What Is a Renal Artery Ultrasound?
Ultrasound is a diagnostic tool that is used to assess the structural and functional characteristics of the main artery supplying the kidney. Renal artery disease correlates with the degree of kidney dysfunction. As the end-organ, the kidney depends on a normal flow through its artery to supply oxygen and nutrients to is cells, as well as to serve as the source of waste products and electrolytes for active and passive filtration, elimination, or re-absorption.
Renal Artery Ultrasound
Renal artery Doppler duplex ultrasound uses the combined approach of B mode ultrasound, which renders an image based on the differences of reflected sound waves from differing tissue densities; and Doppler technology that uses to advantage reflections from red blood cells that render actual flow imagery.
Renal artery disease, i.e., renal artery stenosis, is caused via two mechanisms:
- Atherosclerosis involving the branching point of the renal artery from the aorta or the proximal renal artery.
- Fibromuscular dysplasia involving the distal renal artery or its intrarenal branches.
Renal artery stenosis presents the untoward dynamic of obstruction into the kidney’s physiology, with its resulting hypoperfusion of tissue and impairment of waste elimination. Duplex Doppler ultrasonography can assess function via information gleaned from the renal arteries; is also can provide information on the structure and anatomy. B-mode imaging combined with Doppler measurements can identify both stenotic lesions and arterial flow, and make comparisons to the blood flow of the aorta.
Duplex Doppler ultrasonography can detect both unilateral and bilateral disease and can be used for serial evaluations to observe for progression of disease. It provides a convenient method to follow those who have had surgical angioplasty or other vascular surgery.
Since the kidney is also an important regulator of electrolyte balance and part of the Renin-Angiotensin-Aldosterone system that regulates blood pressure, balanced homeostasis is at risk with renal artery stenosis and other problems. Doppler ultrasonography can evaluate renal vascular flow to evaluate renal artery stenosis, renal vein thrombosis, and renal infarction. CT and magnetic resonance (MR) are required for confirmation, but a screen can be performed using the less expensive ultrasonography.
Renal Artery Ultrasound Diagnosis
The kidneys not only eliminate waste, but are integral to electrolyte balance, homeostatic hydration, and blood pressure control. Renal artery ultrasound is indicated in
- Hypertension: when secondary causes are suspected (i.e., not “essential” hypertension): suspected renovascular hypertension.
- Elevated serum creatinine levels: when stenosis threatens the entire kidney.
- Atherosclerosis: 70-80% occlusions can lead to renal tissue hypoxia.
- Back pain attributable to renal disease
- Electrolyte disturbances: Such as hyperkalemia, hyperphosphatemia, hypocalcemia, etc., and associated metabolic acidosis.
- Computations of renal “resistive index”: A pre-operative measurement comparing systolic blood flow velocity with end-diastolic velocity, which correlates well with a predicted benefit of planned revascularization surgery.
Renal artery ultrasound is also part of a global assessment of kidney anatomy, which can distinguish differences in size between the pair when unilateral kidney disease is present. A perusal for calcifications and obstruction of the urinary tract is easily included when renal artery ultrasound is performed. Because renal artery ultrasound is not as sensitive as confirmatory imaging methods, such as CT and MRI, a negative result is not necessarily reassuring, but indicative of the necessity for the more involved imaging that uses contrast.
Other Imaging Used in Renal Artery Disease
While renal artery ultrasound is useful as a screen or for initial investigation into renal artery disease, from atherosclerotic obstructions to renal artery hypertension, confirmation is via more invasive methods.
- CT Arteriography: Invasive due to its intravenous contrast is highly accurate for and confirmatory of atherosclerotic renovascular disease
- Magnetic Resonance Arteriography: Used less often due to concerns with renal sensitivity to the contrast medium used (gadolinium)
Blood tests are generally non-specific in identifying any exact type of kidney disease, but they can indicate global kidney dysfunction when impairment reaches a threshold beyond which results become worrisome. Some components (e.g., serum creatinine) are useful for serial monitoring of disease stability or progression.
Management and Treatment of Conditions That Are Identified via Renal Artery Ultrasound
Management and treatment of conditions that are identified via renal artery ultrasound are for addressing the following:
- Renal artery atherosclerosis and stenosis resulting in occlusive disease. This creates hypoperfusion and impairs sodium excretion which expands the extracellular fluid volume, contributing to systemic hypertension.
- Suspected renal infarction as a result of arterial atherosclerotic emboli.
- Renal artery hypertension.
- Incidental findings of stony urinary tract obstruction or kidney calcifications indicating chronic renal dysfunction.
In the renal artery, these conditions all represent some type of obstruction that can be addressed in one of three ways:
- Medical therapy: Used alone or in combination to the invasive approaches (below).
- Renal angioplasty: Repair of the diseased arterial segment using a stent that can be inserted via interventional radiological techniques (percutaneously). The stent can reestablish blood flow through the diseased portion of the artery.
- Surgical revascularization
Because kidney hypoperfusion from renal artery disease sets into motion the kidney’s blood pressure maintenance mechanisms as a disadvantage, blocking this very mechanism can help address or prevent the hypertension that results. The renin-angiotensin-aldosterone system is tempered with an angiotensin-converting enzyme inhibitor (ACE inhibitor) or an angiotensin II receptor blocker (ARB) in combination with a diuretic. This is effective enough to realize a reduction in the risk of stroke or myocardial infarction that come from the kidney disease-hypertension-further kidney disease-further hypertension vicious cycle.
Percutaneous Transluminal Renal Angioplasty and Stenting
Although renovascular angioplasty can be done without stenting, the success in addressing renal artery disease is superior when stenting is incorporated into the procedure, whether there is unilateral or bilateral disease.
Compared with percutaneous stenting, the success of surgical revascularization is about the same in addressing hypertension. It is therefore relegated to those with multiple small artery disease, in need of aortic reconstruction, or who could benefit from a bypass of the renal artery to supply blood to the kidney(s).
Renal Artery Ultrasound and Prevention
Prevention of kidney impairment to excretion from hypoperfusion or the development of systemic hypertension secondary to renal artery disease (perpetuating a vicious cycle: renal artery disease engendering hypertension which furthers renal artery disease, and so on) is best addressed simultaneously for the most rational therapeutic approach.
Patients with atherosclerotic renovascular disease typically have widespread (systemic) atherosclerosis and present with a high risk for coronary artery disease. If there is bilateral disease (or unilateral disease with a single kidney), antihypertensive therapy is indicated.
ACE inhibitors and angiotensin II receptor blockers can be used to interrupt the kidneys’ reactive renin-angiotensin-aldosterone system from compensatory overreaction toward secondary hypertension which can further the actual primary renal disease. Diuretics are recommended to impact impaired excretion favorably.
Blood tests can be used to monitor electrolyte balance and prevent or treat imbalances. Serum creatinine can be measured serially to follow any progression of global renal disease upon which to determine whether invasive correction (percutaneous stenting or surgical revascularization) is warranted to eliminate or delay any life-threatening consequences of progressive disease.
Those at risk for all cardiovascular disease, specifically diabetics, should implement strict glycemic control and maintain their targeted glycated hemoglobin A1c. Smokers should quit, using medications available (e.g., Chantix) to increase the odds of success. Patients with unacceptable cholesterol and/or triglyceride levels should be placed on statin drugs to correct these abnormalities in their lipids.
At some point, which is individualized for the patient and by physician preference and experience, more invasive remedies may be indicated, especially with the inherent connection to other atherosclerotic complications such as coronary artery disease and its risk of stroke and myocardial infarction.