The renal resistive index: is it a misnomer?

Progress in digital ultrasound technology and diffusion of Doppler ultrasound evaluation of the kidney enable a widespread non-invasive evaluation of renal haemodynamics. Initially most attention has been paid to the study of extraparenchymal renal arteries, mainly to detect renovascular disease. However, this approach has low reproducibility and accuracy. Therefore, interest has gradually moved towards the duplex evaluation of intrarenal anatomy, where the best and most reliable signals are obtained from the large segmental or interlobar arteries that run directly towards the transducer. Among the sonographic parameters used in the last decade, great emphasis has been placed on the intrarenal resistive index (RRI), which is defined as the dimensionless ratio of the difference between maximum and minimum (end-diastolic) flow velocity to maximum flow velocity. It has been used for a long time for the diagnostic and prognostic assessment of renovascular disease . One of the earliest prospective uses of the RRI was in the prediction of kidney function outcomes following intervention for renal artery stenosis. In the pioneering study of Radermacher et al., an RRI[0.80 is associated with poorer outcomes, when surgery or angioplasty is used to correct renal artery stenosis ..