Abstract
Chest pain is one of the commonest reasons for visiting hospital emergency departments, accounting for 7–10% of all presentations in the USA, with only 23% diagnosed with coronary artery disease (CAD). 1 In the UK, chest pain can account for 25% of all acute medical admissions. 2 In order to reduce the burden on emergency departments, most hospitals run rapid access chest pain (RACP) clinics to efficiently investigate and treat patients with chest pain. We explore the role of cardiac computed tomography angiography (CCTA) in patients with suspected stable angina.
Historically, the exercise tolerance test (ETT) has been the mainstay of investigation, but it has modest sensitivity and specificity, particularly in lower risk populations. The UK 2010 National Institute for Health and Care Excellence (NICE) guideline on chest pain of recent onset acknowledged the limitations of ETT and recommended it no longer be used. 3 Instead, the guideline proposed that investigation selection be determined by pre-test probability (PTP) risk score, calculated with a tool adapted from the Duke Clinical Score, which takes into account clinical history and cardiovascular risk factors. 4 Patients with high PTP risk score should be offered invasive coronary angiography as the first-line investigation. Patients with intermediate PTP risk score should be offered functional imaging tests (stress echocardiography, nuclear scintigraphy or stress magnetic resonance imaging). Patients with low PTP risk score should be offered CCTA.
NICE anticipated that the PTP risk score may overestimate the prevalence of CAD in primary care populations, as the original tool was based on tertiary care populations. We recently examined the prevalence of CAD in our RACP clinic in southeast London. Obstructive CAD was only confirmed in 23% of patients with a high PTP risk score and 9% in those with a moderate PTP risk score. 5 This may reflect a lower incidence of disease in our population, but more likely confirms that the PTP risk score overestimates the true risk. This has now been conclusively demonstrated in the international registry, CONFIRM, in 14,048 patients who underwent CCTA. 6 The incidence of significant CAD in this registry was consistently lower than predicted by PTP in patients with both typical and atypical symptoms, in all participating centres, and across all sex and age groups.
Accepting that most patients presenting with chest pain to RACP clinics do not have CAD, the best way to investigate them is with a low-cost test with excellent negative predictive value (NPV). CCTA has been compared to invasive coronary angiography and was found to have very good sensitivity of 95% and excellent NPV of 99%. 7 CCTA has also been compared to ETT, using invasive angiography as the gold standard. Again, CCTA was found to have an excellent NPV of 100% compared with 64% for ETT. 8 CCTA was also found to be more cost effective than nuclear scintigraphy in a recent clinical trial in the emergency room in the USA. 9 Moreover, CCTA has now been compared to functional imaging tests in a very large multicentre randomised controlled trial (PROMISE), which has completed recruitment of 10,000 subjects and is due to report later this year. 10
To reduce the number of patients found to have normal coronary arteries at diagnostic invasive coronary angiography, which was 39% in a recent large American registry,
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the NICE PTP risk score needs to be refined. The alternative is to reset the thresholds for CCTA, functional imaging tests and invasive coronary angiography within the current PTP risk score model, by expanding the role of CCTA to patients with low and intermediate PTP risk score. This approach is supported by the most recent European
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and American
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guidelines, which both recommend CCTA for low and intermediate PTP (Figure 1). CCTA would rule out CAD in the majority of patients. Those found to have mild atheromatous plaque on CCTA could benefit from secondary prevention while patients found to have severe stenoses can be referred for invasive angiography with a view to revascularisation in the same session. Patients found to have moderate coronary stenoses on CCTA can be investigated for ischemia with functional imaging tests. In the near future, such functional assessment may be possible by fractional flow reserve (FFR) calculated from the cardiac CT data, using an algorithm that measures computational fluid dynamics, which correlated well with the invasive FFR measured invasively
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or by CT stress perfusion.
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Comparison of NICE 2010, ESC 2013 and ACCF/AHA 2012 guidelines. Risk thresholds for choice of investigation in patients with suspected stable angina. NICE: National Institute for Health and Care Excellence; ESC: European Society of Cardiology; ECG: electrocardiography; CT: computed tomography.
In terms of safety, in 2005, CCTA radiation doses were as high as 20 mSv. Since then, dose reduction techniques including prospective gating, 16 iterative reconstruction technology and lowering X-ray tube voltage in non-obese patients 17 have resulted in a dramatic dose reduction to under 2 mSv. This corresponds to approximately 100 chest X-rays but is less than the estimated annual background radiation exposure in the UK (2.7 mSv) and much less than nuclear scintigraphy (15.6 mSv), 18 the most widely used functional imaging test.
Strict application of the current NICE guideline would result in substantial (42–93%) cost increase5,19 because of patients with high PTP risk scores being referred directly for invasive coronary angiography. CCTA is a very fast test, both in terms of acquisition of the images and reporting; hence, its low unit cost per scan (£173) compared with stress echo (£236), nuclear scintigraphy (£293) and invasive angiography (£1052) based on the NICE costing report. 20 Resetting the thresholds to direct patients with intermediate as well as mild PTP risk scores to CCTA would cut costs, but more health economic modelling is needed. Nevertheless, from an economic perspective, expanding the use of CCTA as low-cost, accurate test with a high NPV is attractive in an era of contracting healthcare budgets.