Repeated exposure to high doses of diagnostic radiation for heart disease can prove to be harmful. By Michael Lim
ADVANCES in imaging technology, diagnostic techniques and non-invasive therapeutic techniques have provided physicians with significantly better tools to better diagnose patient diseases and to provide the appropriate treatment. Many of these diagnostic and therapeutic techniques in the area of heart disease involve the use of ionising radiation (harmful radiation) which includes X-rays and radiation from radioactive isotopes injected into the body. While the use of these diagnostic and therapeutic options with ionising radiation in the management of heart diseases have both been indispensable to physicians and beneficial to patients, resulting in decreasing death rates and prolonging survival, repeated exposure to significant doses of radiation may have adverse consequences.
Radiation risks from cardiac procedures
There is no absolute consensus as to what the limit of radiation is that can be deemed harmful to the body although there is complete consensus that high radiation exposure increases the risk of cancer.
Much of the data on the impact of radiation on humans comes from the Life Span study (published in Radiation Research journal in 2007) which examined the survivors of the atomic explosions in Japan and who had a mean radiation dose exposure of 29 mSv; and the 15 Country Study on more than 400,000 radiation workers (published in British Medical Journal in 2005 and Radiation Research journal in 2007) whose average radiation dose exposure was more than 19 mSv. Both these studies demonstrated an incremental risk of cancer of about 2 per cent.
There is also epidemiological data from patients who have suffered a heart attack. In a publication in the American Journal of Cardiology in 2012, Canadian researchers reported on the radiation exposure for patients suffering their first heart attack during a 10-year period.
The paper showed a doubling of radiation doses over the years for patients who were admitted to hospital with an acute heart attack. This was due to an increasing trend to attempt to immediately open blocked heart arteries using balloon and stent techniques upon admission to the hospital – resulting in an average radiation exposure dose of about 12 millisieverts (mSv) within the first month of an acute heart attack.
The increase in radiation exposure is balanced by the life saving benefits of opening a blocked artery as quickly as possible. In the subsequent two years of the acute heart attack, the average cumulative radiation in this study group from heart scans and procedures was about 19 mSv.
In an earlier 2011 publication by the same team of Canadian researchers, from 1996 to 2006, of the close to 64,000 patients in the study who had their first heart attack, slightly more than 12,000 (about 19 per cent) were diagnosed to have new cancers. The authors concluded that increased exposure to radiation from cardiac procedures was associated with an increased risk of subsequent cancer. The authors estimated that for every 10 mSv of harmful radiation, there was a 3 per cent increase in the risk of cancer over a five-year period.
Benefits of cardiac diagnostic procedures
However, the risks of radiation exposure have to be compared to the risk of dying from a heart condition. In a 2012 publication in the European Heart Journal which examined radiation from cardiac imaging, the authors provided estimates of cancer risk from cardiac procedures as compared to risks from underlying heart disease.
For example, males in their sixth decade with no symptoms but who have more than one risk factor for heart disease, the 10-year risk of dying from heart disease is 1,000 per 10,000. For these patients, the lifetime incremental cancer risk resulting from radiation exposure from a low-dose cardiac imaging test, such as a low radiation dose coronary CT angiogram using the appropriate techniques, is about 2 per 10000.
Simply put, in this example, if 10,000 patients with this profile were to undergo a low radiation dose heart scan, 1,000 heart deaths may potentially be prevented at the risk of two additional persons developing cancer. While the calculated risk from radiation is a lifetime risk, the heart risk given in this example is only based on a 10-year risk and hence, will also increase substantially when a longer period is considered.
For patients with chest symptoms, the likelihood of finding significant heart disease is much higher than in those with no symptoms. Hence, the benefits of doing heart scans when compared to radiation risks are even more substantial.
Benefits and risks of therapeutic cardiac procedures
Cardiac procedures that are used to treat heart conditions may sometimes carry significant risks. In a recent 2013 editorial in Circulation Arrhythmia and Electrophysiology journal, the author addressed the issue of treatment for atrial fibrillation (AF), the commonest abnormal heart rhythm in the elderly.
As AF is associated with an increased risk of stroke, one of the techniques currently being used is radiofrequency ablation which involves the insertion of catheters into the heart and burning a ring around the heart tissue to isolate foci which trigger AF. However, in recent years, concerns have been raised about the presence of asymptomatic strokes following radiofrequency ablation, with a reported incidence of asymptomatic stroke as high as 39 per cent. In a worldwide survey by Cappato published in the Journal of the American College of Cardiology in 2009, the major complication rate was 4.5 per cent. For every 10,000 procedures, there were 15 deaths, 23 major strokes and 71 transient strokes. In addition, the success rate is about 2 out of 3 and there is a high rate of recurrence of AF following radiofrequency ablation.
Hence, given the risks of the procedure and the high recurrence rate, medication should be considered first before considering radiofrequency ablation as an option. In contrast, there is very little doubt that opening the blocked heart artery during an acute heart attack with balloon and stent techniques has substantial benefits for the patient as compared to the risks.
When you are required to do a heart test or procedure, do remember a few Golden rules: Firstly, when a diagnostic heart test is required, a non-invasive option which has no harmful radiation is preferred.
Second, if blockage of the heart artery is suspected, always opt for a non-invasive test first before considering an invasive test.
Third, if invasive coronary angiography is proposed to confirm the presence of significant blockage of the heart artery, a CT scan of the heart arteries may be considered as an alternative as it is non-invasive. For young adults and women, a magnetic resonance imaging scan of the heart arteries, if available, will be a good alternative for women and young adults as the procedure carries no radiation risk and does not require any injection.
Fourthly, before undergoing any non-invasive scan of the heart that has harmful radiation, it may be good to understand the amount of radiation exposure from the test.
Lastly, if you are undergoing a heart procedure, you may want to understand the risk of both major complications and the risk of “silent” complications such as “silent” strokes. While the “silent” strokes may not be obvious, they can contribute to memory and cognitive impairment in later years.