Effects of nitrous oxide/oxygen inhalation on the maternal circulation during vaginal delivery

The hemodynamic changes occurring during the first stage of labor were studied in 24 healthy pregnant women during inhalation of different nitrous oxide/oxygen (N2O/O2) gas mixtures (intermittent 70/30, 40/60, 0/100 and continuous 40/60). Cardiac output increased (P less than 0.01) from 6.6 +/- 0.2 l/min between uterine contractions to 8.5 +/- 0.3 l/min during contractions. Heart rate, stroke volume, systolic, diastolic and mean arterial pressures were increased (P less than 0.01) and total peripheral vascular resistance was reduced (P less than 0.01) during contractions compared to measurements performed between contractions. The maternal circulation was influenced by the use of N2O/O2. During intermittent inhalation, higher concentrations of N2O were associated with a decrease (P less than 0.01) in heart rate, cardiac output and arterial pressure as well as an increase (P less than 0.01) in stroke volume. The degree of pain relief also increased (P less than 0.01) with increasing concentrations of inhaled nitrous oxide. The circulatory influence of intermittent inhalation given during uterine contractions was also apparent during the interval between contractions when N2O/O2 was not administered. The most obvious effects on both circulation and pain were demonstrated during continuous inhalation of N2O/O2. There was a close association between analgetic and cardiovascular effects indicating that the latter, at least partly, were due to pain relief. However, it was not possible to exclude or confirm possible direct pharmacological effects of N2O/O2 on maternal circulation.