Minimum inspired oxygen concentration alarm

Abstract

EDITOR: The use of anaesthetic machine and patient monitor alarms, in addition to clinical observation, can enhance patient safety, but it is very important that these limits are accurately set. The importance of these settings is highlighted by a critical incident which occurred recently in one of our theatres. Case report: An 8-yr-old, American Society of Anesthesiologists (ASA) I patient was scheduled for an elective ear, nose and throat procedure. Induction was uneventful and general anaesthesia was maintained with spontaneous ventilation via a laryngeal mask. The patient's head was draped and surgery commenced. An unobserved disconnection occurred at the junction of the circle-system breathing circuit tubing and the filter to which the gas analyser sampling line was attached (Fig. 1). The reduced movement of the reservoir bag was not initially noticed and the capnography trace continued as per normal. The inspired oxygen concentration decreased but the alarm did not sound as its setting had not been changed from the factory default of 18% for oxygen. Had it been set to above 21% it would have been activated. Fortunately the disconnection was discovered and the circuit reconnected before the patient sustained any harm.Figure 1.: Site of disconnection between circle breathing system and filter.The case prompted an audit at two hospitals in our region which demonstrated that the minimum inspired oxygen concentration alarm was changed from the factory reset by less than 10% of anaesthetists. Personal communication with a colleague from a neighbouring region revealed that two identical incidents had recently occurred in their hospital, and the same audit is at present being conducted in their hospital. Fortunately no patient suffered any harm as a result of the disconnections but the potential for patient awareness, movement, adverse airway events and hypoxia is evident. The use of an oxygen analyser is mandatory in accordance with the checklist for anaesthetic apparatus as set out by the Association of Anaesthetists of Great Britain and Ireland and is recommended in many other countries [1]. The Association guidelines however give no advice regarding the values to which the analyser's alarm limits should be set [2]. Oxygen concentrations are usually measured at either the fresh gas outlet or via the end-expired gas sampling line attached to, or near to, the filter. In addition to guarding against the delivery of hypoxic mixtures, the analyser (sampling end-tidal gases) can also help to detect problems with breathing circuit and ventilator integrity, and with alveolar hypoventilation [3,4]. A study by Knaack-Steinegger showed that disconnections were evident within 30 s using expired oxygen concentration alarms [5]. Disconnections are evident within a few breaths when using end-tidal gas monitoring with oxygen concentration alarms set to above 21%. Capnography alarms and the waveform are used by anaesthetists as a monitor for early detection of problems with patients' ventilation, including disconnections in the breathing system. In spontaneously breathing patients, where there is a disconnection proximal to the capnography sampling point, the capnography trace may well continue as per normal and hence this alarm would not detect the disconnection. An anaesthetic agent analyser is also useful to detect disconnections in a breathing system and guard against patient awareness, but factory default settings of 0% would need to be reset to minimum alveolar concentration (MAC) awake or above. The audit performed in our hospital also investigated the alarm settings of the agent analyser and found that in no theatres were the limits changed to above 0%. The oxygen alarms are often not set above 21% as doing so results in the alarm sounding when there is no (enriched) oxygen flowing through the circuit. This can be overcome by leaving the sampling line in continuity with the circuit and allowing a small quantity of oxygen to flow (most modern anaesthetic machines have a minimum oxygen flow preset). An alternate solution is to cap the circuit off at the end of the case. This also helps to prevent wastage of anaesthetic gases and stops the anaesthetic agent analyser alarm sounding (if set above 0%). Ventilator pressure alarms can detect leaks during mechanical ventilation, but they are not foolproof [6]. The minimum inspired oxygen alarm is a useful monitor but should be set above 21%, especially for spontaneously breathing patients and when the circuit is covered. Setting the anaesthetic agent alarm to above 0% further adds to patient safety. M. T. Dale D. Jadhav *Department of Anaesthesia, Critical Care and Pain Management, Leicester Royal Infirmary, Leicester, England