Calculate the Doppler pulsatility index (PI) from peak systolic, end-diastolic and time-averaged maximum velocity, plus resistive index and S/D ratio.
Pulsatility Index Formula
Use velocities from the same arterial Doppler waveform and in the same unit. The four modes calculate PI, PSV = PI × TAMX + EDV, EDV = PSV − PI × TAMX, or TAMX = (PSV − EDV)/PI. Solving for TAMX requires PI > 0; PI = 0 does not determine a unique TAMX when PSV = EDV.
Variables:
- PI is the Pulsatility Index
- PSV is peak systolic velocity from the traced waveform.
- EDV is velocity at end-diastole. It may be negative for reversed end-diastolic flow and need not be the waveform’s lowest velocity.
- TAMX is the time average of the traced maximum-velocity envelope over a complete cardiac cycle, not an average of PSV and EDV or the intensity-weighted mean velocity.
Subtract EDV from PSV, then divide by positive TAMX. Related outputs are RI = (PSV − EDV)/PSV, S/D = PSV/EDV, and peak-to-end-diastolic spread = PSV − EDV. S/D is undefined when EDV = 0; a negative S/D is the signed arithmetic result for reversed EDV and requires waveform context. These indices are dimensionless; the spread is reported in cm/s.
What is a Pulsatility Index?
The Pulsatility Index (PI) describes the spread between peak systolic and end-diastolic velocity relative to the traced time-averaged maximum velocity in a specific arterial Doppler waveform. PI varies with the vessel, organ, physiology, patient condition, and acquisition method. A higher or lower value is not, by itself, evidence of stenosis, vascular disease, fetal distress, or another diagnosis; interpretation requires the application-specific protocol and the original waveform context.
How to Calculate Pulsatility Index?
The following steps outline how to calculate the Pulsatility Index:
- First, obtain peak systolic velocity (PSV) and select the input velocity unit: cm/s, m/s, or mm/s.
- Next, obtain signed end-diastolic velocity (EDV) from the same trace.
- Next, obtain positive TAMX from the traced envelope over the complete cycle.
- Use PI = (PSV − EDV)/TAMX, with all velocities in the same unit.
- Check that PSV > 0, TAMX > 0, EDV ≤ PSV, and TAMX ≤ PSV. EDV is not a universal lower bound for TAMX; inspect the original waveform.
Example Problem:
Use the following variables as an example problem to test your knowledge:
Peak systolic velocity (PSV) = 120 cm/s
End-diastolic velocity (EDV) = 80 cm/s
Traced TAMX = 100 cm/s. PI = (120 − 80)/100 = 0.4; RI = 40/120 ≈ 0.3333; S/D = 120/80 = 1.5; peak-to-end-diastolic spread = 40 cm/s.
