Current Insights into the Significance of the Renal Resistive Index in Kidney and Cardiovascular Disease

Initially, the renal resistive index (RRI) was investigated with the aim of improving diagnosis in kidney diseases, but this goal was not met. Recently, many papers have highlighted the prognostic significance of the RRI in chronic kidney disease: specifically, in estimating the revascularization success of renal artery stenoses or the evolution of the graft and the recipients in renal transplantation. Moreover, the RRI has become significant in the prediction of acute kidney injury in critically ill patients. Studies in renal pathology have revealed correlations of this index with parameters of systemic circulation. The theoretical and experimental premises of this connection were then reconsidered, and studies analyzing the link between RRI and arterial stiffness, central and peripheral pressure, and left ventricular flow were conducted with this purpose. Many data currently indicate that RRI is influenced more by pulse pressure and vascular compliance than by renal vascular resistance-assuming that RRI reflects the complex interplay between systemic circulation and renal microcirculation and should be considered a marker of systemic cardiovascular risk beyond its prognostic relevance for kidney disease. In this review, we overview the clinical research that reveals the implications of RRI in renal and cardiovascular disease.

The clinical significance of renal resistance index (RRI) and renal oxygen saturation (RrSO2) in critically ill children with AKI: a prospective cohort study

OBJECTIVE

The purpose of this study was to look into the clinical significance of the renal resistance index (RRI) and renal oxygen saturation (RrSO2) in predicting the development of acute kidney injury (AKI) in critically ill children. A new non-invasive method for the early detection and prediction of AKI needs to develop.

METHODS

Patients admitted to the pediatric intensive care unit (PICU) affiliated with the capital institute of pediatrics from December 2020 to March 2021 were enrolled consecutively. Data of clinical information, renal Doppler ultrasound, RrSO2, and hemodynamic index within 24 h of admission were prospectively collected. Patients were divided into two groups: the study group was AKI occurred within 72 h, while the control group did not. SPSS (version 25.0) was used to analyze the data, and P < 0.05 was considered a statistical difference.

RESULTS

1) A total of 66 patients were included in this study, and the incidence of AKI was 19.70% (13/66). The presence of risk factors (shock, tumor, severe infection) increased the incidence of AKI by three times. 2) Univariate analysis showed significant differences in length of hospitalization, white blood cells (WBC), C-reactive protein (CRP), renal resistance index (RRI), and ejection fraction (EF) between the study and control groups (P < 0.05). There were no significant differences in renal perfusion semi-quantitative score (P = 0.053), pulsatility index (P = 0.051), pediatric critical illness score (PCIS), and peripheral vascular resistance index (P > 0.05). 3) Receiver operating characteristic (ROC) curve showed that if RRI > 0.635, the sensitivity, specificity, and AUC for predicting AKI were 0.889, 0.552, and 0.751, respectively; if RrSO2 < 43.95%, the values were 0.615, 0.719 and 0.609, respectively; if RRI and RrSO2 were united, they were 0.889, 0.552, and 0.766, respectively.

CONCLUSIONS

The incidence of AKI is high in PICU patients. And infection, RRI, and EF are risk factors for AKI in PICU patients. RRI and RrSO2 have certain clinical significance in the early prediction of AKI and may provide a new non-invasive method for early diagnosis and prediction of AKI.

End-diastolic velocity mediates the relationship between renal resistive index and the risk of death

OBJECTIVE

Renal resistive index predicts the risk of death in many populations but the mechanism linking renal resistive index and death remains elusive. Renal resistive index is derived from end-diastolic velocity (EDV) and peak systolic velocity (PSV). However, the predictive value of EDV or PSV considered alone is unknown.

METHODS

We conducted a retrospective analysis of 2362 consecutive patients who received a kidney transplant from 1985 to 2017. EDV and PSV were measured at 3 months after transplantation, renal resistive index was calculated, and the risk of death was assessed [median follow-up: 6.25 years (0.25-29.15); total observation period: 13 201 patient-years].

