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 [2–4]. 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.
References
Ghijselings IE, De Cooman S, Carette R, Peyton PJ, De Wolf AM, Hendrickx JF (2015) Performance of an active inspired hypoxic guard. J Clin Monit Comput. doi:10.1007/s10877-015-9684-9
Brody S. Bioenergetics and growth. New York: Reinhold; 1945.
Stenqvist O (2001) Clinical window. Review articles, Datex-Ohmeda
Ting PH (2003) Clinical window. Clinical paper, Datex-Ohmeda
Personal contact with Mikael Petrini at Maquet, Sweden
Conflict of interest
The report has not been externally funded, and none of the authors has a conflict of interest.
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Eriksson, S., Bredbacka, S. Letter to the Editor. J Clin Monit Comput 30, 377 (2016). https://doi.org/10.1007/s10877-015-9715-6
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DOI: https://doi.org/10.1007/s10877-015-9715-6