Entonox analgesia--a method of reducing the delay between demand and supply

Mothers in labour using a self-administered 50% N2O and 50% O2 (Entonox) demand system, experience a time lag between making a demand at the onset of a contraction and pain relief. A practical method is described of reducing this time lag by supplementing the intermittent supply from the demand valve system with a continuous nasal flow of N2O and O2. Consequently a basal arterial N2O tension is maintained and when a demand is made, a higher level of N2O is attained in any time interval compared with that using the demand system alone. In six trained volunteers under laboratory conditions, the N2O levels measured at the mouth on a breath-by-breath basis using a mass spectrometer, showed a substantial and statistically significant reduction in the time taken to attain end-tidal N2O concentrations in the range 10-25%. This method seems worthy of a clinical trial.