Diastolic circulatory dynamics in the presence of elevated placental resistance and retrograde diastolic flow in the umbilical artery: a Doppler echographic study in lambs.

To evaluate the effects of elevated placental resistance on the diastolic blood flow in the main arteries of the fetus, placental resistance was mechanically increased in six exteriorized lambs. A string was inserted into the exposed section of the umbilical cord and placed around the veins isolating them from the arteries. Doppler flow velocities waveforms were measured by placing a lightweight acoustic bag on the umbilical cord. Placental resistance was increased by tightening the string until retrograde diastolic flow was observed in the umbilical artery. All measurements were obtained within 10 minutes. Heart rate, PO2, PCO2, and pH before umbilical vein compression were 177 +/- 24 beats/min, 16.4 +/- 1.8 mm Hg, 42.5 +/- 4 mm Hg, and 7.3 +/- 0.05, respectively, and 112 +/- 38 beats/min, 5.9 +/- 1.5 mm Hg, 56.6 +/- 7.0 mm Hg, and 7.2 +/- 0.05, respectively, after umbilical vein compression. The patterns of diastolic flow observed after compression were descending aorta, retrograde; aortic arch, retrograde; ascending aorta, bidirectional; cephalic aorta, forward, which were quite different from their respective baseline patterns. It is concluded that the appearance of reverse diastolic flow in the umbilical artery indicates that (1) the lowest vascular resistance in the fetal circulatory network is no longer at the placental but at the cerebral level, and (2) preplacental blood with low oxygen content from the descending aorta and pulmonary artery is being shifted toward the brain.