The Utility of Duplex Ultrasound Scanning of the Renal Arteries for Diagnosing Significant Renal Artery Stenosis

Renovascular hypertension may account for 1% to 5% of all cases of hypertension. However, atherosclerotic renal artery stenosis is much more frequently encountered, especially in the elderly patient with evidence of atherosclerosis elsewhere [1]. Arteriography has been the gold standard for detecting renal artery stenosis. Although arteriography allows direct visualization of the renal arteries, it is invasive and expensive and does not adequately assess the functional significance of the renal artery lesion. Because azotemia is not uncommon in patients with atherosclerotic renal artery stenosis [2], it may be advantageous to avoid arteriography as a screening test because of the possibility of causing contrast-induced acute renal failure or atheromatous embolization to the kidneys. Because of these limitations of arteriography, investigators are interested in noninvasive screening for renal artery disease [3, 4]. Many of the noninvasive screening tests (such as intravenous urography, renal scintigraphy, determination of plasma renin activity, and the captopril test) have an unacceptably low sensitivity and specificity. Renal scintigraphy with the administration of captopril is safe and noninvasive and has a reasonable sensitivity and specificity in many circumstances. Captopril renography is not as sensitive in patients with bilateral renal artery stenosis or in patients with stenosis to a solitary functioning kidney [4, 5]. Renal scintigraphy with captopril may also not be as sensitive in patients with substantial azotemia, the subgroup of patients most in need of a safe screening test for renovascular disease. The captopril-stimulated renal flow scan does not provide an anatomical assessment of the degree of stenosis, but it is an excellent test for assessing the functional significance of a stenotic lesion. Magnetic resonance angiography is an attractive noninvasive screening method for assessing the renal arteries [6]. Most segments of the renal artery can be adequately visualized, and it is now possible to view the renal arteries three-dimensionally through post-processing reconstruction [7]. Currently, this technique has two major drawbacks. It is expensive and may overestimate the degree of stenosis. One study of three-dimensional, spiral computed tomographic angiography showed excellent imaging of the mesenteric and renal arteries [8], but the procedure required a large amount of contrast material administered in a bolus, thus limiting the utility of this test in patients with azotemia. Duplex ultrasound scanning of the renal arteries is a noninvasive screening test for the detection of renal artery stenosis. It combines direct visualization of the renal arteries (B-mode imaging) with measurement of various hemodynamic factors in the main renal artery and within the kidney (Doppler), thus providing both an anatomical and a functional assessment [9, 10]. Duplex scanning also allows the measurement of kidney size at the same time that the examination is being done. Unlike other noninvasive screening tests, duplex scanning is not affected by medications that the patient may be taking; the level of renal function; or whether the disease is unilateral or bilateral or affects a solitary functioning kidney. Improvements in image resolution, Doppler technology, and processor software have led to the ability to better scan the visceral circulation [11]. We determined the utility of duplex ultrasound of the renal arteries for identifying patients with renal artery stenosis of 60% or more and for excluding patients with either normal renal arteries or renal artery stenosis of less than 60%. Methods Patients We prospectively entered patients into our study who were being evaluated for possible renal artery stenosis. One hundred and two consecutive patients who had both duplex ultrasound scanning of the renal arteries and renal arteriography were included for analysis. Duplex scanning was done before renal arteriography in 60 (58.8%) patients. Only main renal arteries were analyzed in our study. Only 9 patients had accessory renal arteries on arteriography, which was a smaller number than we expected [5]. The cohort of patients that we studied had a high pretest likelihood of disease. Most patients who were studied had either hypertension that was difficult to control or unexplained azotemia (or both), giving them a pretest likelihood of renovascular disease of 70% [1]. A smaller number of patients was evaluated because of associated peripheral vascular disease or abdominal aortic aneurysm. The pretest likelihood in this group is approximately 40% [1]. The persons who did the duplex ultrasound examinations did not have access to the patient's chart or to the results of renal arteriography. All renal arteriograms had at least two views of the renal artery, and the degree of stenosis was estimated by visual examination. Renal arteriograms were both formally