doi: 10.1161/CIRCULATIONAHA.107.670059
(Circulation. 2008;117:1333-1339.)
© 2008 American Heart Association, Inc.
Controversies in Cardiovascular Medicine |
Should all high-risk patients be screened with computed tomography angiography?
All High-Risk Patients Should Not Be Screened With Computed Tomographic Angiography
From the Departments of Medicine and Radiology, University of Virginia Health System, Charlottesville.
Correspondence to Christopher M. Kramer, MD, University of Virginia Health System, Departments of Medicine and Radiology, Lee St, Box 800170, Charlottesville, VA 22908. E-mail ckramer{at}virginia.edu
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Introduction |
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On the basis of the most recent American Heart Association/American College of Cardiology guidelines for assessment of cardiac risk,1 high-risk patients are defined as those with a 10-year coronary heart disease risk of >20% based on Framingham risk criteria or the presence of diabetes mellitus. The major independent risk factors for coronary heart disease comprising the Framingham Risk Score are cigarette smoking, hypertension, elevated total cholesterol and low-density lipoprotein cholesterol, low serum high-density lipoprotein cholesterol, diabetes mellitus, advanced age, and male gender.1 Derived from National Health and Nutrition Examination Survey statistics from 1999 to 2000,2 the prevalence of risk factors in patients aged 20 to 74 years is 17% for total cholesterol
240 mg/dL, 14.9% for hypertension, 26.4% for smoking, and 8% for diabetes (including undiagnosed). Data from the 2003 Behavioral Risk Factor Surveillance System survey of 103 191 adults aged >18 years3 show that >37% of the population surveyed had 2 risk factors for coronary heart disease and thus are considered to be at high risk. These figures together suggest that the number of high-risk patients who are potential candidates for screening programs is quite high.
Response by Gottlieb p 1339
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The Clinical Role of Computed Tomographic Coronary Calcium Scoring |
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Multiple studies over the last 2 decades have confirmed the prognostic utility of coronary artery calcium (CAC) measurements primarily with electron-beam computed tomography (EBCT) and more recently with multidetector CT (MDCT) technology (Figures 1 and 2
). The 2 techniques are fairly equivalent as long as the latter is performed with at least a 4-detector scanner.4,5 The reproducibility of MDCT, however, may not be as good as EBCT at lower calcium scores.4
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CAC is an indicator of atherosclerotic plaque burden, and very high levels confer an increased risk of future cardiac events. The absence of CAC confers a very low but still measurable cardiac risk,6 whereas its presence confers an increased relative risk of hard events. However, absolute event rates are relatively low (1% to 2% per year) even in the highest-risk group, and thus the routine clinical use of CAC scoring has yet to be defined clearly. There is no correlation between CAC and physiological or anatomic significance of a stenosis.7 In addition, there can be significant heterogeneity between the extent of plaque calcification even within an individual subject, independent of age, gender, or number of plaques. Ethnic heterogeneity must also be taken into account when CAC results are interpreted. The Multi-Ethnic Study of Atherosclerosis (MESA) demonstrated that CAC is most prevalent in whites, with a lower risk of calcification (between 23% and 31% lower) for those of black, Hispanic, or Chinese descent.8
Recent studies suggest that the utility of CAC may be highest in patients who are at intermediate risk according to the Framingham risk data; CAC levels can place such patients into higher or lower risk categories6 (Figure 3). For high-risk patients who would be candidates for screening (Framingham Risk Score >20%), a CAC score >300 raised the risk of coronary death or nonfatal myocardial infarction to nearly 20% over 7 years. The St Francis Heart Study, a prospective, population-based EBCT study of 4903 asymptomatic individuals between ages 50 and 70 years,9 showed that after >4 years of follow-up, CAC predicted coronary artery disease events independently of either standard risk factors or CRP and was a better predictor than the Framingham Risk Score. The area under the receiver operating characteristic curve was 0.79±0.03 for CAC versus 0.69±0.03 for Framingham Risk Score (P=0.0006).
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On the basis of these data, recently released appropriateness criteria for CT and magnetic resonance imaging10 stated that using CAC to screen asymptomatic patients was inappropriate for low-risk patients and uncertain for intermediate- and high-risk patients. More data regarding the incremental prognostic value of CAC over risk factor assessment and the benefits of primary prevention in those with high CAC scores may ultimately help to categorize higher-risk patients more appropriately. The latest recommendations from the American College of Cardiology Foundation/American Heart Association 2007 Clinical Expert Consensus Document11 state that "asymptomatic individuals with an intermediate Framingham Risk Score may be reasonable candidates for coronary heart disease testing using CAC as a potential means of modifying risk prediction and altering therapy." The Consensus Document also states that high-risk patients should be treated aggressively on the basis of National Cholesterol Education Panel III guidelines and that they do not need further risk stratification with CAC.
