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 (VO2G) and compared this to that calculated using the reverse Fick method (VO2F). Both VO2G and VO2F increased after cardiopulmonary bypass (P < 0.001), without change in DO2 (i.e. oxygen extraction ration increased). There was a significant relationship between changes in DO2 and VO2F, both before bypass (r = 0.74, P < 0.001) and after bypass (r = 0.69, P < 0.001), while changes in DO2 and VO2G 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 VO2F and VO2G perioperatively. We could not demonstrate supply dependence in elective cardiac surgical patients.