Association Between Smoking Status, Preoperative Exhaled Carbon Monoxide Levels, and Postoperative Surgical Site Infection in Patients Undergoing Elective Surgery

Abstract

Importance: Cigarette smoking is a risk factor for many perioperative complications, including surgical site infection (SSI). The duration of abstinence from smoking required to reduce this risk is unknown.

Objectives: To evaluate if abstinence from smoking on the day of surgery is associated with a decreased frequency of SSI in patients who smoke cigarettes and to confirm that smoking is significantly independently associated with SSI when adjustment is made for potentially relevant covariates, such as body mass index.

Design, setting, and participants: In this observational, nested, matched case-control study, 2 analyses were performed at an academic referral center in the upper Midwest. Cases included all patients undergoing elective surgical procedures at Mayo Clinic, Rochester, Minnesota, between January 1, 2009, and July 31, 2014 (inclusive) who subsequently developed an SSI. Controls for both analyses were matched on age, sex, and type of surgery.

Exposures: Smoking status and preoperative exhaled carbon monoxide level, assessed by nurses in the preoperative holding area. Patients were classified as smoking on the day of surgery if they self-reported smoking or if their preoperative exhaled carbon monoxide level was 10 ppm or higher.

Main outcomes and measures: Surgical site infection after a surgical procedure at Mayo Clinic, Rochester, as identified by routine clinical surveillance using National Healthcare Safety Network criteria.

Results: Of the 6919 patients in the first analysis, 3282 (47%) were men and 3637 (53%) were women; median age (interquartile range) for control and SSI cases was 60 (48-70). Of the 392 patients in the second analysis, 182 (46%) were men and 210 (54%) were women; median age (interquartile range) for controls was 53 (45-49) and for SSI cases was 51 (45-60). During the study period, approximately 2% of surgical patients developed SSI annually. Available for the first analysis (evaluating the influence of current smoking status) were 2452 SSI cases matched to 4467 controls. The odds ratio for smoking and SSI was 1.51 (95% CI, 1.20-1.90; P < .001), which remained statistically significant after adjusting for covariates. In the second analysis (evaluating the influence of smoking on the day of surgery), there were 137 SSI cases matched to 255 controls. The odds ratio for smoking on the day of surgery and SSI was 1.96 (95% CI, 1.23-3.13; P < .001), which remained statistically significant after adjusting for covariates. Preoperative exhaled carbon monoxide level was not associated with the frequency of SSI, suggesting that the association between smoking on the day of surgery and SSI was not related to preoperative exhaled carbon monoxide levels.

Conclusions and relevance: Current smoking is associated with the development of SSI, and smoking on the day of surgery is independently associated with the development of SSI. These data cannot distinguish whether abstinence per se reduces risk or whether it is associated with other factors that may be causative.

Figure 1.. Flow Diagram Showing the Derivation of Cases and Controls for Analyses of Surgical Site Infection (SSI)

The first analysis evaluated the influence of current smoking status on SSI. The second analysis evaluated the influence of smoking on the day of surgery on SSI.

Figure 2.. Distribution of Preoperative Exhaled Carbon Monoxide Levels for Smokers Who Reported Smoking on the Day of Surgery (Yes) and Those Who Reported Abstaining (No)

Data are from all smokers evaluated in the preoperative holding areas between December 18, 2009, and June 24, 2014. IQR indicates interquartile range; SSI, surgical site infection.