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 of residence there.
DESIGN: Cross-sectional study.
SETTING: Antenatal clinics at sea level in Lima, Peru and at high altitude 4300m in Cerro de Pasco, Peru. POPULATION: A total of 304 normal pregnant women between 7 and 41 weeks of gestation 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 and 19 at high altitude) were also studied.
METHODS: Arterialised blood gases, haemoglobin, pulse oximetry and minute ventilation were measured once in each woman. MAIN OUTCOME MEASURES: PO2, PCO2, pH, haemoglobin, oxygen saturation, arterial oxygen content and minute ventilation and their relationship to gestation.
RESULTS: PO2, PCO2, bicarbonate, base excess and oxygen saturation were lower in pregnancies at high altitude compared with sea level; pH, haemoglobin, arterial oxygen content and minute ventilation were higher. At high altitude 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 adequate in the first trimester but declined towards term. However, maternal oxygenation was maintained in those whose family had lived longest at high altitude suggesting a beneficial adaptation to a hypoxic environment, occurring over generations.