·81 4·813 0-97 5-010 0·82 4·825 0·98 5-022 0-83 4·838 0·99 5·034 0-84 4·850 1-00 5·047 0·85 4·863 The rate of oxygen consumption will, of course, be enormously modified in any individual by a number of circumstances: it will be profoundly affected by the degree of muscular activity; by the nature and THE BASAL METABOLIC RATE 31 amount of food taken; by changes in temperature; and by the pr.esence or absence of mental exertion or excitement. In order, therefore, to compare the metabolism of one person with that of another, it is necessary that the circumstances under which the comparison is made should, so far as possible, be identical-that is, we should try to keep those factors which modify the rate of metabolism constant in each case. Such standard conditions may be obtained by keeping the patient in bed at a constant temperature, completely relaxed mentally and physically, and with glandular and peristaltic activity reduced to a mini­ mum by previous starvation. Practically the whole energy expenditure under such conditions will be represented by that necessary to maintain the vital functions, such as the respiratory and cardiac move­ ments, the tonus of the muscles, and the body tem­ perature. Metabolism measured under such conditions is called the basal metabolism, which may therefore be defined as the energy expenditure of an organism in a state of complete physical and mental rest. In order to maintain the body temperature at 37° C. a certain amount of energy must be continually pro­ duced, since heat is always being lost from the surface of the body. The heat produced to replace that lost from the body surface forms, in fact, a large part of the total production under basal conditions. To a considerable extent, therefore, the amount of energy produced by a person under basal conditions depends on his surface area. The smaller the weight of the subject, for example, the greater will be his surface proportionately. Hence combustion will have to takc place at a greater rate in a small man in order to maintain his body temperature constant 34 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE PREPARATION OF THE PATIENT The determination of the B.M.R. is made ideally as soon as possible after the patient has wakened in the morning. He should have taken no exercise of any sort since the preceding evening, not even being allowcd out of bed in thc morning to wash himself or to brush his teeth. Occasionally it may be ncces­ sary to determine the B.M.R. of an out-patient who cannot be taken into the hospital for the night. Such a determination on an ambulant case tends to be unsatisfactory, but the patient may be rendered approximately basal by being brought straight from his house to the hospital in a conveyance as early as possible in the morning, and being put at complete rest, when he gets there, for at least an hour before the test is made. The patient should fast for at least twelve hours before the test so that glandular and peristaltic activity may be reduced to a minimum. He should ·have his temperature takcn before the test, as it has been shown that the metabolic rate rises approxi­ mately 10 per cent. for each rise of 10 C. of body temperature. It would thus be a waste of time to estimate the patient's basal metabolic rate if he were suffering from any degree of fever. On the other hand, the patient should be comfortably warm, to exclude the converse fallacy of a raised metabolic rate due to shivering or to the invisible muscular hypertonus caused by cold. Lastly, the patient should be mentally relaxed, and of all conditions this is the most difficult to attain.' Any unknown test is apt to excite a patient-if his confidence is not fully obtained-and when such a THE BASAL METABOLIC RATE 35 test is associated with a somewhat awe-inspiring para­ phcrnalia of masks and bags his nervousness may become extreme. It is thus essential to have explaincd to the patient the previous day the scope and full details of the test, and if possible to havc allowed him to try on the mask and to become accustomed to breathing through valves. If the test is performed in the ward, screens should be put round the bed, and both the patient and the ward should be kept as quiet as possible until the specimen has been procured. Too often the subject of the test is discovered by the operator engaged in an animated conversation with a friend in the next bed or engrossed in an enthralling novel. It is, indeed, questionable whether all these con­ ditions can usually be procured in ordinary hospital wards. It would be ideal if the determination could always be made in some quiet room adjacent to the laboratory. The patient should be wheeled therc in his bcd, and left for an hour before the test is started, to accustom him to his new surroundings. TECHNIQUE During the last few years numerous methods have been devised for determining the B.M.R .• Most of these have aimed at simplification of technique, and the majority are quite incapable of giving accurate results. Only two methods of proved value need be considered here-an " open" and a " closed" method. In the former the patient expires into a specially oonstructed bag; this exph,ed air is analysed, and ,from the results of the analysis the oxygen consump­ i tion and carbon-dioxide production of the patient 36 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE are calculated. In the latter the subject rebreathes oxygen from a reservoir, carbon dioxide in the expired air is absorbed by means of soda lime, and the oxygen consumption is measured directly by the diminution in the volume of the gas in the reservoir. The closed method possesses certain advantages, and at least one grave defect: it is easy to perform, no knowledge of gas analysis being necessary; only a single piece of portable apparatus is required, so that the complete determination may be made in a ward or even in a private house; lastly, three or four determinations may be carried out at one sitting. Moreover, by using a recording drum attached to the apparatus a complete record of the patient's res­ piration during the tests can be obtained, and any disturbance can be eliminated. On the otlier hand, a very grave criticism of the method is that it allows only a determination of the subject's oxygen con­ sumption, and his carbon-dioxide elimination is not measured. Hence it is necessary to assume an arbitrary value for the respiratory quotient in cal­ culating the calorific value of the oxygen used. We know, however, that the respiratory quotient of even normal individuals under basal conditions may vary considerably. This variation is often greatly ac­ centuated pathologically, and we have already seen how the calorific value of oxygen varies with fluctua­ tions in the respiratory quotient. It is, therefore, not surprising to find that this method, which postulates a fixed respiratory quotient of 0 '82, gives accurate results only in about three-quarters of the cases studied, and in the remaining quarter the results are apt to be fallacious. The open method, on the other hand, though re- TIlE BASAL METABOLIC RATE 30 returned to their original value the patient is con­ nected to the spirometer, and at some convenient moment the exact time is noted by starting a stop­ watch. Simultaneously the volume of oxygcn in the reservoir is read. It is essential that the test 0"'I9cn ChcLmb~l---t--+ Wa.Cer cock - """"'"1....1 __ Outlet valve. -:-:=:::;;~C~~ Illspiratlon rube Evpiro..tion tube. _ ===~ Cotl, for ref,ll,"!] Oa~ge" Lhll,mbtT­ FIG. I.