A comparison of the efficiency of three anesthesia circle systems

Three commonly employed anesthesia circle systems were tested to determine what effect the placement of the inflow port, overflow valve, and inspiratory and expiratory valves, as well as inflow rate, had on the rate of rise of anesthetizing alveolar concentrations of halothane. System efficiency was evaluated on the basis of whether alveolar gases, containing low concentrations of halothane, or fresh and deadspace gases, containing higher concentrations of halothane, were eliminated through the overflow valve. Small potentially significant differences in efficiency were found: greatest efficiency (greatest ejection of alveolar gas) occurred when inspiratory and expiratory valves were close to the endotracheal tube and the overflow valve was immediately distal to the expiratory valve. Increasing inflow rate from 1 to 3 L/min had a large impact on the rate of rise of alveolar halothane concentration. A further increase to 5 L/min had a relatively small effect.