Renal Artery Intervention : Catheter-Based Therapy for Renal Artery Stenosis

Screening for Renal Artery Stenosis

Screening for renal artery stenosis is appropriate in patients at increased risk for this disease ( Table 20-1 ). Whenever possible, screening tests for renal artery stenosis should be performed noninvasively using direct imaging tests such as Doppler ultrasound, computed tomographic angiography (CTA), or magnetic resonance angiography (MRA). Noninvasive imaging has become so sophisticated and accurate that it is seldom necessary to perform catheter-based angiography for the diagnosis of renal artery disease.

The appropriateness of screening angiography for RAS at the time of cardiac or peripheral vascular angiography of other vascular beds has been addressed by recommendations and guidelines endorsed by an expert consensus panel of the American College of Cardiology (ACC) and the American Heart Association (AHA). For patients with risk factors as outlined in Table 20-1 or clinical syndromes suggestive of RAS, aortography is given a Class I indication for screening at the time of angiography performed for other clinical indications. There is published evidence that nonselective, diagnostic, screening renal angiography is safe and is not associated with any incremental risk when performed at the time of cardiac catheterization.

Duplex Ultrasonography

Duplex ultrasonography (DUS) is an excellent test to detect renal artery stenosis but is highly dependent upon the skills of the technician performing the test. It is the least expensive of the imaging modalities and provides useful information about the degree of stenosis, the kidney size and other associated disease processes such as obstruction. The location and degree of stenosis can accurately be determined by duplex ultrasound of the renal artery.

Overall, when compared with angiography, DUS has a sensitivity and specificity of 84% to 98% and 62% to 99%, respectively, when used to diagnose renal artery stenosis. Renal artery duplex is an excellent test for the follow-up of RAS after revascularization. Following endovascular therapy a renal artery duplex should be obtained within the first few weeks to establish a baseline, at 6 months, 12 months, and yearly thereafter.

One drawback of DUS is that the sensitivity is lower for identifying accessory renal arteries (67%) compared with main renal arteries (98%). Therefore, if the patient has hypertension that cannot be adequately controlled with a good regimen, and the DUS fails to demonstrate RAS, another imaging modality may be considered to identify stenosis of an accessory renal artery.

Detecting renal artery in-stent restenosis (ISR) is a potential problem when native vessel parameters are used for diagnosis. Recently a cohort of 132 patients with renal artery stents had angiographic correlation with DUS findings. There was no single peak systolic velocity (PSV) cutoff that would accurately discriminate 60% to 99% from 0% to 59% restenosis in all patients. A PSV <241 cm/s was useful in excluding ISR (negative predictive value 96%): 78 of 81 renal arteries with PSV <241 cm/s had 0% to 59% restenosis. A PSV ≥296 cm/s was accurate in predicting ISR (positive predictive value 94%): 33 of 35 renal arteries with a PSV ≥296 cm/s had ISR by angiography. A PSV between 241 and 295 cm/s represented an indeterminate zone in which renal artery restenosis could not be diagnosed or excluded on the basis of DUS alone.

Resistive Index

The resistive index (RI) is obtained by measuring the peak systolic velocity (PSV) and the end diastolic velocity within the renal parenchyma at the level of the cortical blood vessels. It is an indication of the amount of small vessel arterial disease (i.e., nephrosclerosis) within the renal parenchyma. The renal artery RI has inappropriately been suggested as a method to stratify patients likely to respond to renal intervention. However, a prospective study of renal stent placement by Zeller et al. demonstrated that an elevated RI predicted a favorable blood pressure response and renal functional improvement at 1 year after renal arterial intervention. If there are good clinical reasons to revascularize a kidney, then it should be performed independently of the RI.

Noninvasive Angiography

Computed tomographic angiography (CTA) uses ionizing radiation and iodinated contrast to produce excellent images of the abdominal vasculature ( Figure 20-2 ). CTA has a sensitivity and specificity for detecting RAS of 89% to 100% and specificity of 82% to 100%. Excellent three-dimensional image quality with enhanced resolution can be obtained with multidetector-row CTA technology. The advantages of CTA over magnetic resonance angiography (MRA) includes higher spatial resolution, absence of flow-related phenomena that may overestimate the degree of stenosis, and the capability to visualize calcification and metallic implants such as endovascular stents and stent grafts. CTA is generally well tolerated with an open gantry and thus claustrophobia is not as limiting a factor as it is for MRA. The disadvantages of CTA compared with MRA are exposure to ionizing radiation and the need to administer potentially nephrotoxic iodinated contrast agents.

MRA also provides excellent imaging of the abdominal vasculature and associated anatomical structures. When compared with angiography, MRA has demonstrated a sensitivity of 91% to 100% and a specificity of 71% to 100%. Contrast-enhanced MRA using gadolinium improves image quality when compared with noncontrast studies and shortens imaging time, thereby eliminating some of the artifact created by gross patient movement. However, MRA does not have the same sensitivity and specificity in patients with fibromuscular dysplasia (FMD) and is generally not a good screening test if FMD is suspected.

MRA should not be used in patients with a glomerular filtration rate less than 30 mL/min/1.73 m ² because of the increased likelihood of developing nephrogenic systemic sclerosis. MRA may not be used in patients with metallic (ferromagnetic) implants such as some mechanical heart valves, cerebral aneurysm clips, and electrically activated implants (pacemakers, spinal cord stimulators). At the present time, MRA is not useful in following patients after stent implantation due to artifact produced by the metallic stent.

Invasive Angiography

The “Achilles' heel” of renal stenting is the inaccuracy of the angiographic determination of the severity of renal stenoses. The traditional “gold” standard for determining the severity of renal artery stenosis has been invasive angiography. Even with quantitative measurement, angiography may be unable to discriminate between nonobstructive stenoses and clinically significant ones ( Figure 20-3 ). Most would agree that interventionalists are able to identify “critical” stenoses in renal arteries, but for mild to moderately severe lesions, physiological confirmation is necessary.

Translesional Pressure Gradients

Confirmation of the correlation with hemodynamic evidence of significant renal artery stenosis and renin release has been documented by De Bruyne and colleagues. Other investigators have now established that hemodynamic parameters of significant renal artery stenosis (peak systolic gradient >21 mm Hg, renal fractional flow reserve of ≤0.8, and a dopamine-induced mean translesional gradient ≥20 mm Hg) are associated with clinical improvement after renal stenting in patients with mild to moderate renal artery stenoses.

TIMI Frame Count

Angiographic measurements of renal blood flow by using renal frame counts (RFC) and renal blush grades (RBG) for microvascular flow can differentiate normal patients from patients with FMD. Hypertensive patients with renal artery stenoses have also been shown to have decreased renal perfusion as measured by RFC. Clinical responders tended to have higher baseline RFCs than nonresponders and had greater improvement in their RFC values following RAS. Three-quarters of the hypertensive patients who responded to RAS had a baseline RFC ≥25, and if the RFC improved by >4, then 79% were responders to RAS.