The relationship between peak velocity in the fetal descending aorta and hematocrit in rhesus isoimmunization

To correlate the peak velocity in the fetal descending aorta, as measured by pulsed Doppler ultrasound, with fetal hematocrit values assessed by funipuncture in pregnancies complicated by rhesus isoimmunization.

One hundred twelve consecutive funipunctures were performed on 33 rhesus-negative gravidas of 21-36 weeks' gestation (median 30). Doppler flow, corrected for angle, was measured on the fetal descending aorta with pulsed Doppler equipment immediately before funipuncture. Differences between observed peak velocities and the calculated gestational age-dependent upper confidence limits (delta peak velocities) were compared with corresponding differences between observed hematocrits and the calculated lower confidence limits (delta hematocrits), and a regression analysis on the above paired difference values was performed. In addition, the correlation coefficient between delta peak velocities and delta hematocrits was calculated for the first procedure per pregnancy only.

The mean peak aortic velocity of anemic fetuses was higher than that of unaffected fetuses (P < .001); delta peak aortic velocities correlated negatively with delta hematocrits (r = -0.66, P < .001). The correlation coefficient between delta peak aortic velocities and delta hematocrits for the first procedure peer pregnancy only was r = -0.72 (P < .001). Prediction of fetal anemia by Doppler using gestational age-dependent 95% confidence limits was possible with positive and negative predictive values of 73 and 66%, respectively.

Peak aortic velocity, a noninvasive assessment of fetal anemia, may be used as an additional test for monitoring pregnancies complicated by rhesus isoimmunization. However, the limited predictive capacity hampers its clinical usefulness.