RESULTS

Doppler indices were available in 1721 of 2362 (78.9%) patients (exclusions: 113 who died or returned to dialysis before, 427 with no Doppler studies, 27 with renal artery stenosis, 74 missing values). Among them, 279 (16.4%) had diabetes before transplantation. Mean age was 51.5 ± 14.7, 1097 (63.7%) were male. During follow-up, 217 of 1721 (12.6%) patients died. Renal resistive index and EDV shared many determinants (notably systolic, diastolic and pulse pressure, recipient age and diabetes) unlike renal resistive index and PSV. EDV used as a binary [lowest tertile vs. higher values: (hazard ratio: 2.57 (1.96-3.36), P  < 0.001)] and as a continuous (the lower EDV, the greater the risk of death) variable was significantly associated with the risk of death. This finding was confirmed in multivariable analyses. Prediction of similar magnitude was found for renal resistive index. No association was found between PSV used as a binary or a continuous variable and the risk of death.

CONCLUSION

Low EDV explains high renal resistive index, and the mechanism-linking renal resistive index to the risk of death is through low EDV.

The diagnostic value of Doppler ultrasonography after pediatric kidney transplantation

Ultrasonography (US) plays a major diagnostic role in the pre- and post-transplant evaluation of recipient and donor. In most cases, US remains the only necessary imaging modality. After pediatric kidney transplantation, US can ensure immediate bedside diagnosis of vessel patency and possible postoperative non-vascular complications. Criteria for US diagnosis of kidney vessel thrombosis and stenosis in the transplant will be presented. Non-vascular complications after kidney transplantation include hydronephrosis, hematoma, lymphocele, and abscess. US can detect suggestive, but nevertheless non-specific, acute signs (sudden increase in volume and elevated resistive index), and chronic rejection, which therefore remains a histological diagnosis. US is of little or no help in detection of tubular necrosis or drug toxicity, but it can exclude other differential diagnoses. This educational review provides a practical and systematic approach to a multimodal US investigation of the kidney transplant. It includes a short overview on possible indications for contrast-enhanced ultrasonography (CEUS) in children after kidney transplantation.

The role of renal resistive index as a prognostic tool in kidney transplantation: a systematic review

Background

In kidney transplant recipients (KTRs), observational data have reported conflicting findings about the utility of renal resistive index (RRI) in determining outcomes. We aimed to synthesize the current literature and determine the prognostic role of RRI in KTRs.

Methods

We conducted a systematic review to assess the role of RRI in predicting death, graft failure, graft function, and proteinuria. Of the 934 titles/abstracts reviewed, 26 studies were included. There was significant heterogeneity in RRI measurements and thresholds as well as in analytic methods and a meta-analysis could not be performed.

Results

All included studies were observational and included 7049 KTRs. Eight studies analyzed death, of which five reported a significant association with higher RRI. In the remaining three, small sample sizes and lower/multiple RRI thresholds may have limited detection of a statistically significant difference. Three studies investigated all-cause graft failure and an association with RRI was reported but varied by time of RRI measurement. Three out of five studies that analyzed a composite of patient and graft outcomes reported an association with RRI. Evidence analyzing death-censored graft failure, graft failure (unclear if death-censored or all-cause), measures of graft function, and proteinuria were conflicting. Most studies had a moderate to high risk of bias.

Conclusions

RRI likely has a prognostic role in predicting patient outcomes, reflecting patient systemic vascular disease burden rather than graft hemodynamics. Since cardiovascular diseases are a major cause of death and graft loss, RRI may be explored as a non-invasive tool to risk stratify KTRs.

Kidney Ultrasound for the Nephrologist: A Review

Ultrasound imaging is a key investigatory step in the evaluation of chronic kidney disease and kidney transplantation. It uses nonionizing radiation, is noninvasive, and generates real-time images, making it the ideal initial radiographic test for patients with abnormal kidney function. Ultrasound enables the assessment of both structural (form and size) and functional (perfusion and patency) aspects of kidneys, both of which are especially important as the disease progresses. Ultrasound and its derivatives have been studied for their diagnostic and prognostic significance in chronic kidney disease and kidney transplantation. Ultrasound is rapidly growing more widely accessible and is now available even in handheld formats that allow for bedside ultrasound examinations. Given the trend toward ubiquity, the current use of kidney ultrasound demands a full understanding of its breadth as it and its variants become available. We described the current applications and future directions of ultrasound imaging and its variants in the context of chronic kidney disease and transplantation in this review.

Resistive Renal Index of the Recipient and its Correlation With the Outcome of Kidney Transplantation at the End of the First Year: Preliminary Results

BACKGROUND

Renal resistive index (RRI) of ultrasonography is a useful and potent tool for the assessment of patients who have undergone transplantations. The aim of this study was to evaluate the association of RRI with renal function 1 year after transplant.

