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Ask the doctor: Nuclear stress tests

Q. I recently had a nuclear stress test and the contrast agent got stuck in my gut, so the image couldn’t be read. Is this a common problem, and is there anything that can be done about it?

A. The purpose of a stress test is to identify problems with blood supply to the heart muscle or the coronary arteries that supply it with blood. The standard test involves measuring the heart’s electrical activity with an electrocardiogram (ECG) while the patient “stresses” his or her heart with exercise, which usually means walking on a treadmill or pedaling a stationary bike.

But for the test to be useful, you have to be able to walk or pedal fast enough to get your heart pumping at 85% of its maximum rate for several minutes. People who cannot exercise that hard — if you are on a treadmill, it means walking pretty briskly — can get a nu clear stress test instead. The test involves infusion of a chemical into a vein to mimic the effect that exercise has on the coronary arteries, followed by infusion of a radioactive material — a radiotracer — that allows a special camera and computer to create very detailed images of the heart.

Nuclear stress tests are also used in other situations. They’re more sensitive than the standard stress test, so they’re sometimes better at pinpointing the location of a coronary artery blockage that’s affecting the heart. But they’re a good deal more expensive. A regular stress test costs about $100. A nuclear stress test costs between $600 and $800.

One chemical “stressor” used commonly today is a drug called dipyridamole (the brand name is Persantine). A drug called adenosine is also used. The most commonly used radiotracer is technetium-99m.

In small doses, dipyridamole makes blood clots less likely to happen, so it’s sometimes used to treat strokes or transient ischemic attacks (TIAs) that aren’t responding to other medications. But it’s used in nuclear stress tests because in larger doses that are infused directly into the bloodstream through a vein, dipyridamole expands (dilates) the coronary arteries in the same way exercise does. Healthy coronary arteries expand more freely than those that are lined with atherosclerotic plaque. Doctors can diagnose coronary artery disease by comparing how much of the radiotracer reaches the heart muscle with and without the dipyridamole.

Here’s the trouble: dipyridamole’s artery-expanding effects are not limited to the coronary arteries. Blood vessels in the gut and liver are also affected. So in you and many others, the radiotracer “lights up” those blood vessels, which interferes with getting a clear image of the heart. How often does that happen? I can’t say for sure — maybe about 40% of the time. It’s definitely a common problem.

There are several ways we can try to solve it. People are told to fast before a nuclear stress test because the chemicals can cause nausea and vomiting. But if we’re having a hard time getting a clear image, we’ll sometimes have patients eat or drink and then repeat the imaging. It also sometimes helps to delay the imaging for 30 to 45 minutes. Technetium-99m tends to accumulate in the gallbladder and your gallbladder empties after you eat. That emptying can clear out some of the technetium-99m from the area below the diaphragm, so the image of the heart is clearer. Food also gets the intestines moving, which keeps the radiotracer from pooling.

Typically, people are on their backs when the imaging part of the test is done. Sometimes if patients lie on their stomachs (the prone position), we can get a better image of their hearts. There are also machines today that allow the image to be taken while you are upright. The upright position helps because gravity pulls the stomach and the intestines down a little bit and out of the way of a clear view of the heart.

A third thing we can try is infusing the dipyridamole while patients are exercising very lightly — not nearly as hard as they need to during a regular stress test. Even very light exercise increases blood flow to the arms, hands, legs, and feet, so the radiotracer is less likely to collect in the gut.

If none of this works, we can try using a different substance as the radiotracer. Technetium-99m has mostly replaced thallium in nuclear stress testing because it is less radioactive and helps us get clearer images, but thallium can be used. It tends to collect in the kidney and not so much in the gut, so it’s less likely to interfere with getting a clear image of the heart.

— Sharmila Dorbala, M.D.
Brigham and Women’s Hospital

Posted by: Dr.Health

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