If you are at intermediate to high risk for a heart attack or have chest pain that might be angina, your physician may recommend an exercise stress test to determine whether narrowed coronary arteries are interfering with blood flow to the heart. An exercise stress test is more accurate than a resting ECG. That's because blood flow to the heart may be inadequate only during physical activity, when the heart needs more oxygen to function.
During an exercise stress test, you'll be monitored and evaluated with an ECG while walking on a treadmill or pedaling a stationary bicycle. The intensity of the workout is gradually raised by increasing the speed and incline of the treadmill or resistance of the bicycle.
As you exercise, the workload on your heart will increase and the coronary arteries will widen to allow more blood to reach the heart. Because narrowed coronary arteries do not dilate as much as healthy ones, heart muscle fed by narrowed arteries receives an inadequate amount of blood. When the heart muscle receives too little blood, you may experience chest pain or shortness of breath as well as changes in the heart's electrical activity that can be detected with the ECG.
During the exercise stress test, your doctor will also monitor your heart rate, breathing, and blood pressure. You might also be asked to breathe into a tube to measure how much oxygen your body is using. Depending on your fitness level, the test can last up to 15 minutes. If you become fatigued or experience symptoms (chest pain, shortness of breath, dizziness, or an irregular heartbeat), the exercise stress test will be terminated early.
At the end of the test, your doctor will note the length of time you spent on the treadmill or bicycle (called your exercise capacity) and how much your heart rate declined in the two minutes afterward (known as your heart rate recovery). Recent studies at Johns Hopkins and other institutions show that these measurements can be used to help predict the risk of heart attacks and are better predictors than an ECG alone in individuals without symptoms of heart disease.
You may be concerned about having a heart attack or cardiac arrest during an exercise stress test. However, because you will be carefully monitored and the test will be stopped if you experience dangerous symptoms or ECG changes, the risk is very small—about one in 10,000.
Pharmacological Stress Test
Some people who need an exercise stress test are unable to exercise, because of age, frailty, or a health problem such as arthritis. In these individuals, a pharmacological stress test is performed by injecting a drug that mimics the effects of exercise by increasing the heart's workload. The drug dobutamine is commonly used for this purpose.
Additional Stress Testing
Sometimes the information obtained from an exercise stress test is not definitive enough to allow your doctor to accurately assess your heart attack risk. When this happens, you may undergo additional testing such as a nuclear medicine stress test or stress echocardiogram. As with a regular exercise stress test, there is a very small risk of a heart attack or cardiac arrest during the exercise portion of these tests.
Nuclear Medicine Stress Test
This test starts off like a regular exercise stress test. However, once you reach your maximum level of exercise on the treadmill or bicycle, a mildly radioactive substance (for example, thallium-201) is injected into a vein. The radioactive material travels to the heart, where it is distributed within the heart muscle in proportion to the blood flow (that is, areas of the heart that receive more blood also receive more thallium). You will then lie on an examination table while a special camera takes pictures of your heart to reveal the distribution of the radioactive material, which indicates possible areas of heart injury resulting from narrowing of the coronary arteries.
A nuclear medicine stress test is more accurate than a traditional exercise stress test: It detects coronary heart disease in about 20 percent of people whose resting ECG and exercise stress test are normal despite narrowing of the coronary arteries. The amount of radioactivity from a nuclear medicine stress test is greater than that from an x-ray, so it's important for the benefits to outweigh the risks.
This test involves having an echocardiogram both before and immediately after an exercise stress test. An echocardiogram is a painless test that uses ultrasound waves to generate images of the heart on a video screen.
You will be asked to lie on your left side. A technician or nurse will apply gel to a transducer, a wand-like device that emits and receives reflected ultrasound waves. The transducer is then pressed against the skin of the chest and abdomen, and images of the heart are projected on a video screen.
Echocardiography reveals the size and the shape of the heart's chambers, the activity of the heart's chambers and valves, and how much blood is ejected with each contraction (known as the ejection fraction). This test can also detect blood clots within the heart and, sometimes, scarring. In addition, echocardiography is helpful for diagnosing heart valve disorders, left ventricular hypertrophy (enlargement of the heart muscle), cardiomyopathy (a disease of the heart muscle), chronic bronchitis or emphysema, and congenital heart disorders (heart problems present at birth). Echocardiography is highly accurate.