Cardiopulmonary function during 7 h of constant-dose halothane and methoxyflurane

Cardiopulmonary effects of prolonged, constant-alveolar-dose halothane (HAL) and methoxyflurane (MOF) in O2 anesthesia on spontaneously breathing dogs were determined. One hour after anesthetic induction, end-tidal concentration was set at 1.04% HAL or 0.28% MOF [each representing 1.2 minimum alveolar concentration (MAC) for dogs] and maintained for 7 h. No time-related changes were associated with MOF. However, HAL significantly (P less than 0.05) elevated cardiac output (Q) from 2 to 7 h by increasing stroke volume; mean aortic pressure (MAP) also increased with time (P less than 0.05 beginning at 5 h). Four of these dogs were studied again at least 3 wk later at a constant end-tidal dose of 1.48% HAL in O2 (1.7 MAC). Q and MAP were lower initially during 1.7 MAC than during 1.2 MAC but not after 2 h of anesthesia. The greater HAL dose initially depressed ventilation and elevated arterial partial pressure of CO2 (PaCO2) compared with the lower dose. PaCO2 continued to increase with duration of 1.7 MAC HAL, as did results of ventilatory gas volume and flow measurements.