CARDIORESPIRATORY AND SYMPATHOADRENAL RESPONSES DURING WEANING FROM CONTROLLED VENTILATION

Abstract

Weaning from controlled ventilation was attempted in 20 patients who were ventilated with volume constant ventilators for at least 24 h before study. Measurement of alveolar to arterial O2 tension differences on 100% O2 (AaDO21.0) in patients who failed to wean (nonweaners, n = 10), showed a mean AaDO21.0 at 5 min after beginning to wean of 388 .+-. 56 mm Hg (SEM [standard error of the mean]) and right-to-left shunt (.ovrhdot.Qs/.ovrhdot.Qt) of 21%, values which were significantly different from control (P < 0.025) and from 10 patients who weaned (P < 0.005). Since rises in .ovrhdot.Qs/.ovrhdot.Qt were not accompanied by increased pulmonary capillary wedge pressure, the increased .ovrhdot.Qs/.ovrhdot.Qt was most likely due to acute atelectasis and not left ventricular failure. Patients who fail an initial period of weaning should probably be palced on positive end-expiratory pressure during subsequent weaning attempts. Significant increases in cardiac output and arterial PCO2 [CO2 tension] occurred in both groups. Assessment of urinary catecholamine excretion during weaning suggests that sympathoadrenal stimulation often is intense and usually is greater in those patients who weaned successfully. Increased arterial PCO2 may be the mechanism for sympathoadrenal stimulation and rises in cardiac output (r = 0.39, P < 0.01) during weaning.