Editor, We would like to report an incident of isoflurane overdose due to critical overfilling of a vaporizer in the absence of factors previously associated with this problem. Following gaseous induction with sevoflurane, a 2-year-old child was brought from the anaesthetic room into the operating theatre. For maintenance of anaesthesia, the larygneal mask airway was connected via a paediatric circle system to a Datex-Ohmeda Isotec 5 vaporizer set to deliver 1.5% isoflurane and mounted on a Datex-Ohmeda Aestiva 3000 anaesthetic machine. After a couple of minutes, however, the end-tidal isoflurane concentration, displayed on an associated Hewlett-Packard gas analyser, was 4.3%. We also noticed marked hypotension at 40/25 mmHg and there was T-wave inversion with ST-segment depression on lead II of the monitored ECG. At this point, inspection of the sight glass of the vaporizer revealed that it had been overfilled. We immediately removed the vaporizer from the back bar and, after an interval to allow the isoflurane concentration to fall to within the therapeutic range, switched to a sevoflurane-filled one, which was already mounted on the machine. The patient's blood pressure and ECG returned to normal. Postoperatively, the child was well with a normal 12-lead ECG and no neurological abnormalities. The incident has been reported to the manufacturers and the vaporizer examined with no fault found. The anaesthetic assistant reported that, on filling the vaporizer prior to starting the case, an ‘air lock’ had developed. This had been overcome by rocking the vaporizer in place while still mounted on and locked to the back bar in the standard upright position. For filling, the vaporizer concentration dial was turned to ‘off’ and air entry was prevented at the filling port. Following filling of the vaporizer, the agent level had been inspected by the anaesthetic assistant and also by one of the anaesthetists (J.N.P.) for the presence of volatile agent but not specifically for overfilling. On subsequent testing, we were unable to replicate such an ‘air lock’ condition, but colleagues confirmed that they have experienced this problem previously and dealt with it in a similar way. We were, however, able to overfill the vaporizer by tilting it, and found a similarly elevated concentration of isoflurane on connection to a test circuit. Unrecognized vaporizer overfilling may lead, as here, to overdose of volatile anaesthetic, although failure of rather than excess volatile agent delivery has also been noted under such conditions previously [1] and may reflect the degree of overfilling [2]. In our case, although rocked gently in place, the vaporizer remained in the upright position and locked to the back bar, in accordance both with manufacturer's instructions and with a previous alert from the UK Medicines and Healthcare Products Regulatory Agency [3]. The vaporizer concentration dial was not turned to the ‘on’ position, nor was air entry allowed at the filling port, a double-fault condition found to have resulted in the death of a child in Canada [2] and also in a near miss incident in a London teaching hospital [4]. Moreover, overfilling was found to be both common and frequently unrecognized within each hospital in both reports, and indeed the latter report indicated that many of the hospital's anaesthetic staff were convinced that later generation vaporizers could not be overfilled. Two independent pre-use machine checks failed to reveal the overfilled vaporizer in our case. Inattention may be the most likely reason but there are alternate explanations. A different make of vaporizer was reported to allow an air bubble to become trapped at the top of the sight glass when overfilled, giving the erroneous impression that it was correctly filled [2], but we are uncertain whether this might have occurred in our case. Also, it has previously been noted that the Isotec 5 fill level may rise over a 10 min period when left to stand [1], such that checking the sight glass immediately after topping up the vaporizer would not reveal overfilling. We note also that, because of the design of the sight glass on this model, the liquid level in an overfilled vaporizer is visible only to an anaesthetist or assistant in the sitting position in front of the machine or stooping to inspect it. We believe that this incident illustrates a risk of vaporizer overfilling which is not widely recognized, and which does not specifically violate the manufacturer's instructions or previous guidance [3]. Our experience has highlighted to us the importance of checking each vaporizer specifically for overfilling, as described in the guidelines of the Association of Anaesthetists of Great Britain and Ireland [5], and also reinforced the necessity of end-tidal agent monitoring [6].
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May 25, 2015
Keunyeong Oh + 4
Open Access