OC18.04: Direct measurement of blood flow into the intervillous space and its relationship with uterine artery blood flow

Methods: Based on a power calculation 110 women with low-risk singleton pregnancies were recruited after written consent according to ethical approval. Inner diameter and peak blood velocity were measured in the IVC at the level below the ductus venosus outlet during rest and respiratory activity including peak inspiratory movement during the 3rd trimester. The Bernoulli equation was used to calculate pressure gradients. t-test and 95% CI of the mean were used to document differences. Results: At median gestational week 36 (range 32–38) the IVC was wider at rest than during respiratory activity, 5.3 mm (95% CI 5.1–5.5) vs. 4.7 (4.4–4.9), respectively, and considerably less during high-amplitude inspiratory movement, 1.7 (1.4–1.8). Peak systolic blood velocity increased from 41cm/s (38–47) during rest to 130 (121–140) during high-amplitude inspiration, which corresponds to an increase in pressure gradient from average 0.7 to 6.8 mmHg between the abdominal IVC and the atria. Conclusions: During high-amplitude fetal inspiration the abdominal IVC is almost completely blocked and the pressure gradient to the chest is increased by a factor of 10 supporting the concept that respiratory movements facilitate upper body venous drainage and CO2 washout, and additionally give preference to ductus venosus flow at the expense of IVC drainage.