# Arterial resistivity index

Arterial resistivity index
Medical diagnostics
Purposemeasure of pulsatile blood flow

The arterial resistivity index (also called as Resistance index, abbreviated as RI), developed by Leandre Pourcelot, is a measure of pulsatile blood flow that reflects the resistance to blood flow caused by microvascular bed distal to the site of measurement.

## Calculation

The formula used to calculate resistance index is:[1]

${\displaystyle RI={\frac {v_{systole}-v_{diastole}}{v_{systole}}}}$

## Description

Resistance index Description
0 Continuous flow
1 Systolic flow, but no diastolic flow
>1 Reversed diastolic flow

The RI is altered not by vascular resistance alone but by the combination of vascular resistance and vascular compliance.[2][3]

Normal mean renal artery RI for an adult is 0.6 with 0.7 the upper limit of normal. In children, RI commonly exceeds 0.7 through 12 months of age and can remain above 0.7 through 4 years of age.[4]

## Uses

It is used in ultrasound testing of umbilical artery for placental insufficiency. RI should not exceed 0.60 at 30 weeks of gestation.[5] RI is also commonly used to monitor kidney status, especially following kidney transplant.

Following kidney transplantation, patients with an RI > 0.8 have an increased mortality.[4][6]