The pulsatility index (PI) is a common Doppler ultrasound metric used to describe how strongly blood flow velocity changes during the cardiac cycle. Use the calculator below to quickly compute PI from measured peak systolic velocity, end-diastolic velocity, and mean velocity.
PI Calculator
Formula: PI = (PSV − EDV) / Mean Velocity
Enter all values using the same unit (for example, cm/s or m/s).
What is the pulsatility index?
The pulsatility index is a dimensionless ratio that reflects downstream vascular resistance and arterial compliance. In simple terms, it tells you how “pulsed” the waveform is: higher values generally indicate greater variation between systolic and diastolic flow, while lower values indicate steadier flow through the cardiac cycle.
PI is used in multiple settings, including obstetric Doppler studies (such as umbilical artery or middle cerebral artery), neurovascular evaluation, and peripheral vascular assessments. Exact interpretation always depends on the vessel, patient condition, and reference standards for that context.
PI formula and variables
Standard equation
PI = (PSV − EDV) / Mean Velocity
- PSV: Peak systolic velocity (maximum velocity during systole)
- EDV: End-diastolic velocity (velocity at end diastole)
- Mean Velocity: Time-averaged velocity over one cardiac cycle
Because all terms are velocity values in the same units, the units cancel out. That means PI itself has no units.
How to use this calculator correctly
- Measure PSV, EDV, and mean velocity from the same vessel segment and waveform quality standard.
- Use consistent units for all inputs.
- Enter values into the calculator and click Calculate PI.
- Review the PI result and contextual notes.
Tip: If your waveform quality is poor (motion artifact, angle errors, aliasing), correct the acquisition first. A precise equation cannot fix noisy data.
Worked example
Suppose:
- PSV = 70 cm/s
- EDV = 20 cm/s
- Mean velocity = 35 cm/s
Then:
PI = (70 − 20) / 35 = 50/35 = 1.43
This indicates moderate pulsatility in many contexts, but final interpretation still depends on the specific vessel and clinical scenario.
PI vs. RI vs. S/D ratio
Related Doppler indices
- Resistive Index (RI): (PSV − EDV) / PSV
- S/D Ratio: PSV / EDV
- Pulsatility Index (PI): (PSV − EDV) / Mean velocity
PI includes mean velocity in the denominator, which can make it more sensitive to the overall waveform shape than RI alone.
General interpretation guide (non-diagnostic)
Very broad educational guidance:
- Lower PI can reflect lower vascular resistance or vasodilation.
- Higher PI can reflect higher resistance, reduced compliance, or distal impedance.
- There is no universal normal PI across all vessels, ages, and physiologic states.
Always compare against vessel-specific reference ranges, gestational age charts (in obstetrics), and your institutional protocol.
Common causes of misleading PI values
- Incorrect Doppler angle correction
- Arrhythmias causing beat-to-beat variability
- Sampling near bifurcations or turbulent flow zones
- Using inconsistent cardiac cycles
- Inputting mean velocity from a different acquisition setting
Important clinical note
This tool is intended for education and quick estimation. It is not a diagnostic device and does not replace clinical judgment, radiology standards, or specialist interpretation. If findings are unexpected, always re-check waveform quality and clinical context before making decisions.