Fetal color Doppler ultrasound velocity indices (waveform parameters)
Color Doppler in obstetrics quantifies blood-flow velocity patterns in specific fetal vessels to reflect fetoplacental resistance and fetal cardiovascular adaptation. [1]
The most commonly reported quantitative indices are peak systolic velocity (PSV), end-diastolic velocity (EDV), resistive index (RI), pulsatility index (PI), and systolic-to-diastolic ratio (S/D). [1]
Umbilical artery (UA) color Doppler indices
UA Doppler is obtained from the umbilical artery waveforms and is interpreted primarily as an indicator of placental vascular resistance. [1]
Waveform- and index-derived quantities are based on the systolic and diastolic velocities measured from the Doppler spectrum. [1]
Index definitions:
- S/D ratio = PSV / EDV. [1]
- Resistive index (RI) = (PSV − EDV) / PSV. [1]
- Pulsatility index (PI) = (PSV − EDV) / time-averaged maximum velocity. [1]
With increasing downstream resistance, UA waveform diastolic flow typically decreases, which increases impedance-derived indices such as RI and PI. [1]
Middle cerebral artery (MCA) color Doppler indices
MCA Doppler provides information about fetal cerebral circulation and is used to assess redistribution of blood flow in response to suspected hypoxemia. Dopplervelocimetria
The MCA waveform is summarized using the same Doppler indices (PSV, EDV, PI, RI, S/D) applied to the MCA spectrum. [2]
A lower MCA PI is often interpreted as a “brain-sparing” type physiologic pattern in growth restriction. [3]
Cerebroplacental ratio (CPR) color Doppler parameter
The cerebroplacental ratio (CPR) is calculated from the MCA PI divided by the UA PI to combine information about fetal brain circulation and placental resistance. American College of Radiology ACR Appropriateness Criteria® Growth Disturbances–Risk of Fetal Growth Restriction
A lower CPR is used as a marker of worsening fetoplacental function and more severe adaptive redistribution patterns. American College of Radiology ACR Appropriateness Criteria® Growth Disturbances–Risk of Fetal Growth Restriction
Ductus venosus (DV) color Doppler waveform parameters
DV Doppler evaluates venous flow patterns that reflect fetal cardiovascular status and central hemodynamics. [4]
DV color Doppler is typically interrogated in mid-sagittal or cross-sectional planes after identification of the vessel segment that shows characteristic forward flow/aliasing features. [4]
DV flow velocity is described by A-, S-, and V-wave components (reflecting atrial contraction, ventricular systolic forward flow, and diastolic forward/late filling behavior), which are assessed for abnormal patterns such as A-wave reversal. [4]
A-wave reversal is associated with increased right atrial pressure and is considered a late sign of cardiovascular compromise in the setting of fetal deterioration. [4]
Practical measurement characteristics affecting reported “parameters”
Fetal Doppler indices depend on the correct vessel selection and the Doppler angle relative to flow, while PI and related indices are used clinically because they can be less sensitive to angle than absolute velocities in many settings. [2]
Gestational age strongly influences reference values for UA PI, MCA PI, and CPR, so interpretation commonly uses gestational-age-specific centiles or cutoffs. [3]
Sex- and other physiologic factors can also affect Doppler indices during gestation, which contributes to variability in normal ranges. [5]
Summary of commonly reported fetal color Doppler parameters
- PSV and EDV are measured from Doppler waveforms. [1]
- RI and PI summarize waveform impedance using PSV and EDV (and PI also uses time-averaged maximum velocity). [1]
- S/D ratio summarizes systolic-to-diastolic behavior using PSV and EDV. [1]
- CPR summarizes the relationship between MCA PI and UA PI. American College of Radiology ACR Appropriateness Criteria® Growth Disturbances–Risk of Fetal Growth Restriction
- DV waveform components (A-, S-, V-waves) are assessed for abnormal patterns that reflect fetal cardiovascular status. [4]