Association between oxygen delivery and consumption in patients undergoing cardiac surgery. Is there supply dependence?

We studied the relationship between oxygen delivery (DO₂) and consumption (VO₂) in twenty patients undergoing cardiac surgery, in order to determine if VO₂ was dependent on DO₂ (pathological oxygen supply dependence). We measured VO₂ from expired gas analysis (VO(2G)) and compared this to that calculated using the reverse Fick method (VO(2F)). Both VO(2G) and VO(2F) increased after cardiopulmonary bypass (P < 0.001, without change in DO₂ (i.e. oxygen extraction ratio increased). There was a significant relationship between changes in DO₂ and VO(2F), both before bypass (r = 0.74, P < 0.001) and after bypass (r = 0.69, P < 0.001), while changes in DO₂ and VO(2G) had no such relationship (pre-bypass: r = 0.38, P = 0.094; post-bypass: r = 0.10, P = 0.68). There was poor agreement between VO(2F) and VO(2G) perioperatively. We could not demonstrate supply dependence in elective cardiac surgical patients.