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Accuracy of CT Coronary Angiography |
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CT to perform noninvasive coronary angiography is in rapid evolution. It has long been an excellent technique for diagnosing anomalous coronary arterial anatomy in adults (Figure 4), and its accuracy for detecting stenoses in patients with coronary artery disease has progressed rapidly. Initially, EBCT was the technique of choice because of excellent temporal resolution, but as the number of detectors for MDCT angiography (MDCTA) has increased rapidly in the new millennium, it has all but replaced EBCT. In the first multicenter study of 16-detector scanners, 29% of segments were unable to be evaluated.12 The negative predictive value (99%) compared with x-ray angiography was outstanding, but the positive predictive value was poor because of both the high number of segments that could not be evaluated and false-positives, which were primarily due to the presence of coronary calcification.
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Scanners with 64 detectors (Figure 5) and now dual-source 64-detector scanners13,14 (Figures 6 and 7) are becoming more widely available; thus, temporal and spatial resolutions are steadily improving with a concomitant reduction in unreadable segments and false-positive studies. In an initial published single-center 64-detector study of 70 patients with exclusions for atrial fibrillation but not for calcified arteries, heart rate, or obesity, only 12% of segments were excluded for inadequate image quality.15 Per-segment values were impressive for sensitivity (95%), specificity (86%), and positive (66%) and negative (98%) predictive values. On a per-patient basis, the values were also high for sensitivity (92%), specificity (91%), and positive (80%) and negative (97%) predictive values. Subsequent moderate-size studies (52 to 67 patients) have demonstrated sensitivities on a per-segment basis ranging from 85% to 99%, specificities from 93% to 99%, negative predictive values from 95% to 99%, and positive predictive values from 76% to 97%.16–20 In these studies, the greatest number of segments excluded from analysis was 6%, and some did not exclude any segments. Clearly, 64-detector CT has developed into an excellent test for excluding significant coronary artery disease. Heavily calcified coronary arteries remain the principal cause of false-positive studies.21
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Studies are under way to determine which patient populations are best served by CT angiography (CTA). To date, it has shown to be useful in the following groups: low- to intermediate-risk patients seen in the emergency department with acute chest pain22; patients with left bundle-branch block23; and patients before cardiac valve surgery.24 A positive test in symptomatic patients is predictive of cardiovascular events, primarily revascularization, whereas a negative test in the same patient population is an excellent marker of a good prognosis over 1 year of follow-up.25 The only study in asymptomatic patients completed to date was in patients before cardiac valve surgery.24
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CTA for Identification of Soft Plaque |
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A promising potential use of MDCTA is the differentiation of atherosclerotic plaques based on their density, as measured by Hounsfield units. An early study examined 34 plaques in 15 patients with intravascular ultrasound used as the gold standard and differentiated plaques into categories of soft, intermediate, and calcified.26 Subsequently, larger studies demonstrated significant overlap of the CT attenuation pattern of intravascular ultrasound–defined hypoechoic plaque (lipid-rich soft plaques) and hyperechoic plaques (fibrous plaques).27,28 Thus, MDCT may be best at differentiating calcified from noncalcified plaques, and interobserver variability for this application is excellent.29 More research is required to determine the utility of differentiating lipid-rich from fibrous plaque with the use of MDCT.
In a study of 161 intermediate-risk patients, MDCT with the use of a 64-detector scanner identified noncalcified plaques in 48 (30%) of the patients.30 Noncalcified plaque was the sole manifestation of coronary artery disease in 10 (6%) of the patients. Generally, these soft plaques were nonobstructive. Long-term follow-up of these patients is not yet available to show whether identification of soft plaques is prognostically important. Some authors have suggested that the identification of noncalcified plaque may be most important in patient populations for whom calcium scoring is less accurate, eg, younger patients and those with a history of smoking.31 However, patients without coronary calcification (some of whom have soft plaque) have an extremely low event rate. In the St Francis Heart Study, only 8 of 1504 patients (0.5%) without calcium had a coronary event over 4.3 years, leading to an event rate of 0.1% per year.9 Another large study of >10 000 subjects quantified the risk of those without calcium as 0.4 events per 1000 person-years of observation.32 Thus, further risk stratification in this population does not appear necessary. In a study of younger patients (mean age, 43 years), the event rate in those without calcium was only 0.05% per year,33 further limiting the potential additive value of MDCT in patients without coronary calcification.
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Potential Risks of MDCTA |
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Efforts are being made to limit the radiation dose from MDCTA because its widespread use as a screening examination may expose the population studied to a significant burden of excess radiation. One method for limiting radiation exposure is tube current modulation, which limits the full radiation exposure to certain critical portions of the R-R interval. For comparison purposes, the dose range for invasive x-ray angiography is on the order of 2 to 5 mSv.34,35 For 16-detector MDCT, published radiation doses range from a low of 3 mSv with tube current modulation36 to a high of 15 mSv.35 Early publications with 64-detector scanners showed that radiation dose without tube current modulation was as high as 13 mSV in men and 18 mSV in women.15 More recent estimates in studies using tube current modulation range from 5.4 to 9.4 mSv.37
The risk of radiation exposure in the general population is calculated as 5x10–2 Sv–1 for lifetime cancer mortality.38 According to a recent review, typical doses for MDCTA yield lifetime risks of 0.07% for inducing a fatal cancer in the general (ie, age- and gender-averaged) population.39 This risk has been further quantified on an age basis in a phantom study that estimated a lifetime attributable risk of cancer of 1 in 1911 for a 60-year-old man and 1 in 715 for a 60-year-old woman.40 In a symptomatic patient, this stands in contrast to a 0.1% risk of death, myocardial infarction, or stroke from x-ray angiography.39 In an older population, the lifetime risk is substantially less. However, in an asymptomatic high-risk patient, often a younger individual, this potential risk is substantially higher than for other types of screening examinations.
