Duplex derived intrarenal resistance index correlates with invasive pressure gradient measurements in detecting relevant unilateral renal artery stenosis

Background: The purpose of this study was to assess the correlation between the transstenotic pressure gradient as determined by a pressure wire and the decrease in the intrarenal resistance index (RI) > 0.05 measured by duplex ultrasound in significant unilateral renal artery stenosis (RAS). Intravascular ultrasound (IVUS) was correlated to the angiographic degree of RAS. Patients and methods: In 17 patients, transstenotic pressure gradient measurements, IVUS and RI measurements were obtained before and after stenting. After stenting, graded stenosis was created by progressive balloon catheter inflation. Stenosis severity was expressed as the ratio of the distal pressure (Pd) corrected for aortic pressure (Pa). The balloon inflation pressure was adjusted to create a stenosis with Pd/Pa ratio 1.0 to 0.7. In parallel, RI measurements were performed in the affected kidney and compared to the contralateral kidney for the calculation of the side-difference in RI. IVUS was performed to correlate the cross-sectional degree of stenosis with quantitative angiography (QA). Results: In 60 out 68 measurements, the RI difference (decline > 0.05) correlated with a Pd/Pa ratio < 0.9; this correlation was significant by linear regression analysis (p < 0.001). The preinterventional mean degree of stenosis was 63.4 % + 16.1 (24.6 - 84.6 %) as assessed by QA and 76.7 % + 13.2 % (47 - 92 %) as assessed by IVUS, showing a significant correlation (p < 0.035). Conclusions: In unilateral RAS, a drop in RI > 0.05 as compared to the unaffected kidney correlates to an invasively measured Pd/Pa ratio < 0.9, which is regarded as a significant pressure gradient associated with increased renin production, which promotes renovascular hypertension. QA underestimates RAS severity as compared to IVUS.