Changes in flow velocity patterns of the superior and inferior venae cavae during placental circulatory insufficiency

Reference ranges of fetal superior vena cava blood flow velocities and pulsatility index in the second half of pregnancy: a longitudinal study

Background

Fetal superior vena cava (SVC) is essentially the single vessel returning blood from the upper body to the heart. With approximately 80-85% of SVC blood flow representing cerebral venous return, its interrogation may provide clinically relevant information about fetal brain circulation. However, normal reference values for fetal SVC Doppler velocities and pulsatility index are lacking. Our aim was to establish longitudinal reference intervals for blood flow velocities and pulsatility index of the SVC during the second half of pregnancy.

Methods

This was a prospective study of low-risk singleton pregnancies. Serial Doppler examinations were performed approximately every 4 weeks to obtain fetal SVC blood velocity waveforms during 20–41 weeks. Peak systolic (S) velocity, diastolic (D) velocity, time-averaged maximum velocity (TAMxV), time-averaged intensity-weighted mean velocity (TAMeanV), and end-diastolic velocity during atrial contraction (A-velocity) were measured. Pulsatility index for vein (PIV) was calculated.

Results

SVC blood flow velocities were successfully recorded in the 134 fetuses yielding 510 sets of observations. The velocities increased significantly with advancing gestation: mean S-velocity increased from 24.0 to 39.8 cm/s, D-velocity from 13.0 to 19.0 cm/s, and A-velocity from 4.8 to 7.1 cm/s. Mean TAMxV increased from 12.7 to 23.1 cm/s, and TAMeanV from 6.9 to 11.2 cm/s. The PIV remained stable at 1.5 throughout the second half of pregnancy.

Conclusions

Longitudinal reference intervals of SVC blood flow velocities and PIV were established for the second half of pregnancy. The SVC velocities increased with advancing gestation, while the PIV remained stable from 20 weeks to term.

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

OBJECTIVES

The 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.

METHODS

Based 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.

RESULTS

During the second half of pregnancy, SVC blood flow increased from 57.8 mL/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 0.41 cm (0.40-0.42) to 0.46 (0.44-0.48), peak systolic velocity from 35.9 cm/s (34.9-37.0) to 62.2 (59.1-65.5), and time-averaged maximum velocity from 20.3 cm/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 108.1 mL/min (98.8-117.9) at rest to 205.9 (183.2-230.5) during FBM.

CONCLUSION

The 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 CO(2) in that area, which also contains the chemoreceptors.

The importance of venous Doppler velocimetry for evaluation of intrauterine growth restriction

The management of growth-restricted fetuses requires accurate diagnosis to optimize the timing of delivery. Doppler velocimetry is the only noninvasive method for assessing the fetoplacental hemodynamic status. This review will give a critical overview of the current knowledge on fetal venous blood flow in pregnancies complicated by in-trauterine growth-restricted fetuses. Adaptation of the circulation in intrauterine growth-restricted fetuses is described. Normal and abnormal venous Doppler waveforms are presented. Correlations of abnormal waveforms with the presence of acidemia and perinatal outcomes are emphasized. Limitations of venous Doppler velocimetry for optimizing the time for delivery and the perinatal outcome are also presented.

Fetal growth restriction due to placental disease

Normal fetal growth depends on the genetically predetermined growth potential and its modulation by the health of the fetus, placenta and the mother. Fetuses that are small because of intrauterine growth restriction (IUGR) are at higher risk for poor perinatal and long-term outcome than those who are appropriately grown. Of the many potential underlying processes that may result in IUGR, placental disease is clinically the most relevant. Fetal cardiovascular and behavioral responses to placental insufficiency and the metabolic status are interrelated. The concurrent evaluation of fetal biometry, amniotic fluid volume, heart rate patterns, arterial and venous Doppler, and biophysical variables therefore allow the most comprehensive fetal evaluation in IUGR. In the absence of successful intrauterine therapy, the timing of delivery is perhaps the most critical aspect of the antenatal management. A discussion of the fetal responses to placental insufficiency and a management protocol that accounts for multiple Doppler and biophysical parameters as well as gestational age is provided in this review.