How to detect patients at risk and diagnose coronary artery disease in a simple and safe way?
Risk in asymptomatic patients using CT Coronary Calcium Scoring
CT Coronary Calcium scoring (CAC) has been developed as a way of detecting and quantifying coronary artery atherosclerotic calcification by non-contrast ECG-gated CT. It is superior to any of the traditional risk parameters (Framingham risk score (FRS), IMT and CRP) in predicting cardiovascular events.
A score of 0 has a high prognostic value with very low risk of death (<1% at 10 years).
CAC is most useful in those aged 45-75 years at intermediate risk
using the FRS (10-20% cardiovascular risk over 10 years), to help reclassify these patients into higher or lower risk categories. CAC may be helpful in lower risk groups (6-10% 10-year risk) when there is also diabetes (age 40-60 years) or a family history of premature cardiovascular disease. Several studies have shown about half of intermediate risk patients can be reclassified: 39% into a lower risk group and 16% into a higher group.
Patients with high risk based on FRS should be treated with optimal medical management and there is little benefit in CAC apart from those patients who are averse to statins, where CAC may aid further decision regarding management.
CAC > 400 implies high risk of >20% mortality in 10 years – these patients need optimal risk factor management. Functional imaging (such stress echocardiography or Myocardial Perfusion Imaging) is appropriate since the risk of coronary obstruction is higher.
The radiation dose of CAC is low (at or below 1 mSv), lower than annual environmental radiation in WA which is around 1.5-2 mSv. There is currently no Medicare rebate available in Australia for CAC.
CT Coronary Angiography
A calcium score of 0 does not exclude non-calcified plaque leading to stenosis. Symptomatic patients are best assessed by CT Coronary Angiography (CTCA), functional imaging or invasive angiography.
The strength of CTCA lies in its high negative predictive value of 97-99%, which makes it ideal to rule out significant coronary artery disease in low to intermediate risk patients with stable chest pain and angina symptoms.
Luminal stenosis of >50% needs further investigation: either functional imaging (in cases with 50-69% stenosis) or invasive angiography (usually in cases with >70% stenosis).
CTCA allows plaque characterisation of calcified, non-calcified and mixed plaque. CTCA is emerging as a more advanced risk stratification tool in this respect.
There has been rapid development of CT scanner technology, mainly to reduce radiation dose and avoid motion artefact. Now, the heart can be imaged in a single heartbeat with certain modern cardiac CT scanners. These advances allow us to image patients with arrhythmia, tachycardia and dyspnoea, with much higher image quality. Radiation doses are much lower (<1 mSv is achievable with the most advanced technology).
The specialist referred CTCA can attract a rebate for low to intermediate risk patients with stable symptoms who would have been considered otherwise for invasive angiography.
- CT coronary calcium scoring in asymptomatic intermediate risk patients allows more accurate risk stratification and aids in further management.
- CT coronary angiography is non-invasive, low-dose imaging that excludes coronary artery disease in symptomatic low to intermediate risk patients.
References available on request.
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Author competing interests: nil relevant disclosures.
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