The effect of altitude on umbilical cord blood gases

Analysis of umbilical cord blood acid-base status has proven useful in the immediate care and resuscitation of the newborn and provides an objective measure of the intrapartum fetal environment. Our study was designed to determine whether there are any significant differences in the acid-base status of neonates delivered at a relatively high altitude of 6000 ft compared with sea-level controls.

Our study population consisted of 300 women who had undergone spontaneous vaginal delivery after uncomplicated labor; 150 delivered at an institution located at an elevation of 5900 ft and the remainder at an elevation of 87 ft. Exclusion criteria were hypertensive disease, diabetes, suspected fetal growth retardation, fetal distress, meconium-stained amniotic fluid, and chorioamnionitis. All patients received continuous electronic fetal monitoring, and tracings were reviewed by a physician blinded to the umbilical acid-base results. Umbilical cord arterial and venous samples were collected and analyzed within 30 minutes of delivery.

Compared with sea-level controls, neonates delivered at high altitude had a significantly higher mean pH with lower mean carbon dioxide pressure values in both arterial and venous specimens (P less than .05). The difference between the mean arterial pH values was 0.026 (P less than .05, 95% confidence interval [CI] 0.015-0.037) and mean venous values was 0.037 (P less than .05, 95% CI 0.027-0.047). There were no significant differences in the arterial or venous oxygen pressure or oxygen saturation values between the study and control groups. There was a significant decrease in the mean birth weight of 320 g (P less than .05, 95% CI 199.5-441.3) and higher pre-delivery hematocrit values (2.4%; P less than .05, 95% CI 1.6-3.2) in the high-altitude population compared with the sea-level group.

Neonates delivered at high altitude have significant alterations in umbilical blood acid-base measurements compared with neonates at sea level. Other significant differences include a lower infant birth weight and a higher maternal hematocrit relative to pregnant women at sea level.