The basics of umbilical artery velocimetry

Abstract

 
Introduction
The Doppler phenomenon or Doppler effect was first described by Christian Johanan Doppler (1803- 1853). His field of interest was Astronomy. He described the apparent changes in colour of light (intensity) emitted by stars when they come near or far from earth. The colour or brightness becomes more apparent if the star is coming towards earth, or, it fades away if the star is going away from earth.

This phenomenon was properly explained as change of frequency when there is a change of velocity, later known as the Doppler effect or the Doppler shift. Many examples were later described : the change in sound of the ambulance when it is coming near OR away from the listener. The same applies to a train when it is coming to the station and when it is leaving it.

In the human body each vessel has its own specific waveform.

This depends on few points: vessel length, caliber, wall elasticity and distance from the cardiac pump. Each wave basically has systolic and diastolic components.

Umbilical artery Doppler waveforms provide an estimate of downstream placental vascular resistance and placental blood flow. There is a strong association between reduced end-diastolic umbilical artery blood flow velocity and increased vascular resistance in the umbilical placental microcirculation.

Also, abnormal umbilical artery Doppler waveforms have been associated with an increased risk of fetal acidosis, as measured during cordocentesis, and may improve the performance of the biophysical profile score in predicting fetal acidemia and hypercarbia.

Why does this matter?
Placental insufficiency is the primary cause of intrauterine growth restriction in normally formed fetuses and can be identified using umbilical artery Doppler velocimetry. The use of Doppler during antenatal fetal surveillance has involved assessment of (1) the umbilical arterial and venous flow velocity waveforms, (2) the fetal cerebral circulation, and (3) the fetal venous circulation, in particular the ductus venosus.

Ultrasound machines with the appropriate probe and capability simultaneously display; two images: a B-scan image to locate the vessel needs to be examined and the other displays the Doppler beam to interrogate the site needed to study blood flow. Blood flow can be coloured either red or blue according to its direction to the transducer : coming to OR going away from it.

Colour Doppler studies, in simple words mean : detection and demonstration of blood flow. Indicated Doppler studies may be carried out at earlier trimesters of pregnancies. This write-up will however, focus on studies after 28 weeks.

Umbilical artery waveform
Doppler ultrasound waveforms reflect blood velocity; therefore, they potentially can provide information on various aspects of blood flow in a circulation, including the presence and direction of flow, velocity profile, volume of flow, and impedance to flow. In perinatal practice, Doppler of the umbilical artery has been used extensively for assessing downstream circulatory impedance (i.e., resistance to pulsatile flow).

Choice of indices
Despite its limitation, the S/D ratio remains the most widely used Doppler index for evaluating the fetal circulation, especially for umbilical arterial haemodynamics.

The RI [Resistance Index] values have defined limits of 0-1.0. The limitation of the RI is due to its inability to reflect impedance increases with the reversal of end-diastolic blood flow. Theoretically, the PI provides more haemodynamic information than the RI and S/D ratio, as it includes data on the whole cardiac cycle in the form of its denominator. Furthermore it expresses haemodynamic alterations associated with absent or reversed end-diastolic flow.

In practice, however, computation of the time-averaged value is not as precise as determination of the peak systolic or end-diastolic frequency shifts.

Doppler waveform analysis is usually based upon the following characteristics of the maximum frequency shift envelope:

  • Peak systolic frequency shift value (S)
  • End-diastolic frequency shift value (D)
  • Average frequency shift value over the cardiac cycle (A)

These three parameters are used to develop indices that reflect the pulsatility of the Doppler waveform. A Doppler index (DI) is calculated as a ratio and is, therefore, virtually independent of the angle of insonation, thus obviating the need for measuring the angle of insonation.

The most commonly used obstetrical applications are the peak systolic frequency shift to end-diastolic frequency shift ratio, (S/D) and the resistance index (RI), which represents the difference between the peak systolic and end-diastolic shift divided by the peak systolic shift. The pulsatility index (PI = S-D/A) is also used.

Flow velocity profiles:
S/D ratio = Systole/diastole
RI (Resistance Index)=Systole-diastole/ systole
PI (Pulsatility Index)=Systole-diastole/mean velocity

Analysis of Doppler waveforms from an arterial source yields information about downstream impedance to flow: the Doppler index (DI) worsens with increasing pathology of fetoplacental angiomorphology.

