Oxygen transport and oxygen consumption vs. cardiac output at different haematocrits

Abstract

Oxygen consumption was measured in 10 anaesthetized, surgically instrumented domestic pigs on cardiopulmonary bypass while cardiac output (pump flow rate) was decreased. Oxygen consumption data (calculated by the Fick principle from blood flow rate and arterial and mixed venous content measurements) were plotted against total oxygen transport (TOT=QCaO₂, where Q is pump flow rate and CaO₂ is arterial blood oxygen content). Oxygen consumption (VO₂) measurements were made in each animal at two haematocrits (approximately 30%, which is normal for pigs, and approximately 15%). In five of the animals (Group I) the measurements were made with normal haematocrit first, the blood was then haemodiluted with plasma or Dextran, and the measurements were repeated. In the remaining five animals (Group II), the haematocrit orderwas reversed. The plots showed two regions: above a certain value of TOT which we call critical TOT, VO₂ was relatively independent of TOT, while at lower values of TOT, VO₂ decreased approximately linearly with TOT. At the low haematocrit, the critical TOT (±S.E.M) was significantly lower (P <0.05) than at normal haematocrit (6.9 ± 0.9 vs. 10.7 ± 1.2 ml/min/kg). Below the critical TOT, the curves for the two haematocrit levels were not significantly different. Above the critical TOT, the average VO₂ was lower at the low haematocrit than at the normal haematocrit (6.0 ± 0.6 vs. 8.8 ± 1.1 ml/min/kg).