METHODS

We retrospectively evaluated 93 kidney allograft recipients. Data were collected from transplantations performed in our medical center between January 2014 and September 2018. Patients with acute loss of the renal function and renal survival <1 year were excluded from the study. We longitudinally compared the RRI at the first week after transplant and the third month, with estimated glomerular filtration rate (eGFR, by Chronic Kidney Disease Epidemiology Collaboration equation) at the end of the first year of transplantation. RRI was divided into 2 groups (RRI ≤0.80 or >0.80).

RESULTS

From the total of 86 recipients, 59 (68.6%) were male recipients and 27 female recipients with a mean age of 48.3 ± 12.1 years. No correlations were found between the first week's RRI with sex and age of both donors and recipients (P > .05). Similarly, the first week's RRI was not correlated with delayed graft function (DGF) and 1-year eGFR after transplant (P > .05). On the contrary, RRI performed at the third month was strongly correlated to DGF and eGFR at the end of the first year. Last but not least, neither higher recipient age nor sex affects the value of resistive index in the third month after transplantation (P < .05).

CONCLUSIONS

RRI values <0.80 in the third month after kidney transplantation were related to better annual renal function and a lower incidence of DGF.

Current Status of Ultrasound in Acute Rejection After Renal Transplantation: A Review with a Focus on Contrast-Enhanced Ultrasound

Renal transplantation has developed into the best treatment for end-stage renal disease, but severe cases can even lead to loss of renal allograft function due to rejection and complications caused by surgical procedures. If a series of postoperative complications can be reduced or even avoided, the quality of life of recipients will be significantly improved. Acute rejection in a transplanted kidney is one of the main complications after renal transplantation. Early detection and diagnosis will significantly help the prognosis of transplanted kidney patients. As a seminal morphological and hemodynamic examination method, ultrasound can monitor the tissue structure and arteriovenous blood flow of the transplanted kidney, providing information on the transplanted kidney’s gross shape and blood perfusion. Ultrasound is a commonly used detection method after renal transplantation. At present, two-dimensional ultrasound, color Doppler ultrasound, three-dimensional ultrasound, and contrast-enhanced ultrasound have been applied in the monitoring of complications after renal transplantation. Contrast-enhanced ultrasound, as a non-invasive, radiation-free, and easy to perform examination technique, can qualitatively and quantitatively evaluate the microcirculatory blood perfusion of the transplanted kidney. It can reflect the function of the transplanted kidney more objectively and sensitively. In recent years, contrast-enhanced ultrasound has attracted attention as a new technology that can quantitatively monitor the transplanted kidney’s microcirculation perfusion. A large number of studies have shown that contrast-enhanced ultrasound has unique advantages in monitoring acute rejection after renal transplantation compared with other imaging methods, providing a reliable basis for clinical intervention. This article reviews the current status of and recent research on contrast-enhanced ultrasound in acute rejection after renal transplantation.

Early changes in renal resistive index and mortality in diabetic and nondiabetic kidney transplant recipients: a cohort study

Background

Renal resistive index (RI) predicts mortality in renal transplant recipients (RTR). However, its predictive value may be different according to the time of measurement. We analysed RI changes between 1 month and 3 months after transplantation and its predictive value for death with a functioning graft (DWFG).

Methods

We conducted a retrospective study in 1685 RTR between 1985 and 2017. The long-term predictive value of changes in RI value from 1 month to 3 months was assessed in diabetic and non-diabetic RTR.

Results

Best survival was observed in RTR with RI < 0.70 both at 1 and 3 months, and the worst survival was found in RTR with RI ≥ 0.70 both at 1 and 3 months (HR = 3.77, [2.71–5.24], p < 0.001). The risk of DWFG was intermediate when RI was < 0.70 at 1 month and ≥ 0.70 at 3 months (HR = 2.15 [1.29–3.60], p = 0.003) and when RI was ≥0.70 at 1 month and < 0.70 at 3 months (HR = 1.90 [1.20–3.03], p = 0.006).

In diabetic RTR, RI was significantly associated with an increased risk of death only in those with RI < 0.70 at 1 month and ≥ 0.70 at 3 months (HR = 4.69 [1.07–20.52], p = 0.040). RI considered as a continuous variable at 1 and 3 months was significantly associated with the risk of DWFG in nondiabetic but not in diabetic RTR.