-Sectional view of closod apparatus lor detennination of the Basal Metabolic Rate. l>hould be started and stopped at the same phase of l'espiration, and the best moment to select is probably just at the completion of expiration. After a suitable length of time, and again just when expiration ends, the volume of oxygen is read orf and simultaneously the watch is stopped. Thus the volume of oxygen used in a measured time, measured at atmospheric 40 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE pressure and at the temperature of the spirometer, has been obtained. After repetition of the test the mean of two or three observations is reduced to standard temperature and pressure by means of the Tables supplied with the machine or, less conveniently, by means of the formula : 1 t S T P vol. observed X 273 X press. observed vo. a ... ---:-----:-.---==-___;:.,__..,------ (temp. In °C.+273)X760 The B.M.R. is then calculated thus: Let the volume of oxygen used at S.T.P. be tl? litres, and let the time during which this oxygen is used be T minutes. With an arbitrary n.Q. of 0·82 it will be seen from Table II that 1 litre of oxygen is equivalent to 4'83 calories. Hence the number of calodes pro- duced per hour is 4'83;X60, and the B.M.R. is equal to this number divided .by the body surface in square metres. The surface of the individual is obtained, the height and weight being known, from Du Bois' formula: S=(wt. in kilos.)00425 X (ht. in cm.)Oo725 X 71'84 X 10-4, or from the graph (Fig. 2) constructed from this formula. The result may then be expressed as a per­ centage increase or decrease on the figure for a normal person of that age and sex. The normal figures are given in Table III. "Open" Method.-In determining the B.l\I.R. by the open method a similar mask is employed to that already described. The inlet valve is not connected to any reservoir, so that the patient breathes the ordinary air of the room. The expiratory outlet of the mask is connected by fabric-covered, corrugated THE BASAL ME'l'ABOLIC RATE 4,1 II / / V II / 7 V J / / v ~ .0 ..:t - ~ o o "l / ) / II / / v rl / / [7 1/ I / / v 1/ 1/ /' / V V ,/ ./' +7 ;. /' .:. -.;., o 00 "t""t /' $ /~ 9 /«- I /9 / I· I v ~ I 1\ V I J. / !I v. V I lY. / ~ / v' / I ~ ./ V( /' V, 2......- .., ~~ I~ ,,- ~. ,: ~ Cl t:- II I/f ) II 1 v J / / / v y V / / / ,/ / V / V CIt ~ ~ ~ • '!' .... I~ J V I v· / v V ~ -~ ~ ~ ;:14 v::: i/ ; ,Iv / / / t-V o o ~cn l 00 O)~ .E 44 CLINICAL CIIEMISTltY IN PRACTICAL MEDICINE to the procedure is insufficiently realised and, in consequence, far too much stress is often laid upon a single initial estimation, which is usually much higher than the true B.J.\iI.R. Once the patient has become thoroughly habituated, with skilful technique the experimental error should not be much more than 5 per cent. The basal metabolic rate is significantly r~ised in cases of hyperthyroidism, fevers, and the leukaemias and in functional or organic neurological conditions producing spasticity, tremor, or excitement. It is usually mildly increased in acromegaly and in Paget's disease. It is lowered in hypothyroidism and in the terminal stages of wasting diseases. Much less con­ stantly, or significantly, it is lowered in hypopituitar­ ism. In hydraemic nephritis a low value of the B.M.R. has been fairly frequently recorded and has been made the basis of thyroid therapy in this condition. We believe, however, that such low readings are due to the simple fallacy that the inactive oedema fluid may artificially raise the body weight and, therefore, the estimated .body surface on which the B.M.R. is cal­ culated. Thus, low readings are obtained in spite of the fact that the patient, apart from his oedema, may have a normal rate of metabolism. It is a curious fact in this connection that cases of exogenous obesity, unassociated with gross endocrine disorders, have almost invariably a norm~l B.l\:l.R. Because of the great proportion of inactive fat in the obese, one would have expected a low result, and the normal rate would seem to indicate that the obese patient is actually using up a greater amount of energy than would a healthy person of " ideal" weight, of the same height and sex. THE BASAL METABOLIC RATE 45 The n.M.R. is invariably raised in cases of hyper­ thyroidism, sometimes to as much as 100 per cent. above normal, and the extent of the increase furnishes the most accurate single sign at our disposal for gauging the severity of the disease, besides affording a valuable aid to diagnosis in those cases where the signs observed at the bedside are suggestive but in­ conclusive of the condition. A simple enlargement of the thyroid gland or benign goitre causes no increase, ·but, if anything, a decrease in the n.M.R., which result may be used to differentiate this condition from true hyperthyroidism, if such a differential diagnosis is not entirely apparent clinically. The severity of exophthalmic goitre, as is well known, is not constant or regularly progressive, but tends to assume a cyclical periodicity, in which periods of exacerbation are followed by periods of comparative remission. These fluctuations in the severity of the disease are accurately demonstrated by readings of the B.l\I.R., which may thus aid in furnishing a~ indication of the effects of treatment and of the time at which operative interference may be undertaken with the best chance of success. It may be taken as a general rule that operation is undesirable if the n.M.R. is persistently increased by 40 per cent. or more. Operative procedures should in any case be under­ taken while the B.M.R. is falling rather than rising. When iodine therapy is used as a pre-operative treat­ ment of toxic goitre it is usual to follow the effect of treatment by frequent determinations of the n.M.R. In this way it is possible to choose the optimum time for operation, which is when the B.M.R. is at its lowest and before it has begun to rise again, as it will do with continued administration of iodine. 46 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE This procedure is also of value in differentiating thyrotoxicosis from other conditions in which the B.M.R. is also raised, since only thyrotoxicosis responds to iodine by a fall in the B.M.R. The condition known as toxic adenoma, on the other hand, gets slowly and progressively worse, and the B.M.R. accordingly tends to increase steadily, or at any rate does not show the same periodicity (Fig. 3). It is BMR. +70 +60 +50 +40 +1.0 o II o1ll:ic ~ ~- ~ V,) ~ .- , ? 1\ ;, '.., , J \ / \ / '" I \ / f oP~ h .. laic '-t ... II \ / ~ 1 a :5 + S 6 ., 8 9 10 11 12 :13 :K Months. FIG. 3.-Variations in the Basal Metabolic Rate over a period of eighteen months, (a) in 8 case of exophthalmic goitre, (b) in 8 case of toxic adenoma. claimed by some authorities that a differentiation may thus be made between the two conditions, though it would seem that such a differential diagnosis could be made as effectunlly and less tediously from the history of the case and the clinical examination. Indeed, there is nowadays a strong tendency to consider toxic adenoma and exophthalmic goitre, or Graves' disease, under one heading of "Thyroid enlargement associated with thyrotoxicosis," since CHAPTER IV THE MECHANISM OF NEUTRALITY REGULATION THEORETICAL CONSIDERATIONS THE blood is very slightly alkaline-the arterial blood being somewhat more alkaline than the venous-and in order that the bodily functions may be properly carried out it must be kept in that condition. A small deviation from the normal seems to be per­ missible without the production of immediate ill­ effects, but rather larger deviations in either direction are sufficient to cause serious derangement of many functions. Indeed, it is only within a moderately narrow range of alkalinity that life is possible, and certainly the blood must never be allowed to be­ come acid or even neutral. It is obvious, therefore, that the mechanism for maintaining the blood-and through it the tissues-at the correct reaction must be at the same time delicately balanced and robust. A proper understanding of this mechanism and of the wllys in which it may be strained in disease can best be obtained by a consideration of the chemical prin­ ciples involved. Acids and alkalis are among those substances whose aqueous solutions conduct electricity. Such substances, in solution, are dissociated to a greater or less degree into electrically charged particles called ions. Thus a solution of hydrochloric acid contains, D 40 50 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE besides undissociated moleculcs, positively charged hydrogen ions (H+) and negatively charged chlorine ions (CI-); a solution of sodium hydroxide contains molecules of that substance, and also positively charged sodium ions (N a +) and negatively charged hydroxyl ions (OH-). This dissociation into ions-ionisation­ is reversible, and is never absolutely complete, so that undissociated molecules are always prescnt, and by suitable means, such as cvaporation of the solution, the ions can be made to recombine. All acids, when dissolved in water, produce hydrogen ions, and indeed- the definition of the, term "acid" is based on this property. Since the characteristic property of an acid is the production of hydrogen ions, it follows that the strength of an individual acid de­ pends on its capacity for ionisation. A solution of a strong acid containing 1 g. molecule per litre contains more .hydrogen ions than one of a weak acid which also contains 1 g. molecule of acid per litre of solution. This can only mean that in the case of the strong acid a great many molecules have dissociated and so given rise to free hydrogen ions, while relatively few molecules of the weak acid have dissociated. In an extremc casc a very strong acid would be almost com­ pletely ionised, an exceedingly weak one hardly at all. Hcnce the strength of an acid is determined by its degree of dissociation- that is, the extent to which it is ionised. This conclusion is quite independent of the number of hydrogcn atoms in the acid molecule, and, for instance, hydrochloric acid (HCI), with one atom of hydrogen per molecule, but largely ionised in solution, is a much stronger acid than phosphoric acid, which contains three atoms of hydrogen in cach molecule (H 3P04) r " .~ TIlE MECII'ANISl\1 OF NEUTRALITY REGULATION 51 but is only slightly ionised. Such acids as carbonic (H2C0 3) and phosphoric, which contain more than one atom of hydrogen in the molecule, dissociate in stages. Thus carbonic acid is dissociated, though, bcing a weak acid, only slightly, to H+ and He0 3 -, H2C03~H++HC03 -, and the bicarbonate ion, lICO 3 -, is still further dis­ sociated, though to an almost infinitesimal degree, according to the equation: HC03-~H++C03-· Similarly, phosphoric acid dissociates in thrce stages: H3P04~H++H2PO,~ H2P04-~1I++HPO,­ HP04=~H++PO,;. Phosphoric acid is a weak acid, and even in the first stage the amount of dissociation is small; in the second and third it is extremely slight. In the same way we find that the strength of a base, such as sodium hydroxide, is also dependent on its degree of ionisation, a base being essentially a sub­ stance which, in solution, gives rise to hydroxyl ions {On-}. Before the implications of these considerations and their bearing on the mechanism of neutrality regula­ tion are discussed, it is desirable to explain the nomen­ clature in general use for the quantitative expression of acidity and alkalinity. Purc water, though it conducts electricity very badly, does so slightly in virtue of the fact that a very few molecules are dissociated according to the equation: H20~H++OH-. 54 CLINICAL CIIEMISTRY IN PRACTICAL MEDICINE volume and pressure of a gas, which, according to Boyle's Law, are so connected that the pressure is inversely proportional to the volume. And, just as in the gas the product of the pressure and the volume is constant, so in the solution the product of the con­ centrations of hydrogen ions and hydroxyl ions is constant. Experiment shows that in pure water 1 g. of hydrogen ion is present in 10,000,000 litres of water, so that the concentration of hydrogen ion is 1 , or 10-7• The concentration of hydroxyl 10,000,000 ions is the same, and the product is thercfore 10-1-1. We have seen that this product is constant, and is not altered by the presence of acid (or, on similar reason­ ing, of alkali), so that we arrive at the important con­ clusion that under all circumstances (cone. of H+) X (conc. of OH-)=10-u . If, then, the conccntration of hydrogcn ions is grcater than in pure water, as in an acid solution, the concen­ tration of hydroxyl ions must be less, and conversely, in an alkaline solution, where the concentration of hydroxyl ions is greater than in water, the concentra­ tion of hydrogen ions must be less. The concentration of hydrogen ions, then, affords a quantitative measurc of the degree of acidity or alkalinity, being 10-7 in neutral solutions, greater than 10-7 in acid solutions, and less than 10-7 in alkaline solutions. _ 'rhus a decinormal solution of hydrochloric acid, a strong acid, and therefore almost completely dissociated, contains very nearly 0·1 g. of hydrogen ions per litre, a hydrogen ion concentra- tion of 110 or 10-1 • Similarly a decinormal solution of 'fUE MEClIANISl\[ OF NEUTRALITY ltlWULATlON 55 - - ------------------- the strong base sodium hydroxide contains 0·1 g. moleculcs of hydroxyl ions per litre, so that the hydrogen ion concentration is only 10-13• On the other hand a decinormal solution of acetic acid, which being a weak acid is slightly dissociated (1·36 per cent.), 1·36 1 has a hydrogen ion concentration of only 100 X 10' or I·S6XI0-3, or 10-2:867• In practice it is customary to use only the indices of these figures, under the t('rm "pH," and to say that purc water has a pH of 7, an acid solution a pH less than 7 (thus deci­ normal HCI has a pH of 1, decinormal acetic acid onc of 2·867), and an alkaline solution a pH greater than 7 (thus decinormal sodium hydroxide has a pH of 13). Tl.fJugh this is perhaps the simplest way of regarding the derivation of the term" pH," it is actually defined a"l t'le logarithm to the base 10 of the concentration (If hydrogen ions, the negative sign being omitted. 