Oxygen Consumption and Cardiovascular Function in Children During Profound Intraoperative Normovolemic Hemodilution

Subcutaneous Po2 as an index of the physiological limits for hemodilution in the rat

This study was designed to test the hypothesis that changes in subcutaneous Po2 (PscO2) during progressive hemodilution will reliably predict a “critical point” at which tissue O2 consumption (V̇o2) becomes dependent on O2 delivery (Q̇o2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315–375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 ± 1 to 6 ± 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced ( P < 0.05) after exchange of 45% of the blood volume (Hct = 16 ± 1%). V̇o2 became delivery dependent when Q̇o2 fell below 21 ml·min−1·kg body wt−1 (mean Hct = 13 ± 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3−, or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of Q̇o2 during hemodilution.