Because the kidney continuously filters your blood, it is strategically placed to detect changes in blood pressure. If the kidney senses a decrease in blood pressure or blood volume, it responds by secreting powerful substances to increase the pressure. Renin is one such substance and by acting on other proteins in the body, it results in constriction of the blood vessels, effectively increasing the pressure inside them.

If the blood vessels leading to the kidneys are blocked or narrowed, less blood reaches the kidney. The kidney then “thinks” there is less blood and less pressure than is needed for the body to survive. At this point, renin is secreted and blood pressure rises, even though there is no loss of blood volume. This condition is referred to as renovascular hypertension.

There are several tests available to diagnose renovascular hypertension. They can be grouped into two main types: anatomic imaging and physiologic measurement. Anatomic imaging tests are most commonly used. They include Doppler ultrasonography, magnetic resonance imaging, or MRI, and angiography, which is injection of dye directly into the renal arteries so they will show up clearly on X-ray.

Doppler ultrasonography uses sound waves to image your kidneys and measure the speed of blood flowing through the renal arteries. An abnormal blood flow rate will usually be seen if a significant obstruction exists.

MRI studies allow us to visualize blood flowing through the renal arteries in a different way. This test produces the best images of these vessels, however, not all people are good candidates for an MRI. If you have any metallic device implanted (like a pacemaker or an artificial joint), or if you are severely claustrophobic, this test may not be possible.

Finally, an angiogram can be performed to look directly at the vessels. During this procedure, a very thin hollow tube called a catheter is passed into the femoral (groin) artery, and a special dye, called contrast media, is injected through the catheter into the renal vessels. X-ray pictures are taken to measure the degree of vessel narrowing.

The advantage to the angiogram is that, if necessary, the narrowed area of the vessel can be expanded with a tiny balloon that fits through the catheter. This can provide at least a temporary fix for the problem. The downside is that there are certain risks associated with any invasive procedure. These include bleeding, infection, and damage to the blood vessels and an adverse reaction to the contrast media. You may want to discuss these possibilities with your provider if you are a candidate for an angiogram.

The most common physiologic study used is the captopril test. In this test, the baseline level of renin in the blood is determined by drawing blood. Then an oral dose of captopril is given and, after a period of time, the plasma renin level is determined again. Because captopril blocks the activity of one of the proteins that renin works on, the blood pressure should fall. This decrease in blood pressure is detected by both kidneys, but especially by the one that has a blocked blood supply. This kidney responds by secreting a large amount of renin. Thus an exaggerated renin response after the dose of captopril is suggestive of a renal cause for hypertension. Alternatively, nuclear imaging can be performed during this study and the blood flow to the kidneys measured directly.

The specific test that might be ordered is highly dependent on the opinion of your provider and the capabilities of the health care facilities in your area. It is recommended that patients ask for an explanation of the test ordered if they are interested.