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Myocardial Perfusion Imaging

by Rosalyn Carson-DeWitt, MD

Anatomy and Physiology
Your heart is a muscle responsible for pumping blood throughout your body. It is made up of four chambers: the right and left atria, and the right and left ventricles. Like all of the organs and tissues throughout the body, the heart muscle requires a constant flow of blood to deliver oxygen and nutrients. Normally, the right and left coronary arteries and their branches deliver oxygen-rich blood to keep the heart muscle functioning optimally.

Reasons for Procedure
When the coronary arteries that feed the heart muscle are narrowed or blocked by fatty deposits, called plaques, the heart muscle may become oxygen-starved, especially during times of stress on the heart. The resulting condition is called myocardial ischemia.

Myocardial ischemia often causes a type of intermittent chest pain called angina. Many patients with coronary artery disease experience angina when they exert themselves, as during exercise. Sometimes angina pain can be felt in other locations such as the right arm, jaw, or back. Myocardial ischemia that persists can eventually damage the heart muscle.

Treatments
Tests to evaluate the health of the heart are usually done when a patient has: a strong family history or other serious risk factors of heart disease, a personal history of angina or previous heart attack, already had a procedure to improve blood supply to the heart muscle, such as angioplasty or bypass surgery.

Common tests used to evaluate the heart include: electrocardiogram, known as an EKG, which examines the heart's electrical activity, exercise tolerance test, which uses an EKG to examine the heart's electrical activity during exercise, echocardiogram, which uses ultrasound to produce moving images of the chambers and valves of the beating heart, cardiac catheterization, which involves the use of x-rays to reveal blockages after dye is injected into the coronary arteries, myocardial perfusion imaging, in which one or more radioactive substances, called tracers, are used to assess blood flow to the heart muscle during rest and exercise.

Myocardial perfusion imaging can help determine how much blood is reaching your heart muscle and how well it is functioning. The test is usually performed in two parts: when the heart is responding to exercise or a similar kind of stress, and when the heart is responding to rest.

Procedure
In the days leading up to your procedure, follow your health care provider's directions: You may need to stop eating about 4-6 hours prior to the test. Avoid caffeine and smoking for 48 hours prior to the procedure. The day of the test, wear comfortable exercise clothing and shoes.

When you arrive, you'll have an intravenous line placed in your forearm. This will be used during the test to administer the radioactive tracer, which is a substance that that accumulates in areas of the heart receiving adequate blood flow.

You'll begin exercising, either by walking on a treadmill or by riding an exercise bike. If you are unable to exercise, a medication, usually dobutamine or adenosine, will be given to you instead. These medications will cause your heart to work harder, as if you were exercising.

Monitors will track your breathing, heart rate, and blood pressure. When you're exercising at peak level, or when the drugs that simulate the effect of exercise on your heart have been administered, the radioactive tracer will be injected into the intravenous line. Soon after, you will lie down on a special exam table underneath a gamma camera.

You'll lie very still while the gamma camera passes over your chest in a slow arc. The camera detects gamma rays emitted by the radioactive tracer you were given. It then transmits this information to a computer, which creates an image of blood flow through your heart muscle.

This test is usually repeated later to evaluate blood flow to your heart muscle at rest. For this portion of the test, you will neither exercise nor be given medication to simulate stress on your heart. The purpose is to see if any restrictions in blood flow during exercise return to normal at rest.

Risks and Benefits
Benefits of myocardial perfusion imaging include: ability to make decisions for future testing and treatment based on knowledge of blood and oxygen delivery to the heart muscle; better accuracy than exercise tolerance testing, which uses an electrocardiogram alone.

Risks of myocardial perfusion imaging include: chest pain, or angina, during exercise or after a medication is given to stress your heart muscle, rare allergic reactions to the medication that simulates exercise or to the radioactive tracer, the minimum amount of radiation released from the tracers is not considered harmful.

In myocardial perfusion imaging, or any other procedure, you and your doctor must carefully weigh the risks and benefits to determine whether it's the most appropriate choice for you.

After the Procedure
Myocardial perfusion imaging generally requires no recovery period. You can usually resume your normal activities immediately following the test. Based on the outcome of the test, you and your doctor will discuss the need for future testing and/or treatment. Be sure to call your doctor immediately if you experience chest pain, shortness of breath, or any other worrisome symptoms following your test.

Sources:

  • Nuclear Stress Test. Mayo Clinic Website. Available at: http://www.mayoclinic.com/invoke.cfm?objectid=0186415C-4738-436E-B37DF76F7AEEBA3B. Accessed July 2, 2004
  • Patient Questions and Answers. American Society of Nuclear Cardiology. Available at: http://www.asnc.org/patientinfo/patientfaq.cfm. Accessed July 2, 2004
  • Thallium Stress Test. American Heart Association Website. Available at: http://www.americanheart.org/presenter.jhtml?identifier=4743. Accessed July 2, 2004


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