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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.
The formula used to calculate resistance index is:
|1||Systolic flow, but no diastolic flow|
|>1||Reversed diastolic flow|
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.
It is used in ultrasound testing of umbilical artery for placental insufficiency. RI should not exceed 0.60 at 30 weeks of gestation. RI is also commonly used to monitor renal status, especially following renal transplant.