Renal resistive index-a valid tool to assess renal endothelial function in humans?

Background. In humans, renal endothelial function is assessed by the vasoconstrictive response to L-NG-monomethyl arginine (L-NMMA). We hypothesized that Doppler sonographic measurements of the renal resistive index in response to inhibition of nitric oxide synthase offer a new methodological approach for testing renal endothelial function. Methods. Forty-one patients without nephropathy were included. Para-aminohippurate and inulin clearance were performed under basal conditions and during L-NMMA infusion. In parallel, renal resistive index was assessed by Doppler sonography, and central blood pressure was determined. Results. Following nitric oxide synthase inhibition, renal resistive index increased significantly, and 29% of our patients developed Doppler sonographic diastolic zero flow. Renal plasma flow decreased in response to L-NMMA, and conversely, renal vascular resistance increased. There was no correlation of renal vascular resistance and renal resistive index at baseline and during nitric oxide synthase inhibition. Changes in renal resistive index were not related to changes in renal perfusion or renal vascular resistance. Renal resistive index correlated with central pulse pressure at baseline and during L-NMMA infusion, whereas renal vascular resistance did not correlate with central pulse pressure. Conclusion. Our data do not support the hypothesis that renal resistive index is a tool to test renal endothelial function in humans and should not be used interchangeably with renal vascular resistance.