Respiratory and hemodynamic effects of methadone in pregnant women.

Minute ventilation, end-tidal PCO2, O2 and CO2 concentrations in expired air, pulse rate and arterial blood pressure were measured in the last half of pregnancy in eight women taking methadone daily. Measurements were made with the subjects seated at rest, during the steady state of 50-watt bicycle exercise, and during recovery. Calculations of O2 consumption. CO2 production, alveolar ventilation and oxygen debt were made. Studies were repeated in five subjects postpartum. Methadone diminishes the normal hyperventilation of pregnancy and its effect persists for more than 24 h. When comparisons are made of pregnant and postpartum values, some respiratory stimulation during pregnancy is apparent. Maternal oxygen debt following standard exercise during pregnancy is diminished after the daily dose of methadone and the maternal heart rate response to exercise is diminished concurrently. The maternal hypoventilation induced by methadone and maintained during exercise may be relevant to the low birth weights and high incidence of sudden infant death syndrome observed by others in the offspring of methadone-dependent women.