Intrarenal resistive index correlates with renal pelvis pressure

Elevation in the intrarenal resistive index has been suggested by many to be a physiological parameter useful for detecting functionally significant hydronephrosis. It is currently unknown whether the intrarenal resistive index changes truly reflect the changes in collecting system pressure or whether they are simply a coincidental epiphenomenon. The purpose of this study is to establish the relationship between intrarenal resistive index and collecting system pressure. Between August 1992 and October 1993, 9 patients younger than 1 year underwent a percutaneous pressure-flow study as part of hydronephrosis evaluation. During the pressure-flow study intrarenal resistive index was measured serially with simultaneous renal pelvis pressure readings. In all patients the index increased as the renal pelvis pressure increased. Furthermore, using the experimentally derived proximal tubular pressure of 14 cm. water as the probable threshold for functionally normal collecting system pressure, it was found that all intrarenal indexes of 82% or less corresponded to renal pelvis pressures of less than 14 cm. water, while all of those greater than 82% corresponded to renal pelvis pressures greater than 14 cm. water. By combining several lines of evidence, it appears probable that as maximal diuresis induced by physiological and pharmacological means leads to acute transient elevation in renal pelvis pressure in a functionally obstructed collecting system, the intrarenal resistive index is capable of reflecting this dynamic elevation in renal pelvis pressure and potentially able to distinguish physiologically significant upper urinary tract obstruction from nonobstructive dilatation.