Calculated Kinetics of Distribution of Nitrous Oxide and Methoxyflurane during Intermittent Administration in Obstetrics

Abstract

Concentrations of nitrous oxide or methoxyflurane to be expected in the brain with intermittent administration to a woman in labor have been calculated. Moderate cyclic fluctuations in cerebral concentration of nitrous oxide result from intermittent administration on a two-minute cycle. For obstetric analgesia, nitrous oxide should be given about 50 seconds before the anticipated onset of each contraction for a period lasting approximately half the time between onsets of succeeding contractions. When methoxyflurane is administered intermittently, negligible cyclic variation in brain concentration can be expected when the patient is breathing normally. An explicit analytical model for the concentration effect is presented and its features discussed.