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HEART Insight:
doi: 10.1097/01.HEARTI.0000373312.76567.d4
Features: The Picture of Health

PEEKING INSIDE THE PUMP

VALEO, TOM

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Abstract

Doctors On The Outside Can See Your Heart From The Inside

When the doctor checks your pulse, measures your blood pressure and listens to your heartbeat during a routine physical, he or she can get a pretty good idea of how well your heart is working.

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But if something isn't quite right — your pulse is too fast, your blood pressure is too high or an abnormal murmur is detected — your doctor will likely augment these tried-and-true low-tech diagnostic methods with an array of high-tech imaging tools that show the structure of your heart and create vivid pictures of the organ in action. For instance:

* A small device held to your chest can bounce sound waves off your heart and translate the echoes into a video known as an echocardiogram – it's just like the ultrasound test that a pregnant woman gets – that can show whether your heart is contracting normally and the valves are opening and closing properly to keep blood moving in the right direction.

* Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) pick up signals from a radioactive tracer injected into your bloodstream to allow doctors to measure blood flow to the heart while you walk on a treadmill or after you receive a medication that causes the arteries on the heart to dilate, as well as to see how well the heart pumps.

* Magnetic Resonance Imaging (MRI) reads magnetic signals emanating from the atoms that make up your body tissues, producing stunningly vivid pictures that can reveal signs of scar tissue from a heart attack or inflammation from an infection, as well as structural and functional heart defects.

* Cardiac Computed Tomography (CT) can find calcium embedded in the plaques that grow within artery walls. A computer analysis of how much calcium is deposited in arterial plaques produces a score that indicates heart attack risk over time.

* Coronary Computed Tomographic Angiogram (CCTA) assembles multiple X-rays of your heart into images that pinpoint where a coronary artery is blocked, and whether the obstruction is severe enough to restrict blood flow to the heart muscle.

Seeing the living, beating heart in action can help detect, confirm or exclude a diagnosis. But used carelessly or excessively, cardiac imaging can waste money, provide ambiguous or misleading information, and depending on the imaging technique, needlessly expose patients to radiation.

“All of these tests can be fantastically helpful,” says Ann Bolger, M.D., William Watt Kerr Professor of Clinical Medicine at the University of California, San Francisco. “But the choice of test is a matter of some art. If I don't have information about a patient's history and physical condition, I don't know how to interpret scanning information,” she adds. “Every time I order a test or look at results, it hinges on what I already know about that patient.”

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THE QUICKER TICKER PICKER-UPPER?

Why aren't imaging tests that show the structure of the heart and its pumping ability as routine as blood cholesterol tests? Because for most patients, it's overkill.

“More scanning does not necessarily mean better care,” explains cardiologist Raymond Gibbons, M.D., Professor of Medicine at the Mayo Clinic College of Medicine in Rochester, MN, and a past president of the American Heart Association. “We as a society tend to focus on technology, particularly the latest technology, with an underlying belief that it replaces less sophisticated approaches, and that's simply not the case. Sometimes the simplest and most basic approaches are unfortunately being overlooked in the rush to technology.”

The Framingham Risk Score (see sidebar), developed from information that has been gathered for more than 60 years about the cardiovascular health of residents of a town in Massachusetts, uses age, blood pressure, cholesterol, smoking and blood sugar levels to gauge a patient's risk of developing heart disease.

“By the time I know the answer to those questions, I know how likely I am to trust a positive or negative scan result,” says Bolger. “I can put test results into perspective.”

And just asking a patient a few simple questions — such as, “Have you ever experienced chest pain?” — also provides invaluable information. Patients who have never experienced chest pain and those who have chest pain may not be at equal risk of a heart attack, even if their blood pressure or cholesterol is the same.

“If you tell me you've never had chest pain, and I spend time chatting with you and I learn from you that you're very active — you hike up mountains and run — that's one thing,” says Bolger. “But if you then tell me that your five brothers, uncle and father all had heart attacks between the ages of 50 and 55, and you're 49 and a half, I'm going to be all over your case.”

