Boston, MA - There is an increased risk of ST-segment depression among cardiac patients exposed to pollution, a new study has shown [1]. Investigators report that exposure to ambient particulate matter and black carbon is associated with changes in electrical conductance and that the risk of pollution-associated ST-segment depression is greatest among coronary artery disease patients one month after discharge.

"We're focusing on patients right after they've been in the hospital for myocardial infarction and acute coronary syndrome," said senior investigator Dr Diane Gold (Brigham and Women's Hospital, Boston, MA). "They've received state-of-the-art care, and we're finding subclinical effects of air pollution, with effects being stronger in the first month or so after discharge. It adds to the evidence that traffic and non-traffic-related air pollution increases cardiac risk."

The results of the study, with first author Dr Kai Jen Chuang (Harvard School of Public Health, Boston, MA), are published online September 8, 2008 in Circulation.

Speaking with heartwire, Gold said studies have linked pollution and traffic exposure to cardiovascular morbidity and mortality, including studies showing direct toxic effects on the heart outside the lungs and others showing that traffic exposure can act as a trigger for MI.

"There have been numerous studies demonstrating that after a day or two of high air pollution, hospitalizations for cardiac disease go up, as does the risk of dying from a heart attack or having a sudden death," said Gold. "We were interested in understanding more about the reasons for this and also in specifically looking at patients after hospitalization for acute coronary syndrome or myocardial infarction [to determine] whether the risk was increased within the first month after hospitalization."

One potential mechanism linking pollution to adverse cardiovascular outcomes is the idea that particulate matter induces ischemia. With this study, Gold and colleagues wanted to determine whether exposure to pollution influenced the ST-segment level—ST-segment depression is a sign of myocardial ischemia—in patients with documented coronary disease.

Investigators studied 48 Boston-area patients with documented coronary artery disease. All had been recently hospitalized for MI, unstable angina, or worsening symptoms of stable coronary artery disease. Each patient was observed up to four times within one year with a 24-hour Holter ECG monitoring. The readings were averaged over 30 minutes, providing investigators with nearly 6000 ECG observations. Particulate-matter and black-carbon levels, a measure of traffic exhaust that is produced by the incomplete combustion of fossil fuels, were collected at a monitoring site approximately 18 km from the participants' homes.

Increases in the mean one- to 24-hour fine ambient particulate-matter (PM_2.5) and black-carbon levels predicted a depression in half-hour-averaged ST-segment levels. An increase in black-carbon levels was associated with a 1.5-fold increased risk of ST-segment depression >0.1 mm. An interquartile increase in PM_2.5 resulted in a 22% higher risk of ST-segment depression, a finding bordering on statistical significance.

"What we're looking at here is subclinical ST-segment depression," said Gold. "The changes in the ECG suggest changes in electrical conductance, but they are not changes that were associated with any clinical adverse events. We think they're indicating a process that could signal increased cardiac risk."

The results showed that coronary artery disease patients recently hospitalized may be the most vulnerable to pollution in the first month after they are discharged. The investigators also observed that patients with myocardial injury, those who infarcted, might be at greater risk for pollution-associated ST-segment depression than patients with acute coronary syndrome or those hospitalized for worsening coronary disease symptoms.

Gold told heartwire the findings have clinical significance when placed in the context of previous studies showing a link between air pollution and cardiac risk. "Physicians do want to figure out how cautious to be with their patients. . . . With all the other evidence, it supports the assertion that patients coming out of the hospital for acute coronary syndrome would be more protected if the air-pollution levels were lower."

She noted that the pollution levels, including the levels of PM_2.5 and black carbon, met standards set out by the Environmental Protection Agency. That pollution levels deemed acceptable caused ST-segment depression is not surprising, she added, noting that pollution within these standards has been previously shown to elicit clinical and subclinical changes.

1. Chuang KJ, Coull BA, Zanobetti A, et al. Particulate air pollution as a risk factor for ST-segment depression in patients with coronary artery disease. Circulation 2008; DOI: 10.1161/circulationaha/108.765669.