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 < 0.05) elevated cardiac output (Q̇) from 2 to 7 h by increasing stroke volume; mean aortic pressure (MAP) also increased with time (P < 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). Q̇ 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 (Pa(CO₂)) compared with the lower dose. Pa(CO₂) continued to increase with duration of 1.7 MAC HAL, as did results of ventilatory gas volume and flow measurements.