A new SIMS PneuPac 550 anaesthetic machine (Luton, UK) and Datex AS/3 (Instrumentation Corp, Finland) anaesthetic monitor were commissioned for use in the back-up operating theatre in our maternity unit. This theatre is mostly used for short procedures, e.g. surgical evacuation of the retained products of conception and suturing of vaginal tears, but is occasionally used for␣Caesarean sections when the main obstetric operating theatre is in use. Shortly after the installation of this equipment, it was noticed that the␣monitored concentration of volatile anaesthetic agent delivered to the patient, via a parallel Lack circuit, exceeded that dialled on the vaporiser. The␣problem appeared to resolve after the␣monitor was recalibrated, but appeared to recur a few days later. Over the␣following weeks, the monitor was calibrated again, the vaporiser changed, the monitor serviced and a portable gas analysis␣system used. It was noted that an increase in fresh gas flow resulted in an increase in agent concentration after a short time and it eventually became apparent that the problem lay within the anaesthetic machine. Prior to this realisation, 100 general anaesthetics had been administered using the machine. Fortunately, intravenous maintenance of anaesthesia was used in the vast majority of cases, contributing to the delay in identifying the source of the problem. It was discovered that the back bar of␣the anaesthetic machine had been incorrectly connected, with the input pipe from the flow meters attached to the back bar outlet connection and the back bar inlet pipe attached to the fresh gas outlet pipe. This effectively reversed the flow through the back bar and any attached vaporiser. This fault was the result of an assembly error, as the design of the anaesthetic machine is such that both the inlet and outlet connectors are on the same side of the machine and were unlabelled elbow connectors. The design has since been modified, such that the inlet pipe is attached directly to the back bar and the outlet pipe is attached via a straight connector. The two pipes are of different diameters (10 mm and 8 mm outer diameters) and the screw threads are also different between the two connections (Fig. 5). This should make the potential for reverse connections impossible. In␣addition, the test block, simulating the␣attachment of a vaporiser during assembly and service checks, has been modified to include a one-way valve, blocking gas flow through the test block should reverse flow occur. These modifications have satisfied PneuPac quality control and the Medical Devices Agency that such an error is unlikely to occur again. Inlet and output connectors of the pneuPAC 550 anaesthetic back bar after modifications to prevent future episodes of reverse connection. Reproduced with kind permission of pneuPAC. Twenty-six anaesthetic machines of this specific design had been produced and all were examined for this fault, which was found to be an isolated incident. Reverse flow through a vaporiser may occur with either misconnection of freestanding vaporisers [1, 2] or misconnection during servicing [3]. The delivered agent concentration has been reported as either being unaffected [2] or doubling [1, 3] following a reversal of flow. Our experience with an Ohmeda Tec 4 (West Yorkshire, UK) isoflurane vaporiser is intermediate, as we saw expected delivered concentrations initially, followed by a drift to approximately double the dialled␣concentration. This could potentially lead to problems␣if the volatile anaesthetic concentrations are not monitored in situations where high inspired concentrations are undesirable, e.g. elderly, cardiac disease, obstetrics. This incident highlights the importance of vigilance by anaesthetic personnel in the use of all equipment, especially new equipment and that which has been recently serviced. As we use multiple pieces of increasingly sophisticated equipment, the potential for faults increases, yet it may be difficult to ascertain where the fault lies. The standard anaesthetic equipment check, as published by the Association of Anaesthetists [4], remains essential, but it may not detect assembly faults such as this. Is it time for the development of specific guidelines for the checking of equipment prior to initial commissioning and after each routine service? The assembly fault described may have been detected earlier by the use of vaporisers with non-return valves and has been prevented from recurring by the introduction of changes in the design of the back bar connections, combined with improvements in test equipment and test procedures. This emphasises the importance of simple measures in the overall safety of anaesthesia.
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Feb 28, 2024
Jiwei Liu + 3
Open Access