Early studies reported an elevated S/D ratio was associated with obliteration of small muscular arteries in the tertiary stem villi. Subsequent studies, however, demonstrated sparse, elongated, uncoiled, and less ramified terminal capillary loops, rather than loss of small arteries, as the major lesions in pregnancies complicated by growth restriction and abnormal umbilical arterial Doppler. These observations suggest that fetoplacental vascular maldevelopment results in an increase in impedance which is reflected in the abnormal Doppler waveforms and indices.

An increase in S/D becomes apparent when approximately 30 percent of the fetal villous vasculature is abnormal; absent or reversed flow can occur when 60 to 70 percent of vessels are abnormal.

Obstetrical complications, such as Fetal Growth Restriction and preeclampsia, result in chronic fetal nutritive and respiratory deprivation. As the period of stress intensifies and/or lengthens, the fetus mobilizes defensive responses that include preferential preservation of fetal growth over placental growth, changes in fetal movement pattern, deceleration of the fetal growth rate, and, eventually, chronic hypoxia and acidosis.

The primary fetal hemodynamic response to this deprivation involves redistribution of blood flow to the brain, heart, adrenals, and placenta at the expense of flow to muscles, viscera, skin, and other less critical tissues and organs. Changes in blood flow impedance in the fetal regional circulations underlie this phenomenon. Doppler velocimetry demonstrates the circulatory changes associated with fetal compromise and allows perinatal prognostication.

The sequence of changes in fetal heart rate, Doppler findings, and other biophysical parameters during progressive fetal compromise reflect the fetal homeostatic response to chronic hypoxia. Abnormal elevation of Doppler indices precedes loss of fetal heart rate variability and reactivity, eventually leading to decline and loss of fetal breathing and body movements. Reversed end diastolic velocity and rapid loss of heart rate variability portend a poor outcome. Abnormal signs and death occur if there are no interventions. This progression provides the basis for determining the sequence of fetal testing in clinical practice.

Such decision will be influenced by other assessments of fetal health, such as fetal anatomical and chromosomal anomalies. In cases of prematurity, delivery may be delayed for 48 hours, allowing the maximum fetal benefits of maternal administration of corticosteroids; under such circumstances, continuous or defined intermittent fetal heart rate monitoring until delivery should be considered.

At an earlier gestational age, reduced or absent umbilical artery end-diastolic flow velocity is an indication for increased fetal surveillance, but not necessarily for immediate delivery. The same factors that affect umbilical artery Doppler waveforms can also affect fetal cerebral artery Doppler waveforms.

Indications for UA Doppler studies
Certain fetal and maternal conditions may increase a need for closer monitoring of fetal wellbeing. The common ones include: babies that are small-for-gestational-age by ultrasound biometry, particularly estimated fetal weight and abdominal circumference for gestation [not necessarily uterine-size/date discrepancy]; moderate to severe pregnancy-induced hypertension; significant differences in fetal weights in multiple pregnancies; oligohydramnios and complicated maternal diabetes.

The Royal College of Obstetricians and Gynaecologists specifically recommends UA Doppler studies as the primary surveillance tool especially for fetal growth restriction. On certain occasions however, the skilled sonographer may use his or her discretion to perform the study, even when specifically requested to do so. Before a request, the Clinician should ask: why do I want an UA Doppler study? Will the findings modify my present management? Has the patient had a recent similar study? If I need to deliver, how soon can I do so?

Technique
In order to reduce methodological variability, it is recommended that umbilical artery Doppler waveforms be measured within 5cm of the umbilical cord insertion into the fetal abdomen. The angle of the fetal Doppler insonation should be kept to less than 45o for an optimal umbilical artery Doppler recording. Because of the potential for variability and inaccuracy with fetal Doppler, it is imperative that measurements be undertaken by expert operators who are knowledgeable about the significance of Doppler changes and who practice appropriate techniques. Inaccurate information concerning fetal Doppler studies could lead to inappropriate clinical decisions. Some factors may affect the findings and interpretation of UA Doppler studies. They are listed in Table I.

Table I: Factors affecting UA Doppler flow velocity waveforms

Understanding the UA Doppler Report
Use of the UA Doppler is associated with a reduction in perinatal morbidity and mortality, reduction of maternal antenatal admissions, reduction of the use of antenatal resources such as repeated cardiotocograms, and reduced frequency of induction of labour in high-risk gestations.

The report of an UA Doppler study should be easy to understand, and should contain the following information as a minimum: flow velocity profiles, resistance index, pulsatility index and date and time of the study.

