[Which settings to optimize anaesthetics delivery: fresh gas flow or delivered fraction?]

During volatile closed-circuit anaesthesia, a chosen end-tidal fraction (Fet) could be achieved by setting either delivered fraction (Fd) or fresh gas flow (FGF). This study compared the efficacy of both strategies and the resulting drug consumption.

Sixty patients (10 per group) were administered, after intravenous induction and intubation, desflurane, sevoflurane or isoflurane+50% N(2)O, to achieve a target Fet equal to one minimal alveolar concentration (MAC), according to one strategy: high FGF (HFGF) Fd fixed 20% above target Fet, FGF 10 l/min then 1l/min after achieving the target, FGF opened at 10 l/min at the end of surgery; low FGF (LFGF) FGF fixed at 1l/min, Fd at the maximal value on the vaporizer, then set at target Fet+20% after achieving Fet equal to one MAC, FGF maintained at 1l/min until extubation.

The target Fet was achieved in all patients in LFGF within 2.1+/-0.9 min followed by 15% (isoflurane) to 57% (sevoflurane) overdosage, but only in nine patients out of 30 after 10 min in HFGF. Delays were similar between desflurane and sevoflurane. Volatile consumption was decreased by 75% in LFGF. Fifty percent decrement and extubation times were shorter with HFGF, similarly for the three agents.

Massive overdosage of Fd is the fastest, reproducible and cheapest strategy to achieve (or to increase) a chosen Fet. High FGF is the fastest to decrease Fet during or at the end of anaesthesia. Combining Fd and FGF adjustments in order to maximize Fd/Fet gradients overwrites pharmacokinetic differences between desflurane and sevoflurane and reduces differences with isoflurane. Automatic adjustments based on volatile pharmacockinetics would be helpful to achieve a target Fet without overdosage.