Respiratory timing and depth of breathing in dogs anesthetized with halothane or enflurane

Tidal volume (VT) and inspiratory (TI) and expiratory (TE) times were measured during enflurane and halothane anesthesia in 12 prone dogs before and after bilateral vagotomy. "Elastic" loading or airway occlusion was used to obtain a VT-to-TI relationship in each state and to examine the rate of change in airway pressure. VT, TI, and TE were significantly (P less than 0.05) larger during enflurane than during halothane anesthesia, both before and after bilateral vagotomy. Before vagotomy, the rate of change in airway pressure during airway occlusion was similar for the two agents, as was an index of impedance of the respiratory system (Z'rs). Thus the difference in maximal pressure generated and in VT was most likely due to the difference in TI. Before vagotomy, TI increased as VT decreased with loading during enflurane but not halothane anesthesia, demonstrating a different effect of the two agents on the phasic vagal inspiratory inhibitory mechanism. After bilateral vagotomy, TI was unaltered during elastic loading with both agents but was still significantly longer during enflurane than halothane anesthesia. Thus it was concluded that TI was longer in the enfluraneanesthetized dogs than in the halothane-anesthetized dogs because of a different effect of these two agents on the bulbopontine "pacemaker" mechanism and not because of different effects on the phasic vagal inspiratory inhibitory mechanism.