Anatomy versus Function in Coronary Disease

The investigation of chest pain involves more than determining coronary anatomy. Discussions about whether non-invasive imaging might replace coronary angiography for the display of coronary anatomy must not detract from the fact that clinical assessment and dynamic functional imaging by echocardiography or nuclear imaging are at least equally important. Often, the need for imaging coronary anatomy of any form is obviated by dynamic functional imaging. However, the debate will continue as to whether screening for coronary artery disease may best be met by ‘non-invasive’ anatomical information by computed tomography (CT) or by a functional dynamic stress test, preferably with some form of imaging. It is important to remember that we are proposing replacing invasive imaging by traditional coronary angiography with ‘non-invasive’ anatomical imaging (CT), which means that the term ‘invasive’ is being replaced by ‘radioactive’.

First Steps

The patient must be assessed clinically in terms of the quality of the pain, coronary risk factors and the presence of an alternative cause for chest pain. Non-cardiac pain in a patient with a low risk of coronary disease and another cause for pain does not need further cardiac investigation – either invasive or non-invasive. Treadmill exercise testing remains useful for many categories of patients and can help stratify risk. It is cost-effective and can aid diagnosis in patients at intermediate risk of coronary disease, as long as they can exercise adequately.

Dynamic Imaging

Stress testing with echocardiography is underused in the UK. In the US approximately 3 million stress echoes were performed in 2007,¹ which would equate to about 300,000 per year in the UK. However, the number of centres offering a full service remains limited.

Stress echocardiography can help patients with known or suspected coronary artery disease in many ways. First, this technique can aid in diagnosis of coronary artery disease, with a sensitivity of 80–85% and a specificity of 85–90%.^2,3 Accuracy is relatively lower for single-vessel disease, but higher for multivessel disease in the presence of normal resting wall motion.^4,5 Second, the technique can be used for risk stratification. A normal stress echocardiogram gives an annual risk of cardiac death of 0.4–0.9% based on a total of 9,000 patients;^6,7 in such patients, coronary angiography or other imaging using ionising radiation can safely be avoided.

An abnormal stress echocardiogram is associated with a far higher risk of events, which is directly related to the extent of the wall motion abnormalites. In a study of 3,156 patients followed for nine years,⁸ ischaemia and the extent of abnormal wall motion were independent predictors of cardiac death. In 7,333 patients⁷ an abnormal pharmacological stress echocardiogram provided independent and incremental value over and above clinical data. In a sub-group of 4,037 patients who underwent coronary angiography without intervention, knowledge of the anatomy did not add significant predictive power to the model. Finally, the technique can be used to detect viable myocardium. The sensitivity of low-dose dobutamine echocardiography for predicting recovery of regional function following revascularisation is 84%, with a specificity of 81%;^9,10 revascularising viable myocardium on dobutamine echocardiography improves survival.^11,12 Myocardial necrosis or a lack of dobutamine-induced contractile response is an accurate marker of poor prognosis.¹³

The Future of Stress Echocardiography

Myocardial perfusion imaging using ultrasound contrast agents now allows simultaneous assessment of both function and perfusion,^14,15 with an improvement in diagnostic accuracy for the detection of coronary disease.¹⁶ Contrast ultrasound agents are safe – certainly a lot safer than any other contrast agents used for cardiac imaging.¹⁷ Importantly, myocardial contrast echocardiography allows for quantitation of the ischaemic myocardium,^18,19 and measurement of myocardial velocities using Doppler tissue imaging allows for an objective and reproducible assessment of wall motion abnormalities.²⁰ 3D echocardiography is also expected to further improve the detection of abnormalities of phase or thickening. Recent recommendations of stress echocardiography by the European Association of Echocardiography have been published.²¹

Conclusions

Clinical evidence involving more than 30,000 patients in multiple controlled trials has established stress echocardiography as a robust, cost-effective and truly non-invasive method that does not involve ionising radiation for the diagnosis and risk-stratification of coronary artery disease patients. It makes scientific and economic sense that prior to proceeding to anatomical imaging, which involves significant amounts of radiation, non-invasive and non-radioactive functional assessment of myocardial ischaemia should remain the priority of diagnostic pathways.