Calculate Doppler resistive index (RI) from peak systolic and end diastolic velocity, with optional pulsatility index, or solve PSV or EDV from a known RI.
Doppler Resistance Index Formula
The Doppler resistance index, also called the resistive index or RI, compares peak systolic velocity with end diastolic velocity. It is unitless because both velocity values use the same type of measurement.
- RI = resistance index, unitless
- PSV = peak systolic velocity
- EDV = end diastolic velocity
If the resistance index is known and you need to solve for peak systolic velocity, the calculator rearranges the formula:
If the resistance index is known and you need to solve for end diastolic velocity, the calculator uses:
Choose the value to solve for and enter the other two values. The calculator converts velocity inputs to cm/s, applies the selected equation, then converts a calculated velocity to the chosen output unit.
Velocity Unit Conversions Used for Doppler RI
Peak systolic velocity and end diastolic velocity must be in compatible units before calculating RI. The calculator uses cm/s as the internal base unit.
| Unit | Conversion to cm/s | Example |
|---|---|---|
| cm/s | multiply by 1 | 50 cm/s = 50 cm/s |
| m/s | multiply by 100 | 0.5 m/s = 50 cm/s |
| km/h | multiply by 250/9 (27.777777…) | 1.8 km/h = 50.0000 cm/s |
| mi/h | multiply by 44.704 | 1 mi/h = 44.704 cm/s |
RI Interpretation Limits
This single-waveform calculator reports ratio arithmetic and reverse algebra. RI varies with the vessel, organ, hemodynamics, patient condition, waveform sampling, and Doppler technique. There is no universal low, normal, or high RI range that applies across all clinical uses, and the result alone cannot diagnose a condition.
Algebraically, RI = 0 when EDV = PSV; RI = 1 when EDV = 0; and RI > 1 can occur with negative EDV or reversed diastolic flow.
Use the separate Repeated-Waveform Resistive Index Calculator when you need a descriptive mean, range, and sample standard deviation for three measurements from the same vessel segment and protocol.
Example Calculations
Example 1: Calculate resistance index
You measure a peak systolic velocity of 80 cm/s and an end-diastolic velocity of 20 cm/s.
The Doppler resistance index is 0.75.
Example 2: Calculate end-diastolic velocity
You know the peak systolic velocity is 100 cm/s and the resistance index is 0.70.
The end-diastolic velocity is 30 cm/s.
FAQ
What is the Doppler resistance index?
The Doppler resistance index is a ratio that describes how much blood flow velocity drops from systole to diastole. It is calculated from peak systolic velocity and end-diastolic velocity. A higher RI generally means lower or absent diastolic flow relative to systolic flow.
Does RI have a unit?
No. RI is unitless because it is calculated by dividing one velocity difference by a velocity. You can enter PSV and EDV in cm/s, m/s, km/h, or mi/h, but the two values must be converted to compatible units before the ratio is calculated.
Can RI be greater than 1?
Yes. RI can be greater than 1 if the end-diastolic velocity is negative, which can happen when there is reversed diastolic flow. The clinical meaning depends on the vessel being measured and the full Doppler waveform context.
