Renal Arterial Resistance Index

The kidney is a profusely vascularized organ which, unlike other organs, does not regulate renal blood flow (RBF) mainly by oxygen demand. Reflex (myogenic effect and tubular-glomerular feedback) and neurohormonal mechanisms modulate RBF and renal resistances by regulating the tone of afferent and efferent arterioles as well as that of the major resistance vessels in the kidneys. Arterial renal resistances are also influenced by a number of other pathophysiological factors, such as increased arterial stiffness, arterial atherosclerosis, renal parenchymal abnormalities, and intra-abdominal and central venous pressure. Finally, renal resistances can be permanently increased if a microvascular disease and vascular rarefaction occur as a result of vasoconstriction-related ischemia, endothelial dysfunction, and inflammatory cytokine activity. In this setting, a parameter reflecting renal resistances could offer a useful tool to better characterize renal disease and the risk of progression. This chapter focuses on renal resistance index (RRI), a parameter obtained by pulsed Doppler which reflects renal arterial resistances. The possible clinical usefulness of RRI has been demonstrated in studies which show the pathophysiological correlates and prognostic role in predicting a greater risk of chronic kidney disease progression and cardiovascular events.