Fetal middle cerebral artery flow velocity waveforms--a terminal pattern. Case report

Evaluation of fetal cerebrovascular circulation and brain development: the role of ultrasound and Doppler

The human fetal brain is protected from pressure changes by autoregulation of the cerebral circulation. However, antenatal intrauterine cerebrovascular events are found to play an important role in the pathogenesis of perinatal brain damage. Changes in placental vascular resistance, cardiac contractibility, vessel compliance, and blood viscosity alter the normal dynamics of fetal cerebral circulation. The introduction of various sonographic techniques and the collection of data from the arterial and venous cerebral circulation provide a powerful tool for the evaluation of physiological and pathological hemodynamic events. Anatomic and physiological considerations of cerebral vasculature in healthy and disease states are relevant in studying brain development and variations in fetal brain blood perfusion. The major role of ultrasound and Doppler modalities in the evaluation of fetal cerebrovascular circulation is enabled by implementation of these considerations. Changes in flow characteristics in fetal cerebral vasculature can be used for clinical decision making in situations of fetal compromise, such as growth restriction and anemia.

Signs of fetal brain sparing are not related to umbilical cord blood gases at birth

BACKGROUND

Fetal chronic hypoxia leads to centralization of circulation in order to spare the vital organs brain, adrenals and the heart. This can be documented by Doppler ultrasound. Increased blood velocity in the fetal middle cerebral artery (MCA) is an acknowledged sign of centralization of circulation in chronic hypoxia, and is called brain sparing.

AIM

Our aim was to assess the relationship between signs of brain sparing in the MCA and umbilical cord blood gases at birth.

STUDY DESIGN

A prospective study.

SUBJECTS

Singleton 57 high-risk pregnancies (outcome was compared with 21 normal pregnancies).

METHODS

MCA Doppler was performed within 24 h of elective caesarean section in high-risk pregnancies. Umbilical cord blood gases were analysed at birth.

MAIN OUTCOME MEASURES

Cord blood gases were related to signs of centralization of fetal circulation in the MCA.

RESULTS

No correlation between signs of brain sparing in the MCA and cord blood gases. Apgar score at 5'<7 was seen in three newborns, but only one of these had antenatal signs of brain sparing. Newborns with antenatal brain sparing were admitted more often (p<0.04) and had a longer duration of stay in NICU (p<0.03) compared to newborns without brain sparing.

CONCLUSION

Decreased pulsatility index in MCA is an acknowledged sign of fetal centralization of circulation during chronic hypoxia. However, signs of brain sparing are not related to cord blood gases at birth, which might suggest that redistribution of fetal circulation can maintain normal blood gases for a long time during chronic hypoxia.

Fetal Doppler velocimetry

The introduction of new techniques for evaluating fetal status, particularly fetuses at theoretical risk for hypoxic ischemic encephalopathy, requires the most rigorous evaluation before widespread clinical deployment. The considerations extend beyond clinical value to the significant medicolegal implications of a failure to predict or ascertain compromise. The attitudes to clinical Doppler velocimetry have been shaped to a large extent by these practical concerns and the initial skepticism, which is a necessary component of scientific rigor. Available data strongly indicate, however, that in competent hands umbilical artery Doppler im-proves the clinical management of IUGR pregnancies. Failure to use Doppler may have the undesirable effect of increasing the risk of adverse outcome in the growth-restricted fetus. There is also strong evidence of benefit in the management of the Rh isoimmunization. Although numerous other clinical applications are on the horizon, much more information is needed to determine objectively the benefits and risks of these newer applications.

Evaluation of the fetal coronary circulation

Prenatal ultrasound today allows the detailed study of small caliber vascular beds including the fetal coronary arteries and the coronary sinus. The coronary circulation is unique because of its critical role in myocardial metabolism and function and its ability to adapt in many fetal conditions. The ultrasound examination techniques for the evaluation of the fetal coronary circulation are presented. Evaluation of the coronary arteries is primarily achieved by color flow imaging and pulsed wave Doppler. Conditions such as fetal growth restriction, anemia, ductus arteriosus constriction and bradycardia are associated with evidence of enhanced coronary blood flow. These findings suggest that short-term autoregulation and long-term alterations in myocardial flow reserve are present in the human fetus. At present, examination of coronary sinus blood flow has limited clinical utility, while increases of the coronary sinus diameter or attenuation of coronary sinus dynamics may provide useful markers of abnormalities of central venous drainage. Abnormal vascular connections between the coronary arteries and the ventricular cavities may be observed in outflow tract obstructive cardiac lesions. In these conditions prenatal detection of coronary fistulae may have a potential impact on outcome and therefore counseling and perinatal management.

Abnormal Doppler velocimetry and blood flow volume in the middle cerebral artery in very severe intrauterine growth restriction: is the occurence of reversal of compensatory flow too late?

OBJECTIVES

To determine the time course of quantified volumetric flow and pulsatility index changes in the middle cerebral arteries of severe intrauterine growth restricted fetuses with absent or reversed end diastolic flow velocities (AREDFV) on umbilical artery Doppler with a view to defining the threshold for early intervention before the onset of reversal of adaptation.

