ARTERIAL CONCENTRATIONS OF NITROUS OXIDE DURING INTERMITTENT PATIENT-CONTROLLED INHALATION OF 50% NITROUS OXIDE IN OXYGEN (ENTONOX) DURING THE FIRST STAGE OF LABOUR

Abstract

Measurements were made in 15 mothers of the arterial concentrations of nitrous oxide at the end of periods of inhalation of 50% nitrous oxide. The mean arterial concentrations, from all the patients, would be equivalent to breathing 26.4% nitrous oxide until equilibrium had been reached between the inspired concentration and the blood. The low measured blood levels are related to such factors as the short duration of inhalation, ventilation-perfusion defects in labour and the almost complete elimination of nitrous oxide between each contraction.