Resistive index (vascular ultrasound)

The resistive index (Pourcelot index) is a calculated flow parameter in ultrasound, derived from the maximum, minimum, and mean Doppler frequency shifts during a defined cardiac cycle. Along with the pulsatility index (PI), it is typically used to assess the resistance in a pulsatile vascular system.

• RI = (PSV - EDV) / PSV

​Where PSV = peak systolic velocity and EDV = end-diastolic velocity.

Clinical use

Resistive index is one of the most common vascular ultrasound indices used owing to its simplicity. The RI is proportional to not only vascular resistance but also vascular compliance. As a vessel narrows and resistance to flow increases, the RI will increase.

When the stenotic segment is located distal or downstream to the ultrasound probe, peak systolic velocity (PSV) is slightly decreased and end-diastolic velocity (EDV) decreases more than the PSV, thus resulting in increased resistance index (RI) ².

When the stenotic site is located just beneath the ultrasound probe, both PSV and EDV are raised ².

When the stenotic site is located proximal or upstream to the ultrasound probe, PSV decreases more than the EDV, resulting in decreased RI, producing tardus parvus waveform ².

Different vessels and vascular beds have different flow requirements, so there are different normal RI values depending on the target organ. Blood vessels supplying vital organs such as internal carotid artery, hepatic artery, renal artery, and testicular artery generally have low resistive index (0.55 to 0.70) ². Meanwhile, blood vessels supplying extremities of the body such as external carotid artery, external iliac, axillary, superior and inferior mesenteric arteries (during fasting) have high resistive index (more than 0.7) ².

Specific RI uses include:

• hepatic RI

• renal RI

• delta RI

• umbilical RI