HOUSTON, April 20 -- A large percentage of influenza deaths are the result of disease-triggered acute myocardial infarctions, researchers here reported on the basis of autopsy-proven data.
In a week during a flu epidemic, the chances of dying of a heart attack increased by 30% and of dying of chronic ischemic heart disease by 10%, reported Mohammad Madjid, M.D., of the University of Texas here, and colleagues, online in the European Heart Journal.
Influenza causes up to an estimated 92,000 U.S. deaths a year as a result of myocardial infarction alone, they reported on the basis of their clinical observations and analyses of published trials.
Many of these deaths could be prevented if every high-risk patient had an annual flu shot. Dr. Madjid and colleagues wrote.
Working with Russian colleagues at the St. Petersburg Medical Academy, the researchers studied the impact of influenza on coronary deaths (acute myocardial infarction and chronic ischemic heart disease) in autopsies conducted from 1993 to 2000 in St. Petersburg.
St. Petersburg was chosen because unlike previous studies that relied on death certificates, a high rate of autopsy-proven fatal cases was available (autopsy rate was almost 70%), the researchers said.
Doctors often neglect to list flu on death certificates if their patient died of a heart attack, and, conversely, heart attack symptoms can be missed in patients with flu and pneumonia, they said.
Furthermore at the time of the study, only a small minority (<3%) of patients in St. Petersburg were receiving flu shots and statin drugs, providing a snapshot of the natural history of these coronary deaths.
In an eight-year review of 34,892 autopsies, there were 11,892 deaths of an acute MI and 23,000 of chronic ischemic heart disease. The median age was 75 for women and 65 for men.
In every year, there was a peak of deaths from MI and chronic ischemic heart disease, and these deaths coincided with an influenza epidemic and winter peaks of acute respiratory disease.
A similar pattern was seen for each subgroup of men and women, for those 50 or older, and for those 70 or older, the researchers said.
When comparing the average influenza epidemic weeks to average off-season weeks, the odds for deaths from MI and from chronic ischemic heart disease increased by 1.30 (95% confidence interval, 1.08-1.56) and 1.10 (CI: 0.97-1.26), respectively, the investigators reported.
Cross-correlation analysis found that coronary mortality echoed acute respiratory disease activity during peak seasonal times and across multiple years, the researchers reported.
Among study limitations, the researchers cited the lack of access to person-specific data on medications and risk factors, or other autopsy diagnoses. Meteorological data to assess impact on influenza and cardiac mortality were lacking and were out of the scope of this study. However, they said, other groups have shown that influenza remains a major determinant of winter's excess mortality.
Influenza and other upper respiratory infections may trigger cardiovascular events by causing an acute and severe inflammatory state in the body, which can lead to destabilization and rupture of atherosclerotic plaques and cause heart attacks, the researchers said.
Recent published reports show that deaths attributed to influenza are increasing in the U.S., and the disease is now the fourth major cause of mortality in the U.S, Dr. Madjid said. Accordingly, he said, it has been noted that during influenza epidemics (except for the 1918 Spanish flu epidemic) roughly twice as many persons die of cardiac causes as from influenza.
Recognition of influenza as a trigger for acute coronary events calls for more intensive efforts to increase the vaccination rate in those at risk for coronary heart disease. This may be especially important in influenza pandemics when a high mortality rate may be expected among the elderly and those with heart disease, the researchers wrote.
Vigorous use of heart-protective medications such as statins during pandemics may also prevent may deaths, they added.
The benefits of influenza vaccination for cardiac patients have been recognized by the American Heart Association and the American College of Cardiology.