Fetal venous circulation - an update on hemodynamics

The refinements of modern ultrasound techniques permit a renewed examinatio n of old concepts of fetal circulation. The concept of preferential streami ng of umbilical blood through the foramen ovale is verified by animal exper iments, and ultrasound studies have confirmed that a similar mechanism oper ates in human fetuses. However, the normalized umbilical flow appears to be less in the human than in fetal sheep, and decreases with advancing gestational age (115 ml min(- 1) kg(-1) at 20 and 64 mL min(-1) kg(-1) at 40 weeks). Compared to the 50% shunting of umbilical blood through the ductus venosus found in animal expe riments, the degree of shunting in the human fetus under physiological cond itions is considerably less, 30% at 20 weeks, which decreases to 18% at 32 weeks, suggesting a higher priority of the fetal liver than previously real ized. Augmented pulsatility in the precordial veins, ductus venosus, and umbilica l vein is an important clinical sign that is poorly understood. Recent flui d dynamic studies show that, apart from the pressure generated in the atria , it is the stiffness of the vessel wall, compliance, and notably, impedanc e which modify these waves. Particularly the substantial shift in impedance at the ductus venosus-umbilical vein junction causes wave reflection and r educed transmission of waves, the result being diminished or absent pulsati on in the umbilical vein.