In vitro demonstration of inhibition of retrograde flow in the human umbilical vein

This study was designed to investigate the apparent inhibition of retrograde flow in umbilical veins. While flushing fluid through cord veins for another study it appeared that there existed an impedance to retrograde flow. Simple hydraulic flow and Doppler ultrasound were used to evaluate this observation. First, in vitro studies were carried out on 100 normal umbilical cords to measure the time it took fluid to flow forward through a 30-cm segment of umbilical vein. These data were then compared to the time required ed for flow in the opposite direction. The average time was 8.84 s in the antegrade direction and 9.97 s in the retrograde direction. This difference was statistically significant with a p-value of < 0.001. To evaluate this observation under more controlled conditions the second phase of the study was conducted. During phase two, umbilical cords were collected from term uncomplicated pregnancies. Doppler test fluid was Pumped through the cord at a rate based on newborn weight. The volume and velocity were measured in both directions by Doppler ultrasound. Average flow volume was 369.1 ml/min in the antegrade direction and 174.8 ml/min in the retrograde direction. The velocities were 9.1 cm/s and 5.5 cm/s, respectively. The difference in mean flow volume was 194.3 ml/min. The difference of the mean velocity was 3.58 cm/s. Both are statistically significant with p-values of < 0.01. These data suggest there is an intrinsic inhibition of retrograde flow in the umbilical vein. The etiology and significance of this finding are discussed.