NITROUS-OXIDE INHALATION - EFFECTS ON MATERNAL AND FETAL CIRCULATIONSAT TERM

Objective: To evaluate the hemodynamic effects of nitrous oxide inhala tion in normal term pregnancy. Methods: Twenty healthy term pregnant w omen were given 30% nitrous oxide in pure oxygen for 2 minutes, and th e hemodynamics were assessed by pulsed-wave color Doppler velocimetry of the uterine and internal carotid artery of the mother and the umbil ical and middle cerebral artery of the fetus. Each vessel was assessed separately, allowing a 5-minute wash-out period between the inhalatio ns. The measurements were continued for 2 minutes after the inhalation , and the pulsatility index (PI) was determined at 1-minute intervals. The maternal heart rate and blood pressure (BP) were recorded before and after inhalation; fetal well-being was confirmed with cardiotocogr aphy. Analysis of variance for repeated measurements and paired-sample t test were used for statistical analysis. Results: A significant dec rease in the PI of the maternal internal carotid artery was observed a fter 2-minutes of inhalation (from 0.83 +/- 0.22 to 0.71 +/- 0.20; P < .001). The uterine artery PI and maternal BP and heart rate were not affected by nitrous oxide. A significant decrease was evident even in the fetal middle cerebral artery PI (from 1.37 +/- 0.27 to 1.22 +/- 0. 17; P = .02). The umbilical artery PI remained unchanged. Conclusion: Both maternal and fetal central vascular resistance were decreased by 30% nitrous oxide inhalation. So far, no adverse effects to mother or fetus have been demonstrated in clinical practice. However, preterm fe tuses are susceptible to intracranial hemorrhage, and the cerebral hyp eremia by nitrous oxide might increase the risk of hemorrhage in these fetuses. This hypothesis requires further investigation.