Dead space fraction for each agent was determined empirically by having Excel’s solver function calculate the value of dead space fraction that minimized the sum of the squared differences between dialed F VAP and predicted F VAP. Dead space fraction for sevoflurane, desflurane, and isoflurane was therefore determined empirically from an unpublished data set of 161 patient containing F VAP, F IN, F ET, MV and FGF ranging from 0.25 to 8 L/min delivered via an ADU® (GE, Madison, WI, USA). F IN, F ET, FGF and MV are routinely monitored, but dead space fraction is unknown. Theoretical analysis of agent mass balances in the circle breathing reveals F VAP = /(1-(1 − FGF/MV)). The current study examines how F VAP can be retrospectively calculated from the agent’s inspired (F IN) and end-expired concentration (F ET), FGF, and minute ventilation (MV). Because F VAP of conventional vaporizers is not registered in automated anesthesia records, retrospective agent consumption studies are hampered. Anesthetic agent consumption is often calculated as the product of fresh gas flow (FGF) and vaporizer dial setting (F VAP).
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