Tiny particles in air pollution can harm people with coronary artery disease by crippling the ability of the heart to conduct electrical signals, Harvard University researchers report.

Pollution from cars and trucks and industrial plants has been shown to trigger heart attacks, but exactly how it does that hasn’t been well-known, researchers say.

“We found that elevation in fine particles from non-traffic as well as traffic sources and black carbon, a marker for traffic, predicted depression ST-segment levels,” said lead researcher Dr. Diane R. Gold, an associate professor of medicine and environmental health. “Effects were greatest within the first month after hospitalization and for patients with heart attack during hospitalization or with diabetes.”

The report is published in the Sept. 9 online edition of Circulation.

For the study, Gold’s team collected data on 48 patients from the Boston area with coronary artery disease. The researchers monitored the patients for 24 hours using portable electrocardiograph machines looking for changes in the electrical conductivity of the heart called ST-segment depression. This dysrhythmia can indicate inadequate blood flow to the heart or inflammation of the heart muscle.

All patients had been hospitalized for heart attack, unstable angina or worsening symptoms of coronary artery disease. Forty percent had suffered heart attacks, and 25 percent had diabetes.

Gold’s group also looked at the average 24-hour levels for all pollutants in Boston. They found that these levels were below accepted or proposed National Air Quality Standard thresholds.

The researchers found that increased levels of a pollutant called PM 2.5 and black carbon, which is found in traffic exhaust, was associated with an increase in ST-segment depression.

In addition, sulfur dioxide, which is the product of combustion, but not from cars, was also associated with an increase in ST-segment depression.

Increases in ST-segment depression were particularly higher in patients recovering from a heart attack compared with other patients, the researchers reported.

“If the air pollution-associated ST-segment changes represent either myocardial inflammation or risk of ischemia, then it is possible that reduction in regional traffic and non-traffic associated air pollution may reduce heart attack or risk for either ischemia, arrhythmia or heart failure in patients with coronary artery disease in the period after hospitalization,” Gold said.

For patients who have just been discharged from hospital after a heart attack, guidelines from the American Heart Association and the American College of Cardiology suggest that patients should avoid heavy traffic because of the stress of driving, Gold said.

“Our study provides additional rationale to avoid or reduce heavy traffic exposure after discharge, even for those without a completed myocardial infarction, since traffic exposure involves pollution exposure as well as stress,” Gold said.

Dr. Byron Lee, a cardiologist at the University of California, San Francisco, agrees that patients who have had a recent heart attack should avoid air pollution.

“This study provides very strong evidence that environmental pollution puts added stress on the heart,” Lee said. “Patients who have just had a heart attack should probably do what they can to avoid pollution exposure. This may mean staying indoors during smoggy days or getting out of the city altogether.”

Dr. Samin Sharma, director of interventional cardiology at Mount Sinai Medical Center in New York City, thinks that patients who have suffered a recent heart attack and live in areas where air pollution levels are high and unavoidable need to be monitored more carefully.

“Maybe these patients need close follow-up,” Sharma said. “These patients may need an increase in their heart medications or medications need to be changed. Optimal aggressive medical therapy and optimization of medical care could be the answer for these patients.”

(Source: Diane R. Gold, M.D., M.P.H., associate professor, medicine and environmental health, Harvard University, Boston; Samin Sharma, M.D., director, interventional cardiology, Mount Sinai Medical Center, New York City; Byron Lee, M.D., cardiologist, University of California, San Francisco; Sept. 9, 2008, Circulation, online)