Isoflurane in labour.

A comparison of the analgesic efficacy of Entonox (50% nitrous oxide in oxygen) and Entonox with 0.2% isoflurane was made in 18 consenting mothers during the first stage of labour. The inhalational gases were self-administered during contractions over a 3 h study period using a standard Entonox demand valve and breathing system. Isoflurane 0.2% was added from an Ohmeda Isotec drawover vaporizer and the inspired concentration of isoflurane was analysed continuously using a Datex Ultima analyser. Linear analogue scores for pain were significantly lower (p < 0.001) during the Entonox-isoflurane administration compared with Entonox alone. Drowsiness was not a clinical problem and there was a high acceptance rate for Entonox-isoflurane inhalational analgesia during labour.