Effects of halothane and isoflurane on ventilation and occlusion pressure

Isoflurane has been said to be more ventilatory depressant than halothane. However, data for comparing the respiratory effects of halothane and isoflurane in humans are insufficient at this time. The aim of this study was to extend our understanding of the nature of the central, as opposed to peripheral, ventilatory effect of halothane and isoflurane by comparing them at two concentrations.

Twenty patients were randomly assigned to receive halothane (n = 10) or isoflurane (n = 10). The patients were studied the day before surgery and during anesthesia immediately before surgery. Ventilatory effects were analyzed in terms of breathing pattern, end-tidal carbon dioxide pressure (PETCO2) and inspiratory occlusion pressure. After anesthetic induction and orotracheal intubation with thiopental and succinylcholine patients were allowed to breathe halothane or isoflurane in oxygen spontaneously at 1.2 (low) and 2.0 (high) minimum alveolar concentration (MAC) applied in random order. Inspiratory active impedance during anesthesia was also measured.

Significant reduction of minute ventilation between awake and low MAC states was observed for isoflurane (-34.4%; P < 0.001) but not for halothane. Inspiratory occlusion pressure at 100 ms increased significantly between awake and low MAC states, from 1.43 +/- 0.89 to 2.67 +/- 1.05 cmH2O (P < 0.05) for halothane, representing an 87% increase, whereas a nonsignificant increase (16%) was observed for isoflurane. Both anesthetics showed a dose-related ventilatory depressant effect, not attributable to changes in mechanical properties, reflected by significant reductions in minute ventilation (P < 0.001), tidal volume (P < 0.001), and inspiratory occlusion pressure at 100 ms (P < 0.05) and increases in respiratory rate (P < 0.001) and end-tidal carbon dioxide pressure (P < 0.01) when concentration was increased. However, at the higher concentration a significantly greater reduction of minute ventilation (P < 0.01) was observed for isoflurane (-25.6%) than for halothane (-9.4%). We did not observe differences in respiratory rate between the two anesthetics. Significant differences in inspiratory occlusion pressure wave were observed, characterized by a concave-upward tendency for isoflurane and for high concentration.

Our study confirms the stronger ventilatory depression induced by isoflurane compared with that induced by halothane and indicates that halothane at 1.2 MAC induces significantly less ventilatory depression than expected.