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Correspondence

A massive gas leak: missing O-ring and a plastic band

JAYAMAHA, S.; LIGHT, D.

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European Journal of Anaesthesiology: November 1996 - Volume 13 - Issue 6 - p 627-630

Sir: Although a gas leak as a result of missing or defective O-rings on the vaporizer housing is not a new problem [1-5], this letter highlights another hazard combined with a missing O-ring which led to a massive gas leak.

A 73-year-old, 65 kg woman with no previous medical problems was anaesthetized for an elective left modified radical mastectomy. Anaesthesia was induced and the trachea was intubated in the anaesthetic room. In the operating room, the lungs were ventilated with oxygen 2 Litre min−1, nitrous oxide 4 Litre min−1 and enflurane 1.5% using a Nuffield Penlon 200 (Penlon Ltd, Oxon, UK) ventilator connected to a Bain breathing system. However, the patient developed hypertension with tachycardia and the end-tidal carbon dioxide (ETCO2) continued to rise, but SpO2 remained 98%.

It was decided to ventilate the patient's lungs manually using the Bain breathing system. However, the reservoir bag did not fill adequately even with a maximum fresh gas flow setting on the Rotameters. A fault in the anaesthetic machine was thus suspected and the lungs were ventilated using the oxygen flush device and anaesthesia maintained intravenously (i.v.) until the anaesthetic machine was exchanged with another. During ventilation with the second anaesthetic machine the vital signs returned to normal. Thereafter the period of anaesthesia was uneventful, the operation was successfully completed and the patient made an uneventful recovery.

On inspection of the original anaesthetic machine it was discovered that the enflurane vaporizer was incorrectly seated. The upstream O-ring was missing and on the same side, the vaporizer was sitting on a plastic band or tie which held a multi-outlet electrical distribution board on to the back bar of the anaesthetic machine (Fig. 1). Further examination of the anaesthetic machine was carried out to measure the extent of the leak with (a) a missing downstream O-ring, (b) a missing upstream O-ring, (c) a combination of the presence of plastic tie and the missing upstream O-ring. The amount of the anaesthetic gas delivered at the common gas outlet with each of the above mentioned defects (a), (b), and (c) was measured using a calibrated Gould Godart Pneumotachograph [6].

Fig. 1
Fig. 1:
Electrical distribution board attached to the back bar of the anaesthetic machine. (a) Review view (b) front view showing the plastic tie encroaching into the vaporizer setting.

Measurements were made with different amounts of total fresh gas flows (FGF) and with the vaporizer in the 'off' position and 'on' at 1%. The amount of gas delivered at the common outlet is expressed as a percentage of FGF dialled with respective measurement and illustrated in Fig. 2 and 3.

Fig. 2
Fig. 2:
Percentage of FGF delivered at the outlet with different settings of FGF when the vaporizer setting='off'.
Fig. 3
Fig. 3:
Percentage of FGF delivered at the outlet with different settings of FGF when the vaporizer setting=1%.

Analysis of the extent of gas leak revealed that the combination of missing upstream O-ring and the malfitting of the vaporizer as a result of the plastic tie led to 100% leakage of FGF when then vaporizer was 'on' at 1% with fresh gas flow up to 6 Litres. The leakage was 97% or more with a FGF of 7 and 8 Litres min−1. Whereas when the vaporizer was 'off' 100% leak was observed with FGF only up to 5 Litre min−1.

It was also observed that a missing upstream O-ring causes a greater leak than a missing downstream O-ring. The extent of the gas leak was approximately 60-70% with a missing upstream O-ring and 40% with a missing downstream O-ring when the vaporizer was 'on' at 1%. The corresponding gas leakages were less but still significant when the vaporizer was 'off'.

With FGF of 5 Litre min−1 of oxygen, the resistance to gas flow through the vaporizer is 5 cm water, at 'off' setting, but is 4-5 times higher (21-29 cm H2O) when the vaporizer is 'on' [7]. This explains why the gas leak was greater when the vaporizer was 'on' than when it was 'off'. Higher gas leak with the missing upstream O-ring could be because of the back pressure exerted by the vaporizer. As there was no other vaporizer downstream to the downstream O-ring during our analysis there was no back pressure when the downstream O-ring was missing, hence less gas leakage.

The results indicate that the patient would have been at a greater risk with low flow anaesthesia with a closed circuit. The gas leak may not have been apparent immediately, as in this case, with a ventilator such as a Nuffield Penlon 200 which ventilates the patient's lungs even if there is no fresh gas flow. It ventilates the patient's lungs with whatever the gas in the breathing system by moving it to and fro with bursts of pressurized driving gas. As there is no physical barrier between the driving gas and the fresh gas flow even the driving gas may enter the patient's lungs if the length of the tube which connects the ventilator to the Bain breathing system is less than 1 m or if the ventilator flow rate is higher than the patient's inspiratory flow rate. Ventilators with bellows have a physical barrier between the driving gas and the fresh gas flow and fail to function with a massive leak of FGF.

If a failure of FGF is suspected the patient's lungs can be manually ventilated using the oxygen flush device (provided a 'cross over' with another gas is not suspected) and adequately anaesthetized using i.v. anaesthesia [4]. The availability of an Ambu or other self-inflating bag system is important as a reserve in such circumstances.

The importance of monitoring cannot be stressed enough and in this case the monitoring of anaesthetic vapour concentration and the patient's inspired oxygen concentration should have alerted the anaesthetist early to the possibility of inadequate anaesthesia. Setting the upper alarm limit of the oxygen analyser to less than 100% (e.g. 70%) will alert the anaesthetist to any possibility of dilution of anaesthetic gases and should be a routine precaution.

This report highlights the importance of proper preoperative checking of the anaesthetic machine including the occlusion test for leaks with the vaporizer at 'on' and 'off' settings, a test recommended by the Association of Anaesthetists of Great Britain and Ireland [8]. Quick and easy ways of performing proper checking of the anaesthetic machine have been described [8,9] and should be practised routinely.

Electrically operated patient monitoring equipment and sophisticated lung ventilators have increased the need for electrical sockets in the operating room. Proper placement of electrical distribution boards is necessary to prevent accidents and to comply in the UK with British standard 5724. It has been recommended that electrical distribution boards be placed above the back bar on the anaesthetic machine or near the top of a monitoring trolley, with the face of the sockets vertical [10]. The board should be at least 25 cm above the bar if there is a possibility that a flammable anaesthetic agent may be employed [10]. Incorrect placement of distribution boards are not only an electrical and an explosion hazard but also an anaesthetic hazard. Hence the importance of employing qualified engineers to make modifications to anaesthetic machines when required is to be stressed.

Acknowledgments

We wish to thank Professor A.P. Adams and Dr P. Hewitt for advice and assistance rendered in the preparation of this manuscript.

S. JAYAMAHA

Royal Hull Infirmary, Hull

D. LIGHT

Guys Hospital, London

References

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© 1996 European Academy of Anaesthesiology