Cerebral Doppler blood flow velocimetry and central hemodynamics in the ovine fetus during hypoxemia-acidemia

Evaluation of utero-placental and fetal hemodynamic parameters throughout gestation in pregnant mice using high-frequency ultrasound

Throughout gestation, changes in maternal and fetal Doppler parameters in pregnant mice, similar to those obtained in human fetuses, were detected using high-frequency ultrasound with a 55-MHz linear probe. In the uterine arteries (UtA), fetal umbilical artery (UA) and fetal ductus venosus (DV) peak systolic velocity increased (UtA, p = 0.04; UA, p = 0.0004; DV, p = 0.02), end-diastolic velocity increased (UtA, p < 0.001; UA, p < 0.0001; DV, p = 0.01) and resistance index decreased (UtA, p = 0.0004; UA, p = 0.0001; DV, p = 0.04) toward the end of pregnancy. In the middle cerebral and carotid arteries, end diastolic velocity increased (p = 0.02 and p < 0.0001) and resistance index decreased (both vessels, p < 0.0001). There was a reduction in the pulsatile pattern in the umbilical vein (p < 0.05). The increased velocities and reduced resistance index suggest a progressive increment in blood flow to the fetal mouse toward the end of pregnancy. Fetal and utero-placental vascular parameters in CD-1 mice can be reliably evaluated using high-frequency ultrasound.

Ductus venosus velocimetry in acute fetal acidemia and impending fetal death in a sheep model of increased placental vascular resistance

We investigated whether hypoxemia without acidemia affects ductus venosus (DV) blood velocity waveform pattern in sheep fetuses with intact placenta and whether worsening acidemia and impending fetal death are related to changes in DV velocimetry in fetuses with increased placental vascular resistance. A total of 34 fetuses were instrumented at 115–136/145 days of gestation. Placental embolization was performed in 22 fetuses on the fourth postoperative day, 24 h before the experiment. The control group was comprised of 12 fetuses with intact placenta. The experimental protocol consisted of fetal hypoxemia that was induced by replacing maternal inhaled oxygen with medical air. To further deteriorate fetal oxygenation and blood-gas status, uterine artery volume blood flow was reduced by maternal hypotension. Fetuses that underwent placental embolization were divided into two groups according to fetal outcome. Group 1 consisted of 12 fetuses that completed the experiment, and group 2 comprised 10 fetuses that died during the experiment. DV pulsatility index for veins (PIV) and fetal cardiac outputs (COs) were calculated. Placental volume blood flow, fetal blood pressures, and acid base and lactate values were monitored invasively. On the experimental day, the mean gestational age did not differ significantly between the groups. In groups 1 and 2, the baseline mean DV PIV and fetal COs were not statistically significantly different from the control group. In the control group, the DV PIV values increased significantly with hypoxemia. In groups 1 and 2, the DV PIV values did not change significantly, even with worsening acidemia and imminent fetal death in group 2. During the experiment, the fetal COs remained unchanged. We conclude that fetal hypoxemia increases the pulsatility of DV blood velocity waveform pattern. In fetuses with elevated placental vascular resistance, DV pulsatility does not increase further in the presence of severe and worsening fetal acidemia and impending fetal death.

Fetal brain sparing is strongly related to the degree of increased placental vascular impedance

BACKGROUND

Doppler ultrasound examination has become an established method of clinical surveillance in high-risk pregnancies. Doppler indication of fetal brain sparing (BS) is an acknowledged sign of circulation redistribution during chronic hypoxia.

OBJECTIVES

To evaluate the relationship between placental vascular resistance and signs of BS in middle cerebral artery (MCA) blood flow velocity.

METHODS

The MCA, uterine artery, and umbilical artery (UA) were located by color Doppler ultrasound in 103 high-risk pregnancies at risk for intrauterine fetal hypoxia. The blood velocity spectrum was analyzed for the following parameters: pulsatility index (PI) and signs of notching in the uterine arteries. Z-score was calculated for the MCA PI.

