Beyond the established risk factors of myocardial infarction : lifestyle factors and novel biomarkers

Abstract: Age, male sex, hypertension, smoking, diabetes, dyslipidaemia, and obesity are considered as established risk factors for cardiovascular diseases. Several of these established cardiovascular risk factors are strongly influenced by lifestyle. Novel biomarkers from different mechanistic pathways have been associated with cardiovascular risk, but their clinical utility is still uncertain. The overall objective of the thesis was to evaluate the associations between certain lifestyle factors (physical activity and snuff use), biomarkers reflecting the haemostatic and the inflammatory systems and risk of a future first-ever myocardial infarction.  A prospective incident nested case-control study design was used with a total of 651 cases of myocardial infarction and 2238 matched controls from the population-based Northern Sweden Health and Disease Study.  The effects of commuting activity, occupational and leisure time physical activity on risk of myocardial infarction were studied. A clearly increased risk of myocardial infarction was found for car commuting compared to active commuting (walking, cycling or going by bus). High versus low leisure time physical activity was associated with decreased risk of myocardial infarction. Low occupational physical activity was associated with risk of myocardial infarction in men.  The risk of myocardial infarction or sudden cardiac death was studied in male snuff users compared to non-tobacco users. No increased risk was found for myocardial infarction or sudden cardiac death among snuff users without a previous history of smoking. However, for sudden cardiac death the study did not have statistical power to detect small differences in risk.  Plasma levels of haemostatic markers have previously shown to be associated with risk of myocardial infarction, but as haemostatic markers are also acute-phase reactants, it is not clear if their association with myocardial infarction is independent of inflammatory markers. In the present study, the haemostatic markers D-dimer, von Willebrand factor (VWF), tissue plasminogen activator (t-PA), and tissue plasminogen activator/plasminogen activator inhibitor-1 complex (t-PA/PAI-1 complex) were associated with risk of myocardial infarction after adjustment for established risk factors and the inflammatory markers C-reactive protein (CRP) and interleukin 6 (IL-6). Furthermore, the addition of eight haemostatic and inflammatory markers could improve the predictive ability for future myocardial infarction beyond that of a model utilizing only established risk factors.  Established risk factors and novel biomarkers were explored as potential mediators of the reduced risk of myocardial infarction related to active commuting. A combination of established risk factors, haemostatic and inflammatory markers appeared to explain a substantial proportion (40%) of the difference in risk for myocardial infarction between active commuters and car commuters. IL-6, t-PA, t-PA/PAI-1 complex, apo B/apo A-1 ratio, and BMI seemed to be the largest potential mediators when tested individually. In conclusion, regular physical activity such as active commuting is associated with reduced risk of a first-ever myocardial infarction. This effect could in part be mediated through a beneficial influence on haemostasis and inflammation, as well as a positive impact on established risk factors. Several haemostatic markers are associated with risk of myocardial infarction independent of established risk factors and inflammatory markers. The combination of haemostatic and inflammatory markers may enhance predictive ability beyond established risk factors. Our findings do not support the hypothesis that snuff use increases the risk of myocardial infarction.