Conclusion

RI changes overtime and this impacts differently diabetic and nondiabetic RTR. RI short-term changes have a strong prognosis value and refines the risk of DWFG associated with RI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02263-8.

The association between renal resistive index and premature mortality after kidney transplantation is modified by pre-transplant diabetes status: a cohort study

BACKGROUND

Renal resistive index (RI) predicts mortality in renal transplant recipients, but we do not know whether this is true in diabetic patients. The objective of this study was to analyse the long-term predictive value of RI for death with a functioning graft (DWFG) in renal transplant recipients with or without pre-transplant diabetes.

METHODS

We conducted a retrospective study in 1800 renal transplant recipients between 1985 and 2017 who were followed for up to 30 years (total observation period: 14 202 patient years). Donor and recipient characteristics at time of transplantation and at 3 months were reviewed. The long-term predictive value of RI for DWFG and the age-RI and arterial pressure-RI relationships were assessed.

RESULTS

A total of 284/1800 (15.7%) patients had diabetes mellitus before transplantation. RI was <0.75 in 1327/1800 patients (73.7%). High RI was associated with a higher risk of DWFG in non-diabetic patients [hazard ratio (HR) = 3.39, 95% confidence interval 2.50-4.61; P < 0.001], but not in patients with pre-transplant diabetes (HR = 1.25, 0.70-2.19; P = 0.39), even after multiple adjustments. There was no interaction between diabetes and age. In contrast, there was an interaction between RI and pulse pressure.

CONCLUSION

Our study indicates that RI is not a predictor of DWFG in diabetic renal transplant recipients, in contrast to non-diabetic recipients. These findings could be due to a different age-RI or pulse pressure-RI relationship.

Postoperative Ultrasound in Kidney Transplant Recipients: Association Between Intrarenal Resistance Index and Cardiovascular Events

Doppler ultrasound, including intrarenal resistance index (RI) measurement, is a widely used modality to assess kidney transplantation (KTx) vascularization. The aim of this study is to gain insight in the associations between early postoperative RI measurements and cardiovascular events (CVEs), all-cause mortality, and death-censored graft survival.

Current status of imaging diagnosis in the transplanted kidney. A review of the literature with a special focus on contrast-enhanced ultrasonography

Objectives

Ultrasonographic scanning is currently the most widespread imaging diagnostic procedure. The method provides real-time morphological, vascular and elastographic information in a non-invasive manner. In recent years, harmonic vascular examination has become accessible using intravenous contrast agents. In urological pathology, this procedure is used in the detection and evaluation of vascular and ischemic complications, in the classification of complex cysts according to the Bosniak system, also in the renal lesions with uncertain etiology and in acute pyelonephritis for the detection of abscesses. The contrast agent (SonoVue) is angiospecific and can be used in patients transplanted immediately after surgery without adverse effects or impaired renal function. Thus, it is desirable to be used in the nephrological pathology of the renal graft and to develop diagnostic models based on the evaluation of renal microvascularization, as well as the quantitative data resulting from the graphical representation of the specific parameters. The purpose of this review is to evaluate the current state of the literature regarding the place and role of contrast substance ultrasound in the early diagnosis of acute renal graft dysfunction and to make a differential diagnosis of this pathological entity.

Method

This review quantifies the role of contrast ultrasound in the diagnosis of acute complications of the renal graft. The research was conducted based on the databases PubMed, MedScape, Cochrane, according to the search criteria such as contrast-enhanced ultrasound + kidney transplant, “time intensity curves” + “kidney transplant”, filtered for the period 2004–2018.

Results

In the nephrological pathology of the renal graft, contrast-enhanced ultrasound is a valuable tool, superior to Doppler ultrasound in predicting the evolution of the renal graft, identifying very small early defects in renal microvascularization. A number of studies succeeded in identifying acute graft dysfunction, some of which establish its etiology - humoral rejection versus acute tubular necrosis. On the other hand, the contrast-enhanced ultrasound parameters do not have the ability to distinguish between cellular and humoral rejection.

Conclusions

If, at present, the histopathological examination is the only one that can differentiate with certainty the cause of acute renal graft dysfunction, we consider that contrast-enhanced ultrasound, as a non-invasive imaging technique, opens a favorable perspective for increasing the survival of the renal graft and decreasing the complications in the renal transplant. The combination of other ultrasound techniques, together with contrast-enhanced ultrasound, could lead to the development of new diagnostic models.

