For Referring Physicians
Newsletter

January 2001

Renal Artery Disease

Atherosclerosis is an equal opportunity disease. It strikes all arteries with equal frequency, although its severity varies from one site to another. While severe renal artery disease is not as commonly seen, there are basically two types of patients who should be screened for renovascular disease: those who have severe hypertension and those with progressive azotemia, particularly if it is aggravated by initiation of angiotensin converting enzyme inhibitors. Here is a list of "red flags" which should initiate screening for renovascular disease:

1. New onset of severe hypertension (HTN) in the elderly (>60y/o); or the development of resistant HTN in previously well controlled elderly patients
2. Relatively young age at onset of HTN particularly in women (most likely cause is fibromuscular dysplasia)
3. Resistant HTN
4. Presence of vascular disease in other organ systems in an uncontrolled hypertensive patient
5. Continuous systolic/diastolic abdominal bruit
6. Hypertensive retinopathy
7. Recurrent flash pulmonary edema especially in the absence of severe cardiac disease
8. Renal dysfunction in association with resistant HTN
9. Acute renal failure with the use of an ACE Inhibitor

While a myriad of studies have been used in the diagnosis of renal arterial disease, only a few have high enough positive and negative predicted values to be useful at this time. The renal arteriogram, of course, is the gold standard. This study is invasive, costly (about $900), and because of its dye load has the disadvantage of potential damage to renal function. It is, therefore, used primarily as part of a therapeutic attempt at renal artery angioplasty once the diagnosis has been established otherwise.

Captopril renal scan is minimally invasive (about $300) and is a good screening test as long as there is normal renal function. There is an increase in false positive findings with creatinine levels =2.0. In the presence of renal artery stenosis, glomerular filtration is maintained by angiotensin. Administration of captopril in renal scintigraphy removes this compensatory mechanism and causes a temporary impairment of renal function in the affected kidney. Nuclear tracers can visualize this impairment, thus allowing assessment of the physiologic significance of a renal artery stenosis. The patient must stop ACE inhibitors and diuretics prior to the examination.

Magnetic resonance angiography (MRA) is non-invasive, still expensive (about $700), and has the disadvantage of frequent overestimation of the severity of the lesion (simply because of the physics involved in visualizing the vessels). It is useful, however, in the markedly obese individual where duplex scanning becomes less successful and reliable.

Renal artery duplex scanning (RAD) is non-invasive, least expensive (about $250), and has a reliability in excess of 90%. Unfortunately, it is quite technologist dependent for its reliability. We have three excellent registered technologists performing these studies in our lab and are expecting accreditation by the ICAVL (Intersocietal Commission for the Accreditation of Vascular Laboratories) shortly. This is not only an anatomic study, but a physiologic one as well. Several criteria are used in this study.

1. The velocity of blood flow is measured in the renal arteries. If this velocity is above 200 cm/sec or greater than three and a half times the velocity in the aorta, it correlates with a greater than 60% stenosis of the renal artery with sensitivity and specificity in excess of 90%.
2. The waveform of the flow signal is analyzed in the segmental arteries in the hilum of the kidney for delay in the arrival of the systolic upstroke (a tardus parvus signal) to confirm proximal obstruction.
3. Blood flow is measured in the renal parenchyma to determine resistance to flow which correlates with the presence of nephrosclerosis. The renal parenchyma should be a low resistance bed so that if the diastolic flow is less than 30% of the systolic flow, the existence of renal parenchymal disease is suggested. If it is less than 20%, the parenchymal disease is considered severe.
4. A subjective evaluation by color flow Doppler shows a red flash in systole and a blue flash in diastole that extends all the way out to the cortex of the kidney in normal patients. Depression of these flow characteristics with loss of the diastolic blue flash is highly suggestive of nephrosclerosis.
5. Loss of renal volume giving a disparity in renal size also suggests ischemic disease.

When the diagnosis of renal artery stenosis is established, referral for possible therapeutic intervention is advisable, not only to prevent renovascular hypertension, but also for preservation of renal function. If hemodynamically significant renal artery stenosis is present without evidence of significant nephrosclerosis, a favorable result may be anticipated from angioplasty. However, if renal parenchymal disease is shown, Nephrology consultation may be indicated.

For any additional information on this subject matter, please contact us at the Vascular Laboratory of the York Vascular Institute, 717-741-9345.

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