The relationship between intra-parenchymal renal resistive index variation and renal functional reserve under physiologic and pathologic conditions

Purpose. An increase of glomerular filtration rate (GFR) after protein load, which is due to pre-glomerular vasodilation, represents renal functional reserve (RFR). The assessment of RFR has been proposed for the risk stratification of patients undergoing potentially nephrotoxic procedures, but is cumbersome to measure. We sought to develop a non-invasive, bedside ultrasound (BUS) test to indirectly measure RFR. The BUS test consists in applying a pressure on the subject’s abdomen by means of a weight (fluid-bag 10% of subject’s body weight) and contemporarily recording renal resistive index (RRI) in an interlobular artery. Mechanical abdominal pressure, through compression of renal vessels, decreases blood flow and activates the auto-regulatory mechanism which can be measured by a drop in RRIs. We hypothesized that the maximum reduction of RRIs from baseline (intra-parenchymal renal resistive index variation, IRRIV) during abdominal pressure could be related to the RFR and serve as an easy bedside predictor of RFR. Methods. We conducted three different studies: (i) a pilot study on healthy volunteers; (ii) a validation study on healthy subjects; and (iii) a pilot study on elective cardiac surgery patients. Each group underwent standardized protein loading test and IRRIV test. Pearson and logistic regression analyses were used to assess the correlation between IRRIV and RFR in each group. Results. In 30 healthy volunteers, enrolled in the pilot study, the Pearson correlation coefficient between RFR and IRRIV was 0.74 (p<0.001). In 47 healthy subjects, enrolled in the validation study, the Pearson correlation coefficient between RFR and IRRIV was 0.83 [95% confidence interval (CI) 0.71–0.90; p<0.01]. Among these, concordance between RFR and IRRIV was described in 45 (95.7%) subjects. IRRIV predicts RFR with a ROC-AUC of 0.86 [CI95% 0.68–1]. In 31 cardiac surgery patients, Pearson correlation coefficient between RFR and IRRIV was 0.81, [CI95% 0.63–0.90; p<0.01]. Among these, concordance between RFR and IRRIV was described in 27 (87.1%) patients. IRRIV predicts RFR with a ROC-AUC of 0.80 [CI95% 0.64–0.96]. Conclusion. IRRIV test is a feasible BUS test that significantly predicts the presence of RFR in healthy subjects. Correlation between IRRIV and RFR seems to be also maintained in pathologic conditions.