For those suspected to have coronary artery disease, other than going for an invasive coronary angiogram, alternative diagnostic methods are available. By Michael Lim
MS A had gone for her annual health screening, and, following her treadmill stress test, she was told that the result was abnormal. An invasive coronary angiogram was recommended.
This diagnostic test involves inserting small plastic tubings through the leg or wrist artery, threading these into the opening of the heart arteries and taking X-ray images of the cardiac arteries, which would first have been stained with iodine contrast agents.
Is an invasive coronary angiogram recommended?
Pitfalls of the treadmill test
It has to be said that the treadmill test, though a commonly used screening test by doctors, has many limitations, especially for women.
During the test, doctors look for changes in the electrocardiogram (ECG), which is a recording of the electrical pattern of the heart, to screen for heart disease.
Women are more likely to have baseline ECG changes even before they start walking on the treadmill, making interpretation of ECG changes with exercise difficult; they are also more likely to have more abnormal ECG changes (ST segment depression) with exercise testing even in the absence of significant blockage of the heart arteries.
The situation is made even more perplexing by trial data, which shows no relationship between ST-segment depression with exercise stress testing and death – heart-related or otherwise.
The timing of the treadmill appears to affect the result as well. In pre-menopausal women with no significant heart artery disease, the presence of ST-segment depression during exercise appears to vary with the menstrual cycle. Exercise ST depression is more frequently observed in the luteal phase of the menstrual cycle – the run-up to a woman’s period starting from ovulation – than in the late follicular phase, which is the period leading up to ovulation.
Post-menopausal women receiving oral estrogen therapy are more likely to have exercise-induced ST-segment depression with normal coronary angiograms than their sisters who are not on estrogen replacement.
Even for men, the treadmill test has an accuracy of only 70 per cent; false positives are possible.
However, if the treadmill test is abnormal, it will be usual for the physician to advise the patient to undergo further tests to look for significant coronary artery disease.
So is the coronary angiogram the best option after an abnormal treadmill test?
A study published in the New England Journal of Medicine in 2010, done on almost 400,000 patients undergoing invasive coronary angiogram, concluded that coronary angiography had a “low diagnostic yield”.
Only just more than a third of the patients in the study were found to have significant blockage of the heart arteries.
The authors of the study called for better methods to cut back on unnecessary angiograms for the remaining two-thirds of patients in the study whose coronary angiograms found no significant heart artery disease, and yet exposed them to the dangers of the procedure.
Invasive coronary angiograms carry a 1-in-1,000 risk of major stroke, heart attack or death. They also carry between 5 and 22 per cent risk of silent strokes, based on data from multiple prospective studies using magnetic resonance imaging (MRI) of the brain before and after the angiogram.
Increasingly, alternative diagnostic methods have become available for those suspected to have coronary artery disease. These include nuclear myocardial perfusion scans, rubidium cardiac PET scans, computed tomography of the heart arteries, stress echocardiography, MRI myocardial perfusion scans and MRI scans of the heart arteries.
The first three require exposure to ionising radiation; the last three do not.
But is exposure to low-dose ionising radiation from cardiac imaging and therapeutic procedures safe?
There have been concerns of cancers developing with the increasing use of ionising radiation during diagnostic imaging of heart disease and during the use of catheter-based techniques to open up blocked heart arteries.
It would seem the concern has some basis. In a study published in the Canadian Medical Association Journal last year, a cohort of 82,861 patients who had a heart attack between April 1996 and March 2006 but no history of cancer were assessed for the risk of cancer following exposure to ionising radiation from cardiac procedures. Of the lot, 63,831 were exposed to ionising radiation for cardiac procedures, and 12,020 (19 per cent) had new cancers diagnosed during the follow-up period. The risk of cancer also increased with increased exposure to radiation from cardiac procedures.
The unit of measurement of the biological effect of radiation is the sievert (Sv). Every 10 millisieverts of ionising radiation raised the risk of age- and sex-adjusted cancer by three per cent over a mean follow-up period of five years.
About 27 per cent of the cancers were thoracic cancers, including breast cancers; 42 per cent were abdominal and pelvic cancers.
The authors thus concluded that exposure to ionising radiation had a direct link with increased cancer risk and that forms of MRI imaging which did not involve ionising radiation were not used extensively enough.
As for Ms A, she consulted friends and other doctors, after which she decided against an invasive coronary angiogram; subsequent evaluation found no objective evidence of significant coronary artery disease. She had thus sidestepped the real risks of an invasive coronary angiogram and the potential risk of cancer from exposure to X-ray radiation.
The decision was also lighter on her pocket.
For men, the treadmill test has an accuracy of only 70 per cent; false positives are possible. However, if the treadmill test is abnormal, it will be usual for the physician to advise the patient to undergo further tests to look for significant coronary artery disease.