Resistive index (vascular ultrasound)
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The resistive index (RI), also known as the 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
The resistive index (RI) 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 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 a 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 the internal carotid, hepatic, renal, and testicular arteries generally have a low RI (0.55-0.7) 2. Meanwhile, blood vessels supplying extremities of the body such as the external carotid, external iliac, axillary, superior mesenteric, and inferior mesenteric arteries (during fasting) have a high RI (>0.7) 2.
Specific RI uses include: