Coronary computed tomographic angiography and exercise electrocardiography: a great match or unequal partners?

Coronary computed tomographic angiography (CCTA) is emerging as a powerful non-invasive tool in the diagnosis of coronary artery disease (CAD). However, there is concern that CCTA is being used indiscriminately without sufficient examination of its added value vis-à-vis more established forms of non-invasive testing. With an evergrowing armament of non-invasive tests for the diagnosis and evaluation of CAD, cardiologists must consider the following questions in weighing up the implementation of a new diagnostic modality in the routine clinical setting. Is testing necessary for the diagnosis of CAD in a particular patient group? Will testing guide management decisions and identify patients who may benefit from coronary revascularization? Will the benefits of testing outweigh its potential risks? The study by Mollet et al. provides important insights into the relationship between the findings of CCTA and conventional treadmill exercise electrocardiography (ECG), but also raises important issues regarding the role of both tests in the diagnosis of CAD. The authors examined, in 62 patients with typical angina pectoris, the diagnostic value of exercise ECG and 16-slice multidetector CCTA, alone and in combination, to predict 50% diameter stenoses detected on invasive, selective coronary angiography (SCA) in any coronary artery branch with a calibre of 2 mm. Exercise ECG data from 53 patients and CCTA data from 61 patients were included in the analyses. The sensitivity of exercise ECG was 78%, specificity was 67%, and positive and negative predictive values were 89 and 47%, respectively. The sensitivity of CCTA was 100%, specificity was 87%, and positive and negative predictive values were 96 and 100%, respectively. In an analysis based on Bayesian principles, an abnormal CCTA increased the post-test probability of significant CAD after a positive exercise ECG from 89 to 99% and after a negative exercise ECG from 58 to 91%. A normal CCTA reduced the post-test probability of significant CAD after a negative or a positive exercise ECG to 0%. Clearly, 16-slice multidetector CCTA has very high sensitivity and negative predictive value, and is an accurate non-invasive test for excluding obstructive CAD even in a population with high pre-test likelihood. These findings are in agreement with those of other studies that examined the value of CCTA in lower-likelihood populations. Likewise, the specificity and positive predictive value of CCTA in the current study are similar to those reported in other patient populations. The diagnostic accuracy of exercise ECG in this highlikelihood cohort is in keeping with other studies reported in the literature and further emphasizes the rigorous methodology and competent execution of the current study. Other notable strengths of the study include the careful classification of patients into risk categories based on clinical predictors and the Bayesian analysis of the test results. The use of quantitative coronary angiography to measure the degree of coronary luminal narrowing reduces uncertainties related to the interobserver variability of visually estimating stenosis severity. Based on the discordant diagnostic accuracy of exercise ECG and CCTA and their Bayesian analysis of post-test probabilities, the authors conclude that exercise ECG is of limited diagnostic value for the detection of significant CAD and that a diagnostic work-up that combines exercise ECG and CCTA markedly improved the post-test probability of the presence or absence of significant CAD. These conclusions invite a discussion of the questions raised in the first paragraph of this editorial.