Resistive index
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Aarti Narayan, M.B.B.S [2]
Overview
The arterial resistivity index (RI) is a measure of pulsatile blood flow that reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement. A resistive index of 0 corresponds to continuous flow; a resistive index of 1 corresponds to systolic but no diastolic flow; and a resistive index greater than 1 corresponds to reversed diastolic flow.
Formula = (Peak systolic velocity - End diastolic velocity) / Peak systolic velocity
The RI is altered not by vascular resistance alone but by the combination of vascular resistance and vascular compliance.