Smoking and mortality following acute myocardial infarction: results from the National Registry of Myocardial Infarction 2 (NRMI 2)

BACKGROUND: Current smokers have lower mortality following acute myocardial infarction (AMI) than non-smokers. This is often referred to as the "smokers' paradox". Our study explored possible explanations of this phenomenon. METHODS: From the 510,044 cases of AMI in the NRMI 2 from 1 June 1994, through 30 April 1997, 297,458 cases without hospital transfer were analyzed. Characteristics and treatments of tobacco smokers and non-smokers were compared before and after age-standardization. Multivariate logistic models investigated possible associations with in-hospital mortality using clinically relevant variables and interaction terms. RESULTS: Twenty-four per cent of AMI cases were current smokers. Smokers were 14 years younger than non-smokers (mean age 58 vs. 72 years, p<0.001) and had lower in-hospital mortality (8.0% vs. 16.4%, p<0.001). After age-standardization, smokers were more likely than non-smokers to suffer a Q-wave type of infarction, and were less likely to have a prior history of diabetes, hypertension, AMI, angina, cardiac failure, and coronary interventions. The unadjusted odds ratio (OR) for smoking and mortality was 0.44 (95% confidence interval, CI 0.43-0.45). After adjustment for age the OR was 0.81 (95% CI 0.78-0.83). Additional adjustment for previous medical history/cardiovascular risk factors changed the OR to 0.86 (95% CI 0.83-0.89). Adjustment for additional covariates and interaction terms had little effect. CONCLUSIONS: Smokers with AMI were on average 14 years younger than non-smokers, explaining most of the apparent association of smoking with differences in presentation and treatment, and lower in-hospital mortality. The residual association of smoking and better prognosis, the "smoker's paradox", was not fully explained by measured covariates.