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Making an informed choice for the best heart test

The Siemens Somatom Drive with a dual source CT scanner. With its cutting-edge technology, the Somatom Drive is able to use lower dose of contrast yet produce results with higher resolution, better image quality and higher accuracy.

A PATIENT of mine related how her husband, who had passed his treadmill testing with flying colours during his recent annual health check, died within minutes before her eyes after complaining of shortness of breath and chest tightness. You may wonder how then is it possible to die of a heart attack after having passed a treadmill test?

Treadmill testing belongs to a group of functional heart tests which involves stressing the heart function and trying to ascertain whether there is sufficient blood flow to the heart muscles. As the heart arteries cannot be visualised by functional tests such as treadmill testing, it is not surprising that in some patients with significant blockage of their heart arteries, the treadmill test may be "negative" and is not able to detect the underlying disease.

Functional versus Anatomic Assessment

If you have chest pain and consult your doctor, you may be given a choice of tests to assess the likelihood of narrowing of your heart arteries . The question on everyone's mind is what test should be the first test you should do if you have chest pain?

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Should it be: functional heart tests such as treadmill testing; stress echocardiography (assessing heart function using an ultrasound during stress testing); nuclear myocardial perfusion scan (injecting radioisotopes into the blood and assessing the distribution of radioactive isotopes in the heart muscle); cardiac rubidium PET scan (injecting rubidium radioisotope into the blood and assessing the distribution of rubidium isotopes in the heart muscle)? Or should it be anatomic imaging of the heart arteries using coronary computed tomography angiogram (CCTA)?

Generally, functional stress tests involve the use of exercise or drugs to stimulate the heart and attempt to look for evidence of imbalance of blood flow in the heart muscles caused by blocked arteries which are not present at rest but are present when the heart is stressed. They do not heart arteries visualisation. But CCTA involves the injection of an iodine-based contrast dye into the arm vein and the use of high- speed X-Rays to get 3D images of the heart arteries within seconds.

First line of investigation

Over the last decade , there has been an increasing use of CCTA to diagnose coronary artery disease. In the national healthcare system in the United Kingdom, CCTA is recommended as the first-line investigation for patients with chest pain. The American Heart Association, American College of Cardiology and other international bodies guidelines on Cardiac CT considers it appropriate to use CCTA as an investigation for most chest pains. For the asymptomatic, the UK guidelines recommended the use non-contrast CT scan of the heart arteries to look for calcium deposits (calcium score ).

Generally, there is substantial data to show that increased presence of calcium deposits in the walls of the heart arteries is correlated with increased degeneration and narrowing of the heart arteries. The amount of calcium present is represented as the Agaston score or calcium score. The CG95 recommends that for a calcium score of 1 to 400 (which means mild to moderate amount of calcium deposits), a CCTA should be performed.

However, the British Society of Cardiovascular Imaging takes the view that CT calcium scoring is to be used only as a backup if a patient is unsuitable for a full CCTA. The reason is that studies comparing CCTA with CT calcium scores show that the utility of using a calcium score of 0 as an exclusion of underlying heart artery disease would end up with misdiagnosis or further testing in a significant proportion of patients, even in low-risk groups. Hence , from a cost perspective , CCTA is more cost-effective compared to CT calcium scoring.

Faster diagnosis, less probes, lower cost, better outcomes

A publication in Open Heart, British Medical Journals in 2015 showed that the implementation of the UK guidelines in the University College London Hospital led to a reduction in the average cost of the diagnostic journey per patient and fewer investigations per patient in order to confirm a diagnosis.

In recent years, data from many large studies have shown data consistent with the UK guidelines. All this shows that that CCTA is a safe and effective alternative to stress testing in those with chest pain.

Heart scans versus prediction models

All these trials examined a population with a relatively low heart artery disease prevalence. The probability of heart disease for each individual was severely overestimated by standard prediction models based on formulas.

Hence although the heart artery disease prevalence in one of the trials, called the CRESCENT trial, was 8 per cent, the predicted probability by the widely used prediction model by Diamond and Forrester was 45 per cent. Likewise, the disease prevalence was 8.8 per cent in another trial, PROMISE, but the average predicted likelihood was 53 per cent.

Even if newer risk prediction tools were used as in the 2013 European Society of Cardiology prediction model and was applied to CRESCENT, it only lowered the probability from 45 to 37 per cent; still much higher than the actual 8 per cent prevalence of heart artery disease. Hence, the current disease prediction models significantly overestimated disease burden.

Better test for women

There is substantial evidence to show that women have higher rates of false-positive exercise stress tests and nuclear stress tests as compared with men, and hence there was a need to look for a better alternative.

As CCTA allows direct visualisation of heart arteries, this would reduce the unnecessary stress and additional testing arising from false-positive tests. In ROMICAT-II, women with chest pain who presented to the emergency department and underwent CCTA benefitted from a greater reduction in length of stay as compared to men.

In a sub-analysis of women in the CRESCENT trial, it was shown that compared to stress testing, women who underwent CCTA had higher chest pain resolution within one year (40 per cent CCTA versus 22 per cent stress ), underwent less additional diagnostic testing and had lower downstream diagnostic costs.

What this meant was significant improvements in processes of care and diagnostic efficiency for women who underwent CCTA instead of conventional stress testing. In women, sub-analysis of the PROMISE trial showed that women derived greater prognostic information from an abnormal CCTA than from an abnormal stress test. In comparison , the prognostic information from both testing strategies were similar for men.

Making the best decision

The key practical take-home messages are:

  • If you want faster diagnosis and less downstream testing, current data favour the use of CCTA as the first line choice for chest pain. In the UK, this has shown faster diagnoses, less investigations, and lower costs for diagnosis.
  • If you want the best certainty about the likelihood of underlying heart artery disease from non-invasive testing, CCTA is the only non-invasive test which has been able to demonstrate a consistent 100 per cent negative predictive value for heart artery testing in experienced centres; no other non-invasive test can tell you with 100 per cent certainty that you do not have heart artery disease.
  • Compared to functional testing, CCTA is more likely to result in resolution of chest pain symptoms, provide better prognostic information and lead to better outcomes. Hence, when you see your family doctor for chest pain, you will be able to have better discussion on what the most appropriate choice for you is.
  • Not all CT scans are the same and the experience of the centre is also very important. Recently, I saw a patient who had a CCTA done in a hospital and was advised to undergo invasive coronary angiogram with a view to open up the heart arteries with balloon angioplasty and stenting.

She did not want to do so and was referred by her family doctor to see me for further evaluation. I examined images of the CCTA and was of the view that there were artefacts that affected the accuracy of the scan. After discussion of the various options, it was decided to use another newest generation CT scan whose radiation dose was extremely low to assess the heart arteries. To the delight of the patient and her family, the scan showed that there was no significant blockage of the heart arteries and no further heart evaluation was required.

Therefore, should you ever require a CCTA, discuss with your family doctor to send you to centres which have new generation CT scanners and doctors who have substantial experience in CCTA.

  • This series is produced on alternate Saturdays in collaboration with Singapore Heart, Stroke & Cancer Centre.