Preoperative smoking status and long-term survival after coronary artery bypass grafting: a competing risk analysis

Patients with severe coronary artery disease who undergo coronary artery bypass grafting consistently demonstrate that continued smoking after surgery increases late mortality rates. Smoking may exert its harmful effects through the ongoing chronic process of atherosclerotic progression both in the grafts and the native system. However, it is not clear whether cardiac mortality is primary and solely responsible for the inferior late survival of current smokers. In this retrospective analysis, we included all consecutive patients undergoing primary isolated coronary artery bypass surgery from 1 January 2000 to 30 September 2015 in an Academic Hospital in Northern Portugal. The predictive or independent variable was the patients' smoking history status, a categorical variable with 3 levels: non-smoker (the comparator), ex-smoker for >1 year (exposure 1) and current smoker (exposure 2). The primary end point was long-term all-cause mortality. Secondary outcomes were long-term cause-specific mortality (cardiovascular and noncardiovascular). We fitted overall and Fine and Gray subdistribution hazard models. We identified 5242 eligible patients. Follow-up was 99.7% complete (with 17 patients lost to follow-up). The median follow-up time was 12.79 years (interquartile range, 9.51-16.60). Throughout the study, there were 2049 deaths (39.1%): 877 from cardiovascular causes (16.7%), 727 from noncardiovascular causes (13.9%) and 445 from unknown causes (8.5%). Ex-smokers had an identical long-term survival than non-smokers hazard ratio (HR) 0.99; 95% confidence interval (CI) 0.88, 1.12; P = 0.899. Conversely, current smokers had a 24% increase in late mortality risk (HR 1.24; 95% CI 1.07, 1.44; P = 0.004) as compared to non-smokers. While the current smoker status increased the relative incidence of noncardiac death by 61% (HR 1.61; 95% CI 1.27, 2.05, P < 0.001), it did confer a 25% reduction in the relative incidence of cardiac death (HR 0.75; 95% CI 0.59, 0.97; P = 0.025). Whereas ex-smokers have an identical long-term survival to non-smokers, current smokers exhibit an increase in late all-cause mortality risk at the expense of an increased relative incidence of noncardiac death. By subtracting the inciting risk factor, smoking cessation reduces the relative incidence of cardiac death.