DOWNSTREAM DETERMINANTS OF PULSATILITY OF THE MEAN VELOCITY WAVE-FORMIN THE UMBILICAL ARTERY AS PREDICTED BY A COMPUTER-MODEL

An electrical analog model of the umbilicoplacental circulation that w as based on hemodynamic measurements in fetal sheep was used to predic t the mean blood velocity waveform in the umbilical artery, A 300% inc rease in placental resistance reduced the normally forward diastolic b lood velocity in the umbilical artery nearly to zero, When resistance was normal, variations in umbilical arterial wall thickness (h) and el astic modulus (E) had little effect on the velocity waveform, whereas a 20% increase in radius (a) halved the ratio of the diastolic-to-syst olic blood velocity (D/S), When placental resistance was high, abnorma l umbilical arterial wall properties (a large, or h or E small) furthe r reduced D/S, We conclude that high placental resistance, low umbilic al arterial resistance, or a combination of high placental resistance with abnormal umbilical arterial wall properties are the primary downs tream determinants of elevated pulsatility in umbilical arterial blood velocity.