Potential leaks from anaesthetic machines. Potential leaks through open Rotameter valves and empty cylinder yokes.

Twenty anaesthetic machines were examined to detect leakage of fresh gas through open Rotameter flow control valves and empty cylinder yokes. Significant leakage was demonstrated in most machines. Any leakage in machines which comply with ISO 5358, where oxygen is the last gas into the common gas manifold, is selective for nitrous oxide and, conversely, it is selective for oxygen in machines where oxygen is the first gas into the common gas manifold. Leakage is augmented by back pressure from ventilators that function as minute volume dividers. In ventilated patients the leakage described may contribute to hypoventilation and patient hypoxia or to pollution of the operating theatre and patient awareness.