Analysis of ventilatory depression by enflurane during clinical anesthesia

Tracheal pressures during airway occlusion (an index of ventilatory drive), inspiratory time, alveolar ventilation, and PaCO2 were measured in spontaneously breathing, healthy patients during enflurane:N2O anesthesia at various inspired concentrations of enflurane. The purpose was to describe more clearly the mechanism of intraoperative hypoventilation. Occlusion pressures, inspiratory time, and alveolar ventilation were less, and PaCO2 was higher, at deeper levels of anesthesia. Hypoventilation is due to reduced mechanical transform of neuromuscular drive and to shorter inspiratory time and, therefore, reduced mean inspiratory flow rate without change in breathing frequency. Effective elastance (an index of lung stiffness) was slightly greater at deeper levels of anesthesia.