Continuous recordings of mixed venous oxygen saturation during weaning from mechanical ventilation and the ramifications thereof

To define the importance of hemodynamic performance and global tissue oxygenation in determining weaning outcome, we recorded mixed venous oxygen saturation (S (V) over bar(O2)) continuously in eight ventilator-supported patients who failed a trial of spontaneous breathing and 11 patients who tolerated a trial and were successfully extubated. Immediately before the weaning trial, S (V) over bar(O2), was not statistically different in the two groups (p = 0.28). On discontinuation of the ventilator, S (V) over bar(O2), fell progressively in the failure group (p < 0.01), whereas it did not change in the success group. During the trial of spontaneous breathing, O-2 demand was similar in the two groups, but it differed in the manner with which it was met. The success group demonstrated an increase in cardiac index (p < 0.05) and O-2 transport (p < 0.02). The failure group did not increase O-2 transport, partly because of elevations in right- and left-ventricular afterload, but, instead, increased O-2 extraction ratio (p < 0.02) with a consequent fall in S (V) over bar(O2). In turn, the low S (V) over bar V-O2 combined with greater venous admixture (p < 0.0006) led to rapid arterial desaturation (p < 0.006) and a relative decrease in O-2 being supplied to the tissues. In conclusion, ventilator-supported patients who failed a trial of spontaneous breathing developed a progressive decrease in S (V) over bar(O2), caused by the combination of a relative decrease in convective O-2 transport and an increase in O-2 extraction by the tissues.