Blood gases in pregnancy at sea level and at high altitude

Objective 1. To measure blood gases and minute ventilation in healthy women with normal pregnancies, compare with non-pregnant women both at sea level and at high altitude; 2. to relate the results at altitude with duration o f residence there. Design Cross-sectional study. Setting Antenatal clinics at sea level in Lima, Peru and at high altitude 4 300m in Cerro de Pasco, Peru. Population A total of 304 normal pregnant women between 7 and 41 weeks of g estation were studied, including 112 who lived at sea level and 192 at high altitude (4300m). For comparison 38 non-pregnant women (19 at sea level an d 19 at high altitude) were also studied. Methods Arterialised blood gases, haemoglobin, pulse oximetry and minute ve ntilation were measured once in each woman. Main outcome measures PO2, PCO2, pH, haemoglobin, oxygen saturation, arteri al oxygen content and minute ventilation and their relationship to gestatio n. Results PO2, PCO2, bicarbonate, base excess and oxygen saturation were lowe r in pregnancies at high altitude compared with sea level; pH, haemoglobin, arterial oxygen content and minute ventilation were higher. At high altitu de oxygen saturation and haemoglobin decreased towards term resulting in a fall in arterial oxygen content at the end of pregnancy. Women whose family had lived at high altitude for at least three generations maintained their oxygenation throughout pregnancy better than women whose family had lived there for less than three generations. Conclusions In pregnancy at high altitude maternal adaptation appeared adeq uate in the first trimester but declined towards term. However, maternal ox ygenation was maintained in those whose family had lived longest at high al titude suggesting a beneficial adaptation to a hypoxic environment, occurri ng over generations.