Abstract
BackgroundFresh gas decoupling is a feature of the modern anesthesia workstation, where the fresh gas flow (FGF) is diverted into the reservoir bag and is not added to the delivered tidal volume, which thus remains constant. The present study aimed to investigate the entraining of the atmospheric air into the anesthesia breathing circuit in case the reservoir bag was disconnected.MethodsWe conducted a simulator-based study, where the METI HPS simulator was connected to the anesthesia workstation. The effect of the disconnected reservoir bag was evaluated using oxygen (O2) and air or oxygen and nitrous oxide (N2O) as a carrier gas at different FGF rates. We disconnected the reservoir bag for 10 min during the maintenance phase. We recorded values for inspiratory O2, N2O, and sevoflurane. The time constant of the exponential process was estimated during reservoir bag disconnection.ResultsThe difference of O2, N2O and sevoflurane concentrations, before, during, and after reservoir bag disconnection was statistically significant at 0.5, 1, and 2 L/min of FGF (p < 0.001). The largest decrease of the inspired O2 concentrations (FIO2) was detected in the case of oxygen and air as the carrier gas and an FGF of 1 L/min, when oxygen decreased from median [25th–75th percentile] 55.00% [54.00–56.00] to median 39.50% [38.00–42.50] (p < 0.001). The time constant for FIO2 during reservoir bag disconnection in oxygen and air as the carrier gas, were median 2.5, 2.5, and 1.5 min in FGF of 0.5, 1.0, and 2 L/min respectively.ConclusionsDuring the disconnection of the anesthesia reservoir bag, the process of pharmacokinetics takes place faster compared to the wash-in and wash-out pharmacokinetic properties in the circle breathing system. The time constant was affected by the FGF rate, as well as the gradient of anesthetic gases between the anesthesia circle system and atmospheric air.
Highlights
Fresh gas decoupling is a feature of the modern anesthesia workstation, where the fresh gas flow (FGF) is diverted into the reservoir bag and is not added to the delivered tidal volume, which remains constant
The differences in inspired concentrations of oxygen as well as the differences in sevoflurane concentrations between these three ten minutes time intervals were statistically significant at all FGFs tested (p < 0.001, for all)
This simulator-based study demonstrates that a reservoir bag disconnection leads to a significant decrease of inspired anesthetic gases and oxygen resulting from the entrainment of atmospheric air due to a large leak in the circle breathing system
Summary
Fresh gas decoupling is a feature of the modern anesthesia workstation, where the fresh gas flow (FGF) is diverted into the reservoir bag and is not added to the delivered tidal volume, which remains constant. The fraction of FGF, which is added during the inspiratory phase, increases the delivered tidal volume (VT) and can potentially cause volutrauma and/or barotrauma [1,2,3]. Such FGF set up is called fresh gas coupling. In FGD systems, a decoupling valve diverts the FGF toward the reservoir bag during the inspiratory phase of controlled mechanical ventilation. By varying the FGF, respiratory rate and/or I:E ratio no changes in delivered VT were noted in FGD systems [5, 6]
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