Firefighters and acute myocardial infarction: understanding mechanisms and reducing risk
Hunter, Amanda Louise
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Acute myocardial infarction is the commonest cause of death in firefighters, accounting for 45% of all deaths on duty. Compared with an average life expectancy of 77 years in the general population, the average age of cardiovascular death in firefighters is 50 years suggesting that occupational hazards are responsible for premature disease. The risk of acute myocardial infarction is increased 12- to 136-fold during rescue and firefighting duties, and is likely to reflect a combination of factors including strenuous physical exertion, mental stress, heat and pollutant exposure. Previous studies have established that the duties of a firefighter, in particular fire suppression, put inordinate strain on the cardiovascular system yet the exact mechanisms underlying the increased risk of myocardial infarction remain poorly defined. In a series of studies, I assessed the effect of occupation-specific risk factors on cardiovascular health in a combination of controlled and real-life studies in order to better define these mechanisms, hypothesising that exposure to high temperatures, strenuous physical exertion, psychological stress and air pollution either alone or in combination caused vascular dysfunction and thrombosis. In order to assess if firefighters had a greater cumulative risk of cardiovascular disease due to their occupation at baseline, I assessed the cardiovascular function of group of healthy, off-duty firefighters and compared this to a group of healthy age- and sex-matched off-duty police officers; an occupational group with similar responsibilities but a much lower risk of on-duty cardiovascular events. I was able to demonstrate that traditional cardiovascular risk factors, vascular endothelial function and thrombogenicity were similar in the two groups concluding that the excess of cardiovascular events and deaths in on-duty firefighters are due to the acute and transient effects of strenuous physical exertion, psychological stress, heat and exposure to air pollutants. Having established that off-duty firefighters had no apparent increased risk of cardiovascular events, I then went on to clarify the effects of combustion derived air pollution in the form of wood smoke on the cardiovascular system. The suppression of wildland or forest fires is globally the single most important duty of the fire service. Previous work within our institution has demonstrated the adverse effects of combustion derived air pollution, in the form of diesel exhaust, on the cardiovascular system. In a similar fashion, I assessed the effect of a wood smoke inhalation in a group of healthy off-duty firefighters by performing controlled exposures to wood smoke utilising a unique and well characterised facility. Interestingly, unlike diesel-exhaust, the exposure to wood smoke had no adverse effect on vascular endothelial function or thrombogenicity in this group concluding that cardiovascular events during wildland fire suppression may not be directly related to wood smoke inhalation but instead precipitated by other mechanisms such as strenuous physical exertion or dehydration. Latterly, I proceeded to evaluate the effects of strenuous physical exertion and heat exposure by comprehensively assessing a number of cardiovascular end points following controlled exposure to a fire simulation activity in a group of healthy, off-duty firefighters. I was able to demonstrate that exposure to extreme heat and physical exertion impaired vasomotor function and increased thrombus formation. Moreover, I demonstrated cardiac troponin concentrations increased suggesting that fire suppression activity may cause myocardial injury. These important findings suggest pathogenic mechanisms to explain the association between fire suppression activity and acute myocardial infarction. In the final phase of work, I endeavoured to assess the effects of real-life firefighter activities on the cardiovascular system. In an ambitious study, I attempted to undertake a comprehensive assessment of cardiovascular function in healthy firefighters following three periods of duty: fire suppression, alarm response and non-emergency activity. I was unable to complete enough studies to adequately power an analysis and draw any firm conclusions about the effect of these duties on cardiovascular health. Further work is required in a real-world setting to more clearly define the occupational risk factors underlying the increased risk of cardiovascular events associated with specific firefighter duties Understanding the biological mechanisms and environmental factors that predispose firefighters to cardiovascular events is essential if we are to develop effective methods for the prevention of acute myocardial infarction on-duty. This body of work has greatly improved the understanding of the mechanisms underlying the increased risk of cardiovascular events on duty and calls for the immediate evaluation of current practice in order to minimise risk to firefighters in the future. Examples of where improvements should be made include strategies to ensure adequate hydration and cooling following exposure to heat and physical exertion, change to working patterns to limit the duration of extreme exposures, and education, training and screening programmes to reduce the impact of traditional and occupational cardiovascular risk factors.