Umbilical artery flow velocity waveforms during acute hypoxemia and the relationship with hemodynamic changes in the fetal lamb

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

The purpose of this study was to assess the effects of hypocapnic hypoxia, acidemia and the combination of hypoxia/acidemia on blood flow velocity variables in the fetal cerebral circulation. Chronically instrumented fetal sheep were used and the ewes were induced to breathe a hypoxic gas mixture for about 90 min. This caused an initial period of hypoxemia followed by a period of mixed hypoxemia/acidemia. When the ewe was reoxygenated, the fetus experienced a period of normoxic acidemia. The fetal cerebral circulation was assessed by recording Doppler blood flow velocity waveform variables in a cerebral vessel and the umbilical artery, using standard ultrasound equipment. External carotid artery blood flow was maintained during hypoxic and hypoxic/acidotic periods despite a fall in cardiac output. In the cerebral vessel, mean maximum velocity (time-averaged maximum velocity), minimum diastolic velocity and maximum systolic velocity manifested increases during hypoxic and hypoxic-acidotic periods, but pulsatility index did not change due to the effect of reduced heart rate on pulsatility index. Umbilical artery pulsatility index increased in the hypoxic and hypoxic-acidotic periods, despite unchanged mean maximum velocity, minimum diastolic velocity and maximum systolic velocity. With acute hemodynamic changes, the measurement of pulsatility index can yield misleading results. For clinical and experimental research on the fetal cerebral circulation, more attention should be paid to the individual Doppler variables, especially to the mean maximum velocity, than to the pulsatility index alone. Changes in mean maximum velocity recorded from the cerebral artery seem to reflect changes in the cerebral arterial flow.

Downstream determinants of pulsatility of the mean velocity waveform in the umbilical artery as predicted by a computer model

An electrical analog model of the umbilicoplacental circulation that was based on hemodynamic measurements in fetal sheep was used to predict the mean blood velocity waveform in the umbilical artery. A 300% increase in placental resistance reduced the normally forward diastolic blood velocity in the umbilical artery nearly to zero. When resistance was normal, variations in umbilical arterial wall thickness (h) and elastic modulus (E) had little effect on the velocity waveform, whereas a 20% increase in radius (a) halved the ratio of the diastolic-to-systolic blood velocity (D/S). When placental resistance was high, abnormal umbilical arterial wall properties (a large, or h or E small) further reduced D/S. We conclude that high placental resistance, low umbilical arterial resistance, or a combination of high placental resistance with abnormal umbilical arterial wall properties are the primary downstream determinants of elevated pulsatility in umbilical arterial blood velocity.

Acute hemodynamic effects induced by chorionic villus sampling: a preliminary investigation

OBJECTIVE

Our purpose was to evaluate the effects of chorionic villus sampling on the fetal heart rate, the fetal umbilical artery pulsatility index, and the maternal arcuate artery resistance index.

STUDY DESIGN

M-mode fetal heart rate, pulsed Doppler fetal umbilical artery pulsatility index, and maternal arcuate artery resistance index measurements were obtained before and immediately after chorionic villus sampling in 50 patients and were compared with 50 procedure-free, gestational age--matched controls. Comparisons within groups were performed with paired T tests and mean changes between groups with two-sample T tests, and variance comparisons between groups were performed with F tests.

RESULTS

The mean changes (SD) in fetal heart rate, umbilical artery pulsatility index, and arcuate artery resistance index in patients undergoing chorionic villus sampling and in controls were not statistically different from zero or from each other. However, the variance around the mean fetal heart rate change was 17.1 times larger in the chorionic villus sampling group than the control group (F 17.1, degrees of freedom 49,49, p < 0.0001), and the variance around the mean pulsatility index change was 2.7 times larger in the chorionic villus sampling group than in the control group (F 2.7, degrees of freedom 49,49, p = 0.0007). There was no significant difference in mean resistance index change or variance around the mean resistance index change between groups.

CONCLUSION

Chorionic villus sampling induces significant but unpredictable fluctuations in fetal heart rate and umbilical artery pulsatility index but does not affect maternal arcuate artery resistance index.

Fetal Doppler velocimetry in the internal carotid and umbilical artery during Braxton Hicks' contractions

Using Doppler ultrasound, previous studies revealed a considerable increase in vascular resistance in the uteroplacental circulation during Braxton Hicks' contractions. Consequently, uteroplacental blood flow is reduced and this affects placental oxygen transfer to the fetus, causing a fall in fetal arterial PO2. In view of the important role of arterial PO2 in the regulation of cerebral blood flow in the fetus, we hypothesised that Braxton Hicks' contractions cause a decrease in cerebral vascular resistance. A study was undertaken in 16 healthy near term pregnancies, using pulsed-wave Doppler ultrasound to evaluate the influence of Braxton Hicks' contractions on cerebral vascular resistance of the fetus. Flow velocity waveforms (FVWs) were recorded of the fetal internal carotid and umbilical artery and the Pulsatility Index (PI) was calculated. During Braxton Hicks' contractions the PI in the recorded vessels did not change. Fetal heart rate showed also no changes during Braxton Hicks' contractions. These findings indicate that resistance to blood flow downstream of these arteries, is not significantly altered, suggesting that Braxton Hicks' contractions have little or no effect on fetal haemodynamics and on fetal oxygenation in the healthy near term fetus.

Mathematical modeling of the human fetal arterial blood circulation

A mathematical model of the human fetal arterial circulation based on mass and momentum conservation for one-dimensional flow is presented. We simplified the fetal arterial vascular system from the heart to the placenta, defined 16 anatomical segments and studied the characteristics of the vascular system in relation to changes in morphology and hemodynamics. The two-step Lax-Wendroff finite difference scheme was used to solve the system of equations, after introducing the rheological constants, the diameter and length of the segments measured by two-dimensional imaging and the mean arterial velocity at the inlet segments obtained by pulsed Doppler. The model was validated by comparing the numerical results to our non-invasive ultrasound direct measurements and to previous published data.

The acute response of the umbilical artery pulsatility index to changes in blood volume in fetal sheep

This experimental study was designed to test the effects of acute changes in fetal circulating blood volume on the umbilical artery pulsatility index (PI). Six fetal sheep were provided with an electromagnetic flow meter for measurement of umbilical venous blood flow, with catheters for determination of arterial blood pressure and umbilical venous pressure, and with a 5 MHz Doppler transducer around one umbilical artery for flow velocity waveform analysis. A catheter in the inferior vena cava was used to infuse 50 ml of maternal blood (hypervolemia) into the fetal circulation or to withdraw 50 ml of fetal blood (hypovolemia) after volume correction. Hypervolemia resulted in a rise in arterial pressure and umbilical venous pressure, without an effect on PI, umbilical blood flow or placental vascular resistance. Hypovolemia resulted in a decrease in fetal heart rate, arterial pressure, umbilical venous pressure and umbilical blood flow. Calculated placental vascular resistance was not changed, whereas the PI increased by 42%. We conclude that volume loading with 10-15% of fetal circulating volume does not affect the umbilical artery PI, whereas acute reduction of fetal blood volume with the same amount is associated with an increase in the umbilical artery PI, without changes in calculated placental vascular resistance.

Fetal hypertension induced by norepinephrine infusion and umbilical artery flow velocity waveforms in fetal sheep

This study was designed to examine the effects of fetal hypertension on the umbilical artery pulsatility index. Fetal arterial blood pressure and umbilical venous pressure were measured in eight sheep, 3 to 5 days after surgery. Umbilical blood flow was measured with an electromagnetic flowmeter around the common umbilical vein. Umbilical artery flow velocity waveforms were obtained either by an indwelling 5 MHz pulsed Doppler device (n = 4) or transcutaneously by a 4 MHz continuous-wave Doppler device (n = 4). Fetal blood pressure was raised by intravenous infusion of norepinephrine 10 micrograms/min during 5 minutes. Norepinephrine infusion resulted in elevated arterial and umbilical venous pressures, accompanied by a bradycardia during the first 3 minutes. Umbilical blood flow, calculated placental vascular resistance, and umbilical artery pulsatility index did not change. After atropine administration, the norepinephrine-induced elevated arterial and umbilical venous pressures were accompanied by tachycardia, increased umbilical blood flow, and no change in placental vascular resistance and umbilical artery pulsatility index. It is concluded that fetal arterial hypertension provoked by norepinephrine infusion has no effect on placental vascular resistance, umbilical blood flow, and umbilical artery pulsatility index.

Evaluation of changes in umbilical blood flow in the fetal lamb by Doppler waveform analysis

Doppler velocity waveform analysis from descending aortic and umbilical artery signals was performed in 14 instrumented fetal lambs when umbilical blood flow was altered acutely. Blood flow was increased and reduced (by cord occlusion or placental embolization) and ranged 14% to 168% and 8% to 132% from baseline values in the descending aorta and the umbilical artery, respectively. When flow was severely reduced (less than 50% of baseline value), mean placental resistance increased almost fourfold and mean heart rate fell by 30% from baseline levels. There were only weak correlations between Doppler waveform indices and actual flow, which were usually not significantly different from the correlations between Doppler indices and actual heart rate. With both cord occlusion and placental embolization, significant changes of index values from baseline values were seen only when flow was severely reduced. We conclude that Doppler waveform analysis is not sensitive enough to detect acute mild to moderate reductions in placental flow. Increased index values with severely reduced flow may be explained in part by the concomitant fall in heart rate.

Pulsed wave Doppler velocimetry in the descending aorta of chronically instrumented fetal sheep

Pulsed wave Doppler sonography was used to determine peak systolic velocity (Vpeak), mean maximum velocity (Vmean) and Pulsatility Index (PI) in the descending aorta of chronically instrumented, unanesthetized fetal sheep (days 120-130 of pregnancy). Measurements were done 1-4 days after surgery using a duplex mechanical sector scanner (4.5 MHz) and offset attached pulsed wave Doppler probe (2.25 MHz) during periods of control, intrauterine cooling and rewarming. Mean values derived from the envelope curve of maximum shift frequencies were (controls): thoracic: Vmean 60.9 +/- 9.1 cm/sec, Vpeak 118.9 +/- 17.8 cm/sec, PI 1.57 +/- 0.24; abdominal: Vmean 49.6 +/- 10.9 cm/sec, Vpeak 98.9 +/- 20.4 cm/sec, PI 1.59 +/- 0.41. Vpeak was positively correlated to arterial pressure (r = 0.23) and PI showed an inverse correlation to heart rate (r = -0.22). During cold stress (reduction of fetal core temperature by 2.7 degrees C) and consecutive rewarming Vpeak and Vmean were positively correlated to fetal core temperature and arterial pressure (r = 0.2-0.26). PI revealed a weak negative dependency from core temperature and heart rate (r = -0.19 and -0.18 respectively).