ABNORMAL WAVEFORMS IN THE UTERINE CIRCULATION

1. Elevated resistance indices (>2 SD).
2. Persistent diastolic notching beyond 24-26 weeks gestation.
3. Systolic notching.
4. Major left to right variance in indices varying with placental site.
5. Transition of high to low impedance.

• Disappearance of the end diastolic notch usually occurs by the end of the second trimester.
• Chan and coworkers (1) estimate that 4.2% of high-risk women would have both an abnormal resistance index and a diastolic notch in both uterine arteries at 20 weeks gestation. This combination is thought to be the most accurate indicator in predicting severe pregnancy complications.
  Women are almost eight times more likely to develop either:
• Clinically significant hypertension.
• Deliver prior to 32 weeks.
• Perinatal demise.
• Infant with a birthweight of less than 1500 grams.
• In an attempt to place an objective quantification of the early diastolic notch, a notch index (NI) has been proposed, however it has been found to be less sensitive than the PI for prediction of hypertensive disorders of pregnancy (2).
• Gestational age of screening – 20-24 weeks is suggested as the best time (3). Antsaklis and co-workers suggest that any notch (unilateral or bilateral) has a sensitivity of 76% and specificity of 95% for pre-eclampsia. For pre-eclampsia that requires delivery prior to 34 weeks, the sensitivity is over 90%. Screening at 20 weeks has a higher sensitivity than screening at 24 weeks (81% versus 76%), but lower specificity (87% versus 95%).
• Placental localization also plays an important role in the interpretation of doppler waveforms. Only about 25% of women have a fully lateral placenta (i.e. supplied by one uterine artery). In cases of fully lateral placenta, the sensitivity of unilateral or bilateral notches for pre-eclampsia reduced significantly from 88% for a mid placenta to 33%. If bilateral notches are considered on their own, the sensitivity for pre-eclampsia was no different irrespective of placental localization: 33% for a fully lateral placenta and 50% if the placenta was not fully lateral. The flow through the placental side unilateral artery is more important a determinant of uteroplacental flow, and hence outcome, than the contralateral uterine artery.
• Sensitivity for adverse outcome is very high if both unilateral and bilateral notches are included, however specificity is low. This is because unilateral notches are common in women who have normal outcome, especially before 20 weeks.
• PI values become more reliable if one uses cut off values 1.4 – 64 % (2); 90% sensitivity for severe or pre-eclampsia prior to 34 weeks (3).

REFERENCES

1. Chan FY, Pun TC, Lam C et.al. Pregnancy screening by uterine artery doppler velocimetry - which criterion performs the best. Obstet Gynecol 1995;85:696-602.
2. Aardema M, De Wolf B, Saro M et.al. Quantification of the diastolic notch in Doppler ultrasound screening of the uterine arteries. Ultrasound Obstet Gynecol 2000;16:630-634.
3. Antsaklis A, Daskalakis G, Tzortzis E et.al. The effect of gestational age and placental location on the prediction of pre-eclampsia on uterine artery doppler velocimetry in low-risk nulliparous women. Ultrasound Obstet Gyencol 2000;16:635-639.