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.