The Effects of Methoxyflurane on Ventilation in Man

Minute volume, tidal volume, respiratory frequency, end-tidal carbon dioxide, and ventilatory response to carbon dioxide inhalation were studied during anesthesia at three different concentrations of end-tidal methoxyflurane, 0.19, 0.37, and 0.82 per cent, in 12 unpremedicated patients prior to surgery. Minute volume rose during light anesthesia and fell with increasing concentrations. Tidal volume decreased progressively from 0.49 liters awake, to 0.22 liters at the highest concentration, while respiratory rate rose from 11 to 22 per minute. End-tidal carbon dioxide rose progressively from 38.6 torr while awake, to 43.4 torr during deep anesthesia. More striking was the change in slope of the carbon dioxide response curve, which decreased progressively with increasing depth of anesthesia from 56 per cent of the control response in light anesthesia, through 28 to 9 per cent during deep anesthesia. An estimate of MAC for methoxyflurane in this study was 0.17 per cent, which agrees with previous reports despite employment of different criteria.