What’s the best test to detect blockage of the heart arteries? The answer can be complicated. By Michael Lim
THEY both came late with serious heart disease. Both had had their hearts checked by the injection of a dye into their heart arteries (a coronary angiography) on the same day in January but only one survived his heart disease.
Mr A, a gentleman in his early 80s, had chest pain for six months and in the three days preceding his hospital admission, he had constant chest pain. He came to the emergency department in the early hours of the morning when his chest pain became unbearable. He had had a heart attack and despite the opening of one of his blocked arteries with a stent to improve blood flow to the heart, he eventually succumbed to the heart attack as his heart muscle had already been severely damaged.
Mr B, who had risk factors for heart disease but did not have chest pain, came with friends for an assessment of his heart. He was visibly shocked when he was told that of his three major heart arteries, two were completely blocked and one was 90 per cent blocked. He underwent heart bypass surgery successfully.
For those with chest pain or have multiple risk factors for heart disease such as high blood pressure, high cholesterol, smoking, diabetes mellitus or family history of heart disease, they should not wait till the heart condition is potentially life-threatening. The common question that many patients ask is, what is the right test for them? The answer is not as simple as it seems.
For decades, the “gold” standard for assessment of the heart arteries has been coronary angiography. It is used as the confirmatory test for blockage of heart arteries. Despite being the gold standard, it is usually not the initial test of choice because it is invasive and has potential complications. At a scientific meeting years ago, I was surprised that studies using an MRI scan of the brain before and after coronary angiography showed a high incidence of silent strokes (15 per cent to 22 per cent) post-procedure. In highly experienced centres, the risk of symptomatic stroke, heart attack or death is about one to two in 1,000. Then, there are other potential complications such as damage to kidneys, temporary blindness, tearing of the aorta, and bleeding.
A recent large US study of almost 400,000 patients undergoing coronary angiography showed that only about slightly more than one-third had significant blockage of the heart arteries. Almost four out of every 10 patients were found to have normal arteries. Almost two-thirds of those undergoing coronary angiography were found to have no significant heart disease. What this meant was that better screening tests were required before subjecting a patient to a test with potential complications.
Surprisingly, in this study, patients with an abnormal non-invasive screening test result were only slightly more likely to have significant heart disease than those who did not undergo any testing (41 per cent vs 35 per cent) – a mere 6 per cent difference. This low accuracy of the conventional non-invasive screening test in this large study shows that in the real world, the accuracy can be very different from published studies.
Compare this with data from a US study that gave an accuracy of 70 per cent or more for treadmill testing, nuclear perfusion scans (injecting radioisotope to assess the flow distribution in the heart muscle), stress echocardiography (ultrasound assessment of left heart chamber function) and Calcium Score (measuring heart artery calcification with X-rays). Hence, published data does not reflect the real world experience where accuracy varies from centre to centre.
It is now known that many of those who develop a heart attack may not have a significant blockage. Last month, a study on the natural progression of heart disease showed that almost 12 out of every 100 patients who had minor narrowing of the heart arteries developed a heart attack or complication after three years. In another study, almost one in 10 with minor “soft” cholesterol rich plaques died during the average follow up period of 7.5 years.
Advances in medical science have provided doctors with more choices in identifying those who have a “soft” plaque and are at a higher risk of developing a heart attack. Newer tests that have been increasingly used are the CT scan and MRI of the heart arteries. Unlike the tests mentioned above which are unable to image the heart arteries, these tests are non-invasive, allow three-dimensional visualisation of the heart arteries and detection of cholesterol plaques. The main concern with the CT scan is the radiation dose but with the newer generation of scanners, a scan of the heart arteries can be done within a few seconds with a radiation dose that is lower than that of invasive coronary angiography. However, the accuracy and radiation dose will vary from centre to centre.
There is no perfect diagnostic test and every test has its strengths and weaknesses. The right test for one person may not be so for another. Ask your own doctor which is the right test for you as he will know your medical condition best.