During a routine list, we were a little surprised for two reasons, when we noticed an end-tidal concentration of 0.9% halothane (>1 minimum alveolar concentration) appear on the monitor. First, the Datex-Ohmeda Avance (GE Medical Systems Ltd, Hatfield, UK) machines had only recently been installed and had never had halothane delivered through them. Second, the procedure was performed under spinal anaesthesia with a target controlled infusion of propofol for sedation. The gas sampling line had been inserted into the patient's facemask using a cannula. The monitor registered an end-tidal concentration of halothane for several minutes until it returned to zero. Even after finishing the case (see picture), the monitor still read ‘Et Hal %’, though at a concentration of zero.
On consulting with colleagues there was awareness that combinations of volatile agents, such as isoflurane and sevoflurane, can cause similar errors. This prompted a literature search, which revealed the following.
Finn et al.1 in fact attributed a very similar incident to the detection of methane in exhaled gases.2 According to Cloarec et al.,3 exhaled methane is actually present in 30 to 50% of healthy adults and it has been suggested that the infrared monitors used in most modern anaesthetic machines to measure the concentration of inhaled volatile anaesthetic agents can confuse methane with halothane. The infrared absorption spectrum of methane has been measured from 2470 to 3200 cm−1.4 The gas analysers incorporated into our machines are nondispersive infrared, side-stream analysers that measure the gas sample at seven different wavelengths, selected using optical narrow band filters. The infrared detectors are thermopiles (many thermocouples connected in parallel). Identification of the anaesthetic agents and calculation of their concentration occurs by measurement of absorbance at five wavelengths in the 8-9 micron band, which does not overlap the infrared absorption spectrum of methane quoted above.5 However, halothane does absorb infrared light at 3.3 microns6 and this overlap between the absorption spectra of halothane and methane has been highlighted by Mortier et al.7 as a potential source of erroneous anaesthetic agent readings.
In retrospect, the factitious reading of halothane described above was possibly due to the detection of methane and not a mixture of isolfurane and sevoflurane, based on the fact that no volatile agent was being used in this case. However, we cannot be certain of the exact cause of this anomaly. Hawkes8 described a similar case in which the detection of halothane in a patient who was known to be malignant hyperthermia susceptible and who was anaesthetised with total intravenous anaesthesia.
We believe our case is another example of the need for vigilance when using monitoring, especially when this does not correlate with the clinical scenario. Clinical judgement and questioning of unusual observations remains paramount in this technological age.
Acknowledgements relating to this article
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Conflicts of interest: none.
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