Umbilical venous velocity pulsations are related to atrial contraction pressure waveforms in fetal lambs

Objective: To identify the source of umbilical venous velocity pulsations, times of transmission from the atrial contraction pressure waveform to velocity waves in the inferior vena cava, ductus venosus, intra- abdominal umbilical vein, and intra-amniotic umbilical vein were examined. Methods: Five lamb fetuses at 125-135 days' gestation were instrumented with solid state pressure transducers in the inferior vena cava, fluid-filled catheters in the inferior vena cava and descending aorta, and epicardial pacemakers. Three to 5 days postoperatively, inferior vena cava, ductus venosus, and umbilical vein velocities were examined with Doppler ultrasound. Normal saline was administered until umbilical vein velocity pulsations developed (180 ± 60 mL). In three fetuses, premature atrial contractions were induced under baseline conditions and after umbilical vein velocity pulsations developed. Results: Times of transmission from the atrial contraction pressure waveform until velocity decreases in the fetal venous system were significantly different in the inferior vena cava, ductus venosus, intra-abdominal umbilical vein, and intra-amniotic umbilical vein (P < .001). Times increased with the distance from the atrium. Inferior vena cava pressure increased with fluid administration from 3.7 ± 4.7 mmHg to 9.3 ± 2.3 mmHg (P < .01). Time from increased pressure waveforms with induced premature atrial contractions to the nadir of subsequent umbilical vein velocity waves decreased from 0.123 ± 0.047 seconds before saline administration to 0.072 ± 0.039 seconds after saline administration (P < .001). Conclusion: Transmission time of atrial pressure into the venous circulation increases with distance from the atrium and decreases with volume loading. Umbilical venous velocity pulsations derive from atrial pressure changes transmitted in a retrograde fashion.