Venous return in the fetal-placental cardiovascular system.

For a quantitative understanding of the venous Doppler sonograms, a model of the fetal cardiovascular system is proposed which stresses the significance of flow. The model comprises three major components: the heart which provides constant power and dynamic capacitance, the peripheral fetal and placental vascular beds with flow limiting conductances (1/resistance), and the resistance of the hepato-ductal system. As long as cardiac power is maintained, the conductances control the flow rate and, depending upon hepato-ductal and ventricular filling resistance, the umbilical venous as well as the central venous pressure. According to the model, the mean flow velocity of venous return is total organ flow rate divided by cross sectional areas in the cardiac input system, i.e. the central veins, both atria and both AV valves respectively. The velocity pattern of venous return is related to cardiac performance and hepato-ductal resistance as follows: The E wave in early diastole describes the filling (driven by the central-venous pressure) of the relaxed ventricle. The time constant equals the ratio of elasticity over resistance in the ventricular wall. The A wave mirrors the contractility of atrial myocardium which is related to central venous pressure. The ratio of the forward A wave in the tricuspid valve to the backward A wave in the caval veins depends on ventricular capacitance. The S wave is related to systolic valvular shift. The velocity pulsations in the ductus venosus are transmitted backwards from the heart. The continuous velocity component in the ductus venosus and the non-pulsatile velocity in the umbilical vein are due to hepato-ductal and placental vascular resistance.