Calculate Doppler resistance index, peak systolic velocity, or end diastolic velocity from any two values with built-in unit conversions.
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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:
The calculator lets you enter any two values and leaves the third blank. It first converts velocity inputs to cm/s, applies the appropriate formula, then converts the calculated velocity back to the unit selected for the missing field.
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 27.7778 | 1.8 km/h = 50.0000 cm/s |
| mi/h | multiply by 44.704 | 1 mi/h = 44.704 cm/s |
General RI Result Ranges
RI interpretation depends on the vessel, organ, patient condition, and measurement technique. Use the ranges below only as a general guide, not as a diagnostic cutoff.
| RI Value | General Meaning |
|---|---|
| Near 0 | End diastolic flow is close to peak systolic flow, suggesting low downstream resistance. |
| Around 0.5 to 0.8 | Common range for many Doppler measurements, depending on the vascular bed. |
| Near 1 | End diastolic flow is very low or absent, suggesting higher downstream resistance. |
| Greater than 1 | Can occur if end diastolic velocity is negative, such as with flow reversal. |
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.
