Low pressure leakage in anaesthetic machines. Evaluation by positive and negative pressure tests.

Positive and negative pressure tests for checking the low pressure section of 36 anaesthetic machines in order to detect gas leaks were prospectively compared. The low pressure section was checked by the soap bubble test as a 'gold standard'. The leakage rate was 86.1%. The sensitivity and specificity of the positive pressure tests were 92% and 100% respectively, whilst the sensitivity and specificity of the negative pressure test were both 100%. However, seven machines revealed a leak which was only detected by the positive pressure test: this problem arose through defects in the emergency oxygen flush devices which allowed oxygen to leak around the valve even though it was in the 'off' position. This might reduce the specificity of the negative pressure test to 41.7%. For security, both tests should be carried out in series; the positive pressure test followed by the negative pressure test. The possibility of gaseous leakage around the oxygen flush device is not generally recognised. This study emphasises the need for skilled servicing of anaesthetic machines on a regular basis.