Assessment of flow events at the ductus venosus-inferior vena cava junction and at the foramen ovale in fetal sheep by use of multimodal ultrasound.

Abstract

Previous techniques for the study of the fetal circulation did not permit assessment of phasic events associated with the cardiac cycle. We used multimodal ultrasound techniques to examine flow events that occur in the major veins and across the foramen ovale in the circulation of the fetal lamb. We studied eight fetal lambs instrumented with catheters in the superior and inferior venae cavae and a peripheral umbilical vein and performed ultrasound studies that included M-mode and two-dimensional imaging, pulsed and Doppler color flow ultrasound, and contrast echocardiography to evaluate flow in the ductus venosus, in both venae cavae, and through the foramen ovale. Two blood streams of different flow velocities were identified within the cephalic portion of the inferior vena cava. The stream that originated from the narrowed ductus venosus had a higher velocity than that from the caudal inferior vena cava (mean velocity, 57 +/- 13 versus 16 +/- 3 cm/s; P < .0002). Facilitated by the eustachian valve and the septum primum, the ductus venosus stream preferentially passed through the foramen ovale to the left atrium. This flow occurred during most of the cardiac cycle, except for 19.6 +/- 2.3% of the cycle when the foramen ovale was closed during atrial contraction. Superior vena cava flow passed almost exclusively into the right atrium and tricuspid valve; a small amount that was refluxed from the right atrium into the inferior vena cava subsequently passed through the foramen into the left atrium. Visualization of fetal circulatory streaming at the venous sites by ultrasound techniques aids in understanding the function of the fetal circulation and may be helpful in detecting the human fetus that is hemodynamically compromised.