Renal arterial resistive index

The renal arterial resistive index (RI) is a sonographic index of intrarenal arteries defined as (peak systolic velocity - end-diastolic velocity) / peak systolic velocity. The normal range is 0.50-0.70. Elevated values are associated with poorer prognosis in various renal disorders and renal transplant.

Technique

The resistive index (RI) is measured using spectral Doppler at the arcuate arteries (at the corticomedullary junction) or interlobar arteries (adjacent to medullary pyramids).

Significance

The resistive index is thought to reflect central hemodynamic (cardiac or aortic) characteristics rather than properties of the kidney itself ^10,11. Some have proposed that since it reflects pulsatility and vascular compliance, it would be more appropriately called an impedance index ¹³.

The renal resistive index is a nonspecific prognostic marker in vascular diseases that affect the kidney. High resistive indices (>0.8) in native kidneys are associated with renal dysfunction and adverse cardiovascular events ^7,8. In renal transplant recipients, high resistive indices (>0.8) are associated with increased risk of graft loss and death ⁹. There is thought to be little correlation between the resistive indices and the quantitative extent of renal dysfunction (measured by serum creatinine values) ².

Differential diagnosis

Reasons for elevated values

• medical renal disease ¹⁵
• ureteric obstruction
• extreme hypotension
• very young children
• perinephric fluid collection
• abdominal compartment syndrome

Reasons for elevated values in a transplant kidney

• acute tubular necrosis (ATN)
• acute or chronic transplant rejection
• renal vein thrombosis
• drug toxicity
• ureteric obstruction
• perinephric fluid collection

Reasons for decreased values

• renal artery stenosis ¹⁴