THE CHARACTERISTICS OF INDIVIDUAL WAVEFORMS

THE CHARACTERISTICS OF INDIVIDUAL WAVEFORMS

UTERINE ARTERIES:

          Uterine artery blood reflects hemodynamic changes that occur on the maternal side of the placenta (Trudinger et aL, 1985).

          Pregnancy results in marked changes in the uterine artery waveform. The resistance to blood flow in both uterine arteries decrease gradually from early pregnancy until the end of the second trimester. Hence, the systolic-diastolic values do not change from 28 week.

Indices of Uterine Artery Waveform Analysis:

          Uterine artery Doppler waveforms are most commonly analyzed by simple semiquantitative techniques. They are based on maximum Doppler shift frequencies, which vary during the cardiac cycle. Evaluation of the change in maximal Doppler shift over time provides information on the impedance of the circulatory bed.

1. Pulsatility Index (PI): Which is the most complex of the three. It is calculated by peak systole minus end-diastole divided by mean value of the area under the curie over one cardiac cycle (S-D/mean velocity). It requires a computer program that calculates the area under the curve. Peak systole divided by end diastole (S/D ratio) or (A/B ratio).
2. Resistance index (RI) or Pourcelot Ratio: Peak systole minus end diastole divided by peak systole (S-D/S).

          Impedance to blood glow in the uterine arteries may increase in pregnancies complicated by hypertension (Fleischer et at. 1986; Trudinger et al., 1990).

          As shown by Fleischer et at. (1986), when the uterine artery systolic! Diastolic (S/D) ratio was >2.6 during the third trimester, the birth weight at delivery was lower than normal and the incidence of fetal distress (low Apgar score and still birth) higher. Therefore increased uterine resistance in hypertensive pregnant women should indicate that the fetus is significantly compromised.

          Color flow mapping has allowed us to interrogate specific sites of the uterine circulation like arcuate, radial and the spiral arteries.

          Studies have shown that there is a drop in resistance from the proximal to the distal branches of the uterine circulation, the uterine artery having the highest impedance and the spiral arteries the feast.

Uterine Artery Waveform Notch:

          The uterine artery Doppler waveform has an early diastolic notch in the non-pregnant state. It also persists in the pregnant state until 20-28 weeks of gestation (Schulman et al, 1986).

          Presence of a notch is a significantly better predictor of poor pregnancy outcome than the S/D ratio or the resistance index (RI). It has been reported that an increased uterine artery RI without a uterine notch poorly correlates with adverse perinatal outcome (Thaler et al., 1992).

          The complexity of the uteroplacental circulation makes accurate identification of the vessel under study difficult with either continuous wave or duplex Doppler ultrasound. Flow velocity waveforms are obtained from the lateral lower quadrants of the uterus, angling the transducer on either side of the uterus towards the cervix.

          Signals achieved in this way are assumed to be originating from the uterine arteries. The uterine arteries are more accurately identified using color Doppler: the region lateral to the lower uterus is examined and the external iliac artery and the adjacent vein are identified, the uterine artery crosses the external iliac artery on its course from the internal iliac artery to the body of the uterus. It is important to angle the transducer to improve the angle of insonation while maintaining vessel identification on the color Doppler. Spectral waveforms are obtained by placing the pulse Dopplar range gate within the vessel at this point.

         

          In the non pregnant state, the uterine artery is a high resistance vessel. It shows a high resistance waveform. Low diastolic flow and early diastolic notching is a normal feature of the non-pregnant uterine circulation.

          With pregnancy, trophoblastic invasion of the spiral arteries takes place converting the high resistance flow pattern into that of a low resistance pattern characterized by increase in diastolic flow and disappearance of the notch.

          It is essential to study both the uterine arteries because of variations In placentatlon. In cases of a laterally located placenta, the placental side uterine artery is the main supplier, that it has a lower resistance as compared to the opposite uterine artery. Examination of both the uterine arteries is an indispensible element of Doppler examination to assess placental performance and risk to the fetus (Joern H & RathW 1998).

          When the placenta is centrally located, there is equal contribution by both the uterine arteries. In case of a unilateral placenta, if the degree of contribution by the opposite uterine artery is deficient, it can facilitate the development of pm eclampsia, IUGR or both. Therefore before studying the uterine artery it is imperative to know the side of the placenta and determine the placental side uterine artery”.

          Color Doppler ultrasound examination for hemodynamical changes of uterine arteries, umbilical artery and umbilical vein revealed utero-piacental-fetal circulation function directly. It is one of the best methods for monitoring IUGR and used for early diagnosis of IUGR. The main pathophyslological changes of IUGR are utero-placental-fetal circulation obstruction and placental dysfunction (XU J & Wen L 1998).