The iodinated contrast dye used in MDCTA poses an additional risk. Nonionic contrast media cause severe allergic reactions in 0.2% to 0.7% of patients.41 Nephrotoxicity is yet another potential risk, one that can be lowered by the use of nonionic low-osmolar contrast media and by avoiding a dose >100 mL of contrast.42,43 Another risk that is difficult to quantify is the risk of further x-ray angiography and interventional procedures that may not be necessary or indicated, which are triggered by a screening examination.
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Does Screening High-Risk Patients Change Behavior? |
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An important question in regard to any screening test is as follows: Does the test change either behavior or outcome in an individual patient? This question has been addressed in several studies of CAC, and the findings are somewhat at odds. In 1 study of 450 asymptomatic active-duty US Army personnel (aged 39 to 45 years), 15% of whom had CAC, the finding of CAC did not motivate them to modify known cardiac risk factors over the course of 1 year.44 In contrast, a study of patients already on statin therapy demonstrated that knowledge of higher baseline CAC scores was associated with improved statin compliance on multivariable analysis.45 It is unclear whether knowledge of the extent of underlying coronary artery disease based on MDCTA will be more of a motivational factor than the presence of CAC alone. An incomplete understanding of the difference between CAC scoring and MDCTA in the eyes of the lay public could mitigate any improvement in patient behavior based on the latter.
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Does Screening High-Risk Patients Change Outcome? |
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Whether screening examinations can be used to monitor effects of therapies to reduce atherosclerosis burden and improve outcomes remains an open question. A nonrandomized study suggested that lower low-density lipoprotein levels in patients on statins was associated with lowering of CAC scores.46 Other studies suggest little relationship between CAC progression and changes in lipid status.47 Recent work using intravascular ultrasound suggests that noncalcified plaques respond better to therapies aimed at plaque regression than do calcified plaques.48 This suggests the theoretical possibility that identifying noncalcified plaques with the use of MDCTA might lead to a greater ability to identify patients with the type of atherosclerosis that may respond to therapy. However, no studies exist to date that link improved clinical outcomes with response to atherosclerosis therapies applied as a result of CAC or MDCTA.
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Is Screening High-Risk Patients Cost-Effective? |
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The most recent American College of Cardiology Foundation/American Heart Association Consensus Document on CAC scoring11 states that data were lacking to apply to cost-effectiveness models. The incremental cost-effectiveness ratio is dependent on the annualized risk of the patient being screened and the effectiveness of primary prevention strategies, both of which use multiple assumptions and are thus problematic. One study suggested a range from $500 000 for a patient with a risk of 0.6 events per year (intermediate risk) to $30 742 if the event rate was 2% per year (high risk).49 The incremental cost-effectiveness ratio would certainly be higher for MDCTA than for CAC scoring given the higher cost of the procedure as well as the additive potential risks of increased radiation burden of angiography and the use of iodinated contrast.
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Why CTA Should Not Be Used for Screening |
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Much of this discussion leads to the conclusion that, in its present form, MDCTA should not be used for screening asymptomatic high-risk patients. To summarize:
- CAC scoring alone, which is associated with lower cost, lower radiation exposure, and no contrast dye, has not been endorsed in the latest guidelines for high-risk patients11 because they should have their risk factors treated aggressively regardless of outcome of further screening.
- Patients with calcium scores of 0 have an extraordinarily low risk of events, and thus the theoretical value of identifying noncalcified plaque appears quite limited.
- The risks of MDCTA may outweigh the potential benefits in asymptomatic patients.
- Even for CAC scoring, it is not entirely clear that knowledge of the result changes behavior in terms of risk factor modification.
- No study to date has demonstrated an association between change in coronary plaque burden by MDCTA and improved outcome.
- Given the prevalence of individuals being assessed as high risk on the basis of risk factors alone and the high cost of MDCTA relative to other potential screening techniques, the cost-effectiveness of screening with this technique is likely to be poor.
Indeed, a recent president of the Society for Cardiovascular Computed Tomography has stated that "screening applications of coronary CTA in asymptomatic individuals currently are not backed by clinical data."50 An article by another prominent individual in the field has stated that "the use of contrast CT for risk stratification of the asymptomatic patient is problematic."31 In summary, in 2008, MDCTA should not be used for screening asymptomatic high-risk individuals.
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Acknowledgments |
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Sources of Funding
Dr Kramer is supported by the National Institutes of Health, National Heart, Lung, and Blood Institute, grant RO1 HL075792.
Disclosures
None.
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References |
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Footnotes |
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The opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.
This article is Part II of a 2-part article. Part I appears on page 1318.
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