People with heart disease were about three times more likely to have ischemia (decreased oxygen supply to heart muscle) during exercise testing after periods of high level air pollution than when they were tested after periods of negligible air pollution, says Juha Pekkanen, M.D., a senior researcher at the National Public Health Institute, Unit of Environmental Epidemiology in Kuopio, Finland.
Although many researchers have reported an association between pollution and increased heart attacks and deaths from heart disease, Pekkanen says this new study is the first to look at myocardial strain and to show ischemia associated with particulate air pollution. Pekkanen says the association between increased risk for ST segment depression, which is measured by electrocardiogram (ECG) and is associated with ischemia, occurred after periods of increased fine and ultra-fine particles in the air.
The researchers findings are based on data collected from subjects living in Helsinki, Finland during the winter of 1998-1999. Volunteers with a history of coronary artery disease agreed to participate in biweekly clinical visits for six months. Each visit consisted of a six-minute, ECG-monitored, submaximal exercise session with a bicycle ergometer. Forty-five heart patients (21 women), average age 68, were in the study. The goal of each exercise session was to achieve a heart rate of 90-100 beats per minute.
Researchers analyzed the data from 342 of the 417 exercise tests conducted. They recorded 72 instances of exercise-induced ST segment depression among the 45 subjects. Twenty-three patients experienced exercise-associated symptoms when air pollution was high two days before a clinic visit. The remaining 22 subjects either had no episodes of ischemia or had episodes at every visit regardless of air quality so were unable to provide information about the relationship between air pollution and myocardial ischemia.
Fine particles - those smaller than 2.5 micrometers (referred to as PM2.5) - represent the type of pollution mainly associated with emissions from factory smokestacks. Ultra-fine particles have a diameter less than 0.1 micrometers. They are the type of pollutants that spew from exhaust pipes. Both types of particulate air pollution increased the risk of ST segment depression about three-fold, says Pekkanen. Ultra fine particles increased risk for ST segment depression 3.29 times, while PM2.5 was associated with 2.84 times the risk compared to no pollution.
The new study "highlights myocardial ischemia as a significant potential mechanism responsible for adverse cardiac outcomes associated with poor air quality," says an accompanying editorial by Richard L. Verrier, Ph.D., associate professor of medicine at Beth Israel Deaconess Medical Center, Harvard Medical School in Boston; Murray A. Mittleman, M.D., Ph.D., director of cardiovascular epidemiology at Beth Israel; and Peter H. Stone, M.D., of Brigham and Women's Hospital and Harvard.
Mittleman says there is very little monitoring of ultra-fine particle pollution. "These ultra-fine particles don't stay around very long because once they are in the atmosphere they tend to grow into larger particles." Nonetheless, he says the association between ultra-fine particles and ischemia is important because "it may help to explain regional patterns of pollution and increased coronary events."
Verrier says the link between air quality and heart disease is more subtle than one between pollution and respiratory ailments such as asthma. But the new findings reveal the importance of avoiding outdoor exercise on hazy days. Mittleman says a better option is to exercise in air-conditioned settings since "most central air conditioning systems can greatly reduce this type of pollution."
The editorial authors also say that the study is focused on patients with known heart disease. "It is unclear whether the risk is present only in those patients with provokable ischemia during exercise testing," they write. They also say that it will be important to determine whether other populations are susceptible.
"The problem of particulate air pollution is pervasive and growing," they write. "An even greater concomitant impact on public health from this insidious contributor to cardiac disease can be anticipated. The American Heart Association, National Institutes of Health and regulatory agencies have an important opportunity through education, funding and regulation of clean air standards to exert a major impact on cardiovascular health."
The co-authors on the study were Annette Peters, Ph.D.; Gerard Hoek, Ph.D.; Pekka Tiittanen, M.Sc.; Bert Brunekreef, Ph.D.; Jeroen de Hartog, M.Sc.; Joachim Heinrich, Ph.D.; Angela Ibald-Mulli, M.Sc.; Wolfgang G. Kreyling, Ph.D.; Timo Lanki, M.Sc.; Kirsi L. Timonen, M.D.; and Esko Vanninen, M.D.
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