Commentary on the Renal Resistive Index

Vol. 164,922,September 1996 Printed in U S A

This Month in Investigative Urology COMMENTARY ON THE RENAL RESISTIVE INDEX 4 -that & phssiolosic b reavrpascnlar. . tal

M of -t ~ M v urnpathy e is a decrease in renal blood flow or an increase in in animnlR have documentedthat, following acute unilateral ureteral occlusion, there is an initial p e r i O d a ' a % & i eubeequentcharacteristic hemodynamic re%ponse is vasoeodction, as &own by actual measmenta of renal blood flw,micropMeture data, or injected caeta of the renal vaseulature. In bilateral ureteral obstruction, while there is a deereare in 4 blood flow and incrsase in reaovescular resistanCa, then? is a maintenance of an elevated ureteral pressure efsersnt arteriolar d - m W e m m between the two modela is a release of atrial natriuretic factor due to the volume -that OCEUCB with a totpl bilateralocclusion, which is known to cause vasodilatation ofthe afTerent renal arteriole.* However, taken in &&I d or partial unilateral and bilateralureteral occlusion,the physiologic characteristic is a decrease in renal M o o d ~ d a n ~ i n n m o v a e c u l a r ~ . TbsnldmmBm . c f h w i n rBaDpBBcLI]Br-bee been the eubjeetofintenseinvestigationover thepast two decades. Unfortunately, the* * m e d w n g t h e * w~ r e m a i n ~ . S t u d i e g h a v e r e v e a l e d a n r r m b e r o f ~ d i ~ t e m e d i a t o r s p r o d u c i n g thebmmdymm *c+w==-which hae been &axdied in mme detail is augiotedn II. Data have been conflicting,but there is widenoecfinereesedmtAraMl Ofaqbtmnh II hingnnilateralaretaalobetructionwhich eubeequenty leadsto a down-regulation dtbAT,-RmRNAintbeabetruetedW.'M-,tbead-. * * ltionofa&okminII antagonist or amvertingwzymeinhibitora has 8 h o w n * m ofthe deueaae in renalblood flow aUough, in mod etudies,the renalflow remains sigoificantlybelowcontrol values, albeit mme!whatimprovedIn addition,tbere have been numemua studies eqhhgtherole ofthe pmrteglandinayatemfollowingureteral obstruction, a n d i t i s f a i r l y w e n Q n u n e n t e d t h a t t h e i n i t i a l ~ t a t i o n i s m e d i a t e d b y w s o d i l a.t o.4l ~ However, the role of thrumboxane, a potsnt, i n t h e m a i n t e n a u c e o f t h e mnovaacular ~ rearstanee, is more controvembl There are studies in support of thia -ae amediator, a d &udiea whichhave failed to ehowthatthromboxanebloegaderevenresthe vasoconstriction. Thue,a nMinPasme test to identi@an increase in rewvaeculatresietanee is of obvious clinical use to differentiate patients with anatomic hydrooephmeie without physiologic Bignificance from patient. with true obatruetive uropathy. The paper in this issue by Ulrich and arworLeregiveeadditiOnal'on m r n i n gthe me of intramnal duplex Doppler sonography as a method to measure renovascular reeistaneein the kidney. They ehow a gradual riee in the reeietance index value during their acute ehdy with considerable variability from animalto animal Wderiug the diecnseionabove, the opposingforces of differentialmediator release could explain variability of responses. Ceutddy. one wwldnot anticipate arise inreeietiveindex initialy during the v d a t o r y period. Renal Doppler sonography was initially used for the evduation of renal traneplant CbnsfUnetioIl U n t i i t a l y , in a number of areas, the technique has not been proven to have sutheint BBnBitivtyand epeeifieity to distinguiehclearly between Merent pathologic conditions.The major problem with the technique hae beentechnical -, either related to the technology used or the experkm ofthe operator. The clinical qmstionthat neede to be addressed is when should it be aeed and how could it help us in our diagnosticarmamentarium. The ~-tal data presented in this paper,and our knowledge dthe pathophysiology of obatruetion, would make it likely that the technique will have m e variability if it is used to evaluate patients with acute renal obstruction. Indeed, the role of renal Doppler in the evaluation of& renal obetruetion hae generated a vigorous commentary in the radiologic literatum.6.6 In general, it would be my feethat there is probaMy a minimal role for Doppler eonography in the evaluation of acute ureteral obstruction. The amain which I believe thie technique w i l l be ofmoat use will be to delineate anatomic chronic hydronephrosis from cases of actual obetruetive nropathy with renalimpairment There are a number of papera in the literature supporting this concept, and 1 will cite several which m a b apeci6cpoint.. In a pair of papere, Dr.Palmer and coworkers have shown an increase in renal resistance in a p u p of children who lmbeeqnentlyanderwentreconetrnctiol17Fortunately, they were able to follow theae children and, 2 years later in a second publications they d e m o d d d that, of the 9 kidneys meamred 3 month postoperatively, all had a decrease in resistive index values less than 75. the changes* in reeietive index related to pelvic preseure, Dr.Fung and coworkers recently presented an In the chmnic. . Ofthe resietiveindexwith renal pelvic presswe. In my mind, it is much more likely that there &wingadired ma indoorin the chmmcally obetructedmodel as opposedto the acutely obstructed model, because inthat eitastioqtbelmrmodmapomendeacrhdabove are blunted. Indeed,tbis obemvah ' n supporta the idea that this technique will be more d i n the chrcmic model ofthe ofcontroversg is the exact cutoff point to define obstruction. This has generated heated debeteinhliterattrrr .Tbeneeofapmvocehve teet,eueh as the fumsemide-etimulatsd study, may decrease the variability. As the authora It

