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

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Abstract
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 < 0.05) larger during enflurane than during halothane anesthesia, before and after bilateral vagotomy. Before vagotomy, the rate of change in airway pressure during airway occlusion was similar for the 2 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 2 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. TI was longer in the enflurane-anesthetized dogs than in the halothane-anesthetized dogs because of a different effect of the 2 agents on the bulbopontine pacemaker mechanism and not because of different effects on the phasic vagal inspiratory inhibitory mechanism.