It’s all due to limitations of coronary angiography – the gold standard in testing for blocked heart arteries
YOU have chest pain and have undergone testing for your heart arteries and were told that you have a significantly blocked artery (at least 50 per cent diameter narrowing of the artery). You decided to do another test to confirm the finding and were then told by another doctor that you do not have significant narrowing of the heart artery. You are confused now and wonder whether one of the tests is inaccurate. You will be surprised when told that both tests were correctly interpreted. How then can both tests be correctly interpreted – and there are two totally different results?
Limitations of the gold standard
To unravel this mystery, one needs to understand the limitations of the tests being used to diagnose the presence of narrowed heart arteries. Coronary angiography (CAG) is currently the gold standard for the assessment of narrowing of the heart arteries and it involves the insertion of a plastic tubing through the wrist or groin artery under local anaesthesia into the opening of the heart arteries and injecting iodine based contrast agents into the heart arteries and taking X-ray images of the heart arteries.
Despite it being used as the gold standard, CAG provides only a two dimensional view of the lumen or channel of the heart arteries and does not allow any visualisation of the wall of the artery, which can sometimes lead to a failure to recognise the presence of significant plaque accumulation in a long stretch of the artery as the diffuse disease may make the residual lumen appear evenly narrowed or “small” in appearance. With the availability of alternative and highly accurate options for the assessment of the heart arteries, the limitations and inaccuracies of CAG are increasingly being recognised.
Physical narrowing of heart arteries
The main purpose of assessment to detect the presence of significant narrowing of the heart artery is to prevent the occurrence of heart attack and sudden death resulting from severe heart artery disease. The basic premise is that if there is narrowing of the heart arteries, heart attacks are more likely to occur.
Studies on the mechanism of a heart attack have shown that in the large majority, the heart attack occurs when the internal lining of the narrowed segment of the heart artery is torn as a result of shear forces when blood flows at a higher speed and pressure through a significantly narrowed lumen. It is akin to putting your thumb at the opening of a running tap and the water will gush out at a higher pressure and a faster speed. The tearing of the lining, also termed “plaque rupture” triggers a series of biological events that culminate in the formation of a blood clot to seal the tear in the lining. If the blood clot is large and occludes the narrowed residual opening of the narrowed heart artery segment, there will be cessation of blood supply to the affected heart muscle segment and a heart attack occurs, resulting in death of heart muscle tissue.
Narrowed segments of the heart artery which are vulnerable to “plaque rupture” are termed “vulnerable plaques” and often contain abundant soft material such as cholesterol deposits and dead cells. The soft material makes it easier for the internal lining of the artery to be torn just as our skin is more likely to tear when fluid accumulates under the skin during the formation of a blister. It is generally believed that the narrower the artery segments, the more likely “plaque rupture” will occur and by convention, a narrowing of at least 50 per cent of the heart artery diameter is considered as significant.
Functional narrowing of heart arteries
Another approach used to determine the significance of a narrowed arterial segment is to determine whether the physical narrowing has functional significance, that is whether there is presence of “myocardial ischaemia” or insufficient blood supply to the heart muscle as a result of narrowed heart arteries.
Non-invasive tests such as treadmill testing, stress echocardiogram (heart ultrasound) tests and heart testing using radioactive isotopes used to assess “myocardial ischaemia” (MIBI scans, Thallium scans, SPECT, rubidium PET CT) have been plagued by low levels of accuracy in real world situations and can be highly operator dependent with varying degrees of accuracy.
In addition, tests using radioactive isotopes mean that the entire body is exposed to harmful or ionising radiation which can sometimes be very high. Although rubidium PET CT scans have been touted to have low radiation doses, the Food and Drug Administration (FDA) in USA had reported instances where those who have had rubidium PET CT scans were found to have extremely high doses of radiation which were persistent for months after the scan.
Unlike X-ray exposure which is a one-off exposure to radiation for a specific region of the body, the injection of radioactive isotopes into the body means that the entire body is exposed to radiation and the radiation persists as long as the isotope remains in the body. The reality is that, in the real world, we never really know how high the radiation doses are for isotope scans as no centre routinely measures the radiation dose your body is exposed to when you have completed the test and leave the centre.