In general, doctors don't prescribe imaging tests for patients without symptoms whose Framingham Risk Score is low, because the results are unlikely to provide more information than what the doctor already knows from your medical and family history. Imaging tests become useful, however, when a patient develops symptoms or crosses over to the intermediate risk category, with age plus elevated cholesterol, blood pressure and blood sugar combining to jeopardize cardiovascular health.

Most imaging tests are non-invasive. One exception is catheter-based angiography, a type of X-ray that entails the injection of a contrast agent with an iodine base into the circulation via a thin tube that has been inserted into a vein or artery in the arm or groin and threaded to the heart. A new study published by the New England Journal of Medicine suggests invasive angiography is not always the best method to find blockages of the coronary arteries when a patient is showing signs that blood flow to the heart may be obstructed. Studies are planned to determine which non-invasive imaging techniques might be better alternatives to invasive angiography to diagnose heart disease early and thus help to prevent heart attacks, especially for patients who have high cholesterol and other risk factors but have not developed any symptoms.

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PROS AND CONS

Some imaging techniques expose patients to radiation, which carries a small but significant risk — especially with repeated testing. The amount of radiation involved is not trivial. A 2009 study by the Mayo Clinic published in the Journal of the American Medical Association (JAMA) calculated that a CT scan could subject a patient to an amount of radiation equal to about 600 chest X-rays. And a recent study in the Archives of Internal Medicine calculates that if men were scanned every five years from the ages of 45 to 75, and women every five years from 55 to 75, approximately 42 additional cases of cancer for every 100,000 men, and 62 additional cases per 100,000 women, would occur.

The FDA has launched a new initiative to reduce unnecessary radiation exposure from CT and other imaging tests, that aims to ensure that “each patient get[s] the right imaging exam, at the right time, with the right radiation dose.” To achieve this goal, the regulatory agency is taking a three-pronged approach:

1. Requiring manufacturers of imaging devices to develop and build in safety devices that can alert technicians when the radiation dose exceeds a safe level;

2. Enhancing technician training and accreditation standards for imaging facilities; and

3. Encouraging professional medical associations and healthcare providers to develop and adopt criteria for appropriate use of imaging tests. The agency will also roll out a public awareness campaign that will give patients tools to help them and their doctors keep track of their radiation exposure from imaging tests.

The American Heart Association issued an advisory for doctors in 2009 that encouraged doctors to limit CT use to patients at significant risk of certain types of heart disease. Most doctors consider the radiation risk acceptable if it helps a patient avoid a heart attack. A middle-aged patient with a high calcium score, for example, may be at elevated risk over 10 years for a heart attack even if current myocardial perfusion imaging with exercise (see sidebar) shows that the heart is receiving an ample supply of blood. More than one imaging test might be needed to assess his or her potential for a heart attack and how aggressive treatment should be to prevent it. The appropriateness criteria being developed by the AHA and other organizations will help your doctor apply the right test.

For example, if a patient had a level of LDL (“bad”) cholesterol of 120, prescribing a statin to bring it down to below 100 might be appropriate, according to cardiologist John Mahmarian, M.D., of The Methodist Hospital in Houston. “But if the patient had a high calcium score of 400, I'd try to lower the LDL even more,” he said. “Knowing those results would make a dramatic change in how aggressively I would try to modify the patient's risk factors.”

“I'm an imager,” says Bolger. “I don't know what I would do without these tests. But I'm very cautious with these tools. We have to know how they fit into less expensive, less risky strategies.” But she adds that patients shouldn't worry if the doctor doesn't order a particular imaging test — or pressure the doctor to do so. “The question is not which test should you get, but what combination of things you and your doctor should put together. The picture builds up gradually, layer by layer.” HI

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SNAPSHOTS OF IMAGING TECHNIQUES

Echocardiogram

HOW IT WORKS: High-frequency sound waves are bounced off the wall and the inner structures of the heart, revealing their size, shape, thickness and function.

