Uteroplacental blood flow assessment

Uteroplacental blood flow assessment is an important part of fetal well-being assessment and evaluates Doppler flow in the uterine arteries and rarely the ovarian arteries.

Pathology

In a non-gravid state and at the very start of pregnancy the flow in the uterine artery is of high pulsatility with a high systolic flow and low diastolic flow. A physiological early diastolic notch may be present.

Resistance to blood flow gradually drops during gestation as a greater trophoblastic invasion of the myometrium takes place. An abnormally high resistance can persist in pre-eclampsia and IUGR. If resistance is low, it has an excellent negative predictive value with a less than 1% chance of developing either pre-eclampsia or having IUGR . A high resistance often equates to a 70% chance of pre-eclampsia and 30% chance of IUGR.

Radiographic features

Ultrasound

The parameters used in the assessment of uteroplacental blood flow include:

• RI = resistive index
• PI = pulsatility index
• presence of persistent diastolic notching

Resistive index (RI)

This is calculated by the following equation:

RI = (PSV-EDV) / PSV = (peak(peak systolic velocity - end diastolic-diastolic velocity) / peak systolic velocity 

• normal (low resistance) RI <0.55
• high resistance
  • bilateral notches RI >0.55
  • unilateral notches RI >0.65

Pulsatility index (PI)

This is calculated by the following equation:

• PI = (peak systolic velocity - end diastolic-diastolic velocity) / time averaged-averaged velocity = (PSV - EDV) / TAV

Abnormal patterns include

• persistence of a high resistance flow throughout pregnancy
• persistence of notching throughout pregnancy
• reversal of diastolic flow throughout pregnancy: severe state​