PULSATILITY INDEX (PI ) IN TRANSCRANIAL DOPPLER (TCD) MEASUREMENTS: PROBLEMS OF CONSISTENCY
By Colin Deane, King’s College London
The Pulsatility Index (PI) is an established measure of flow waveform shape that has been used extensively in peripheral vascular, obstetric, abdominal and transcranial Doppler (TCD) ultrasound.
PI in TCD has been used, amongst other applications, as a measure of raised intracranial pressure (ICP). Unfortunately, there is a lack of unanimity in descriptions of PI in TCD literature. Commonly, the minimum diastolic velocity Vmin is described as the end diastolic velocity (EDV) which is incorrect in very elevated resistance. Other work describes the time averaged peak velocity (TAPV) as 1/3 peak systolic velocity (PSV) + 2/3 EDV. While this is an approximation for normal TCD flow waveforms, it is inaccurate when there is raised ICP. The approximation derives from a review of TCD applications and has managed to gain traction in the literature.
In response to these measures, at least one ultrasound scanner manufacturer has responded by introducing a PI TCD based solely on PSV and EDV, in effect a variation of the resistive index (RI). They also allow measurement of TCD mean as 1/3 PSV + 2/3 EDV rather than a calculation based on the maximum velocity trace.
We show that the introduction of a different measure of PI mitigates against development of TCD in applications where the flow waveform differs from the normal. Errors are shown in the hope that the larger community works towards using the original PI and TCD mean velocity measurements for the sake of consistency and clarity.