Plasma Tissues and platelets
Prothrombin Ca salts Thrombokinase
Fibrinogen Thrombin
Fibrin
The clot consists of fibrin in which blood corpuscles are entangled. It is clear that if any one of the reacting agents can be removed or rendered inert the clotting cannot take place. There is only one inorganic substance taking part in the reaction, and it is this factor that is more easily removed than any of the others. Calcium is precipitated in an insoluble form by various chemical reagents, but it is obvious that for purposes of transfusion the formation of an insoluble precipitate is not permissible. It is therefore necessary to use a substance which will form a soluble compound with the calcium and which is at the same time harmless when introduced into the circulation. The only substance which has been found at present to possess both these properties is citrate of sodium. This forms with calcium a soluble double salt, in which calcium is rendered inert. It is usually held that the calcium to be active must be present in the ionized form, but recent investigations by Vines into the rôle of calcium tend to modify slightly the accepted view of its action. He has shown that calcium is present in the blood in two forms, ionized and combined, and that both take part in the coagulation reaction. He has, in addition, demonstrated that a quantity of anticoagulant sufficient to combine with the whole of the calcium present in a given quantity of blood is not enough to prevent coagulation. It seems, therefore, that the anticoagulant acts by combining with a large organic molecule of which calcium is only one constituent, and not merely by combining with ionized calcium. The organic complex with which the calcium is associated possibly corresponds to the thrombokinase of the theory.
About the time that the use of the citrated blood was introduced by Lewisohn, some investigations upon animals were carried out by Salant and Wise in order to determine how sodium citrate was dealt with and eliminated by the body. These observers found that it very quickly disappeared from the circulation, nearly 90 per cent. of the salt having been got rid of within ten minutes of its intravenous injection. Part of the citrate is destroyed by oxidation, and the rest, 30 to 40 per cent., is eliminated by the kidneys, the urine being rendered alkaline. It was also shown that if a very large dose was given, so large that toxic symptoms resulted, the effect was rapidly obtained; but that if the toxic dose were not fatal, no remote effects followed. Its injection never resulted in any albuminuria.
Lewisohn showed by experiment on the human subject that up to 5 grammes of sodium citrate in the form of a 0·2 per cent. solution could be injected intravenously without any harmful results. It was also shown that this concentration of the salt was sufficient to prevent clotting outside the body, and that the microscopic appearance of the blood cells was not altered by the admixture of this solution.
Theoretically, therefore, the amount of citrate that should be used as an anticoagulant should be 2 grammes for 1,000 cc. of blood, or 100 cc. of 2 per cent. solution for 900 cc. of blood. In practice it is better to err on the side of safety and to use a slight excess of citrate. This amount of citrate should be used for the 750 cc. of blood which constitutes the ordinary maximum amount of blood used in a transfusion. For smaller quantities of blood the amount of citrate may be correspondingly reduced.
The use of citrated blood was introduced to the British Army in France in 1917 by Oswald Robertson, who recommended the use of a larger amount of citrate than this. His object in increasing the amount was to produce a solution which, when diluted with the correct amount of blood, would be isotonic with it. It was thought that a hypotonic solution might result in some damage to the red corpuscles by osmosis, and Robertson therefore recommended the use of 160 cc. of a 3·8 per cent. solution of citrate, which, when mixed with 750 cc. of blood, will give a solution of which the osmotic pressure equals that of 0·9 per cent. saline solution. It may be doubted, however, whether this consideration is of more than theoretical importance. There can be little doubt that in practice the effect of a slightly hypotonic solution, such as is given by the 100 cc. of 2 per cent. solution of citrate, is negligible as regards destruction of corpuscles. If, however, it be thought necessary, an isotonic solution may be produced by the addition of sodium chloride. Other considerations, as will be seen shortly, weigh in favour of giving the smaller amount of citrate. The dosage to be recommended, therefore, on practical and experimental grounds is 2 grammes of citrate in 100 cc. of water for 900 cc. of blood, or 1 gramme of citrate in 50 cc. of water for 450 cc. of blood or less. These proportions need not be observed very accurately. Latitude may be used in either direction without harming either the transfused blood or the patient.
It has been stated above that sodium citrate introduced into the circulation in small quantities, such as are sufficient for anticoagulant purposes, is non-toxic to man. In the light, however, of the extended experience of the last four years, it is seen to be possible that this statement may not be quite literally true. Probably there is an individual variation in the tolerance of different people to sodium citrate. Certainly in some cases a reaction follows the injection of citrated blood. The symptoms of this reaction are a slight headache, a rise in temperature to two or three degrees above normal, sometimes accompanied by a rigor or a sensation of chill, and an increase in the pulse rate. The effect is, however, always very transitory, lasting only two or three hours, and is never, in my own experience, attended by any symptoms which need give rise to anxiety for the patient’s welfare; nor does it in any way prejudice the therapeutic results of the transfusion.
That the reaction is caused by the citrate and not by another constituent of the transfused blood has been believed by several observers. In a case seen by the writer a slight citrate reaction occurred in a youth who acted as blood donor. The transfusion was carried out by a modification of the syringe method, which involved the injection at intervals of a syringeful of citrate solution into the donor’s circulation. The possibility that the reaction was produced by another factor was therefore not present in this instance.
Nevertheless, it must be admitted that citrate has not yet been absolutely proved to be the cause of this slight reaction in all the cases in which it occurs. Evidence has, indeed, been brought forward by Lewisohn and by Meleney to show that citrate is definitely not responsible for the reaction. The statement is made that some reaction occurs after 10 per cent. of all transfusions, and that this percentage is unaffected whether whole blood or citrated blood is used. Lewisohn has himself investigated the effects in a long series of parallel cases in which different methods were employed, and he reports that the results following the use of citrated blood were as good as with any other method. Drinker states that reactions follow the use of citrated blood slightly more often than they do that of whole blood, but this has not been confirmed. He was unable to find any impurity in the citrate that might be held responsible. It is quite possible that all the reactions observed are in reality caused by the “minor agglutinins” mentioned on p. 73. Meleney has noticed that the blood of some donors is more likely to produce a reaction than that of others; this suggests that the responsibility rests with the blood and not with the citrate. The occurrence of a toxic reaction constitutes the only real objection to the use of citrated blood that has yet been brought forward, but even this has not yet been fully substantiated; in any case, the reaction is of so little importance that it is greatly outweighed by the numerous advantages that are conferred by the use of citrate. The possibility that a citrate reaction does sometimes occur may be taken as an indication in favour of using the smaller amount recommended by Lewisohn rather than the larger dose used by Robertson. The experience of a great many observers has established the fact that citrated blood is quite as effective as whole blood in its therapeutic effects.
It is convenient to have the sodium citrate in a form ready for immediate use. I have therefore been in the habit of keeping it in the solid form in small stoppered bottles, each containing 1 gramme of the salt. These are sterilized at 130° C., and can be kept indefinitely until wanted. If 450 cc. of blood or less are to be drawn, the contents of one bottle is shaken into the transfusion flask; 50 cc. (approximately 2 oz.) of sterile warm water are added, in which the citrate will rapidly dissolve. If more than 450 cc. of blood is to be used, the contents of two bottles must be dissolved in 100 cc. or 4 ozs. of water. Alternatively a concentrated solution of citrate may be kept in sealed ampoules, but the salt is less stable in solution, and I prefer to keep it in the solid form.