SETTING

Fetal growth clinic of a large UK teaching hospital.

SAMPLE

Seventeen severe intrauterine growth restricted (AC < third centile for gestation and gender) fetuses with abnormal umbilical artery Doppler velocimetry and oligohydramnios (amniotic fluid index <third centile for gestation).

METHODS

One to three daily Doppler velocimetry and quantified volume flow of the middle cerebral artery Doppler velocimetry of the umbilical arteries and daily cardiotocography.

RESULTS

Reversal of adaptation occurred in eight fetuses, four of which ended as stillbirths and four as neonatal deaths. In two of these fetuses, this had already occurred before the diagnosis of intrauterine growth restriction. In the others, the onset was rapid and difficult to predict. Volume flow in the middle cerebral arteries decreased in those fetuses exhibiting reversal of adaptation but rose in the nine in which compensatory Doppler indices persisted. The decrease in volume flow occurred before the onset of reversal of adaptation in the pulsatility index. In the fetuses exhibiting reversal of adaptation, the rise in the pulsatility index was consistent over 48 hours compared with the physiological fluctuations in pulsatility index in some fetuses where the rise was only for 24 hours followed by a fall. Quantified volume flow fell in the fetuses before reversal of adaptation set-in. The umbilical artery Doppler indices fluctuated between absent and reversed end diastolic velocities within the same fetus irrespective of the changes in the middle cerebral artery Doppler indices.

CONCLUSION

Reversal of adaptation is of sudden onset and is associated with poor prognosis. Predicting its occurrence on Doppler indices is difficult as the changes may be very rapid. However, volume flow changes appear to be slower and of earlier onset than the pulsatility index changes. This may therefore be a more useful tool in predicting imminent reversal of adaptation and therefore early delivery.

Middle cerebral artery Doppler in severe intrauterine growth restriction

OBJECTIVE

To examine longitudinal changes in middle cerebral artery blood flow assessed by Doppler in severely growth restricted fetuses.

METHODS

Eighteen structurally normal singleton pregnancies complicated by suspected intrauterine growth restriction were monitored by serial measurement of the pulsatility index of the middle cerebral artery over 7 to 72 days. Outcome measures included indication for delivery, umbilical venous pH and admission to and length of stay in neonatal intensive care.

RESULTS

Thirteen fetuses demonstrated severe intrauterine growth restriction based on subsequent birth weights being below the 2.5th centile, two had intrauterine growth restriction (birth weights between the 2.5th and 5th centiles), and three had birth weights between the 5th and 50th centiles. The middle cerebral artery pulsatility index showed rapid and sharp changes between examinations in those severely growth restricted fetuses which required delivery before 34 weeks. This pattern was not obvious in severely growth restricted fetuses delivered after 34 weeks, or in those less severely growth restricted, regardless of the gestation at delivery. Changes in middle cerebral artery pulsatility index contributed to the decision to deliver in three cases. The middle cerebral artery pulsatility index demonstrated greater variation in those fetuses with cord pHs of less than 7.25. The length of stay in neonatal intensive care decreased with increasing gestational age and birth weight.

CONCLUSIONS

The difference in the pattern of change in middle cerebral artery pulsatility index in intrauterine growth restricted fetuses may be a reflection of maturity in addition to the degree of fetal compromise. The decision to deliver was multifactorial. The middle cerebral artery pulsatility index only influenced the decision to deliver when changes in other parameters were evident.

The application of individualised fetal growth curves

Individually adjusted or 'customised' growth charts aim to optimise the assessment of fetal growth by taking individual variation into account, and by projecting an optimal curve which delineates the potential weight gain in each pregnancy. This results in an increased detection rate of true growth restriction and a reduction in false positive diagnoses for IUGR. An adjustable standard can apply across geographical boundaries, as individual variation exceeds that between different maternity populations.

Prenatal ultrasonographic assessment of the middle cerebral artery: a review

Published reports, case studies, and articles regarding ultrasonographic morphology, physiology, and pathophysiology of the fetal middle cerebral artery obtained from a MEDLINE search from 1966 through January 1997 were reviewed. Both transabdominal and transvaginal color Doppler ultrasonographic modalities may be used to assess fetal middle cerebral artery flow hemodynamics. Altered middle cerebral artery flow velocities may be noted in various medical conditions that include various behavioral states, term and preterm labor, maternal medications (anesthesia, tocolytics), fetal compromise (growth restriction and hypoxia), twin-twin transfusion syndrome, invasive diagnostic procedures (amniocentesis and fetal blood sampling), fetal anemia and transfusion, in addition to intracranial fetal lesions (congenital anomalies and hemorrhage). In summary, knowledge of Doppler flow velocity of the fetal middle cerebral artery may assist prenatal diagnosis and management of complicated pregnancies.

