• Jeffrey H Shuhaiber, Kimberley Goldsmith, and Samer A M Nashef.
• Papworth Hospital NHS Trust, Cambridge University Teaching Hospital, Cambridge, United Kingdom. Jeffrey01@mac.com
• J. Thorac. Cardiovasc. Surg. 2008 Oct 1;136(4):894-9.

ObjectiveThe effect of seasonal variation on cardiac surgery outcomes is unknown. We investigated the effect of season on risk-adjusted hospital mortality and length of stay.MethodsProspectively collected data from cardiac operations at one center between April 1996 and March 2006 were analyzed. Seasonal variation in outcomes was studied by using multiple regression models that included EuroSCORE and year of operation to adjust for risk profile and changes over time. Analysis was performed for 2 separate surgical groups: patients having coronary artery bypass grafting only and patients having other cardiac procedures with or without coronary artery bypass grafting.ResultsThere were 16,290 patients who had a first record of cardiac surgery in the study period between April 1, 1996, and March 31, 2006, with 10,263 patients having coronary artery bypass grafting only and 6027 patients having another procedure with or without coronary artery bypass grafting. There were increased odds of hospital mortality in patients having operations in winter compared with the average across all seasons for both surgical groups, although this was only significant in the coronary artery bypass grafting-only group (odds ratio, 1.29; 95% confidence interval, 1.01-1.63; P = .04). There were decreased odds of death in the coronary artery bypass grafting-only group in summer (odds ratio, 0.76; 95% confidence interval, 0.60-0.96; P = .02). Intensive care unit stay was 4% (95% confidence interval, 1%-6%) longer in the coronary artery bypass grafting-only group in winter and 3% (95% confidence interval, 1%-5%) shorter in summer than the average stay (P = .003 and .006, respectively). There were no differences in intensive care unit stay in the combined surgery group by season and no differences in total length of stay for either group (coronary artery bypass grafting only and coronary artery bypass grafting with other cardiac procedures).ConclusionsCardiac surgery outcomes are influenced by the time of year. Hospital mortality and intensive care unit stay after coronary artery bypass grafting were increased during the winter season compared with the rest of the year.

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