Some failings of pulsatility index and damping factor.

Pulsatility index (PI) is a commonly used method of objective assessment of the Doppler waveform. PI falls with increasing proximal stenosis and is raised by increasing peripheral resistance. Damping factor (DF) for an arterial segment is calculated by dividing the proximal by the distal PI. DF rises with increasing severity of disease of the arterial segment. DF is not, however, sufficiently accurate to be used alone but is usually combined with transit time measurements to provide information of diagnostic use. Both PI and DF have been examined in a canine model of combined segment disease. With increasing stenosis, distal PI falls as expected but so also does proximal PI. Such a stenosis is, in effect, a flow-throttling resistance so that although the characteristics of blood flow are altered by its presence, similar changes are observed both above and below the stenosis. The reduction of PI by a stenosis distal to the insonation site may result in the false interpretation of a low PI as indicating disease proximal to the insonation site. The observed similarity between PI proximal and distal to a stenosis reduces the usefulness of pulsatility index damping factor, particularly in the assessment of the femoro-popliteal segment in combined segment disease.