Twenty-day cerebral and umbilical Doppler monitoring on a growth retarded and hypoxic fetus

In one growth retarded and hypoxic fetus, the cerebral and umbilical hemodynamic changes were assessed (by Doppler), daily over 20 days. The fetal brain was investigated by magnetic resonance imaging (MRI) close to the delivery, and because the fetus died at delivery we performed an anatomical study of the fetal brain. The evolution of the fetal hemodynamics (day by day) was interpreted according to the MRI findings and the clinical findings. During the period of observation (under sustained hypoxia) the fetal deterioration was characterized by: (a) the progressive development of the oligohydramnios (190d), (b) the disappearance of the vascular reactivity (eight successive cerebral resistance index (RI) constant at 194d), (c) the occurrence of fetal heart rate decelerations (199d), and finally (d) the increase of the cerebral vascular resistances with reduction of the brain perfusion (204d). The anatomical study of the brain showed a periventricular congestion however the histology revealed hypoxic lesions like gliosis and a marked vasodilation of the anterior and middle cerebral arteries. Finally in addition to single Doppler measurements performed 1 week before delivery (for prediction of fetal outcome), one can suggest to use the 'loss of fluctuation of the cerebral RI' to identify the beginning of the period of very high risk for the fetus. Such hypothesis may have to be confirmed on a larger number of pathological pregnancies.

Longitudinal study of fetal middle cerebral artery flow velocity waveforms preceding fetal death

OBJECTIVE

To assess longitudinally fetal cerebral vasodilatation in small-for-gestational age fetuses to investigate whether intrauterine death might be predictable.

DESIGN

Prospective observational study.

SETTING

Ultrasound department in a university hospital.

SUBJECTS

Five pregnancies with ultrasonographically confirmed small fetuses (abdominal circumference less than the 3rd centile) monitored longitudinally until time of intrauterine death.

MAIN OUTCOME MEASURE

Time between last ultrasound examination and diagnosis of intrauterine death, and variation in middle cerebral artery pulsitility index prior to death.

RESULTS

Two of the five fetuses showed a reversal of adaptation (as indicated by an elevation of the middle cerebral artery pulsitility index) within 48 hours of intrauterine death. The other three had their final ultrasound examination 3 to 7 days before death and showed no such reversal of adaptation.

CONCLUSION

Reversal of adaptation in fetal hypoxaemia as indicated by a rise in the middle cerebral artery pulsitility index may be a predictor of intrauterine death within 48 hours. Whether delivery after reversal of adaptation would result in salvage of neurologically intact babies needs to be investigated.

Central and peripheral hemodynamic changes in fetuses with absent end-diastolic velocity in umbilical artery: correlation with computerized fetal heart rate pattern

OBJECTIVES

Our purpose was to study hemodynamic changes in peripheral and central blood vessels and to correlate these changes with the computerized fetal heart rate pattern in fetuses with absent end-diastolic velocity in the umbilical artery.

STUDY DESIGN

Doppler studies of the umbilical artery, the middle cerebral artery, and aortic and pulmonic outflow, together with computerized fetal heart rate monitoring, were performed every 2 to 4 days until delivery in 13 fetuses with absent end-diastolic velocity in the umbilical artery. The pulsatility index was calculated from the flow velocity waveforms obtained from the umbilical and middle cerebral arteries. The velocity time integral (an index of cardiac output) and the heart rate were calculated from the flow velocity waveforms obtained from the aortic and pulmonic outflow.

RESULTS

Two fetuses were delivered immediately after the first examination because of repetitive fetal heart rate decelerations. One fetus was excluded from the study because of major malformations. Ten had three to eight tests each. Six had a biphasic change of the middle cerebral artery, which consisted of a decrease (p < 0.001) followed by an increase in the pulsatility index (p < 0.05). When the middle cerebral artery lost its vasodilation, there was an increase in the middle cerebral artery/umbilical artery pulsatility index ratio (p < 0.05). Left cardiac output decreased (p < 0.05), resulting in an increase in the pulmonary/aortic velocity time integral x heart rate ratio (p < 0.05). Reduced fetal heart rate variation (< 30 msec) developed in all six fetuses, and they were delivered because of repetitive fetal heart rate decelerations. Four fetuses with only a decrease in the middle cerebral artery pulsatility index did not have reduced fetal heart rate variation or decelerations, the aortic velocity time integral x heart rate didn't decrease, and the pulmonic/aortic velocity time integral x heart rate ratio didn't increase. These fetuses were delivered for reasons other than fetal distress. The middle cerebral artery pulsatility index correlated with the aortic velocity time integral x heart rate (r = -0.53, p < 0.0001), and the middle cerebral artery/umbilical artery pulsatility index correlated with the pulmonic/aortic velocity time integral x heart rate (r = 0.56, p < 0.0001).

CONCLUSION

Abnormal fetal heart rate patterns occur in fetuses with absent end-diastolic velocity in the umbilical artery when the middle cerebral artery begins to lose its compensatory maximal dilation. The increase in the middle cerebral artery pulsatility index is associated with a significant reduction in left ventricular output without significant changes in right ventricular function. Thus it appears that a loss of autonomic reactivity occurs in the brain first and is followed within a few days by a similar response in the heart, as shown by the decreased fetal heart rate variation.