The pulsatility index is a calculated flow parameter in ultrasound, derived from the maximum, minimum, and mean Doppler frequency shifts during a defined cardiac cycle. Along with the resistivity index (RI), it is typically used to assess the resistance in a vascular system.
Pulsatility is an intrinsic property of the cardiovascular system, governed by the resistance differential across the arteriolar bed, which allows the potential energy stored in the elastic, proximal arteries to propagate throughout the microcirculation at a mean pressure consistent with adequate perfusion.
When evaluated as a derived flow parameter using pulsed wave Doppler, it is calculated by one of the following equations:
PI = (vmax - vmin) / (vmean)
PI = (peak systolic flow - peak diastolic flow) / (mean flow)
The operator typically recognizes and demarcates maximum (vmax) and minimum (vmin) velocities, while the mean velocity (vmean) is calculated by the ultrasound machine.
Because the calculation of these flow parameters is based on the same Doppler spectrum, possible instrument-dependent errors, or an inappropriate angle of insonation by the user, are mitigated somewhat 1.
Clinical scenarios in which a pulsatility index are calculated include: