Renal Ultrasound (and Doppler Sonography) in Hypertension: An Update

Abstract

Ultrasound (US) allows the non-invasive evaluation of morphological changes of kidney structure (by means of B-Mode) and patterns of renal and extrarenal vascularization (by means of color-Doppler and contrast-enhanced US). In hypertensive subjects it offers a relevant contribution to the diagnosis of early renal damage, acute or chronic nephropathies and nephrovascular disease. However, morphological changes are often detected late and non-specific and in recent years evidence has increased regarding the clinical relevance of renal resistive index (RRI) for the study of vascular and renal parenchymal renal abnormalities. RRI is measured by Doppler sonography in an intrarenal artery, as the difference between the peak systolic and end-diastolic blood velocities divided by the peak systolic velocity. At first RRI was proved to be a marker of renal disease onset and progression; later the influence of systemic vascular properties on RRI was shown and authors claimed its use as an independent predictor of cardiovascular risk rather than of renal damage. Indeed, renal vascular resistance is only one of several renal (vascular compliance, interstitial and venous pressure), and extrarenal (heart rate, pulse pressure) determinants that concur to determine RRI individual values but not the most important one. The clinical relevance of RRI measurement as a surrogate endpoint of specific renal damage or/and as surrogate endpoint of atherosclerotic diffuse vascular damage is still debated.To summarize, from the literature: (a) In hypertensives with normal renal function and no albuminuria, especially in younger people, RRI is an early marker of renal damage that is especially useful when hypertension and diabetes concur in the same subjects. In these subjects RRI could improve current clinical scores used to stratify early renal damage. In older subjects RRI increases in accordance with the increase in systemic vascular stiffness and, because of this close relationship, RRI is also a marker of systemic atherosclerotic burden and the role of renal determinants can weaken. The clinical relevance was not specifically investigated. (b) In transplant kidney and in chronic renal disease high (>0.80) RRI values can independently predict renal failure. The recent claim that systemic (pulse pressure) rather than renal hemodynamic determinants sustain this predictive role of RRI, does not significantly reduce this predictive role of RRI. (c) Doppler ultrasound allows diagnosis and grading of renal stenosis in both fibromuscolar dysplastic and atherosclerotic diseases. Moreover, by RRI assay Doppler ultrasound can indirectly measure the hemodynamic impact of renal artery stenosis on the homolateral kidney, by virtue of the stenosis-related decrease in pulse pressure. However, in elderly subjects with atherosclerotic renal artery stenosis coexisting renal diseases can independently increase RRI by the augmentation in renal vascular stiffness and tubulo-interstitial pressure and hidden changes due to renal artery stenosis.

Renal resistive index as a novel biomarker for cardiovascular and kidney risk reduction in type II diabetes

“…However, there are some limitations that need to be considered especially under the setting both intrarenal and systemic hemodynamic derangements. Because of its complex interaction between both the kidney as well as extra renal hemodynamic factors as well as vascular structural changes, significance of RRI most often includes determinate of both damage to kidney and systemic organs and vasculature Moreover, opinions differ for its use as surrogate marker for kidney injury in atherosclerotic heart disease . Although there are some limitations in the study as iterated by the authors, the results highlight the utility of utilizing RRI as independent prognostic biomarker for progressive kidney disease as well as CVD in patients with metabolic conditions such as obesity, the CRS, and diabetes.…”

“…Although there are some limitations in the study as iterated by the authors, the results highlight the utility of utilizing RRI as independent prognostic biomarker for progressive kidney disease as well as CVD in patients with metabolic conditions such as obesity, the CRS, and diabetes. Moreover, the microvascular resistance and tissue impedance as determined by the RRI has been linked to kidney parenchymal injury and fibrosis …”

Renal resistive index in hypertensive patients

“…The mean value of three measurements at each kidney is usually taken under consideration. 3 However, it is generally recommended that sampling for RRI should be done at the level of the arcuate or interlobar arteries, as resistance to blood flow progressively increases from the hilar arteries toward peripheral parenchymal vessels. 2 However, this threshold increases with age and a recent population-based study has established reference values for RRI, according to age and sex.…”

“…2 RRI is evaluated in segmental or interlobar arteries. 3,6 Moreover, bradycardia increases RRI as there is more time for the diastolic flow to decrease. 4 Several factors can influence the value of RRI (Table 1).…”

“…3 However, it is generally recommended that sampling for RRI should be done at the level of the arcuate or interlobar arteries, as resistance to blood flow progressively increases from the hilar arteries toward peripheral parenchymal vessels. 3,6 An animal study provides evidence that RRI is dramatically affected by pulse pressure and by renal interstitial pressure. A number of studies indicate that this index actually reflects systemic hemodynamics and depends on the aortic pulse pressure, which is affected by parameters like age, presence of hypertension (HTN), or diabetes.…”