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62 Multislice CT in Coronary Heart Disease KCE Reports 82 8.2 DIAGNOSTIC ACCURACY Most published clinical trials, are dealing with the diagnostic accuracy of 64-SCT as an imaging tool (for coronary arteries), referring to CCA as the gold standard and considering a coronary stenosis ≥50% on CCA as clinically relevant. In all published 64- SCT studies, in populations at intermediate or high pre-test probability of obstructive CAD (mostly >35% and up to 100%), test sensitivity is good and ranges between 95 and 100%, indicating a very good negative predictive value. At present, the main value of MSCT seems to be its use to rule out obstructiuve CAD. Test specificity on the other hand is less good. In Abdulla’s SR, it was 91% (87.5-94), 66 and in our meta-analysis it was 82.3% (78.5-85.7) in recent studies and 85.5% (80.4-89.7) in dual-source 64-SCT studies. Positive and negative likelihood ratios in recent studies were 5.0 (3.5-7.4) and 0.03 (0.02-0.06) respectively. In the large Meijboom gender trial (n=402), test performance was compared in women vs. men: whereas sensitivity was excellent in both sexes (93-100), specificity was acceptable in men (90%; 81-95) but poor in women (75%; 95% CI: 62-85). 69 All studies enrolled patients with a high probability of CAD that were selected based on several additional parameters, including regular and controlled heart rate, preserved renal function, breath-hold capacity, and hemodynamic status. Virtually all patients were already scheduled for an invasive CCA before entry in the study. Whether the performance of MSCT can be reproduced in less selected patients at lower prevalence of CAD remains to be assessed, questioning the external validity of the results. 158 Moreover, selection bias may have played a role in published trials, by exclusion of patients in whom calcified coronary arteries were expected (elderly, diabetes, noncardiac atheromatous disease, renal failure) or in whom calcified coronary arteries had been documented by prior CCA. Finally, results of studies may also be biased by the fact that investigators had a better experience compared with the real-life centres which usually examine larger and more broad-spectrum populations and may be less experienced. 66 8.3 DIAGNOSTIC THINKING This level of diagnostic efficacy is concerned with the assessment of the effect of a test result on diagnostic reasoning and disease categorization, or in other words its role in clinical decision making. Few empirical evidence is available on this subject. In a patient with a negative MSCT, i.e. in which no >50% stenosis is detected, a physician will decide that the chest pain symptoms for which the test was performed, most probably were not provoked by CAD. The question remains to what extent this test result changes the initial diagnosis put forward by the attending physician (i.e. pretest likelihood). In a patient population such as that described by Goldstein, 67 the pretest probability of ACS was very low because of a profound pre-selection of patients. In these cases, further testing becomes irrelevant because diagnosis is almost certain, and additional testing will predominantly lead to false positives. In the study by Rubinshtein et al., MSCT reportedly was useful in the diagnostic work-up of patients with chest pain and an inconclusive stress test. 80 However, in a substantial proportion of patients (20 out of 71, 28%) that tested negative with MSCT, treating physicians later on still proceeded to CCA, “because of clinical reasons” (sic), despite previous trials indicating a high negative predictive value of MSCT. In this trial, MSCT clearly did not alter the diagnostic path followed by the physician. The “2006 appropriateness review for MSCT”, issued under the auspices of the American College of Cardiology Foundation, confirms that limited data are available supporting the use of MSCT coronary angiography within patient care algorithms. 108 8.4 THERAPEUTIC IMPACT MSCT of the coronary arteries up to now does nothing more than providing an anatomical image of the coronary tree. In several trials it has been shown that MSCT reliably can be used to rule out the presence of CAD in subsets of patients. In contrast to other imaging techniques such as MPS and DSE, MSCT does however not provide
KCE Reports 82 Multislice CT in Coronary Heart Disease 63 insight in the functional importance of the coronary lesion thus documented. Some studies indicated that less than half of the significant lesions on MSCT have hemodynamic consequences. 111, 159 The lack of functional information being provided by both CCA and MSCT is a major limitation. Ideally, any patient with (atypical) chest pain in whom diagnostic imaging is deemed appropriate (i.e. a patient in whom one expects - if CAD is present - that revascularisation will improve symptoms or prognosis), should undergo a functional test before proceeding to an anatomic imaging test. 160 If the functional examination leads to the decision that revascularisation is appropriate, one should go for invasive CCA, enabling immediate therapeutic intervention, and obviating the need for MSCT. If the functional exam leads to a decision that revascularisation is not appropriate, no further diagnostic imaging steps are needed. If noninvasive functional tests are impossible to be performed or inconclusive, MSCT may be efficacious, but these populations have not been studied so far. Yet, invasive functional evaluation (functional flow reserve) will often be needed. 28 In summary, because MSCT has limited ability to define myocardium jeopardized by ischemia, its potential for predicting benefit from revascularisation is limited. 111 8.5 PATIENT OUTCOMES The real issue of diagnosing CAD is not to correctly identify coronary artery stenoses but to help in predicting and improving patients’ outcome. In this respect, coronary artery imaging may be misleading: significant though prognostic benign lesions may be identified and lead to inappropriate interventions, because they do not affect blood supply to the myocardium or they can be left untreated (i.e. not revascularised). In patients with stable angina pectoris, PCI as the first therapeutic option does not reduce the risk of death, myocardial infarction or other major cardiovascular events when added to optimal medical therapy. 2 This means that myocardial revascularisation (and hence both invasive or noninvasive angiography) can be safely deferred in these patients and can be restricted to those in whom medical therapy does not lead to symptom control. In the latter event, cardiologists will immediately proceed to CCA, enabling them to intervene during the same procedure, obviating the need for a preliminary MSCT. Coronary artery imaging can also be misleading because low grade stenoses may be prone to plaque rupture and may lead to serious clinical events, yet they may be 2, 152 regarded as innocent. 8.6 COST-EFFECTIVENESS Economic evaluations of interventions with unproven clinical effectiveness are not very useful. An intervention should prove clinically effective first before it can be considered cost-effective. Several researchers, however, have attempted to build an economic model to assess the cost-effectiveness of MSCT relative to a suitable comparator. All of these models suffer from lack of evidence about the relevance of MSCT for improving patient outcomes. Therefore, they are necessarily limited to an assessment of the costper-case detected. And even these results can be questioned, as sensitivity and specificity of MSCT has not yet been tested in real-world populations where the prevalence of clinically significant CAD is low. Nevertheless, a cost-outcome description by Goldstein et al. is frequently cited to demonstrate the technology’s cost-effectiveness. While this study did include clinical parameters about therapeutic impact, these were not taken into account in the costeffectiveness analysis. A simple exercise, extending the cost and outcome (QALY) calculation to include treatments following the diagnosis of CAD showed that from hospitalisation up to 6 months of follow-up, a diagnostic strategy with MSCT was more costly and led to a higher loss in QALYs than a standard diagnostic strategy without MSCT. The results need to be interpreted with caution, as this RCT was actually underpowered to draw full economic conclusions. Further studies on the diagnostic efficacy of MSCT are needed. Until then, results of cost-effectiveness analyses remain inconclusive.
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