interpreted by the person doing the study and reviewed by one of the authors. If a difference of opinion existed between the formal reading and the review, a third investigator evaluated the arteriogram without knowledge of the two previous evaluations and the majority ruled. Those interpreting the arteriograms were blinded to the results of the person doing the study and to the results of the duplex ultrasound. Renal Duplex Scanning Technique All patients were scanned with an Ultramark 9 HDI ultrasound machine (Advanced Technology Laboratories, Bothell, Washington) using a C3 40R transducer. This curved array transducer has a 3.0-MHz imaging probe and a 2.5-MHz pulsed Doppler with a focal zone in the vicinity of the depth of the renal arteries. Patients were studied after a 12-hour fast. Ninety percent of the patients had an adequate examination when studied in the fasting state. If excess bowel gas was present, the patients were studied the following morning or at the next available convenient time for the patient. Patients were studied in the anterior, lateral decubitus, and, at times, the prone position so that all portions of the renal artery from the takeoff of the aorta to where the renal artery enters the kidney were visualized. The hilar approaches were particularly useful in patients with excess bowel gas and in obese persons. The renal arteries were first visualized and identified. The ability to use color duplex imaging may allow the ultrasonographer to directly visualize turbulent blood flow if it is present. Once the renal arteries were correctly identified, the Doppler signature was taken at as close to a 60-degree angle as possible. Under no circumstances was the angle more than 60 degrees. Doppler samples were taken at the origin of the renal artery, the proximal renal artery, the midportion of the renal artery, and the distal renal artery. The renal-aortic ratio was calculated by dividing the peak systolic velocity in the renal artery by the peak systolic velocity in the aorta. The peak systolic velocity is a measure of the maximum velocity of blood flow during systole. Using Doppler principles, as an artery narrows, the velocity of blood flow increases. If the renal-aortic ratio was 3.5 or more, it indicated 60% to 99% stenosis of the renal artery. End-diastolic velocities were also measured in all arterial segments. The end-diastolic velocity is a measure of the velocity of blood flow at the end of diastole. In patients with severe degrees of arterial narrowing, the velocity of diastolic flow may be increased. After studies were done using an anterior approach, the patient was placed in the lateral decubitus or prone position (or both), and renal artery velocities were again sampled. The kidney size was measured at its longest axis. Parenchymal flow samples from the kidney itself were obtained, and the resistive index was calculated by using the formula of the peak systolic velocity minus the end-diastolic velocity divided by the peak systolic velocity. The average time for completion of a renal duplex scan was approximately 1 hour. The most common reasons for technical failure were excess bowel gas and obesity. Identification of accessory renal arteries was sometimes difficult. The criteria used for classifying patients were as follows: 1) A renal-aortic ratio of less than 3.5 and a peak systolic velocity of less than 200 cm/s identified patients with 0% to 59% renal artery stenosis; 2) a renal-aortic ratio of 3.5 or more or a peak systolic velocity of more than 200 cm/s [or both] identified patients with 60% to 99% renal artery stenosis; and 3) occlusion of the renal artery was diagnosed by the absence of a flow signal in the renal artery and by a low-amplitude parenchymal signal. The renal artery duplex scans were done by one of four technicians and were reviewed by one of the attending staff in the vascular laboratory. Technician A did 39 (38%) studies, technician B did 41 (40%) studies, technician C did 12 (12%) studies, and technician D did 10 (10%) studies. The technicians often sought the help of their colleagues for patients who were difficult to study. No difference was noted in the accuracy of the results or in the quality of the studies among the four technicians who participated in our study. Statistical Analysis The sensitivity of ultrasound relative to arteriography was calculated as the proportion of positive arteriograms that were positive by ultrasound. The specificity was calculated as the proportion of negative arteriograms that were negative by ultrasound. The positive predictive value was defined as the proportion of positive ultrasounds that were positive by arteriography, and the negative predictive value was defined as the portion of negative ultrasounds that were negative by arteriography. Comparison of hemodynamic variables between the various levels of stenosis was done using analysis of variance techniques. Values are expressed as mean SD. When the overall difference was significant (P < 0.05), this significance l