[Measurements of ventilatory parameters during closed circuit anesthesia--comparisons with semiclosed circuit,...

Since induction of anesthesia using a closed-circuit technique is difficult with most modern anesthesia machines, it is a common clinical practice to convert semiclosed, high-flow anesthesia (SCA) to closed circuit anesthesia (CCA) during the maintenance phase by decreasing the fresh gas flow. The purpose of the current studies was to determine if such changes in the fresh gas flow influence parameters of ventilatory dynamics. The tidal volume, airway pressure, and a ratio of expiratory volume in one second to a tidal volume (EV1.0/VT) were measured in six patients and in a lung simulator during CCA with a fresh gas flow of 200 ml.min-1 and SCA with a flow of 6 l.min-1. The ventilator settings were constant throughout the experiments. Dynamic compliance was calculated as (measured VT)/(peak airway pressure-positive end-expiratory pressure). Both in patients and in a lung simulator, dynamic compliance and EV1.0/VT were higher during CCA as compared to SCA while the tidal volume and airway pressure were lower. It was also suggested that these differences were secondary to differences in the fresh gas flow and not to the circuit situation (closed vs. semiclosed) per se. The results emphasize that the fresh gas flow should be taken into consideration in interpreting the data of ventilatory dynamics during anesthesia. This is especially so during CCA because CCA employs a minimum fresh gas flow.