Dilatation of the ductus venosus in human fetuses: ultrasonographic evidence and mathematical modeling.

Autonomic regulation of blood flow through the fetal ductus venosus has been suggested, but the existence of a sphincter at the ductal entrance in human fetuses has yet to be established. In this paper two cases of apparent ductus venosus dilatation in two growth-restricted human fetuses are reported. Prolonged ultrasonographic analysis (45 min) showed rapid and substantial changes (>80%) of ductal diameters. Pulsed Doppler analysis was used to investigate flow velocity in the ductus venosus and umbilical vein for both normal and dilated conditions. Dilated conditions caused manifest modifications of velocity tracings. Systolic peak velocity in the ductus did not change visibly, whereas velocity at the atrial contraction showed evident reduction; consequently, pulsatility indexes increased. Furthermore, the umbilical vein presented flow velocity pulsations. The mean blood flow rate through the ductus seemed to increase substantially (>70%) for high dilatation. To investigate these findings further, we performed simulations of ductal dilatation by means of a lumped-parameter mathematical model of the human fetal circulation. Model results agreed with clinical evidence and confirmed the relationship between ductal dilatation and the observed velocity alterations. Simulated systolic peak velocity slightly increased for small dilatation (<30%), whereas atrial velocity was reduced when the ductus dilated. Furthermore, the model indicated that umbilical venous pressure decreases for increasing dilatation, whereas no change occurs in the central venous pressure. The present results seem to indicate the presence of active dilatation of the ductus venosus in human fetuses.