'I'hue; decinormal acctic acid, 1·36 per cent. dissociated, ha<:; 1·36 1 1 COllC. ofH+=--x -=1·36X-- 100 10 1000· :Sow, 1 LoglO 1·36=0·133, and LoglO 1000=-3. lIence, Log10(conc. of H+)=0·133+( -3)= -2·867, i.e. pH=2·S67. The use of the pH system of mcasurcment suffers from the drawback that it makes a change in hydrogen ion concentration look less than it really is. It must be remembered that a pH decrease from 7 to 6·7 56 CLINICAL ClIEMISTRY IN PRACTICAL MEDICINE -i.e. a decrease of 0·3-involves a doubling of the hydrogen ion concentration. At pH 7 the hydrogen ion concentration is 10 -7. Twice this concentration, 2 X 10-7, has Log10 equal to Log102+Log101O-7--i.e. 0·301+( -7)-i.e.-6·7-the pH being therefore 6·7. Similarly a decrease of 1 in the pH means that the hydrogen ion concentration has been multiplied by 10. This small change in the pH (the number representing the hydrogen ion concentration) produced by a rela­ tively large change in the actual concentration of hydrogen ions has the disadvantage that it is liable to obscure the real magnitude of changes in alkalinity or acidity. Thus, the pH of arterial blood varies from about 7·3 to 7·5. The range in pH is only 0·2, but actually this means that the concentration of hydrogen ions is more than 50 per cent. greater at one extreme of the range than at the other. A change of 50 per cent. in the blood sugar concentration would be regarded as considerable, yet the peculiarity of the pH system has led to the quite unjustifiable statement that the hydrogen ion concentration of the blood varies only very slightly. Actually it may vary to quite a considerable extent; the pH, however, does vary only slightly. The addition of a small amount of acid to pure water causes a very considerable alteration in the pH. Thus 0·0365 g. of hydrochloric acid, which can liberate 0·001 g. of hydrogen ions, when dissolved in 1li_tre of water gives a solution of pH 3 (i.e. a hydrogen ion concentration of _1_, or 10-8). Yet this amount of 1000 acid added to blood would produce a very much smaller change in pH, and therein lies the problem to TIlE MECHANlSlII OF NEUTRALIT~ REGULATION 59 blood the liberation of one gram molecule of oxygcn involves the setting free of nearly 0'7 of a gram atom of potassium (which can combine with 0·7 of a gram molecule of carbon dioxide). Under normal meta­ bolic conditions the R.Q. (the ratio of CO2 produced to oxygen used) is about 0'8, so that without any change of pH about 1- of thc CO2 produced is neutralised by the potassium libcratcd during the rcduction of oxyhaemoglobin. The remainder, together with the" fixcd" acids (i.e. those other than carbonic acid, which dccomposes to water and CO2) of metabolism and those present in the absorbed foodstuffs (uric acid, phosphoric, sulphuric, etc.) is left to the remaining mechanism­ the buffering power of the blood. In blood there are three buffer -systems present in sufficiently high concentration to be quantitatively important, and of these sodium bicarbonate is one. In addition, phosphates are present, and act as buffers, since phosphoric acid is weak and slightly dissociated. Not only is this so, howevcr, but phosphoric acid contains three hydrogen atoms in its molecule, and can therefore give rise to three different sodium salts which dissociate in different ways. Two of these salts are of practical importance, sodium dihydrogen phos­ phat<r-NaH2PO,-which ionises to Na + and H 2PO,-, and disodium hydrogen phosphate-Na2HPO,-which ionises to 2Na +and HPO, =. Solutions of the former arc slightly acid, since H 2PO, - tends to ionise further to lIPO, = or PO ,= and H +; while solutions of the latter are slightly alkaline, since the tendency of the HPO,= to dissociate further is overcome by its tendency, in the presence of many sodium ions, to combine with hydrogeh ions to form H 2PO, - . The 60 CLlNICAL CHEMISTRY IN l'RACTICAL MEDICINE hydrogen ions for this action are obtained from the dissociation of water, and necessarily, of course, leave an excess of hydroxyl ions. Evidently addition of acid to a solution of disodium hydrogen phosphate does not cause a very great fall in pH, since the dihydrogen phosphate is formed: HCI~H++CI­ Na2HPO 4 ~2N a + + HPO 4 = HCI+Na2HP04~2Na++CI +H2P04 Conversely, addition of alkali to a solution of sodium dihydrogen phosphate produces a relatively small increase in pH on account of the formation of the only slightly alkaline disodium salt : NaOH~Na++OH­ NaH2P04 ~Na + +H2P04- NaOH+NaH2P04 ~2Na + +HP04 - +HsO· Although the actual amount of phosphate in blood is small, and its direct buffering action is correspond­ ingly small, phosphates are of considerable importance in the excretion of acid-the second line of defence which we have to consider shortly. The third buffer substance is protein. The blood proteins include albumin and globulin (mainly in the plasma) and, of course, haemoglobin, which ca:Q. share in neutrality regulation as a general buffer as well as by virtue of the special property previously dis­ cussed. Protein is built up from amino-acids, which are acids since they contain the carboxyl group, -COOH, and can give rise to hydrogen ions. They also contain the basic amino group, - NH2, which can combine with water (just as ammonia does to form TIlE MECHANISM OF NEUTRALITY REGULATION 61 ammonium hydroxide) and then give rise to hydroxyl ions. They are, in fact, like water-amphoteric sub­ stances. In the protein some of these carboxyl and amino groups remain free, although many are con­ cerned in the linkages between the amino-acids, and so proteins themselves are amphoteric. Whether they behave as bases or as acids depends on circumstances, but at the pH of blood they are functioning as weak acids, and exist partly as salts (of sodium in the plasma, of potassium in the red cells). It has been estimated that, per litre of blood, the various buffers are able to neutralise the following amount of strong acid under physiological conditions: c.c. of N. acid neutralised Bicarbonate Haeml'globin . St'rwn protein . . . . Other buffers (mainly phosphate) per litre of blood 18·0 8·0 1·7 ·3 TOTAL 28·0 These various buffer systems are, of course, inter­ dependent and, except for haemoglobin, are not con­ fined to the blood, but are present in tissues also (where, however, the protein salts are probably the most important). Since, so far as the blood is con­ cerned, the buffers are partly in the plasma and partly in the cells, the diffusion of acid from tissues into the plasma will, in practice, set in motion a some­ what complex series of transferences of ions from the pla8ma to the cells and vice versa. Nevertheless, the brief outline of the neutrality regulating mechanisms given above, lacking though it is in detail, is suf­ ficient for a gener~l understanding of the processes and of the ways in w~ich they may be overtaxed under abnormal conditions. 