Ventilatory responses to inspiratory resistive loading before and after gastrectomy during isoflurane anesthesia

We tested the hypothesis that steady-state responses to inspiratory-flow-resistive loading would be preserved immediately after upper abdominal surgery in anesthetized patients. Twenty patients were studied immediately before and after gastrectomy under 1 minimum alveolar anesthetic concentration (MAC) of isoflurane anesthesia. Ventilation, airway occlusion pressure, and Paco2 were measured before and during inspiratory-flow-resistive breathing lasting from 6 to 7 min. Ten of 20 subjects were tested with resistance of 51.8 cm H2O.L-1.s-1 (Load 1) and the remaining 10 subjects were tested with resistance of 83.3 cm H2O.L-1.s-1 (Load 2). Ventilatory variables obtained immediately before and after surgery were compared in each group. Baseline ventilation increased postoperatively with greater frequency of breathing and comparable tidal volume (VT). Immediately after the application of resistive load, minute ventilation (VI) significantly decreased both pre- and postoperatively, due primarily to the decrease of VT. During sustained loading, VI gradually increased and reached steady state in 2-3 min. After 5 min of loading, Paco2 returned to the control level with Load 1 whereas with Load 2, it was higher than the control value. The magnitude and time course of reduced ventilation in response to resistive load were identical between pre- and postoperative conditions. We conclude that the ability of maintaining ventilation to imposed inspiratory-flow-resistive loading is well preserved during 1 MAC of isoflurane anesthesia before and after gastrectomy.