Site of fresh gas inlet and ratios of the delivered fraction and inspired fraction of inhaled isoflurane and sevoflurane in low-flow anesthesia

Abstract

The use of low-flow anesthesia causes a discrepancy between the delivered fraction (FD) and the inspired fraction (FI) of inhaled gases. We compared the FI/FD ratios of a new circle (fresh gas inlet located between the inspiratory valve and the patient) to those of the conventional circle (fresh gas inlet located between the inspiratory valve and the CO2 absorber) in low-flow isoflurane and sevoflurane anesthesia, using three anesthetic machines (Dräger NM-GS, Dräger Fabius-GS, and ACOMA KMA-1300-III). Eighty-two patients were randomly assigned to three experimental groups. For experiment 1, 32 patients were allocated to the NM-GS conventional/new, NM-GS new/conventional, ACOMA conventional/new, and ACOMA new/conventional groups. For experiment 2, 14 patients were allocated to ACOMA conventional/conventional and ACOMA new/new groups to measure isoflurane FI/FD ratios. For experiment 3, 36 patients were allocated to ACOMA conventional/conventional, ACOMA new/new, Fabius conventional/conventional, and Fabius new/new to measure sevoflurane FI/FD ratios. In experiment 1, the NM-GS showed no significant changes in the FI/FD ratios. However, in the ACOMA, the new circle improved the FI/FD ratio. In experiment 2, the isoflurane FI/FD ratios in the new circle of the ACOMA were significantly higher than those in the conventional circle. In experiment 3, the sevoflurane FI/FD ratios in the new circle of both the ACOMA and the Fabius were significantly higher than those in the conventional circles. The positioning of the fresh gas inlet between the inspiratory valve and the patient improved the FI/FD ratios of both isoflurane and sevoflurane during low-flow anesthesia in two decoupling-style anesthetic machines (ACOMA and Fabius).