64 CLINICAl. CHEMISTRY IN PRACTICAL MEDICINE pH on the addition of acid that the term acidosis is applicd, and the term therefore does not necessarily imply an actual demonstrnble changc in pH. In acidosis the pH of the blood and tissues may be still within the normal range, and actually is so unless the acidosis be severe. Either through abnormal retcn­ tion of acid, whereby the buffcrs have bcen partly used up, or through actual loss of buffer, as in the attempt to excrete the excess acid, the amount of buffer remaining available is decreased. The contrary condition, in which abnormally large amounts of base are available for the neutralisation of acid, is called alkalosis. Here again there is not necessarily any easily measurable increase in the pH-that is, any abnormal alkalinity of the blood, though such a change occurs more readily than in acidosis, since the blood buffers are more efficient against a decrease than an increase of pH. Decrease in the amount of available buffer sub­ stances (the alkali reserve) does theoretically, of course, bring about some decrease in the blood pH, but the accurate measurement involves the use of expensive and not always accessible apparatus, together with great care to avoid loss of CO2 during the obtaining of the blood sample. Direct measurement of the blood pH is not, therefore, a method of grcat use in routine examinations for acidosis or alkalosis. It has been stated that the kidney can, and in health does, attempt to compensate for an alkalosis or acidosis by altering the pH of its secretion and by varying its production of ammonia. The amount of adaptation which can be obtained by these methods alone is, however, limited. In order to excrete ex­ cessive amounts of acid, the kidney cnn use still another THE MECHANISM OF NEUTRALITY REGULATION 65 method-it can increase the total volume of urine. This involves either an increase in the water intake or an excessive withdrawal of water from the tissues. Since the clinical conditions resulting in acidosis fre­ quently involve reduction of fluid intake or excessive fluid loss in other ways, it follows that dehydration is a frequent concomitant of disturbances of the neutrality regulation. Indeed, an adequate supply of fluid to make good the dehydration and to allow the kidney to do its share in adjusting the alkali reserve of the body is often an important factor (and one which is not seldom overlooked) in the treatment of such conditions. Since, further, an increased water excretion involves an increased output of sodium chloride, the body may be depleted of salt in either acidosis or alkalosis, and its administration also is necessary as part of a rational treatment. MEASUREMENT OF THE ALKALI RESERVE The defence of the body against acid thus lies essentially in a supply of available base, chiefly sodium and potassium, with which the acid may be neu­ tralised. The available base -is that part of the total base which corresponds to the weak acidic radicles, the bicarbonate, phosphate, ~nd proteinate, which can combine with hydrogen ions to form molecules which are only slightly dissociated. Hence a measure of the available base, the so-c~lled alkali reserve, arIords a measurc of the power of the body to resist acid. In order to estimate the alkali reserve Haldane intro­ duced the method of determining the percentage of carbon dioxide in the alveolar air, a method based on the princip~e that in acidosis the power of the blood to E 66 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE carry carbon dioxide is reduced, ,vith a proportional reduction in the carbon-diqxide content of the alveolar air. This method, however, is by no means easy to carry out and is open to many objcctions, both theo­ retical and practical. Other indirect methods of calculating the blood pH (and so gauging its buffering power) indirectly by determining the ratio of free carbonic acid to combined bicarbonate ions have bcen evolved, but involvc the use of arterial blood and arc too difficult and laborious for routine use. Actually, since very considerable variations in buffering power from the normal mean are found in clinical conditions of acidosis and alkalosis, it is usually sufficient to determine the CO2 content of a sample of venous blood, drawn with precautions to prevent loss of CO2, or even more simply, that of venous blood drawn and re-equilibrated with alveolar air. The latter method is founded on the fact that blood ex­ posed to an atmosphere containing a definite amount of carbon dioxide absorbs that gas in proportion to its supply of available base. Thus this method, due to Van Slyke, measures the ,carbon-dioxide combining power of the blood, and not directly its alkali reserve. With certain reservations to be discussed later, how­ ever, the terms are interchangeable. Although in experimental work, where very small alterations in the carbon-dioxide combining power of the blood may be of importance, a number of pre­ cautions in obtaining the sample must be observed, the requirements of clinical work are satisfied by the use of a fresh sample of oxalated venous blood ob­ tained in the ordinary way. A thin layer of blood is exposed at room temperature in a suitable flask to an atmosphere containing a 5·5 per cent. concentration of THE MECHANIS?I OF NEUTltALI'l'Y REGULATION GO with considerable quantities of acetone in the urine, but with no diminution in the carbon-dioxide com­ bining power of the blood. None the less such an appearance of ketone bodies in the urine must be treated as a danger-signal. Their continued pro­ duction will in time lead to the development of an acidosis, and treatment should be directed, just as in the presence of actual acidosis, towards their oxidative removal. The administration of alkalis in these cir­ cumstances can at best be only the most temporary of expedients, since it does not greatly aid the elimination of the acid substances and does not at all prevent their continued production. A determination of the carbon­ dioxide combining power, therefore, distinguishes between cases of ketonuria, since those cases in imme­ diate danger of coma show a carbon-dioxide combining power below 53 vols. per cent., whereas less urgent cases do not show any s~gnificant variation from normal. Those cases which already exhibit symptoms of coma have a very considerable reduction in the carbon-dioxide combining power of the blood, and the severity of the condition may be assessed by deter­ mining the extent of this reduction. The approximate values of the carbon-dioxide combining power to be expected in these various conditions are shown below (Tablc V). TABLE V CARBON-DIOXIDE COMBINING POWER IN DIABETES MELLITUS Ketonuria, but no immediate danger of coma. • Ketonuria, with immediate danger of coma, but few or no symptoms • . . . . . . Ketonuria, with slight but de~ite symptoms of coma Ketonuria, cema, but fair prognosis . Ketonuria, coma, bad prognosis Vola. CO. per cent. 50 or over 40-50 35-40 20-35 below 20 70 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE The carbon-dioxide combining power in hypo­ glycaemic as opposed to hyperglycaemic coma is of course normal. The excessive production of acids, which brings about the state of diabetic coma, may also contributc to the production of acidosis in other pathological conditions. In starvation, where, the availablc gly­ cogen supplies having been used up, carbohydrate metabolism is reduccd to a minimum, thc complete oxidation of fat is prevented. Hence the ketone acids are produced just as in diabetes, though to a .less extent. This factor, combined with the loss of ayail­ able base in the urine-a loss which is not being made good by the ingestion of salts-may cause such a lowering of the alkali reserve as to render it difficult for the patient to resist the extra strain thrown on the acid-base regulating mechanism by the acidosis of anaesthesia. The practice, therefore, which was once common, of starving the patient prior to opera­ tion is to be deprecated, and, where any cause pre­ disposing to acidosis is suspected, a determination of the carbon-dioxidc combining power should be made, in order that, if necessary, pre-operative treatment may be given to raise the alkali reserve to a safe level. (b) The Failure to eliminate Acid.