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The use of renal resistive indices (RRIs) for the study of renal microcirculation has in the past been proposed for the identification of renal organ damage or even to specifically identify injury to some areas of the renal parenchyma. Nevertheless, according to the most recent evidences from literature this organ-based conception of RRIs has been proven to be partial and unable to explain the RRIs variations in clinical settings of sepsis or combined organ failure of primitively extrarenal origin or, more generally, the deep connection between RRIs and hemodynamic factors such as compliance and pulsatility of the large vessels. The aim of this review is to explain the physiopathological basis of RRIs determination and the most common interpretative errors in their analysis. Moreover, through a comprehensive vision of these Doppler indices, the traditional and emerging clinical application fields for RRIs are discussed.

From growing data from the literature emerges that so-called ecoDoppler "Renal" Resistive Indexes (RRI) are not only specific markers of kidney damage and indicators of renal functional prognosis but they always express more clearly a strong link with the systemic circulation. In fact, some cardiovascular parameters such as aortic pulse pressure, aortic stiffness, abdominal aortic calcifications and clinical settings for instance heart failure progression have shown a great association with RRI. Relationships between renal and inflammatory indexes in hypertensive patients have been found and data from the literature on kidney transplants underline the capability of the graft RRI to change according to host hemodynamic features. The purpose of this article is to explain the emerging pathophysiological aspects of this relationship between renal microcirculation and cardiovascular system by emphasizing the importance for the ultrasonographer and for the nephrologist of not overlooking a global vision of the patient before interpreting the RRI.

In renal diagnosis, the B-mode ultrasound is used to provide an accurate study of the renal morphology, whereas the colour and power Doppler are of strategic importance in providing qualitative and quantitative information about the renal vasculature, which can also be obtained through the assessment of the resistive index (RI). To date, this is one of the most sensitive parameters in the study of kidney diseases and allows us to quantify the changes in renal plasma flow. If a proper Doppler ultrasound examination is carried out and a critical analysis of the values obtained is performed, the RI measurement at the interlobar artery level has been suggested in the differential diagnosis between nephropathies. The aim of this review is to highlight the pathological conditions in which the study of intrarenal RI provides useful information about the pathophysiology of renal diseases in both the native and the transplanted kidneys.