Discrepancy between body and venous hematocrits. Dilution curves of simultaneously administered I¹³¹-HSA and Cr⁵¹-labeled erythrocytes in man

¹ Georgetown University Medical Division, District of Columbia General Hospital, Washington, D.C.

The difference between direct and indirect plasma volumes was studied in man by the analysis of detailed dilution curves following simultaneous administration of I¹³¹-HSA and Cr⁵¹-RBC-labeled erythrocytes. These results showed: 1) the discrepancy between the apparent direct (I¹³¹-HSA) and indirect (Cr⁵¹-RBC) plasma volumes became evident with the first appearance of recirculation. 2) The apparent ratio of body to anticubital vein hematocrits remained approximately the same during the remainder of the 2-hour procedure. Two hypotheses were considered as possible explanations for the rapid appearance of the greater direct plasma volume: a) a rapidly equilibrating extravascular space entirely permeable to plasma protein but not to red blood cells and b) intravascular areas of low hematocrits. The latter hypothesis was considered the more probable.