CT: predicting cardiac deaths
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Health care professionals must balance potential risks against potential benefits
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THE AMERICAN Heart Association (AHA) recently published a scientific statement on the utility and limitations of cardiac computed tomography (CT) systems for imaging of coronary heart disease (Circulation, October 17, 2006) after reviewing 245 scientific papers on the topic. This is very timely for many reasons.
Those who belittle physicians who practise alternate medicine for their anecdotal approach have themselves started following such an approach! Physicians publicise CT scan at the web site of Oprah Winfrey.
They use stories of Bill Clinton, Bush and Whoopy Goldberg getting cardiac care to whip up interest in the technology, which has known limitations.
Coronary block
Coronary arteries may develop blockages due to the formation of plaques, which consist of fat and other substances including calcium.
CT scanning provides information on the location and extent of calcified plaque. The current AHA statement addresses the clinical utility of detection of coronary artery calcified plaque.
The AHA's earlier statement published in 2000, concluded that the Electron Beam Computed Tomography testing for coronary artery calcium was not superior to alternative non invasive imaging techniques in diagnosing coronary artery disease; one could not recommend it for this purpose (heartwire, October, 5, 2006).
Risk prediction
In the latest statement, the AHA conceded that Cardiac CT for calcium may improve risk prediction, particularly for individuals at intermediate risk as decided by conventional criteria; the physician treating them can alter the decision on prevention strategies based on test results.
The procedure can play a significant role in predicting cardiac deaths and making treatment decisions for the millions of people in the middle range of coronary risk (EurekAlert, October 10, 2006). The study concluded that the majority of published studies reported that coronary artery calcium plaque is both independent of and incremental with respect to traditional risk factors in the prediction of cardiac events.
Physicians should consider patients with intermediate risks with an elevated coronary calcium score to belong to high-risk group.
The CT information may help the physician to choose prevention strategies.
Physicians should assess asymptomatic persons for their cardiovascular risk with such tools as the Framingham Risk Score (FRS). (FRS helps to determine 10-year risk for developing coronary heart disease using risk factors such as age, total cholesterol, blood pressure, medication for BP, smoking and the like.)
Individuals belonging to low-risk group or high-risk group do not benefit from coronary calcium assessment.
The AHA strongly recommends that physicians must use low radiation dose technique to assess coronary artery calcium plaque burden. It did not recommend CT coronary angiography of asymptomatic patients for assessing occult coronary artery disease.
The Association examined the role of cardiac CT in measuring clinically meaningful changes in calcified plaque over time and concluded that serial imaging to assess progression of calcification is not indicated at this time.
Risks Vs benefits
The AHA observed thus on radiation risks: "Although all individuals are exposed to ionising radiation from natural sources on a daily basis, health care professionals involved in medical imaging must understand the potential risks of a test and balance them against the potential benefits."
"A CT examination with an effective dose of 10 mSv may be associated with an increase in the possibility of fatal cancer of approximately 1 chance in 2000.
This increase in the possibility of a fatal cancer may be compared with the natural incidence of fatal cancer in the US population of about 1 chance in 5". "... .this small increase in radiation-associated cancer risk for an individual can become a public health concern, if large numbers of the population undergo increased numbers of CT procedures for screening purposes," AHA cautioned.
Extensive survey
The radiation doses from various CT procedures included in the statement based on extensive literature survey varied from 0.6 mSv to 18 mSv.
Regrettably, many physicians tend to ignore the radiation protection aspects of this unique diagnostic tool.
We may compare the admirably cautious and conservative approach of the American Heart Association with that of those who advertise and recommend CT screening of everyone including symptom-less persons. An earlier article argued that the latter approach is unjustified (The Hindu, January 5, 2006).
K.S. PARTHASARATHY
Former Secretary, AERB
(ksparth@yahoo.co.uk)
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