-We have already seen that the kidney plays a considerable part in the excretion of acids, either as such, or togethcr with basic radicles such as sodium, potassium, and ammonium. When this function of the kidney fails, as it may do in cases of acute or subacute nephritis, or in the terminal stages of chronic ncphritis, there is a retention of aeid in the blood, and therefore a diminution of thc available base and a lowering of the carbon-dioxide combining power. THE MECHANISM OF NEUTRAI.ITY REGULATIO~ 71 This retention of acid, although it may at times be considerable, is only one factor, and perhaps not the most important, in the production of an acidosis due to impaired renal function. Probably the most im­ portant factor is the inability of the damaged kidney to manufacture ammonia and so spare the available bases of the body fluids. This failure of ammonia production is particularly marked in acute nephritis and in the terminal stages of chronic interstitial nephritis; it does not occur to any appreciable extent in nephrosis, a condition which is (probably in consequence) not characterised by lowering of the alkali reserve to any great extent. Most cases of chronic interstitial nephritis show a carbon-dioxide combining power within the normal range, and it is only in the terminal stages of the disease, with c6>mmencing uraemic manifestations, that it is lowered much below 53 vols. per cent. The onset of chronic uraemia, however, is frequently accompanied by symptoms which are so varied, and which so often simulate those of many other con­ ditions, that it may be difficult to decide whether they arc due to uraemia or to other possible causes. In these circumstances the discovery of a lowered carbon-dioxide combining power may be of some significance. When uraemic coma supervcnes the carbon-dioxide combining power of the blood is always markedly lowered. This m~y become a valuable diagnostic sign, for, even with coma present, the diagnosis of uraemia may not be perfectly obvious in those cases where the patient has not previously been under observation. . For instance, some cases of cerebral haemorrhage, 'with albuminuria, may present many features in common with uraemic coma, but show 7,j. CLINICAL CHEMISTRY IN PRACTICAL MEDICINE sodium bicarbonate together with water to replace that lost in the motions will often abolish these symptoms. A determination of the carbon-dioxide combining power of the blood in these cases throws light on the condition and suggests the necessary treatment. A combining power of less than 30 vols. per cent. is a very grave prognostic sign. Allwlosis Just as a state Qf acidosis may bc said to exist when the carbon-dioxide combining power of the blood falls below 53 vols. per cent., so an alkalosis exists when it is raised above 77 vols. per cent. Such an alkalosis may be due to excessive intake of alkali or to an excessive loss of acid. (a) Excessive Intake of Alkali.-Though the car­ bon-dioxide combining power of the blood may un­ doubtedly be raised by the administration of alkalis such as sodium bicarbonate, it is usually raised only within normal limits, and a true alkalosis ra1;ely occurs clinically from this cause. Occasionally, how­ ever, this does occur in patients undergoing the intensive alkali treatment for gastric or duodenal ulcer, especially when there is some degree of pyloric stenosis with vomiting and loss of gastric juice. Should symptoms of prostration, shallow breathing, headache, tetany, and vomiting supervene during the course of such treatment it is well to determine the carbon-dioxide combining power of the blood, and if this gives a reading over 77 vols. per cent. a reduc­ tion of the alkali intake should be advised. (b) Excessive Elimination of Acid.-Excessive elimi­ nation of carbon dioxide, which, as we have seen, behaves as a weak acid, may cause an increase in the THE lIECHANISM OF NEUTRALITY REGULATION 75 alkali reserve of the blood. Such a washing out of carbon dioxide may be produced clinically by the hyperpnoea seen in light ether anaesthesia, or in surgical shock, when the loss of carbon dioxide may be so great that the patient may stop breathing. Treatment of the condition by prompt inhalation of carbon dioxide under artificial respiration is so effi­ cacious that it may be said that no surgical theatre ought to be without its cylinder of carbon dioxide for emergency purposes. A similar alkalosis, due to washing out of carbon dioxide, is seen in mountain­ sickness, carbon-monoxide poisoning, and in some cases of hysteria associated with hyperpnoea. It must be remembered that gaseous alkalosis, like gaseous acidosis, cannot be detected by determination of the CO2 combining power, though it may be shown by the much more complicated procedure of determining the free carbonic acid of thc plasma. The most important clinical manifestation of alka­ losis, however, occurs in cases of excessive vomiting such as may be produced by a high intestinal obstruc­ tion or by pyloric stenosis. An examination of the carbon-dioxide combining power of the blood in such a case shows it to be greatly increased. Readings as high as 120 vols. per cent. have been obtained in cases of high obstruction just before death, and there is little doubt that this alkalosis, with the simultaneous dehydration, is the actual cause of death in such cases. The alkalosis is readily understandable when we re­ member that owing to the loss of acid stomach con­ tents in the vomited matter the concentr_ation of chloride ions, which is normally the chief acid factor in the plasma, may be reduced to about a third of its usual value. In consequence there is a considerable 76 CLINICAL CIIEMISTItY IN PItACTICAL l\IEDICINE increase in the concentration of bicarbonate ions in replacement of the loss of chloride ions, bringing about an extensive increase in the alkali reserve and carbon-dioxide combining power of the blood. By the simple expedient of supplying salt solution abundantly in such cases the bicarbonate concentra­ tion may be temporarily corrected (and the fluid loss made good) and the patient brought into a sufficiently satisfactory condition to stand an operation which might otherwise have proved fatal. A determination of the carbon-dioxide combining power is thus of service in verifying the presence and extent of such an alkalosis, and in those cases where thc carbon­ dioxide combining power is found to be significantly