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Thursday, October 20, 2005

Heart Scans: Fact and Fiction

Greg, a 51-year old pharmaceutical representative, prided himself on his healthy lifestyle. At 5 feet 8 inches, he weighed a slender 147 lbs, not much more than he'd weighed as a champion high-school wrestler. He felt great and tackled his work and home life with enthusiasm and energy. He was also proud of his cholesterol numbers, held in check for the past 10 years since he'd worked in the pharmaceutical industry and paid closer attention to his health. His most recent panel: HDL 55 mg/dl, LDL 94 mg/dl, triglycerides 123, all excellent according to his family physician. Greg's mom and dad lived into their early 80s without an encounter with heart disease, and Greg was confident that he was free of risk.

Unexpectedly, Greg's older sister informed him that she'd developed unstable chest pain symptoms recently, requiring hospitalization. She'd undergone a heart catheterization and received two coronary stents, narrowly averting an open heart procedure. This unsettled Greg's complacency, and he now became concerned about his own health. He underwent a heart scan, which yielded a score of 1387, in the 99th percentile for men his age. This score suggested that, far from being low risk, Greg¡¦s risk was, in fact, an alarming 25% per year for heart attack and death.

Can cholesterol tell you whether you have hidden plaque lining your coronary arteries?

No. Cholesterol does not tell you whether or not coronary plaque is present. It's just a blood fat that is one among many causes of coronary plaque, and provides an indirect statistical measure of the likelihood of heart disease. Can you have a heart attack with low cholesterol? Absolutely. Can you have high cholesterol yet survive to age 95 with 18 great-grandchildren and never have a stitch of heart disease? No doubt about it.

Does an EKG show hidden coronary plaque? No. EKG's are just simple electrical measures that may show heart attack while it's in progress or much after the fact.

How about stress tests? Don't they detect hidden coronary plaque?

No, again. Stress tests are a measure of blood flow to the heart, abnormal only when flow is substantially reduced. Blockages that occupy 80-90% of the diameter of arteries, for instance, would be detectable on a stress test. The problem is that 90% of people with hidden (asymptomatic) coronary plaque will have entirely normal stress tests, yet are still at risk for heart attack. In fact, most of the people fated to suffer a heart attack in the next year have normal stress tests. So how can you detect and measure hidden coronary plaque easily, inexpensively, and safely? We don't want invasive procedures that involve scalpels and catheters. We need a test that precisely detects hidden plaque without pain, with virtually no risk, that nearly anybody can get. That's precisely what a heart scan can do. And it does it better, faster, and easier than any other test available.
"We now know that 95% to 99% of the heart disease occurs at sites WITHOUT artery narrowing. Thus, the old tests we perform to detect narrowing and blockages have really misled us. We miss over 95% of the heart disease that causes heart attacks." Dr. Steve Nissen The Cleveland Clinic

What exactly is a heart scan?

If you've undergone a heart scan, you already know that it's among the easiest and fastest health tests available: hold your breath for 30 seconds and you¡¦re done. It's about as simple as any test can get. No poking, prodding or pushing, no IV's, no pre-medication, no preparation. Most people are surprised by how easy it is. The most common comment after the scan is finished is, "Is that it?" With this small effort, you'll be provided with the most powerful piece of information you can get about your heart's future.

A heart scan is really 30-V40 cross-sectional images (varying depending on your height) of the heart from top to bottom, all obtained in the few seconds you hold your breath. (Holding your breath eliminates motion of the heart due to expansion of the adjacent lungs.) Within each of the 30 or so images, a "slice" of your three coronary arteries can be easily seen. Because each slice overlaps with those above and below it, the scan provides, in effect, a three-dimensional survey of the chest contents. Each scan is reviewed and a computer applies specific criteria to help decide whether a selected area within your coronary arteries is truly coronary plaque. The area (in square millimeters) is multiplied by the density of the plaque, and this yields a "score" for this specific plaque. All plaques in every image slice are scored and all the scores added up. This yields a total score, the one reported to you. You will sometimes hear the total score called an "Agatston" score, named after Dr. Arthur Agatston from the University of Miami, who first developed this method of scoring. The Agatston score is now one of the standard calculations performed on all heart scans. (This is the same Dr. Arthur Agatston, by the way, who authored the hugely successful South Beach Diet.)

If you haven't yet undergone a heart scan, how do you decide whether you should get one in the first place? Many authorities recommend that men over age 40, women 50 and over be scanned. This is based on the simple observation, in tens of thousands of people, that substantial numbers of men start showing hidden plaque (scores >0) age 40 and older, women 50 and over. Prior to these ages, scores above zero for either sex are unusual. Women are advised to get scans later than men because the development of plaque lags behind men approximately 10 years.

Beyond age, there are no useful criteria to decide who should and who shouldn't be scanned. We could, for instance, use LDL cholesterol to decide whom to scan. If we chose anyone with LDL cholesterol >130 mg/dl, we would miss half the people with heart disease. (In other words, approximately 50% of people with measurable coronary plaque have LDL cholesterols <130 mg/dl.) Likewise, if low HDL(<40) is our cut-off, you will again miss about half the people with heart disease. Similar failures occur for any other screening parameter you can devise.

Consider this: Can we screen you for the presence of heart disease by screening you with another test first? You may begin to see the absurdity in this approach. A study by Drs. Arad and Guerci at the St. Francis Medical Center in New York, for instance, showed that 67% of people classified by the widely-used Framingham risk scoring system (using cholesterol, blood pressure, smoking history, and sex) were mis-classified: people labeled low-risk were actually high-risk; people labeled high-risk were actually low-risk.

Another study by Dr. Stephan Achenbach of the Massachusetts General Hospital showed a very poor 28% correlation between risk factor scoring systems like the Framingham system and heart scan scores. Most concerningly, significant numbers of people classified as low risk by the Framingham scoring system had extensive coronary plaque.