Sonography of the renal allograft: Correlation between doppler sonographic resistance index (RI) and histopathology

INTRODUCTION

Allograft rejection (AR), chronic allograft injury (CAI) and acute tubular necrosis (ATN) can lead to renal allograft dysfunction after kidney transplantation. Interstitial fibrosis/tubular atrophy (Banff classification 2005) describes chronic allograft injury with no specific etiology, thus explaining the common final endpoint of various (immunologic and non immunologic) etiologies. The aim of this study was to evaluate correlations between the Doppler sonographic RI-values and histopathological changes of renal allografts biopsies during rejection, acute tubular necrosis and chronic allograft injury as well as the influence of donor and recipient features on the intrarenal RI-values.

METHODS

102 allograft biopsies and ultrasound reports of 69 patients with kidney transplantation performed at the hospital Klinikum rechts der Isar (Technische Universität München, Germany) between 2009 and 2013 were analyzed retrospectively (41 biopsies of living donors, 61 biopsies of deceased donors). Chronic allograft injury was described using the IFTA (interstitial fibrosis and tubular atrophy) or the ECAI score (extended chronic allograft injury score). The ECAI score was built out of the chronic histological lesions glomerulopathy, interstitial fibrosis, tubular atrophy and fibrous intimal thickening (cg + ci + ct + cv) of the BANFF scoring.

RESULTS

Intrarenal RI-values were significantly higher in patients with allograft rejection than without rejection (median 0,79 vs. 0,73; inter quartile range: 0,20 vs. 0,13; p = 0,018). The same was found for T-and non-T cell mediated rejection (median 0,78 vs. 0,73; inter quartile range 0,20 vs. 0,13; p = 0,039). There were no significant differences in the RI-values between the subtypes of T-cell mediated rejection (type IA-IIB). Furthermore, there were no significant differences of RI-values regarding antibody-mediated rejection (present vs. not present) or type of rejection (T-cell- vs. antibody mediated rejection). Patients with rejection and simultaneously chronic allograft injury showed significantly higher RI-values than patients with only chronic allograft injury. Analyses using the IFTA or the ECAI score showed comparable results (IFTA p = 0,043; Score p = 0,021). The intrarenal RI-value was neither able to detect chronic allograft injury nor to distinguish between acute tubular necrosis and rejection. The intrarenal RI-value showed a significant correlation with recipient age (p < 0,001) but not with donor features.

CONCLUSION

In summary, the intrarenal RI-value can indicate a rejection but gives no clear hint to acute tubular necrosis and cannot differentiate from it. Since patients with rejection can have normal RI-values, a biopsy should always be performed in case of suspected rejection. The intrarenal RI-value has no unambiguous validity to determine intrinsic values of the renal allograft, but should rather be understood and interpreted as a systemic parameter influenced by multiple factors.

A Novel CNN-Based CAD System for Early Assessment of Transplanted Kidney Dysfunction

This paper introduces a deep-learning based computer-aided diagnostic (CAD) system for the early detection of acute renal transplant rejection. For noninvasive detection of kidney rejection at an early stage, the proposed CAD system is based on the fusion of both imaging markers and clinical biomarkers. The former are derived from diffusion-weighted magnetic resonance imaging (DW-MRI) by estimating the apparent diffusion coefficients (ADC) representing the perfusion of the blood and the diffusion of the water inside the transplanted kidney. The clinical biomarkers, namely: creatinine clearance (CrCl) and serum plasma creatinine (SPCr), are integrated into the proposed CAD system as kidney functionality indexes to enhance its diagnostic performance. The ADC maps are estimated for a user-defined region of interest (ROI) that encompasses the whole kidney. The estimated ADCs are fused with the clinical biomarkers and the fused data is then used as an input to train and test a convolutional neural network (CNN) based classifier. The CAD system is tested on DW-MRI scans collected from 56 subjects from geographically diverse populations and different scanner types/image collection protocols. The overall accuracy of the proposed system is 92.9% with 93.3% sensitivity and 92.3% specificity in distinguishing non-rejected kidney transplants from rejected ones. These results demonstrate the potential of the proposed system for a reliable non-invasive diagnosis of renal transplant status for any DW-MRI scans, regardless of the geographical differences and/or imaging protocol.

