Their report is published (Tuesday 14 February) in Europe's leading cardiology journal European Heart Journal, which is the journal of the European Society of Cardiology.
The authors, from the Department of Epidemiology & Biostatistics at Erasmus Medical Centre in Rotterdam, say their findings suggest that the role of ECGs in existing cardiovascular prevention programmes should be evaluated.
The results come from an analysis of a large proportion of the men and women involved in the Rotterdam Study, a prospective population study investigating chronic disabling diseases. A total of 5,148 participants with no evidence of prevalent myocardial infarction (MI) were enrolled from 1990-93. They underwent a baseline ECG and examination. Data from clinically recognised MIs (i.e. heart attacks that were formally diagnosed) over the years that followed were analysed. The 4,187 of the total who had at least one repeat ECG during two rounds of follow up assessment between 1993-96 and 1997-99, were analysed for clinically unrecognised MI (heart attacks not diagnosed at the time of occurrence).
Senior author Dr Jacqueline Witteman, Associate Professor of Epidemiology, said: "Over our median follow up time of more than 6 years, we found an incidence rate of nine heart attacks per 1,000 person years. There were around 12 heart attacks per 1,000 person years in men (8.4 recognised and 4.2 unrecognised) and around seven per 1,000 person years in women (3.1 recognised and 3.6 unrecognised). Additionally, in men as well as in women, there was one sudden death per 1,000 person years.
"Overall, 43% of the total heart attacks had been clinically unrecognised – a third of the male heart attacks and more than a half of the female heart attacks. This is a significant proportion of all the MIs."
Dr Witteman said that in each of the age bands between 55 and 80, men had a higher incidence of recognised MIs than women and a similar incidence of unrecognised MIs. This provided the evidence that heart attacks are less often recognised in women, irrespective of characteristics that have previously been associated with MI.
According to co-author Dr Eric Boersma, Associate Professor of Clinical Cardiovascular Epidemiology, heart attacks may go unrecognised because of atypical symptoms, and the explanation for the worse figures for unrecognised heart attacks in women was not straightforward.
"There are likely to have been multiple factors. Men and women experience chest pain in different ways. MIs can occur without typical symptoms in women (also in people with diabetes and the elderly). They may sense shoulder pain instead of chest pain, they may think they have severe flu that is taken a long time to recover from, and those with an inferior-wall infarction may complain of stomach pain. So women may hold back from reporting symptoms and doctors may also be in doubt whether or not to consider heart disease as a source of the complaints. It is also a problem that women and their doctors have traditionally worried more about death from breast and gynaecological cancer, than from heart disease."
Dr Boersma said that although the study was conducted in the Netherlands the results were likely to be equally applicable to any other developed country.
He said the findings of the Rotterdam Study suggest that the role of ECGs in existing cardiovascular prevention programmes should be evaluated.
"Patients with a history of MI are at increased risk of repeat cardiovascular complications, irrespective of their awareness. Therefore, people with unrecognised infarctions may also benefit from effective preventive treatment. By that I mean preventive drugs, including aspirin, beta-blockers and statins, and specific lifestyle advice. In most developed countries cardiovascular prevention programmes are installed, which aim to identify high-risk individuals on the basis of classical risk factors, including smoking and obesity, and co- conditions, such as diabetes mellitus. Our findings indicate that these programmes might be enriched with an ECG."
ECG-systems are readily available and ECG measurements are easily obtained; their interpretation might be facilitated by computer software. Even so, the researchers emphasise that formal cost-benefit studies are needed before definite conclusions on the role of ECGs in prevention programmes can be drawn.
 Incidence of recognized and unrecognized myocardial infarction in men and women aged 55 and older: the Rotterdam Study. doi:10.1093/eurheartj/ehi707.
[2,3] The incidence rate was determined using the cumulative amount of person years as determined in five-year strata from age 55 to 80. Incidence rates were then presented as the number of MIs per 1,000 person years. The initial group of 5,148 available for recognised MI contributed 28,121 person years to the study. The 4,187 eligible for unrecognised MI analysis contributed 23,505 person years.
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