Intraparenchymal Renal Resistive Index: The Basic of Interpretation and Common Misconceptions

Renal resistive indices (RRIs) express the percentage reduction of the end diastolic blood flow in renal vessels in relation to the maximum systolic blood flow and can be obtained through the Doppler spectrum analysis from segmental and/or interlobar arteries. RRIs have had a great diffusion in clinical practice as markers of purely renal organ damage, providing a useful noninvasive evaluation of renal microcirculation. However, studies in the past that have tried to correlate RRIs with the type of renal histological damage (glomerular, mesangial, or tubular-interstitial) have given conflicting results. Growing data from the literature demonstrate that RRIs are not only derived from specific renal components (such as the renal capillary wedge pressure), but are also the result of complex interactions between the vascular tree and systemic hemodynamic factors, including the pulse pressure, systemic vascular compliance, and cardiac function. The purpose of this chapter is to explain the emerging pathophysiological aspects of the relationship between renal microcirculation and cardiovascular system and to answer some of the most frequent questions about RRI applicability and utility in renal and non-primarily renal clinical scenarios.