Fetal Doppler velocimetry at high altitude

To determine impedance and velocity characteristics of the fetal circulation using Doppler ultrasound, at extremely high altitude (4300 m) in the Peruvian Andes compared to an ethnically similar population at sea level.

This was a cross-sectional study of 196 women resident at high altitude (Cerro de Pasco, 4300 m above sea level) and 196 women resident at sea level (Lima) with normal singleton pregnancies. Pulsatility index (PI), maximum velocity (Vmax) and minimum velocity (Vmin) in the umbilical artery, the descending aorta, the middle cerebral artery and the ductus venosus were compared between the two populations using fractional polynomial regression analysis.

The PI was higher at high altitude than at sea level in the umbilical artery (regression coefficient = 0.112, P < 0.001), and not significantly different in the descending aorta, middle cerebral artery and ductus venosus. Vmax was lower at high altitude than at sea level in all three arterial vessels assessed; Vmin was lower in two: the umbilical artery and the descending aorta. The high-altitude/sea-level ratios for umbilical artery Vmax and Vmin were 0.93 and 0.82, respectively (P < 0.001 for each), the ratios for descending aorta Vmax and Vmin were 0.93 and 0.89, respectively (P = 0.003 and P < 0.001, respectively), and the regression coefficient for the middle cerebral artery Vmax was -2.844 (P = 0.003). There was no significant difference in the middle cerebral artery Vmin or in the ductus venosus Vmax and Vmin.

Despite the lower ambient oxygen at high altitude and an increase in umbilical artery PI, the fetal circulation does not exhibit a 'brain sparing effect'. This and the overall decrease in blood flow velocities in the fetal circulation at high altitude may be due to the increased fetal hematocrit, which will result in increased blood viscosity.