Raise the alarm

We have raised the default oxygen alarms on our Datex–Engstrom (GE Heathcare, Chalfont St Giles, Bucks, UK) anaesthetic machines with circle absorbers, from 18% to 24% following a case of unrecognised disconnection of the circle absorber from the common gas outlet. Detection of low oxygen mixtures is via the carbon dioxide analyser. The common gas outlet connector had been removed to give oxygen via a Hudson facemask and oxygen tubing to the previous patient. In our case the alarm sounded when the oxygen concentration had dropped to 17% and no harm came to the patient. As well as raising the alarm level, we ask that the circle absorber proximal intake be connected to the distal patient end during supplementary oxygen use, so that the next anaesthetic cannot be given without realising the error. Three similar cases have been described, and non-reattachment of the common gas outlet was considered to be a major contributor to a maternal death [1, 2]. Hypoxic gas mixtures (under 21%) can be breathed from a circle absorber after disconnection during re-breathing, or if an oxygen ⁄ air mixture is selected and only air is used. For instance, 6% sevoflurane in air can be delivered without alarms because there is no hypoxic guard. A hypoxic guard prevents delivery of < 25% oxygen with nitrous oxide so our figure of 24% for our current inspired oxygen alarm is a reasonable, though arbitrary, figure. Our Datex–Engstrom machines had a factory set inspired oxygen default of 18% and, following a casual survey, the majority of other departments have this alarm level. When our machines with a 24% oxygen alarm are idle and with no fresh gas flow, an advisory square box saying ‘low FiO2’ appears on the top left of the screen without an audible alert, because the carbon dioxide analyser is sucking in 21% oxygen in air.