Ventilatory compensation for inspiratory resistive loads during anaesthesia with halothane or isoflurane

Abstract

We have analysed the ventilatory response to sustained inspiratory resistive loads in 14 patients, while awake and during halothane (n = 7) or isoflurane (n = 7) anaesthesia. Patients breathed halothane or isoflurane in oxygen at 1.2 minimum alveolar concentration (MAC). Inspiratory resistances of 0, 12 and 37 cm H2O litre-1 s were applied. Tidal volume (VT) was maintained with the greater loads. At the greatest resistance, a significant reduction in minute ventilation occurred in both awake (-18.9%) and anaesthetized states, with both halothane (-10.4%) and isoflurane (-14.5%). Ventilatory frequency decreased significantly from mean 14.6 (SD 4.7) to 12.5 (4.3) bpm in the awake state and during anaesthesia, with increasing inspiratory resistance (29.5 (3.6) to 23.7 (7.2) bpm and 25.8 (3.3) to 23.4 (4.0) bpm, respectively, for halothane and isoflurane) because inspiratory time (TI) was significantly longer (P < 0.01). End-tidal PCO2 increased by 0.3 kPa, on average, from baseline to the highest level of resistance (P < 0.01). Inspiratory occlusion pressure at 100 ms increased significantly with increased loading in all situations (P < 0.001). We found a similar pattern of ventilatory adaptation to sustained inspiratory flow resistive loads both in awake and anaesthetized states. VT was maintained at increased loads because of an increase in inspiratory neuromuscular output and inspiratory duration.