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m,

7ez*preclsurew ~intbeir~,comprVingthereeietiVeindexofthe~bstructedlridneytothecontralateralkjdn~willprobablybeofvalue, ashas previously

beem mggededby Dr.Robert WeisS and colhgu@&Taken alk@br, there is an expanding body of information which suggests that Doppler m x q ? q h y will be-fo UBB in idantifying & i d l y sisnifieant obetmtive umpathy. Moreover,in the future,we may be able to couple the teeimqwwiththe-

tum ofvamactive drugsto look for a reversal ofa high reeistive index to predict reanrery followingcorrective surgery.

E. Darramtt Vaughn, Jr. Department of urology The New York Hospital-CornellMedical Center

REFERENCES

New York,New York

1. menwater, J. Y.:The pathoph~ologyof urinary tract obstntction. In: Campbell's Urology. Edited by P. C. Walsh, A. B. Retik, T. A. Stamey and E.D. Vaughan. Jr. Philadelphia: W.B. Saunders Co., 1992. 2. Gdmi, F.A,Matthew%G. J., Marion, D., von L u t h ~ t t iN. , and Vaughan, E. D., Jr.: Volume expansion enhances recovery of renal

function in prolonged diurmk and natriumsis after release of bilateral ureteral obstruction: possible role for atrial natriuretic peptide. J. Urol., 16%1276,1995. 3. F'imenbl, J. L,Jr., Wmg, 5.and Martinez-Maldonado, M.:Regulation of the renal angiotensin I1 receptor gene in acute unilateral ureteral obshetion. Kidney Int., 4& 1614,1994. 4. Allen, J. T.,Vauehan, E.D., Jr. and Gillenwater,J. Y.:The effectofindomethacinon renal blood flow and ureteral pressure in unilateral obetruction in awake dogs. Invest. Urol., 1 6 324,1978. 5. Cronan, J. J. and Tublin, M.E.: Role of the resistive index in the evaluation of acute renal obstruction. AJR, 164: 377, 1995. 6. Platt, J. F., E W ,J. H.and Rubin, J. M.:Role of renal Doppler imaging in the evaluation of acute renal obstruction.MR, 164: 379. 1995. 7. Palmer, J. M,Indfoas,K K, R C. and Marder. D. h k Diuretic mmgraphyin postnabl hydronephrosis.J. Uml., 146: 605,1591. 8. Ordorim, R. C., Lindfm, K K and Palmer, J. M.: Diuretic Doppler sonography following successful repair of renal obstruction in children.J. Urol., 1M):774, 1993. 9. h g . L C. T.,Stechler, R. E.,Khoury, A. E., McLorie, G. A, Chait, P. G. and Churchill, B.: Intrarenal resistive index corre1atr.s with renal pelvia pressure. J. Urol., 16% 607,1994. 922