Evaluation of blood volume measurement techniques.

OUR STUDIES on the seeming loss of plasma from the circulation after transfusion and the mobilization of plasma often seen after hemorrhage or drug infusions have been handicapped by a large variation in measured responses between animals. Conversations with other workers on blood volume regulation have indicated that such variability is not uncommon. The necessity of using large groups of animals to test the effect of any experimental change makes for tedious research. It also raises the question as to the quantitative accuracy of the techniques used for measuring blood volume. We have, therefore, made several consecutive injections of T 1824 and Cr-tagged red cells in the same dogs to measure the ability of the two techniques to reproduce the same volume. This was then followed by a study of the ability of the two to follow known reductions in blood volume produced by stepwise hemorrhage. We have shown that after an injection of tagged cells into dogs under morphine-sodium pentobarbital anesthesia, the blood activity level falls to a fairly stable value in about 10 minutes in splenectomized animals and in 20 to 30 minutes in intact dogs. Variations of individual samples, when corrected for any hematocrit change, about the average plateau value have a standard deviation of 3 per cent. Since the hematocrit technique shows a standard deviation of 2 per cent, errors attributable to an uneven distribution of tagged cells in the circulation would appear to be small. Injected T 1824 does not, of course, reach a stable concentration after mixing is completed, but continues to disappear through the capillary membranes at what has been claimed to be an exponential rate.' If the