Does the measurement of the difference of resistive indexes in spleen and kidney might be used for characterization of intrarenal tardus parvus phenomenon in chronic kidney disease?

Doppler sonography is used as a routine test for detection of renal artery stenosis (RAS). Although increased peak systolic velocity at the site of the narrowing is a widely accepted method for assessing main renal artery stenosis, tardus parvus phenomenon detected on the downstream blood flow has been used as an alternative to direct insonation of the main artery. However, the uncertainty about the best Doppler parameter to be used for characterization of tardus parvus has yielded a variety of studies. Recently, the difference between the resistive index of the spleen and kidney (RISK) has been proposed as a potential marker of kidney damage and Doppler criterion for RAS in hypertensive patients without chronic kidney disease (CKD). The hypothesis of this study was to speculate further and propose the RISK as valuable new Doppler diagnostic criteria for the tardus parvus in CKD too. Data from 183 control patients and 135 chronic kidney disease patients (CKD) were included in the study to test the feasibility of the method and plausibility of the hypothesis. Criteria inclusion was a color Doppler ultrasound measurement of the renal (RRI) and spleen resistive index (SRI). Serum creatinine (0.83 ± 0.20 vs 3.27 ± 1.74), RRI (0.60 ± 0.06 vs 0.69 ± 0.09), SRI (0.55 ± 0.06 vs 0.58 ± 0.09), RISK (0.06 ± 0.04 vs 0.11 ± 0.08), and spleen size (94.1 ± 13.2 vs 100.8 ± 16.5) were higher in CKD patients (p < 0.05). RRI correlated with SRI in both groups; however, the coefficient of determination was different (Z = 2.29, p = 0.022). In the CKD group, RISK correlated inversely with age (r = -0.276; p = 0.001) and positively with creatinine (r = 0.509; p < 0.001). In the multivariate analysis, creatinine remained an independent predictor of the high RISK (Wald = 13.07; p < 0.001; odds ratio = 1.87; 95% confidence interval 1.33-2.62). Since extraneous factors similarly influence RRI and SRI, the SRI can be considered the correction factor that can estimate the cardiovascular burden on the RRI. Consequently, according to the suggested hypothesis, RISK helps to distinguish the tardus parvus related to RAS from tardus parvus related to systemic influences on Doppler morphology. The identification of the underlying mechanisms of tardus parvus, would help a more accurate characterization of the phenomenon and improve screening and diagnostic test for RAS in CKD patients.

Impact on renal resistive index of diabetes in renal transplant donors and recipients: A retrospective analysis of 1827 kidney transplant recipients

High renal resistive index (RI) is observed in diabetes and is associated with poor patient survival, but whether it is primarily due to renal vascular resistance or systemic vascular alterations is unclear. The respective impact of kidney transplant from diabetic donors or to diabetic recipients on RI would shed some light on this issue. The objective of the study was to analyze the impact of donor and recipient diabetes on RI in order to understand the respective impact of the kidney and the vascular environment. The authors conducted a retrospective study in 1827 renal transplant recipients who received a kidney between 1985 and 2017, and had Doppler measurements at 3 months after transplant. Donor and recipient characteristics at the time of transplant and at 3 months were reviewed. Both donor diabetes and recipient diabetes were associated with RI in univariate analysis, but only recipient diabetes remained significantly associated in stepwise multivariate analyses (effect estimate on RI: +0.03 ± 0.005, P < 0.001). These findings were confirmed when RI was expressed as a binary variable using a cutoff of 0.75 (OR = 2.50 [1.77, 3.54], P < 0.001). Other determinants of RI were recipient characteristics (age, sex, systolic and diastolic blood pressure, and duration of dialysis). Donor characteristics were not associated with RI. Our results suggest that high RI observed in diabetic recipients shortly after transplant is primarily due to the new vascular environment, rather than to characteristics of the transplanted kidney. Therefore, RI reflects systemic rather than intra-renal changes.

Renal intraparenchymal resistive index: the ultrasonographic answer to many clinical questions

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.

Determinants and Prognostic Significance of the Renal Resistive Index

BACKGROUND

The noninvasive assessment of renal hemodynamics is currently possible by assessing the renal resistive index (RRI) derived from intrarenal Doppler arterial waveforms as (peak systolic velocity - end-diastolic velocity)/peak systolic velocity. In this review, we outline the important determinants of the RRI to clarify the true identity of the RRI and highlight its potential diagnostic and prognostic value in renal and cardiovascular pathology.

