Cr⁵¹-labeled red cell, I¹³¹-fibrinogen, and T-1824 dilution spaces

¹ Department of Physiology, Medical College of Georgia, Augusta, Georgia

The differences in dilution spaces of Cr⁵¹-labeled red cells, I¹³¹-labeled dog fibrinogen, and T-1824 were studied. All three indicators were injected simultaneously into anesthetized, acutely splenectomized dogs. Arterial samples were taken at 10, 20, 30, 50, and 70 min postinjection. Another injection of the indicators was made, samples were taken, and a third injection was made. Cr⁵¹ and I¹³¹ were separated in the samples by determining the counts per minute on the day of the experiment and again 10 days later, then solving equations utilizing the concepts of differential decay. The Cr⁵¹-labeled red cell space averaged 63.9 ml/kg and was the most reliably reproduced. The average BV_fibrinogen (67.9 ml/kg) was significantly less than the average BV_T-1824 (73.8 ml/kg). BV_fibrinogen is more reliably reproduced than BV_T-1824 but less so than BV_cells. The T-1824 space apparently includes more of the extravascular space than does the fibrinogen space and indicates that the volume of "extra plasma" is considerably less than previously believed.