Blood flow in the foetal superior vena cava and the effect of foetal breathing movements

Abstract

ObjectivesThe superior vena cava (SVC) drains venous blood from the upper foetal body, mainly the head. Data on the human foetus is scarce. Here we present reference values for the blood flow during the second half of pregnancy, and test the hypothesis that foetal breathing movements (FBM) enhance this flow. MethodsBased on a power calculation, 110 women with low-risk singleton pregnancies were recruited to a longitudinal study that included three sets of observations during the second half of pregnancy. Ultrasound was used to determine inner diameter, peak systolic blood velocity and time-average maximum velocities in the SVC during rest and respiratory activity. ResultsDuring the second half of pregnancy, SVC blood flow increased from 57.8mL/min (95% CI 51.7–64.3) to 221.5 (204.5–239.3). Based on 558 sets of observations obtained during foetal rest and FBM, we found an overall increase in diameter from 0.41cm (0.40–0.42) to 0.46 (0.44–0.48), peak systolic velocity from 35.9cm/s (34.9–37.0) to 62.2 (59.1–65.5), and time-averaged maximum velocity from 20.3cm/s (19.7–20.8) to 27.3 (26.1–28.6). This resulted in an overall 90% increase in mean SVC blood flow, from 108.1mL/min (98.8–117.9) at rest to 205.9 (183.2–230.5) during FBM. ConclusionThe blood flow in the SVC increases during the second half of pregnancy and is substantially augmented during FBM. Since high-amplitude FBM additionally reduces flow in the inferior vena cava, the net effect is a prioritised venous drainage from the foetal head enhancing the washout of CO2 in that area, which also contains the chemoreceptors.