[Index resistance in renal ultrasound: what is the clinical significance?]

In the area of renal diagnosis, B-mode ultrasound allows for the study of renal morphology, while power color Doppler is of strategic importance not only for the qualitative and quantitative information it provides on renal vasculature, but also because it allows for the measurement of 'Index Resistance'. This is the ratio between the peak systolic speed [VPS] minus the telediastolic speed [VTD] and the VPS: [VPS-VTD/VTD]), now one of the most sensitive parameters in the study of renal pathologies through the quantification of changes in renal plasma flow. The reliability of the measurement of IR is dependent on accurate methodology, such as sampling at the level of the interlobar or arcuate arteries of 3 to 5 waves of similar dimensions in three different areas of the kidney, etc. Reliability also depends on careful analysis of the value obtained, owing to the confounding influence of peripheral resistance in addition to many other factors, including tachy-brady-arrhythmias, severe hypotension, and perirenal or subcapsular fluid collections. In adults an average IR of < 0.70 is considered normal, although this figure varies with age, giving higher values in children in the first years of life and in the elderly. The color Doppler measurement of IR at the level of the interlobar artery has been proposed as an indicator for differential diagnosis of acute or chronic nephropathies: for example, the ratio was higher in acute pathologies with vascular and tubulo-interstitial involvement, but not in those with glomerular involvement. This review aims to highlight clinical situations in which the study of intrarenal IR can provide useful information on the physiopathology of renal disease in both the native and in the transplanted kidney, as illustrated by the alterations of the morphology of the Doppler wave that are caused by variations in vascular resistance, hydrostatic capillary pressure and pressure inside the urinary tract.