Performance of an active inspired hypoxic guard.

Current hypoxic guards systems fail to maintain the inspired O2 concentration (FIO2) ≥ 21 % across the entire fresh gas flow (FGF) range when a second carrier gas is used (N2O or air). We examined the performance of the Maquet O2 Guard(®), a smart hypoxic guard that increases O2 delivery if an inspired hypoxic mixture is formed. After obtaining IRB approval and informed consent, 12 ASA I-II patients were enrolled. During anesthesia with sevoflurane in O2/air, the O2 Guard(®) was tested by administering O2/air at the following delivered hypoxic guard limits [expressed as (total FGF in L min(-1); FDO2 in %)] for 4 min each: [0.3;67], [0.4;50], [0.6;34], [0.8;25], [1.0;21], [1.2;21], [1.5;21], [2;21], [3;21], and [5;21]. The following data were collected: (1) time from FIO2 = 30 to 20 %; (2) time from FIO2 = 20 % to O2 Guard(®) activation; (3) time from O2 Guard(®) activation to FIO2 = 25 %; (4) FGF and FDO2 used by the O2 Guard. If SpO2 was <90 % for 10 s or longer at any time, the patient was excluded. Three patients were excluded for low SpO2. The incidence of FIO2 < 21 % was 100 % within the 1-2 L min(-1) FGF range. The O2 Guard(®) was activated within 20 s after FIO2 became 20 %, except in one patient where FIO2 oscillated between 20 and 21 %. FDO2 was increased to 60 % and FGF to 1 L min(-1) (the latter only if it was lower than 1 L min(-1) prior to activation of the O2 Guard). FIO2 increased to 25 % within 55 s after O2 Guard activation in all patients. The O2 Guard(®), an active inspired hypoxic guard, rapidly reverses and limits the duration of inspired hypoxic episodes when the delivered hypoxic guard fails to do so.