Phasic flow events at the aortic isthmus-ductus arteriosus junction and branch pulmonary artery evaluated by multimodal ultrasonography in fetal lambs.

Abstract

We assessed the phasic flow and interaction between the pulmonary trunk and aortic isthmus flow at their junction in the lamb fetus in late gestation and also assessed the interaction of the left pulmonary artery branch. With echocardiographic and Doppler ultrasonographic and saline-contrast techniques, we studied 7 fetal lambs with arterial and venous catheters in place to assess direction, velocity, and timing of flow at the aortic isthmus, ductus arteriosus, and proximal left pulmonary artery. At the isthmus-ductus junction, ductus systolic flow occurred later (0.048 +/- 0.006 second, mean +/- SD) and accelerated more slowly than isthmus flow but with higher velocities (peak 70.7 +/- 7.1 vs 63.1 +/- 6.3 cm/s, velocity time integral 5.7 +/- 1.2 vs 4.5 +/- 1.3 cm, respectively; P <.001). There was a small degree of late systolic flow reversal and admixture from both sources. Signals from the left pulmonary arterial branch showed a sharp, brief systolic forward flow with a peak velocity of 48.8 +/- 9.1 cm/s, followed by late systolic and diastolic flow reversal with a peak velocity of 23.5 +/- 8.7 cm/s. The differences in the flow timing may be the result of different timing of ventricular contraction, resistances in the vascular beds, and ductus constriction, both anatomic and physiologic.