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 systolic velocity - end diastolic velocity) / peak systolic velocity
- normal (low resistance) RI < 0.55
- high resistance
Pulsatility index (PI)
This is calculated by the following equation:
- PI = (peak systolic velocity - end diastolic velocity) / time averaged velocity = (PSV - EDV) / TAV
Abnormal patterns include
- persistence of a high resistance flows throughout pregnancy
- persistence of notching throughout pregnancy
- reversal of diastolic flow throughout pregnancy: severe state
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