raised, operation should be deferred until the ex­ cessive alkalosis has been corrected by the supply of salts and water. Co-existence of Alkalosis and Ketosis Since the production of ketone bodies is the causative factor in the acidosis of diabetes and starvation, their frequent presence in the urine and occasional presence in the breath in cases of severe vomiting quite naturally lead many people to suppose that an acidosis must be present. Cases of cyclical vomiting in children, of severe gastro-enteritis, of hyperemesis gravidarum, and of post-operative vomiting frequently show such signs of acidosis. It is, therefore, something of a surprise to find that instead of a reduction there may sometimes be an increase in the carbon-dioxide com­ bining power of the blood of such patients. There seems to be no doubt that there are two processes at work in these cases: a tendency to ketosis due to starvation, to an anaesthetic or to bacterial infection, GLYCOSURIA 79 with the smell of acetone and his urine loaded with sugar, further investigation is unnecessary to establish the fact that he is suffering from diabetes mellitus. There remain, however, a large group of cases where symptoms are absent or equivocal-mostly cases of functional glycosuria, but a few with mild diabetes­ where an accurate diagnosis is impossible without bio­ chemical methods. INVESTIGATION OF A CASE OF GLYCOSURIA BY TilE CARBOHYDRATE TOLERANCE TEST The detection of sugar in the urine by anyone of the tests available should, as a general rule, be followed immediately by an examination of the blood. In symptomless glycosuria, this is of paramount im­ portance in order to determine whether the patient is, or is not, suffering from diabetes. Further examina­ tion of urinary specimens while fasting and at varying times after a meal, which is recommended by some writers, is simply a waste of time. In some cases of mild diabetes, for instance, sugar is by no means invariably present in the urine, while certain cases of purely functional glycosuria may excrete sugar in every specimen of urine, not excluding the fasting specimen. The slight saving of trouble in avoiding the blood analysis is not worth the risk of missing a case of mild diabetes which, neglected, becomes more severe, but which, caught in time, is easily treated. On the other hand, it is well worth the very temporary inconvenience of blood examination to save a case of functional glycosuria from being regarded as one of diabetes with all the trouble and distress that such a diagnosis involves. . Even where a condition of true 80 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE diabetes is obviolls clinically, an examination of the blood is useful, not necessarily from the point of view of diagnosis, but rather as an aid to gauging the severity of the case and as a guide to the type of treatment to be adopted. Amongst the circumstances in which the finding of glycosuria does not demand blood-sugar estimations as the next step are those in which the urine sugar may be, not glucose, but lactose-that is to say, in pregnancy or lactation. Excessive production of lac­ tose during these periods may cause the appearance of that substance in the blood ann. so produce lactosuria, for which the kidney threshold is very low. Lactose responds to the usual qualitative tests in -a manner similar to glucose, and therefore the hvo are readily confused. In theory, the best way of distinguishing them is by their reaction with phenyl hydrazine, whereby both form osazones which crystallise in ·rather different forms and can be distinguished by microscopical examination. In practice, however, it is difficult to prepare pure crystalline osazones from urine, and the difficulty is intensified when, as not uncommonly happens, the two sugars are found together. The presence of lactose can be shown by submitting the urine to the mucic acid test, but it is important to know, not merely whether lactose is present, but if glucose is also present. If the latter sugar is being excreted a positive response to the mucic acid test does not negative the diagnosis of possible diabetes. A more valuable method of differentiation, therefore, is the quantitative method (Appendix), which not only detects and estimates lactose, but shows whether or not glucose is present as well. Glycosuria is a fairly common accompaniment of GLYCOSURIA 81 hyperactivity of the thyroid and pituitary glands, so that in the presence of well-marked signs of these conditions the presence of sugar in the urine does not as a rule call for further investigation. In an investigation of the blood sugar for the purpose of differentiating diabetes mellitus from other possible causes of glycosuria we believe it desirable, both as an ultimate saving of time to the physician and as a saving of inconvenience to the patient, to proceed at once with a carbohydrate tolerance test­ that is, the construction of a blood-sugar curve. In this way information is obtained both as to the fasting-level of the blood sugar and as to the reaction of the blood towards glucose ingestion, and both in diagnosis and subsequent treatment the latter information may prove to be the more valuable. The procedure to be adopted is as follows. The test should be begun early in the morning with the patient fasting since the preceding evening. The in­ gestion of glucose, and therefore of any carbohydrate­ containing meal, raises the blood sugar above the fairly constant fasting-level, and though normally this increase disappears in the course of about two hours it sometimes lasts much longer, and a fast of twelve hours at least is necessary before one can be sure that the blood sugar has returned to the fasting­ levf'l. A sample of blood is withdrawn for determina­ tion of the fasting sugar content, and the bladder is emptied. Fifty grammes of glucose are then given in concentrated .solution, flavoured, if desired, with lemon juice. Thereafter samples of blood and urine are obtained at intervals of thirty minutes for two hours. The urine samples are tested qualitatively for sugar, and the sugar content of the blood is deter- 84 CLINICAL CHEMISTRY IN PRACTICAL MEDICINE that the blood sugar passes the normal threshold when a healthy person ingests 50 g. of glucose, so that if the sample of urine obtained at the end of the test contains more than a mere trace of glucose we are inclined to regard it as evidence of some lowering of the renal threshold, though this, if it is thc sale abnormality, is of little or no importance. The normal blood sugar curve varies somewhat according to the age of the patient. Young adults, and particularly children, show a greater sugar toler­ ance than old people. In consequence the normal blood sugar curve in youth is flatter and returns more rapidly to the fasting-level than in old age. Abnormal Responses The appearance of sugar in the urine may obviously be due either to the raising of the blood sugar above the threshold or to a lowering of the threshold--or, of course, to a combination of these causes. If even the fasting, blood sugar is above the threshold, sugar will be present in all specimens of urine; otherwise it will be present only after the ingestion of sufficient carbohydrate to raise the blood sugar above the critical point. Similarly the lowering of the threshold may be slight, with consequent intermittent glyco­ suria, or it may be so great as to cause continuous excretion of sugar. It is this dual mechanism which renders it so difficult to tell from urine examination alone whether or not a glycosuria is an indication of diabetes mellitus. Diabetes Mellitus.