The resistance of Entonox valves.

The resistances of thirteen Entonox valves were measured. Although there was a wide variation between them, this is unlikely to interfere with their performance when used in labour. During the passage of continuous high flows, freezing occurred in two valves, causing a reduction in flow after 5 minutes. It is very unlikely that this would occur in clinical use. Measurements of the resistances of eight of the valves were compared using a static tube or a wide-bore needle to measure pressure changes. There was no significant difference between the two methods at flows of up to 100 l./min.