Renal Artery Duplex

Michael A. Ricci, MD, RVT

 

Background:
The diagnosis and treatment of Renal Arterial Disease has evolved over time, especially over the last several years. Although Bright first called attention to the association between hypertension, renal disease, and cardiac hypertrophy in 1836, it was another century before the long-lasting effects of renal ischemia became evident through the classic experiments of Goldblatt. Nephrectomy later became a popular treatment for hypertension, until Smith, in 1956, demonstrated only a 26% cure rate with that procedure. Although revascularization began to be utilized in the 1960's, it soon became apparent that even this procedure was successful only 50% of the time. However, through the persistent efforts of Dean and others, it became apparent that careful selection could result in 90% of patients cured or improved following renal revascularization. Of late, it has become apparent that a number of patients with renal insufficiency may also benefit from revascularization, perhaps even eliminating the need for dialysis. The need for a reliable and reproducible screening test is apparent, since the selection of patients that might benefit from surgery must come from a much larger population with hypertension or renal insufficiency.

Clinical Indicators:
Certain clinical characteristics may suggest the diagnosis of renovascular disease. Fibromuscular dysplasia, which is responsible for one quarter of renovascular hypertension, generally should be considered in a younger patient with the "sudden" onset of hypertension, particularly accelerated or malignant hypertension. This condition occurs earlier than the atherosclerotic variety (average age 33 years vs. 46 years with atherosclerosis) and is more frequent in women (81% are female).

Most patients, however, have renovascular hypertension from atherosclerosis, although it is not uncommon for a patient with long-standing, "essential" hypertension to present with a sudden worsening of blood pressure (BP) from a progressive Renal Artery lesion. Of patents who have hypertension, about half will present after the age of 50 years, and of those 39% will have renovascular hypertension. The level of the initial BP, particularly the diastolic BP, may be an important indicator of renovascular disease:

Diastolic BP	Essential Hypertension	Renovascular Hypertension
90-117 mmHg	100%	0
> 118 mmHg	74%	26%
Accelerated	77%	23%

It has been suggested that all patients presenting with a diastolic BP greater than 105 mmHg should undergo a work-up for renovascular disease.

Recently, it has become apparent that some patients with renal failure have Renal Artery disease leading to "ischemic nephropathy". Patients with rapidly progressive renal dysfunction or recent worsening of previously stable insufficiency should also undergo investigation.

Indications for Renal Artery Duplex Scanning:
Renal Artery Duplex Scanning (RADS) will be most useful as a screening test among a "selected" group of patients with some clinical indicator rather than the population at large.

The following have been suggested by Vaughan as candidates for screening:

1. Malignant or accelerated hypertension
2. Young patients with hypertension
3. Presence of a flank bruit
4. Decreased serum potassium
5. Azotemia

Technique of Renal Artery Duplex Scanning:
Strandness has repeatedly pointed out that RADS is the most difficult test that we perform in the Vascular Laboratory and should only be carried out by experienced individuals who have the dedication, persistence, and time to carry out a complete exam. In our lab, these studies are carried out by a single experienced sonographer who has had additional training and course work in this area. The test usually takes 1-1½ hours to complete. It is our preference to carry out this exam in the early morning after an overnight fast. If we cannot complete the exam because of bowel gas or a full stomach, we will have the patient return on a different day for another attempt.

The exam is started with the patient supine and a pillow under their head. Scanning is carried out with a low frequency transducer (2.5-3.0 MHz). Scanning begins in the midline anteriorly with identification of the abdominal aorta and the velocity within this vessel is recorded. The left renal vein overlying the aorta is then identified and serves as an important reference point. The transducer is rotated 90° to identify the origins of the renal arteries. The velocity is recorded and the vessels are followed distally, with the aid of the velocity signals, as far distally as possible. It is not always possible to visualize the entire length of the Renal Artery although every attempt is made to do so. The angle of insonation is also difficult to pinpoint, which is the reason Strandness and others have advocated the Renal-Aortic Ratio as a criteria for diagnosis (see below). The patient may also have a pillow rotated slightly and a pillow placed under their back so that a flank approach may be utilized. This is usually necessary for visualization of the distal Renal Arteries as well as intralobar and arcuate vessels (where the velocity is also recorded). The technologist also searches for accessory Renal Arteries before finally measuring kidney size. We do not calculate renal volume from length, width, and thickness measurements but we do find that the length is a clinically useful and important measurement. A renal size less than 9 cm should prompt a search for an occluded Renal Artery as recommended by Strandness.

Criteria for Interpretation:

Normal: Peak Systolic Velocity (PSV) < 180 cm/sec
Renal-Aortic Ratio (RAR) < 3.5 (Renal PSV/Aortic PSV)
End Diastolic Ratio (EDR) >0.2 (End Diastolic Velocity/PSV)
Abnormal: PSV > 180 cm/sec and RAR > 3.5 indicates >60% diameter-reducing stenosis
PSV > 180 cm/sec and RAR < 3.5 indicates <60% diameter-reducing stenosis
EDR < 0.2 suggests intrinsic renal parenchymal disease
No flow = Occlusion
Results:Using the criteria above, Strandness (1993) has reported the following results:

Duplex		Arteriography
	< 60% Stenosis	> 60% Stenosis	Occluded	Total
RAR < 3.5	15	3	1	19
RAR > 3.5	11	45	0	56
Occluded	0	0	10	10
Total	26	48	11	85

Sensitivity, 92%; Specificity, 58%; Accuracy, 81%

In the 11 cases with a RAR >3.5 with an arteriographic reading <60%, the disagreement was in regard to the severity of the stenosis.

The EDR is a gross indicator of the diastolic flow which should normally be high in the normal, low resistance kidney. Intrinsic renal disease should cause an elevation of the resistance, decreasing diastolic flow. Early results seem to suggest that the EDR may be an indicator of parenchymal disease in the face of normal renal arteries. It may turn out to be a predictor of the results of revascularization.

Schreiber has followed a number of patients (85) over an 52 month period with RADS and repeat arteriograms. That study found progression in 44%, including progression to total occlusion in 14 patients (most common with an initial stenosis >75%). Renal function worsened in those with progression. Taylor and Strandness have also followed 19 patients with a stenosis >60% and found a decrease in renal size over the 13 month study, suggesting that loss of renal mass may be a good indicator of renal function. More study is necessary, however, before this conclusion can be reached.

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