Right and left atrial pressures were measured in eight chronically instrumented fetal and neonatal lambs. Flows were measured with a combination of electromagnetic flow sensor and microsphere techniques. Three of the fetuses were ventilated in utero during the measurements. Four fetuses were studied as neonates immediately after spontaneous term delivery and one was studied as a normal fetus in utero. Data from these preparations were augmented with seven sets of previously reported data from normal fetuses in utero for analysis. Linear least‐squares regression analysis demonstrated that inferior caval vein flow into the right atrium was inversely related to right atrial pressure. This flow could not be demonstrated to depend on the velocity of blood in the inferior caval vein. Non‐linear least‐squares regression analysis of foramen ovale flow as a function of a power of the flow in the inferior caval vein revealed that the square of the velocity of blood in the inferior caval vein predicted foramen ovale flow. Of the two forces that determine foramen ovale flow in the fetus, pressure difference and kinetic energy, the latter was far larger than the former. These results support the theory that the fetal foramen ovale is maintained in an open position by the kinetic energy of the blood in the inferior caval vein.
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