Supply-dependent oxygen consumption and mixed venous oxyhemoglobin saturation during isovolemic hemodilution in pigs.

To assess the responsiveness of mixed venous oximetry under conditions of supply-independent and supply-dependent oxygen consumption, we subjected ten pigs to graded isovolemic hemodilution while measuring cardiopulmonary variables, including oxygen consumption (VO2), oxygen delivery (DO2), and mixed venous oxyhemoglobin saturation (SvO2). Supply-dependent VO2 was reached at an average hemoglobin concentration of 3.9 +/- 0.7 g.dl-1, an SvO2 of 38.2 +/- 8.1 percent, and an oxygen extraction ratio of 0.55 +/- 0.10. The change in SvO2 corresponding to a 100-ml decrease in DO2 was -9.2 +/- 3.7 percent when VO2 was supply-independent and -7.6 +/- 5.2 percent when it was supply-dependent (NS). However, the rate of decline in SvO2 in the supply-dependent range was significantly slower than the rate calculated assuming unchanged VO2 (-32.6 +/- 16.3 percent; p less than 0.01). The results indicate that appearance of supply-dependence of VO2 does not render mixed venous oximetry unresponsive to further deterioration of oxygen supply during isovolemic hemodilution.