-In diabetes the glycosuria is due primarily to a raising of the blood sugar, though in some cases, particularly those of long duration, there may be either lowering or raising' of the renal GLYCOSURIA 85 threshold, a fact of which some account must be taken later. The raising of the blood sugar even during fasting, when the supply of insulin is deficient, is due to a combination of causes. The diabetic apparently cannot oxidise glucose as can the normal person, nor can he use it for synthesis of glycogen. Hence the two means by which glucose is ordinarily removed from the blood have failed to an extent which is seldom if ever complete, but is proportional to the severity of the diabetes. Besides failing to remove glucose from the blood, the diabetic organism, again to an extent depending on the severity of the condition, manufactures glucose from protein (and possibly fat) in a vain effort to supply the carbo­ hydrate for which his tissues are starving, but which they cannot utilise. There is thus a tendency for sugar to accumulate in the blood even when no carbohydrate has been ingested for some time, and this accumulation may be great enough to raise the fasting blood sugar above the kidney threshold, in which case sugar will be found in all urine passed. Consideration of the factors involved makes it obvious that the height to which the fasting blood sugar is raised above the normal indicates to some extent the severity of the diabetes in an untreated case. It is not unusual to obtain somewhat high values for the fasting blood sugar in non-diabetic persons, especially if they are of a nervous disposition or fear the slight pain of veQ.epuncture. In these cases par­ ticularly the blood-sugar curve, which, of course, is normal, is valuable in leading to the correct diagnosis. Even when the increase in the fasting blood sugar is gross the curve is useful in affording further in­ formation for gauging the severity of the diabetes, and, 86 CLINICAL CllEMIS'rRY IN PRAC'rICAL MEDICINE an aid in future treatment, in determining the renal threshold if this is higher than the fasting blood level. 55'0r-__ r--__ ..---_r--_ _, 1250 ~ ~----t=====~-----r----~ ~ o ~200r__+--~----+_--~~~~~ l ... d 0- ::I ~150~~~~~~~~~~~~~ 1!: 50~ __ ~~ __ ~~ __ ~~ __ ~ o Y.a 1 lY.e 2 Hours FIG. 5.-The blood sugar, in diabetes, after ingestion of 50 g. of glucose. In diabetes, as in health, ingestion of glucose causes an increase in the blood sugar. The very factors GLYCOSUnIA 89 slowly in the tissues, and the effect of a single injection is much more prolonged. Under these circumstances the blood sugar is kept much more constant, as is shown in Fig. 7. Thyroid and Pituitary Dysmnction.-In over- ZOO~~~~~--~--_,--~--~--~~ "tI o ~160~~~~~~+-+---~~~--+--F+-1 ~ ~ g1401~~--~--~~~-4.~~~~~~+-i ... ,.. ~120HL+---~--~--++-4~~--~~+---;-~ u en ". ~10~~---+---+---&~-+---+---+---+-; i!: &O~ __ ~---L~~--~--~~~~~ 10 12 + 8 12 4 8 12 + 6 -J~--a.m .--J'--__ p. m--....J ''--a m.- FIG. 7.-The dotted line shows the cours~ of the blood sugar in B patient receiving two doses (each of 20 units) of insulin per day. The insulin was given at the points marked I, and meals were taken at the times M. On this particular day there was a hypoglycaemic reaction at A. The continuous line shows the blood sugar in. the sa.me patient ro­ ceiving, for the sixth successive day, 50 units of zinc-protamine insulinate once per 24 hours (I). Meals Qre marked as before. activity of the thyroid and pituitary glands, both conditions in which glycosuria may occur, the blood picture may simulate. that of mild diabetes. The fasting blood sugar is raised to some extent above the normal, and the response to glucose ingestion re­ sembles that of diabetes' in showing a rather greater DO CLINICAL ClIEl\IlS'£RY IN PRAC'£lCAL MEDICINE increase in the blood sugar than the normal, and a slow fall of the blood sugar from the maximal concen­ tration, instead of the normal rapid return to the fasting-level. Usually, however, the maximum con­ centration of sugar in the blood is reached in very little more than the normal time, but in practice it is not casy to use this as a means of differentiating uncom­ plicated exophthalmic goitre or acromegaly from the same conditions complicated by the presence of mild diabetes. Those cases of diabetes which give fasting blood-sugar values of the order sometimes found in disturbance of thyroid or pituitary function (100 to 130 mg. per 100 c.c. of blood) are so mild that their blood -sugar curves do not show departures from the normal sufficiently marked for very fine distinctions to he drawn. In such cases the physician is thrown largely on his own clinical resources. Although they are not accompanied by glycosuria, the opposite pathological condition of the thyroid and pituitary glands should perhaps be mentioned here for the sake of completeness. In myxoedema and hypopituitarism the fasting blood sugar may be abnor­ mally low. There is also an increased tolerance of carbohydrate, which disappears from the blood almost as rapidly as it is absorbed and is followed by little or no increase in the blood sugar. The curve obtained from a sugar tolerance test in these conditions is thus much flatter than is normally the case, and in severe myx­ oedema it may approximate to a straight line parllllel with the base-line. Typical curves from cases of thyroid and pituitary disturbances are given in Fig. 8. Liver Deftciency.-It is held by some writers on the subject that the conversion of glucose to glycogen docs not commence immediately the blood sugar GLYCOSURIA 91 begins to rise after carbohydrate ingestion, but that a certain " head " of glucose is necessary before the reaction can proceed, at any rate with more than minimal velocity. This is the explanation given of the fact that in the normal person the blood sugar reaches its maximum so early as three-quarters of an hour after ingestion of glucose, when absorption cannot be completed. It is supposed that at this "'d o o ...- ~ l00r-----~----~------~----~ a 150 i--+-+---+---""Idr----I «::) ~ i a 100~--~~~~~~-+----~ en J; ~ 50~--~.-~--~----~----~ o :y", .1 1~ 2 Hours FIG. 8.-I3lood·sugar curvos in cases of hyper- and hypo-thyroidism. In pituitary dysfunction the curves are very similar. time, when the blood-sugar concentration has reached about 140 mg. per 100 c.c., sufficient" head" of sugar is present to stimulate glycogen synthesis, and that this synthesis accordingly becomes the dominating factor, taking place at a much greater rate than absorption, so that the blood sugar.begins to fall again. On this supposition it is easy to understand that a deficiency in the glycogen-storing mechanism, which is seated principally in the liver, may well have the effect of requiring an abnormally great" head" of glucose to be present before glycogen synthesis can pro(,ecd.