          In abnormal pregnancies due to lack of trophoblastic invasion, there is increased resistance in the placental bed. This increased resistance is reflected back in the uterine arteries. These uterine arteries are characterized by

1. Low diastolic flow.
2. An early diastolic notch.
3. Biphasic deceleration slope.
4. High indices.

          During the course of pregnancy the uterine artery changes from a high resistance pattern with no notches. Disappearance of notch will happen first in the uterine artery which is directly under the placenta. Normally the notch is seen until 28 weeks of gestation. Persistance of notch after 28 weeks is an indicator of hypertensive complications! IUGR or both. Persistance of notch indicates unaltered vasospasm. After delivery, the uterine artery doesn’t return to its pre pregnant level for 4-6 weeks (K.J.W. Taylor 1995).

          Various Indices have been used to describe the uterine artery waveform eg:systollc/ diastolic ratio, RI and PI. These indices In due course of pregnancy should show a fall in their values. Failure of trophoblastic invasion results In increased resistance to flow, a low diastolic velocity and consequent increase in indices causing uteroplacental Insufficiency. Abnormal Doppler values indicative of a failure to modify the uterine circulation in early pregnancy are associated with preterm delivery, development of protelnuric PIN and SGA fetus. (Hanington 1997).

          The upper limit of systolic! Diastolic ratio is approximately 2.6 the systoli dIastolic ratio more than 2.6 after 26 weeks of gestation suggests that full trophoblastic Invasion has not occurred and mother and fetus are at risk for adverse outcome.

          The upper limit of RI value is about 0.58. Increased values of RI indicate that there is increased risk of IUGR. The RI value will be lower from placental side and from distal accurate than proximal uterine arteries (Campbell S & Cooper 1998).

          The difference between the systolic! Diastolic ratio of both uterine arteries should not exceed 1 normally. A difference of more than 1 indicates pre eclampsia and IUGR. (Farmakides and Schulman 1992).

          Pathological Doppler velocimetry of the uterine and uteroplacentat circulation is a powerful predictor of proteinuric PIH and I or IUGR in high risk pregnancies (Zimmermann P 1997). Doppler examination of uterine and umbilical arteries can detect at mid pregnancy, the severe forms of PIH and SGA fetuses (Todrost T 1995).

UMBILICAL ARTERY:

          Umbilical artery flow velocity waveform correlates the hernodynamic changes in the fetoplacental circulation or in other words reflect downstream (placental) vascular resistance. With the aid of color i9ow, the umbilical arteries can be detected as early as 8-10 weeks.

          A characteristic umbilical artery FVW has a rapid upstroke during systote and a gradual decline during diastole, while maintaining a continuous forward flow. A decrease in peripheral resistance results in an increased end diastolic velocity. Conversely, increased peripheral resistance (as seen in cases of placental insufficiency) likely results in decreased, absent or even reversed enddiastolic flow.

Site of Doppler interrogation:

          Several studies have shown that the location of Doppler sampling site in the umbilical cord affects the Doppler waveform and is reflected in the Doppler indices.

          Abnormal umbilical artery FVW5 represents a failure of development of adequate placental vasculature, with normal umbilical artery FVWs (seven to eight arterioles per high power field),

          About 40% of hypertensive pregnancies have increased resistance in the umbilical artery (>25D of normal values).

          An extreme situation of impaired umbilical blood flow is an absence of end-diastolic flow velocity. There is an ominous association between the absent end-diastolic velocity and adverse perintal outcome.

          Perinatal morbidity Indudes preterm birth and low birth weight Infants, which are mostly attributable to fetal or maternal complications mandating early intervention. In these conditions, fetal heart rate monitoring and Doppler studies of the middle cerebral arteries may pmvlde more Information regarding the fetal state.

          Karsdrop et al. (1994) noted that the evidence of developing AEDV/REDV was lesser In pregnancies complicated by hypertension alone, than If pregnancies were complicated by fetal growth retardation and hypertension together.

          The Insonation angle should be less than 60°. Approximately twelve waveforms should be averaged; the Indices should be ideally taken at the mid cord or at placental Insertion. The indices are highest at the fetal abdomen and lowest at the cord insertion into the placenta. The wall filter should be set at 100 MHz. If there is absent end diastolic velocity, It may lowered to 50 MHz.

