Difference Between Resistance Index and Pulsatility Index in the Umbilical Artery
Both the resistance index (RI) and pulsatility index (PI) are quantitative measures of umbilical artery Doppler waveforms that assess fetoplacental vascular resistance, but they differ in their mathematical formulas: RI = (peak systolic velocity - end-diastolic velocity) / peak systolic velocity, while PI = (peak systolic velocity - end-diastolic velocity) / mean velocity over the cardiac cycle. 1
Mathematical Formulas and Calculation
Resistance Index (RI) is calculated as: RI = (A - B) / A, where A is peak systolic velocity and B is end-diastolic velocity 2
Pulsatility Index (PI) is calculated as: PI = (A - B) / mean velocity, where A is peak systolic velocity, B is end-diastolic velocity, and mean refers to the time-averaged mean velocity throughout the cardiac cycle 2
The key distinction is that RI uses only the peak systolic value as the denominator, while PI incorporates the mean velocity across the entire cardiac cycle in its denominator 2
Clinical Utility and Practical Differences
Both indices are equally acceptable for managing most cases of suspected intrauterine growth restriction (IUGR), with either the systolic-to-diastolic (S/D) ratio or PI being sufficient 1
When end-diastolic flow is absent, the S/D ratio becomes immeasurable (division by zero), but PI can still be calculated and used 1—this represents a critical practical advantage of PI in advanced placental compromise
Both RI and PI demonstrate high correlation with each other (Spearman correlation coefficients approximately 1.0), indicating they have essentially the same relationship to IUGR 2
No evidence supports preferring one index over the other in terms of sensitivity or specificity for detecting growth restriction 2
Physiologic Interpretation
Both indices reflect downstream placental vascular resistance, with higher values indicating increased resistance in the fetoplacental unit 1, 2
Progressive placental compromise causes decreased end-diastolic flow, which increases both RI and PI values 2
Both indices normally decrease with advancing gestational age in the third trimester, reflecting progressive reduction in placental vascular resistance 3
The flow pulsatility index (PI) is determined by the ratio of total umbilico-placental vascular resistance divided by fundamental impedance, multiplied by the pulsatility index of arterial pressure 4
Technical Considerations
Both RI and PI should be obtained at the abdominal cord insertion for optimal reproducibility, though any segment along the umbilical cord is acceptable 1
Measurements must be taken in the absence of fetal breathing movements and when the waveform is uniform 1
Values obtained near the fetal end of the cord are higher than those at the placental end for both RI and PI, with differences inversely related to gestational age 5
In clinical practice, averaging multiple values of S/D ratios or PIs is unnecessary 1
Common Pitfalls
Do not assume one index is inherently superior—both RI and PI have equivalent clinical validity for predicting IUGR when using appropriate gestational age-specific reference ranges 2
Remember that PI remains calculable when end-diastolic flow is absent, whereas S/D ratio and RI become problematic in this scenario 1
Be aware that sensitivity for predicting growth restriction is modest (approximately 53%) even with acceptable specificity (88%) for both indices, so normal values do not exclude placental insufficiency 2