Cholesterol values and conventional risk factors cannot be reliably used to decide whether or not to have a scan. We therefore rely on age as a guide.

If there is some high-risk measure in your life - parent with heart attack before age 55; diabetes, cholesterol >300, smoking - then you might consider having your scan 5 years earlier (35 for men, 45 for women).

What's the best heart scan score? Zero, or no detectable plaque in any of your scan images. What's terrible? Just like playing golf, the higher your score, the worse it is. But just like asking "what's a terrible golf score?", the answer for what constitutes a terrible heart scan score is "It depends". Certainly, the higher your score, the more plaque you have in your coronary arteries, the greater your heart attack risk. Scores of 1000 or greater (that's a lot of plaque!) carry a risk for heart attack of 25% per year (if no preventive therapies are initiated), as it did unexpectedly for Greg, above. Scores between 0 and 1000 are much more common and carry various degrees of risk in-between.

Why is it called a "calcium score"?

What precisely is measured on a heart scan? Children's arteries are flexible, thin-walled tubes, free of plaque, with lining tissue that measures a millimeter or so in thickness. The years take their toll, particularly if there are genetic reasons or noxious factors like smoking, high cholesterol, high fat meals, high blood sugar, etc. The lining tissue of arteries is delicate and easily injured, and reflect that injury by thickening. Within the thickened lining, fibrous structural material, calcium (like that in bone), and inflammatory cells also accumulate. The gruel-like material that results is called "atherosclerotic plaque".

Many plaque components can't be easily measured in a living human being, such as inflammatory cells or structural tissue. Remember, we're trying to accurately measure plaque without scalpels, catheters, or other invasive methods. Calcium collects within plaque and can be measured accurately and easily. Interestingly, microscopic studies of atherosclerotic plaque have shown that calcium consistently occupies 20% of the total volume of plaque. This proportion remains true for women, men, young, old, and whether or not you¡¦ve already had a heart attack. In other words, calcium provides an indirect though accurate means to measure total plaque volume. If we were to measure 2 cubic millimeters of plaque, for instance, then you have a total of 2 x 5= 10 cubic millimeters of total plaque. The rule of coronary calcium scoring is easy: the higher your score, the more plaque lines your coronary arteries. The coronary calcium score predicts your risk of heart attack Study after study has shown that, the higher your heart scan score, the greater your risk for heart attack. This is because the more plaque that lines your coronary arteries, the more opportunity there is for plaque rupture and heart attack, even if the plaque is only a 20% blockage. This remains true even if you feel great, your cholesterol is low, you're a long-distance jogger, etc. A heart scan score is, by far, the most powerful predictor of your heart¡¦s future. Of course, this does not mean that you can't change your future. In fact, that's exactly what knowledge of your heart scan should prompt you to do. If you find out that you have a high score, the next step is to start or improve your prevention program. Your goal: to lower your risk for heart attack to as close to zero as possible.

Unlike cholesterol, your score represents the sum total of factors contributing to coronary plaque growth up until the day of your scan. Let's say, for instance, that you spent the years between age 30 and 40 overweight, sedentary, smoking, and indulging in unhealthy eating habits. A cholesterol panel during those 12 years may have reflected your unhealthy lifestyle. But, at age 41, you lose 60 lbs., begin an exercise program, follow a healthy diet, and stop smoking. A cholesterol panel after you¡¦ve achieved all this would be much improved„oeven if you had extensive plaque in your coronary arteries. A heart scan at this point, however, would likely reveal a high score, as it reflects the sum total of influences in your life. Your score would not falsely reassure you, as a cholesterol panel would have. Is it "hard" or "soft" plaque?

A common misconception is that, since calcium is a hard substance, the coronary calcium score obtained on your heart scan only provides a measure of hard plaque. Not true. Calcium measured is a reflection of total plaque, both soft and hard.

Actually, the great majority of people with coronary plaque have a mixture of hard and soft plaque, and this can even change day to day, week to week, since plaque is a dynamic, living tissue. In other words, a soft plaque today can develop hard elements tomorrow, and a hard plaque today can evolve to develop soft parts any time in future. Most plaques, in truth, are both. That's why your heart scan score is such a great measure of hidden, total, plaque.

The most important health test you can get

Heart scans are simple and inexpensive. Yet they reveal, with great accuracy, the number one killer of men and women in the U.S. It is a highly accurate, simple, 30-second test that makes heart disease detection a snap. They key is to get your "score" before danger sets in. The heart scan is, first and foremost, a tool for prevention. If you know your score, you know that a vigorous effort at prevention is in order.

Selected References Rationale for coronary calcium measurement as a measure of total coronary plaque

Detrano R, Tang W, Kang X, Mahaisavariya P, McCrae M, Garner D, et al. Accurate coronary calcium phosphate mass measurements from electron beam computed tomograms. Am J Cardiac Imag 1995; 9:167-173.

Janowitz WR, Agatston AS, Kaplan G, Viamonte J Jr. Differences in prevalence and extent of coronary artery calcium detected by ultrafast computed tomography in asymptomatic men and women. Am J Cardiol 1993; 72:247-254.

Rumberger JA, Simons DB, Fitzpatrick LA, Sheedy PF, Schwartz RS. Coronary artery calcium areas by electron beam computed tomography and coronary atherosclerotic plaque area: a histopathologic correlative study. Circulation 1995;92:2157-2162.

Simons DB, Schwartz RS, Edwards WD, Sheedy PF, Breen JF. Noninvasive definition of anatomic coronary artery disease by ultrafast computed tomographic scanning: a quantitative pathologic comparison study. J Am Coll Cardiol 1992;20:1118-1126.

by Dr. William Davis


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