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 oxyge nation in determining weaning outcome, we recorded mixed venous oxygen satu ration (S (V) over bar(O2)) continuously in eight ventilator-supported pati ents who failed a trial of spontaneous breathing and 11 patients who tolera ted a trial and were successfully extubated. Immediately before the weaning trial, S (V) over bar(O2), was not statistically different in the two grou ps (p = 0.28). On discontinuation of the ventilator, S (V) over bar(O2), fe ll 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 deman d was similar in the two groups, but it differed in the manner with which i t 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 aft erload, but, instead, increased O-2 extraction ratio (p < 0.02) with a cons equent fall in S (V) over bar(O2). In turn, the low S (V) over bar V-O2 com bined with greater venous admixture (p < 0.0006) led to rapid arterial desa turation (p < 0.006) and a relative decrease in O-2 being supplied to the t issues. 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 trans port and an increase in O-2 extraction by the tissues.