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WHAT IT'S USED FOR: Produces moving images of the beating heart, and shows whether valves are opening and closing correctly, or whether blood is refluxing backwards or “regurgitating” through valves that don't close completely. Shows whether heart function is normal or impaired.

DRAWBACKS: Accuracy of results depends on skill of the technician performing the scan, the size of the patient, and the cardiologist interpreting it.

COST: Medicare reimbursement ranges from $35–$177.

INSURANCE: Covered.

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Myocardial Perfusion Imaging

HOW IT WORKS: A radioactive tracer is injected into the bloodstream. As you walk on a treadmill, your heart absorbs the tracer circulating in your blood and a scanner reveals how much tracer reaches the heart wall. Areas of the heart wall that have taken up only a small amount of the tracer are not getting enough blood, usually due to clogged coronary arteries. Areas that have taken up no tracer at all may be dead, indicating damage from a previous heart attack.

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WHAT IT'S USED FOR: To determine how well the coronary arteries are supplying blood to the heart muscle, and to identify areas of the heart wall that are not receiving an adequate blood supply. There are two main types of myocardial perfusion imaging: Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT). They provide similar information — how much blood is getting to the heart muscle — but use different tracers.

DRAWBACKS: Patients are injected with a small amount of radiation.

COST: Medicare reimbursement ranges from $186–$210.

INSURANCE: Covered.

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Magnetic Resonance Imaging

HOW IT WORKS: Powerful magnets line up all the water molecules in your body's cells in the same direction, and when the magnetic field is shut off the molecules return to their random patterns — a process that gives off energy that is detected by a scanner and used to create 2-dimensional images of a cross-section of your heart, or 3-dimensional images that show the heart wall, valves and chambers, as well as blood flow through your heart and blood vessels. No radiation is involved.

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WHAT IT'S USED FOR: To create vivid images of the heart and major blood vessels.

DRAWBACKS: Some people experience claustrophobia while lying inside the scanning tube; “open” MRI designs help offset this problem. MRI scanning also requires patients to remain perfectly still for several minutes at a time, or the scan may need to be repeated.

COST: Medicare reimbursement ranges from $517–$853, depending on which type of MRI is done.

INSURANCE: Covered.

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Coronary Computed Tomographic Angiogram

HOW IT WORKS: X-rays of cross-sections of the heart are combined to produce a vivid image of the coronary arteries. There are two types. A cardiac CT scan shows the presence of calcium in the lining of coronary arteries. The Coronary Computed Tomographic Angiogram (CCTA) locates blockages in the arteries.

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WHAT IT'S USED FOR: CT can indicate build-up of plaque that constricts blood flow. In some instances, along with other testing, CCTA can help determine whether an invasive study is needed to place a stent to prop an artery open, or bypass surgery to reroute blood flow.

DRAWBACKS: Exposes patients to some radiation, which could increase risk of cancer, especially in young women.

COST: Varies widely, from $50 to about $350 or more.

INSURANCE: Covered.

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RISKY BUSINESS

The Framingham Heart Study, a public health study that began in 1948 with 5,209 participants, has provided invaluable data allowing doctors to determine the risk factors associated with heart disease — such as smoking, obesity, elevated cholesterol — by comparing those who had heart attacks, strokes and other cardiovascular illness over the years to those who did not. Data from the study have also allowed doctors to “weight” various risk factors in terms of how damaging they are to cardiovascular health, and to add up a person's risk to determine a score that predicts the likelihood of a heart attack or dying of coronary heart disease within the next 10 years. The lower your Framingham Risk Score, the higher your odds of dodging heart disease. To find out more about the study, visit: www.framinghamheartstudy.org. To take a free online risk assessment based on the Framingham Risk Score, visit www.americanheart.org/RiskAssessment.

© 2010 American Heart Association, Inc.

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