Arterial resistivity index

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:

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

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.

Uses

• It is used in ultrasound testing of umbilical artery for placental insufficiency. RI should not exceed 0.60 at 30 weeks of gestation.

See also

• Pulsatility index
• Leandre Pourcelot (French)

External links

• Resistivity index (English/Spanish)
• Pulsatility index (Spanish)