RESULTS

Signs of BS in the MCA were correlated to increased placental vascular impedance. The degree of BS in the MCA expressed as Z-scores was correlated to increasing vascular impedance, both in the umbilical and uterine arteries, and to adverse outcome of pregnancy. The greatest deviation in MCA PI Z-scores was seen in preterm pregnancies.

CONCLUSION

A clear correlation exists between increasing placental vascular impedance and BS in the MCA. Preterm pregnancies express the greatest deviation from the mean MCA PI.

Abnormal arterial flows by a distributed model of the fetal circulation

Modeling the propagation of blood pressure and flow along the fetoplacental arterial tree may improve interpretation of abnormal flow velocity waveforms in fetuses. The current models, however, either do not include a wide range of gestational ages or do not account for variation in anatomical, vascular, or rheological parameters. We developed a mathematical model of the pulsating fetoumbilical arterial circulation using Womersley's oscillatory flow theory and viscoelastic arterial wall properties. Arterial flow waves are calculated at different arterial locations from which the pulsatility index (PI) can be determined. We varied blood viscosity, placental and brain resistances, placental compliance, heart rate, stiffness of the arterial wall, and length of the umbilical arteries. The PI increases in the umbilical artery and decreases in the cerebral arteries, as a result of increasing placental resistance or decreasing brain resistance. Both changes in resistance decrease the flow through the placenta. An increased arterial stiffness increases the PIs in the entire fetoplacental circulation. Blood viscosity and peripheral bed compliance have limited influence on the flow profiles. Bradycardia and tachycardia increase and decrease the PI in all arteries, respectively. Umbilical arterial length has limited influence on the PI but affects the mean arterial pressure at the placental cord insertion. The model may improve the interpretation of arterial flow pulsations and thus may advance both the understanding of pathophysiological processes and clinical management.

Abnormal fetal cerebral and umbilical Doppler measurements in fetuses with intrauterine growth restriction predicts the severity of perinatal morbidity

PURPOSE

This prospective study was performed to determine if the ratio of the middle cerebral artery (MCA) S/D ratio (ratio of peak systolic blood flow velocity to diastolic velocity) to the umbilical artery (UA) S/D ratio (MCA/UA S/D ratio) predicts the degree of neonatal morbidity in fetuses suspected of having intrauterine growth restriction (IUGR).

METHODS

Sixty-one fetuses were identified prospectively by sonography as having an estimated fetal weight below the 10th percentile for gestational age. The 61 fetuses underwent Doppler sonography in the third trimester and then were stratified into 3 groups based on the MCA/UA S/D ratio: group A, MCA/UA S/D ratio > 1.0 (controls; n = 37); group B, MCA/UA S/D ratio < or = 1.0 (intracerebral blood flow redistribution; n = 16); and group C, reversed or absent UA diastolic flow (n = 8). Outcome variables assessed included gestational age at delivery, birth weight, UA pH, mode of delivery, respiratory distress syndrome requiring intubation, and intracranial hemorrhage.

RESULTS

The mean MCA/UA S/D ratios in groups A and B were 1.69 + /- 0.61 and 0.59 + /- 0.24, respectively (p < 0.01). The mean gestational ages at delivery for groups A, B, and C were 34.7, 33.2, and 29.0 weeks, respectively. The mean birth weights were below the fifth percentile for age for groups B and C and significantly related to the severity of abnormal Doppler findings (p < 0.01) after correction for age. Mean UA pHs were 7.25 + /- 0.01, 7.19 + /- 0.01, and 7.14 + /- 0.13 for groups A, B, and C, respectively, with significant differences between groups A and B (p < 0.05) and groups A and C (p < 0.05). Respiratory distress syndrome and intracranial hemorrhage were not associated with abnormal Doppler findings after correction for gestational age. The interval between the abnormal Doppler examination and delivery (p < 0.001) and the occurrence of fetal distress requiring cesarean section (p < 0.001) were significantly related to the severity of Doppler findings.

CONCLUSIONS

In fetuses with suspected IUGR, abnormal MCA/UA S/D ratios are strongly associated with low gestational age at delivery, low birth weight, and low UA pH. Abnormal MCA/UA S/D ratios are also significantly associated with shorter interval to delivery and the need for emergent delivery.