Maternal hyperventilation helps preserve arterial oxygenation during high-altitude pregnancy.

We examined arterial oxygenation during pregnancy and 3 mo postpartum in 35 nonsmoking residents of Leadville, CO (elevation 3,100 m) to determine how well and by what mechanisms maternal arterial oxygenation was maintained during pregnancy at high altitude. Hyperventilation raised arterial O2 saturation above that in the nonpregnant state. Respiratory alkalosis persisted throughout pregnancy, shifting the O2-hemoglobin dissociation curve to the left, further facilitating O2 loading in the lung. However, a decrease in blood hemoglobin concentration and a slight increase in the alveolar-arterial O2 gradient in the late pregnancy caused the arterial O2 content to fall below that in the nonpregnant state. Compared to published sea level values, the Leadville women had higher ventilation and hemoglobin values, yielding arterial O2 contents as high as in pregnant women at sea level. Thus, ventilation and hemoglobin concentration were important variables contributing to O2 ttransport during pregnancy at high altitude.