Wednesday September 10 2008
A link between air pollution and heart attack has been suggested before
“Air pollution from traffic hinders the heart's ability to conduct electrical signals”, BBC News reported. It said that in a study of 48 patients hospitalised for heart attack or similar conditions, exposure to small chemical particles produced by burning fossil fuels caused ‘worrying changes’ that were picked up by ECG monitoring. It continued that The American Heart Association already recommend that some heart patients avoid driving after leaving hospital because of the stress it creates.
The study tested levels of atmospheric pollution in the areas where the participants lived and looked at how this was related to changes in their ECG throughout a 24-hour period. Although the study was well conducted, the accuracy of this method in measuring each individual’s exposure to pollution is unclear. Also, the relevance of the changes in the patients’ ECG readings is questionable, as they were very small and it is unknown if the patients actually suffered from any angina pains at these times. Although further research is needed, there is no harm in people who have had heart attacks and who are concerned, to aim to reduce their exposure to traffic pollution during their immediate recovery period.
Where did the story come from?
Dr Kai Jen Chuang and colleagues from the Department of Environmental Health, Harvard School of Public Health, Harvard Medical School and Brigham and Women’s hospital, Boston, carried out the research. The study was funded by the National Institute of Environment Health Sciences, the Environmental Protection Agency, and the National Science Council. The study was published in the peer-reviewed medical journal: Circulation.
What kind of scientific study was this?
This was a cohort study designed to investigate the possibility that traffic exposure is a trigger for heart attack. A link between increased levels of atmospheric pollution and damage to the cardiovascular system has been suggested previously, but there is limited electrophysiological evidence to date.
The researchers recruited 48 patients in the greater Boston area. All patients had heart disease, had suffered from heart attack or other heart-attack-like conditions and had all undergone a procedure to open the blocked arteries around the heart (percutaneous coronary intervention). The patients were seen at home within two to four weeks of hospital discharge. The researchers excluded patients who had known abnormalities of heart conduction (e.g. atrial fibrillation) as this might have affected interpretation of the outcomes on the heart traces. Also excluded was anyone who had recently had coronary artery bypass graft surgery, those with psychiatric illness and those currently smoking.
Participants completed a questionnaire at the beginning of the study about their relevant medical history and medications. They then had their heart activity monitored (traced) over a 24-hour period using an ambulatory ECG monitor. Repeat questionnaires and 24-hour ECGs were then taken on three further occasions, each three months apart. An experienced analyst reviewed the 24-hour ECG traces in half-hourly periods, with particular attention to the ST segment of the trace, as depression of this segment is usually seen when the heart muscle has reduced oxygen supply, as in angina.
While the participants wore the ECG monitors, two types of pollutants - atmospheric fine particle matter (PM) and black carbon (BC) - were monitored at a site an average of 17.6km from the participants’ homes. Hourly measurements of carbon monoxide, ozone, nitrogen dioxide, and sulphur dioxide were also taken from five state monitoring sites in Boston, and hourly temperature change was obtained from the National Weather Station. The researchers then analysed the association between half-hourly ST segment levels on the ECG and change in air pollutants.
What were the results of the study?
Participants in this study were 81% male and had an average age of 57 years old. They had all suffered from heart attack, related conditions or worsening angina and were taking typical medications used for heart disease.
Analysis showed that atmospheric pollutants were at their peak levels between seven and eight in the morning while ST segments were at their lowest levels between three and four in the afternoon. Levels of PM and BC were positively correlated with each other (i.e. when there were high levels of one, there were also high levels of the other, and the same for low levels), and an increase in both predicted a decrease in the half-hourly ST segment levels. This link with ST depression persisted even when the researchers took into account heart rate, hour of the day, hourly temperature, day of the week, and order of the visit.
A specific increase in BC levels in the last 24 hours was associated with patients having a 50% increased risk of having an ST depression of 0.1mm or greater (95% confidence interval 1.19 to 1.89). The average ST depression associated with this increase in BC level was estimated as -0.031mm (95% confidence interval -0.042 to -0.019). The associations between PM increase and ST depression of 0.1mm or greater were not significant.
The researchers also found that increases in nitrogen dioxide and sulphur dioxide both significantly increased risk of 0.1mm or greater ST depression (risk increase 51% and 41% respectively; estimated average ST change -0.029mm and -0.033mm respectively).
Individual medical factors affected the extent of ST depression with rise in BC and PM. Patients who had suffered a heart attack (rather than related conditions) showed significantly greater ST depression than patients who had not had a heart attack. There was also a significantly greater ST depression when it was the patient’s first visit after a heart attack compared to the second to fourth visits.
What interpretations did the researchers draw from these results?
The researchers conclude that in the first month following hospitalisation for treatment of coronary artery disease, patients may be most vulnerable to the ischaemic effects of air pollution (principally from black carbon). People who have suffered from heart attack may be at the greatest risk for this pollution-associated ST depression.
What does the NHS Knowledge Service make of this study?
This is a well-designed study that examined the electrophysiological effects of air pollution upon the heart. However, there are important limitations to consider:
- The study tested general atmospheric levels of pollution recorded at some distance from the participants’ homes and looked at how this related to changes in their ECG readings over a 24-hour period. The accuracy of this test is unclear for several reasons. Firstly, it is unknown where the patients were during times of higher pollution; it is reasonable to expect that those outside in traffic would be more exposed than those who were indoors with the windows closed. Secondly, what the patients were doing when ST segment depression was recorded could have had an effect. If they were active at this time then it could explain the reason for some heart muscle ischaemia, or conversely if they were resting at the time then other reasons for ischaemia, such as pollution, may be responsible.
- The amount of ST depression was very small, in most cases only -0.01 to -0.03mm. Depression is usually only considered significant at 1mm decrease. It is also unknown whether the patients themselves actually suffered from any angina pains associated with the ST depression.
- All of the participants were of a particular subgroup with significant heart disease, who had recently undergone invasive treatment for the condition (percutaneous coronary intervention) and are therefore not comparable to the general population.
- Pollutant levels in Boston may be different from those in the UK or elsewhere.
If people who have had heart attacks are concerned, they could try to reduce exposure to heavy traffic pollution in the immediate recovery period until further research is carried out.
Sir Muir Gray adds...
Pollution always causes harm, but people with heart disease should not be put off walking; 30 minutes a day, even in city streets does more good than harm.