SUMMARY

Although the RRI was initially considered to reflect intrarenal vascular pathological processes, this index is actually a product of a complex interaction between renal and systemic vascular wall properties and hemodynamic factors. Indeed, studies in patients and general populations consistently demonstrated a significant and direct association between the RRI and central or peripheral pulse pressure independent of other covariables. Moreover, studies in renal transplant patients also showed that the RRI mainly reflects characteristics of the recipient but not those of the graft. Thus, the major influence of the systemic hemodynamics on the intrarenal arterial waveforms excludes RRI as a specific marker of renal vascular pathology. On the other hand, because the RRI reflects pulsatility in renal arteries, it might be useful for the early detection of renal microvascular damage. Future longitudinal studies are still needed to clarify whether the detection of Doppler changes in intrarenal arteries might yield an improvement in the adverse cardiovascular and renal outcome.

CONCLUSION

Published studies on RRI imply that the interaction between the systemic hemodynamics and peripheral circulation in the kidney is a complex physiological phenomenon. In addition to renal vascular properties, the central hemodynamic factors significantly influence the intrarenal arterial Doppler waveform patterns. Previous research also suggested an important role of the RRI for the evaluation of renal target organ damage, particularly in patients with increased pulsatility of the intrarenal blood flow.

The internist and the renal resistive index: truths and doubts

The renal resistive index (RRI) is measured by Doppler sonography in an intrarenal artery, and is the difference between the peak systolic and end-diastolic blood velocities divided by the peak systolic velocity. The RRI is used for the study of vascular and renal parenchymal renal abnormalities, but growing evidence indicates that it is also a dynamic marker of systemic vascular properties. Renal vascular resistance is only one of several renal (vascular compliance, interstitial and venous pressure), and extrarenal (heart rate, aortic stiffness, pulse pressure) determinants that combine to determine the RRI values, and not the most important one. RRI cannot always be considered a specific marker of renal disease. To summarize from the literature: (1) hydronephrosis, abdominal hypertension, renal vein thrombosis and acute kidney injury are all associated with an acute increase in interstitial and venous pressure that determine RRI values. In all these conditions, RRI is a reliable marker of the severity of renal damage. (2) The hemodynamic impact of renal artery stenosis can be assayed by the RRI decrease in the homolateral kidney by virtue of decreasing pulse pressure. However, renal diseases that often coexist, increase renal vascular stiffness and hide the hemodynamic effect of renal stenosis. (3) In transplant kidney and in chronic renal disease, high RRI values (>0.80) can independently predict renal and clinical outcomes, but systemic (pulse pressure) rather than renal hemodynamic determinants sustain the predictive role of RRI. (4) Higher RRI detects target renal organ damage in hypertension and diabetes when renal function is still preserved, as a marker of systemic atherosclerotic burden. Is this the fact? We attempt to answer.

Contrast-enhanced ultrasonography in the early period after kidney transplantation predicts long-term allograft function

INTRODUCTION

Real-time contrast-enhanced sonography (CES) can assess microvascular tissue perfusion using gas-filled microbubbles. The purpose of the study was to evaluate the feasibility of early CES in predicting long-term kidney allograft function in comparison to color Doppler ultrasonography (CDUS).

METHODS

We prospectively studied 68 consecutive kidney transplant recipients using CES and conventional CDUS investigation 1 week after transplantation. Transplant tissue perfusion imaging was performed by low-power imaging during intravenous administration of the sonocontrast SonoVue. Renal tissue perfusion was assessed quantitatively using flash replenishment kinetics of microbubbles to estimate renal blood flow (RBF). The obtained sonography values were correlated with clinical data 1 week up to 1 year after transplantation.

RESULTS

In contrast with conventional CDUS resistive indices, RBF estimated by CES 1 week posttransplantation significantly correlated with kidney function after 1 year (r = 0.67; P < .001). Determination of RBF by CES revealed a significant correlation with donor age but not recipient age, whereas conventional CDUS resistive index was significantly correlated to recipient age (r = 0.54; P < .001) but not donor age. Furthermore RBF was associated with vascular fibrosis and intimal thickening of the engraftment biopsies.

CONCLUSION

This is the first prospective study demonstrating the prognostic value of CES early after kidney transplantation. In contrast with CDUS, CES reveals information about kidney allograft perfusion independent of recipient vascular compliance.