          The umbilical artery waveform is a simple waveform with no systolic or diastolic notching and with a large amount of diastolic flow velocity. During the normal course of pregnancy there is an overall increase in placental size with resulting increase in number of tertiry stem villi. The number of small aterial channels in these tertiary villi keeps on increasing causing a decrease in resistance to diastolic flow, This decrease in resistance is reflected as an increase in diastolic velocity. Before 16-18 weeks of gestation, the placental circulation is a high resistance bed and the indices are on the higher side.

          During the latter half of gestation, there should be a normal fall in the indices. In abnormal pregnancies, because of high resistance in the placental bed there will be reduced, absent or reversed end diastolic flow velocity. Absent end diastolic velocity is clearly abnormal. Most abnormal umbilical flow velocity wave forms represent a failure of development of an adequate placental vasculature. Fetuses that are growth retarded with abnormal flow velocity have half as many tertiary arterioles.

          The most commonly used indices for the umbilical artery are systolic! diastolic, RI and PI. The normal value of systolic! Diastolic ratio at 28 weeks of gestation is 3±0.6 and at 30 weeks of gestation it is reduced to 2.5±0.4, A systolic! diastolic ratio greater than 4 is considered as abnormal. The normal values of RI at 28 and 30 weeks of gestation is 1.2±0.5 and at 30 weeks of gestation is 1±0.4.

          An abnormal Doppler wave form analysis of umbilical artery (systolic! diastolic ratio more than 4)is the more accurate predictor of poor out come in SGA fetuses (Dubinsky T and Powell F 1997).

          Hypoplastic umbilical vessels are associated with an increase in placental vascular resistance that may be the consequence of under development in response to a chronic reduction in placental blood flow. (Burch JF & Sibony 0 1997).

          In patients with IUGR fetuses, those with normal umbilical artery findings does not require any intervention (Nienhius 1997).

          The absence of end diastolic velocity in the umbilical artery is a distinctly abnormal finding that identifies a high resistance feto placental circulation rather than the normally low resistance system and adverse fetal outcome (Katherine D Wenstrom 1991).

          Absent end diastolic flow uniformly heralded adverse perinatal outcome and in severity it Is next to reverse flow, which is rare. Reverse flow Is a morbid event and Is the most severe velocity flow abnormality seen. Fetuses with absent end diastolic flow have low birth weight, will be born earlier and have a high Incidence of IUGR, PIH and a low APGAR score. Absent end diastolic velocity may precede clinical signs of stress by long periods of time. Absent end diastolic velocities are also seen In SGA fetus, fetal trisomles and fetal distress etc.

Factors influencing the umbilical wave form:

1. Site of sampling: Highest Indices are there in the cord near the Insertion into the fetal abdomen and lowest at the placental end of the cor4. In contrast to normal fetuses, IUGR fetuses with Doppler measurement had significant differences in PI and RI between the two ends of the cord (Skoll MA1997).

2. Gestation age: With advancing pregnancy, the wave form demonstrates a progressive increase in the diastolic component with decrease In the Indices.

3. Fetal heart rate: With bradycardia, the diastolic phase increase and end diastolic velocity decreases. These changes leao to an increase in indices, the converse changes occur with tachycardia (Petter W Callen). However after 28 weeks of gestation and over the normal range of fetal heart rates (110- 160 beats! mm), there is no significant effect on indices.

4. Fetal breathing: Fetal breathing changes the maximum frequency shifts from one cardiac cycle to the next. It is therefore recommended that the indices should be measured only during fetal apnea,

5. Fetal behavioral state: Results indicate that aortic and internal carotid artery waveforms are altered during fetal activity. No such changes were noted in the umbilical artery flow on the Doppler indices.

6. Maternal cigarette smoking: The effect of smoking on blood flow has been somewhat controversial with researchers reporting either no effect or a significant effect from cigarette smoking. However, because of the dispute and the potential for cigarette smoking to alter flow patterns in chronic smokers these authors advocate that patients should not smoke for at least one hour prior to a Doppler study (Morrow JW and Bull SB 1998).

7. Subotimal Insonation angle: Optimization of the angle of insonation to the umbilical vessels improves clarity of signal reproduction. However the Doppler indices relate systolic to diastolic components of flow, optimal angle of insonation is less important.

8. Improper gain settings.

9. Intra and inter observation variation.

10. Maternal diabetes: Although the resistance index is increased in the uterine arteries of diabetics with a morphological vasculopathy, there is no relationship with short or long term diabetic control and it is not a good predictor of diabetes related fetal morbidity, presumably because these changes are reflecting the risk of acidosis as a result of hypoxia rather than metabolic acidosis.