Resistance Index: [see above]
Pulsatility Index: As pregnancy progresses, there is a gradual decrease in the Pulsatility Index, ranging between 1 to 1.56 at 22 weeks to between 0.9 to 1.4 at 42 weeks. An abnormal systolic/diastolic ratio for UA Doppler is defined as a value greater than or equal to 3.

EDF present: Once UA Doppler shows the presence of EDF, planned delivery may be safely postponed until term (37 completed weeks and beyond), as long as other surveillance parameters are reassuring.

Absent EDF: An absence of end-diastolic arterial blood flow is correlated with increased perinatal mortality, fetal acidosis in labour and a need for admission into the Neonatal Intensive Care Unit. Most Obstetricians will initiate procedures to deliver the baby with this finding, although Perinatal and Fetal Medicine experts may be understandably comfortable to watch things for a while. Additional insonation of the umbilical vein has been reported as a useful step towards risk evaluation. There has been some mention of the use of biophysical profile testing as a supplement, but this is not commonly undertaken in the United Kingdom.

Reversed EDF: It is important to have a management plan in place, following an UA Doppler report. When EDF is absent or reversed, it is reasonable to administer maternal corticosteroids and arrange delivery, once the baby is above a gestational age of 34 weeks. Even in low-risk pregnancies, this is the gestational watershed at which cerebral haemorrhage constitutes a significant neonatal problem.

Limitations of the UA Doppler study
A normal UA Doppler study does not rule out the likelihood of fetal death from all causes, for example, placental abruption.

Controversial and selective indications for UA Doppler studies:

  • Previous unexplained SGA or unexplained stillbirths beyond 28 weeks;
  • Maternal vascular dysfunction e.g. IDDM with or without vasculopathy; Cocaine use;
  • Maternal age above 40yrs, with a risk factor;
  • Unexplained occasional ‘suspicious CTG’ before labour, which reverts to normal;
  • Surveillance in mothers who do not want IOL after 42 completed weeks of pregnancy.

Recent advances in UA Doppler
Two important developments in the practice of UA Doppler studies are: the finding that fetal birthweight, in the presence of an abnormal UA Doppler, is an important predictor of neonatal outcome. This is not surprising, considering that the majority of babies studied are SGA. The other important finding is that, uterine artery Doppler findings are comparable to UA Doppler’s in predicting perinatal outcomes in babies with fetal growth restriction. Interestingly, the later study answers one important question raised in the first: that including uterine artery Doppler in the overall evaluation of fetuses with ultrasound-confirmed growth restriction, may identify a subset of pregnancies in which there is significant risk, despite a normal UA Doppler.

Further to this, it is worthwhile to point out that in selected patients, umbilical vein Doppler’s are more reliable predictors of impending fetal death or severe acidosis. A discussion on that is beyond the scope of this article.

It is probably possible to diagnose a possible ‘tight’ nuchal encirclement of the umbilical cord, or of an intermittent ‘strain’ of a true-knot of the umbilical cord, in the process of performing UA Doppler studies.

Conclusions and recommendations
The Society of Obstetricians and Gynaecologists of Canada recommends that each [clinical care setting] organizes annual inter-disciplinary fetal health surveillance updates, or that all care providers participate in a fetal health surveillance update course every few years.

CTG appreciation and teaching is regularly taught, but not the UA Doppler. The easy alternative to this is to request that all UA Doppler reports be validated by a Fetal Medicine specialist, who would hopefully outline a plan of management. At the very minimum, the following practice points are worth noting:

Practice points on the UA Doppler

  1. There may be a significant difference in inter-operator findings on UA Doppler studies. This may be because of differences in intensity and standards of training.
  2. The sonographer needs all the relevant antenatal information that may affect the interpretation of findings.
  3. There is no value in repeating the UA Doppler study in less than 10 to 14 days, provided that all other parameters of antenatal fetal surveillance are normal.
  4. Even when an UA Doppler shows reassuring features, a later change in clinical features, such as vaginal bleeding with severe abdominal pain should mandate at least a CTG trace, to ascertain immediate fetal status.

* Interested readers may want to access a free on-line course on Antenatal Surveillance on: www.fetalmedicine.com.

** There is also an on-line tool for calculating the Resistance index and Pulsatility index for the 50th and 95th centile at: www.perinatology.com/calculators/umbilicalartery.htm

*** The UA Doppler images shown were accessed from a PowerPoint presentation by Dr Mohammed Abdalla , on the obgyn.net site, and from other open-domains on Google images.

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