Objective: To test the following two hypotheses during weaning failure: (a) mixed venous oxygen saturation (SvO2) does not decrease in patients whose oxygen consumption does not increase, and (b) blood lactate may increase in patients who demonstrate substantial decreases in SvO2.
Design and setting: A prospective observational and physiological study in a 30-bed university intensive care unit.
Patients and participants: 18 patients who failed weaning and 12 patients who succeeded weaning (controls).
Measurements and results: Hemodynamics, global tissue oxygenation, cardiovascular response (cardiac index/oxygen extraction diagram), and blood lactate were measured in ventilator-supported patients undergoing a spontaneous breathing trial. In patients who failed without having increased their oxygen consumption (n=9) the increase in oxygen delivery was accompanied by a decrease in oxygen extraction (by 15+/-4%). In patients who failed (n=9) having increased their oxygen consumption (by>10%) this increase was met mainly by an increase in oxygen extraction (by 30+/-7%). SvO2 increased by 2+/-1% in the former patients, whereas it decreased by 20+/-5% in the latter. Arterial lactate increased (range 2.3-3.1 mM/l) in only three patients who failed to have increased oxygen consumption and exhibited heart failure and the highest decreases in SvO2 (by 12-39%).
Conclusions: Patients whose SvO2 does not decrease during weaning failure do not have increased oxygen consumption probably due to respiratory center depression in some of them. Patients whose SvO2 decreases have increased oxygen consumption.