Recently, Idris E. Ghijselings et al described the performance of a new inspired hypoxic guard [1]. We have found a theoretical risk for overdosage of vapor when working with very low fresh gas flows (FGF) in manual mode.

An O2/air FGF of 0.3 L/min with a FDO2 (delivered O2 fraction) of 67 % is a FGF-FDO2 combination that the hypoxic guard allows to be used [1]; this corresponds to a delivery of 200 mL O2 plus 100 mL N2/min. O2 uptake (VO2) for a patient with a body weight (BW) of 75 kg under anesthesia is 200 mL/min (calculated according to Brody’s formula as (10 × BW 0.75) × 0.8 [24]. This mixture will theoretically cause a hypoxic FIO2 (inspired oxygen concentration) because all delivered O2 will be consumed: nitrogen will accumulate in the circuit until the O2 concentration reaches zero percent.

The new inspired hypoxic guard, the O2 Guard, will intervene once the FIO2 decreases to 20 %, increasing any FGF below 1.0–1.0 L/min with a FDO2 of 60 % [1]. According to the FLOW-i manual there is no action taken to adjust the vapor concentration in accordance with the increased FGF [5]. In our scenario this means a 3.3 time increase of the amount of vapor fed to the circuit.

A Gas Man® simulation with an induction for 8 min with a 1 L/min FGF and 8 % sevoflurane vaporizer setting followed by a maintenance FGF of 0.3 L/min and 7 % sevoflurane vaporizer setting, shows that after 1 h of anesthesia the end-expired sevoflurane concentration will be 2.17 %. If at that time the O2 Guard increases the FGF from 0.3–1.0 L/min, the end-expired sevoflurane concentration will increase to 3.05 and 3.41 % after 5 and 10 min, respectively, constituting a 47 and 57 % increase after 5 and 10 min, respectively.

This simulation indicates that activation of the O2 Guard may allow an unannounced change of the end-expired sevoflurane concentration of the order 47 % in 5 min. There may also be a risk for not detecting this as alerts and the anesthesiologist’s actions probably focus on the hypoxic situation.