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Title: Smoking and risk of myocardial infarction in women and men: longitudinal population study

Authors: Prescott E, Hippe M, Hein HO, Vestbo J

Reference: BMJ April 4 1998;316:1043-1046

Reviewer: Riaz R. Rabbani

Problem addressed: Smoking is a major contributor to morbidity and mortality due to ischemic heart disease.  With the increasing incidence of smoking among women, the female smoking habits have become similar to those of men.  This allows for a more fair comparison of the contribution of smoking to myocardial infarction between the two sexes. 

Purpose of study: To compare the risk of myocardial infarction associated with smoking in men and women. 

Location of study: Denmark

Methods: Three separate databases (Copenhagen City Heart Study, the Glostrup population studies, and the Copenhagen Male Study) were utilized.  Patients with previous myocardial infarction were excluded.  Mean follow-up was 12.3 years.  A self-administered questionnaire was used to assess cardiovascular risk factors. Nearly 11500 women and 13200 men were included in the analysis.

Tobacco consumption was categorized into: never smokers; ex-smoker; non-inhaling current smokers; inhaling current smokers of 1-14gm, 15-24 gm or more than 25gm of tobacco per day (a cigarette=1gm, a cigar=5gm). Type of tobacco use (cigarette, cigar, pipe, etc.) was also documented.  Blood pressure, lipid status, body mass index, education level, alcohol consumption, physical activity, and diabetic status were determined.  Subjects were followed for fatal and nonfatal myocardial infarction.

Results: With the exception of alcohol consumption and physical activity men had a more disadvantageous cardiovascular risk profile.  Female current smokers had a relative risk of 2.24 and male smokers 1.43 relative to non-smokers for myocardial infarction.  The difference remained unchanged even after multiple adjustments for other risk factors.  For current smokers, there was a definite dose response curve.  All relative risks were higher in women than in men and were not affected by adjustment for other risk factors.

Discussion: This prospective study reveals that relative risk for myocardial infarction in smokers is 50% higher for women than men after multivariate analysis.  In this study, the relative risks of myocardial infarction did not vary between men and women for any of the risk factors examined except for tobacco use.  Thus, the authors conclude that the sex differences cannot simply be attributed to differing baseline rates.

Myocardial infarction was more strongly associated with current tobacco use than with accumulated exposure to tobacco.  This finding lends credence to the assertion that the short-term hemodynamic effects of tobacco are more important than the long-term chronic effects, at least as it related to coronary thrombosis.

One possible biological mechanism that may explain the relatively greater importance of the contribution of smoking to myocardial infarction among women is estrogen deficiency. Smoking decreases level of estrogen (smokers have earlier menopause, decreased incidence of endometrial cancer, osteoporosis, and uterine fibroids). This estrogen deficiency leads to increased burden of ischemic heart disease.