Estimation of plasma volume from hematocrit and plasma oncotic pressure during volume expansion in dogs

Changes in hematocrit and plasma oncotic pressure were measured continuously during and after isotonic volume expansion in splenectomized dogs in order to test the potential of these types of measurements to predict changes in plasma volume. A volume of Ringer's solution amounting to 15% of the initial blood volume was infused over 10 min. At the end of the infusion, 54% of infused fluid remained within the intravascular space and 9% of the infused volume was retained within the intravascular space at 50 min after the end of the infusion. Hematocrit and plasma oncotic pressure decreased by 2.4% and 2.4 mmHg, respectively, at the end of infusion and then variables returned to their control levels gradually. Changes in plasma volume were estimated indirectly from hematocrit and plasma oncotic pressure based on the dilution of the erythrocytes and the protein. Highly significant correlations were observed between the measured plasma volume (Y) and the plasma volume (X) calculated from hematocrit (Y = 0.95X + 0.22, r = 0.96) and the plasma oncotic pressure (Y = 0.86X + 0.46, r = 0.91). We therefore conclude that either hematocrit and plasma oncotic pressure measured continuously are reliable parameters for predicting the time course of the plasma volume change during an isotonic volume expansion of up to 15% of the initial blood volume.