Eastern Cardiothoracic Surgical Society

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Seasonality of Postoperative Pneumonia after Coronary Artery Bypass Grafting
Thomas J. Martin1, Adam E. M. Eltorai1, Kevin F. Kennedy2, Frank Sellke3, Afshin Ehsan3.
1The Warren Alpert Medical School of Brown University, Providence, RI, USA, 2Department of Cardiology, St Luke's Mid America Heart Institute, Kansas City, MO, USA, 3Division of Cardiothoracic Surgery, Department of Surgery, Brown University, Providence, RI, USA.

Objective: Postoperative pneumonia is a common source of morbidity and mortality following coronary artery bypass grafting (CABG); however, the effect of seasonal risk factors remains unknown. We hypothesized that patients receiving CABG in Fall and Winter months are at increased risk for developing postoperative pneumonia.
Methods: A retrospective cohort study of patients receiving CABG within the National Inpatient Sample between 2005 and 2015 was performed. Baseline characteristics were abstracted, and patients were clustered by month of procedure: Fall/Winter (October-March) and Spring/Summer (April-September). The primary outcome was defined as all-cause pneumonia; secondary outcomes included influenza and pneumonia secondary to H. influenzae type B, S. pneumoniae, B. pertussis, varicella, or rubeola. Outcomes with significant differences between groups were analyzed by time series decomposition; odds ratios were generated for standard calendar quarters and adjusted for 29 AHRQ comorbidities.
Results: 238,757 and 277,941 patients receiving CABG during Fall/Winter and Spring/Summer, respectively, were identified (Table 1). There were significantly higher rates of all-cause pneumonia, influenza, H. influenzae pneumonia, and S. pneumoniae pneumonia among patients in Fall/Winter vs Spring/Summer (Table 2). On time series decomposition (Figure 1), patients receiving CABG during Q1 (January-March) had significantly increased risk for developing all-cause pneumonia, influenza, H. influenzae, and S. pneumoniae pneumonia compared to Q3 (July-September) (Tables 3 and 4).
Conclusions: These findings demonstrate a significant within-year seasonality for the development of postoperative pneumonia after CABG. Preoperative immunization against influenza, H. influenzae, and S. pneumoniae may reduce risk of postoperative pneumonia for patients receiving CABG during Fall and Winter months.

Study Participant Characteristics (Abbreviated)
VariableTotal n=516698Fall/Winter (Oct-Mar) n=238757Spring/Summer (Apr-Sep) n=277941
Female, N(%)143215 (27.7)66197 (27.7)77018 (27.7)
Age, mean (SD)66.1 (10.8)66.0 (10.8)66.1 (10.8)
Elective Admission, N(%)248564 (48.1)113988 (47.7)134576 (48.4)
Patient Disposition N(%) (Uniform)
Routine225359 (43.6)106129 (44.5)119230 (42.9)
Transfer to Short-term Hospital4009 (0.8)1945 (0.8)2064 (0.7)
Transfer to Other (SNF, ICF, etc.)100544 (19.5)46593 (19.5)53951 (19.4)
Home Health Care172164 (33.3)77162 (32.3)95002 (34.2)
Died during Admission13807 (2.7)6595 (2.8)7212 (2.6)

OutcomeTotal n=516698Fall/Winter (Oct-Mar) n=238757Spring/Summer (Apr-Sep) n=277941P ValueStd Diff (%)
All-cause pneumonia, N(%)28270 (5.47)13715 (5.74)14555 (5.24)<0.0012.2
influenza, N(%)183 (0.035)139 (0.058)44 (0.016)<0.0012.2
H. influenzae type B, N(%)472 (0.091)249 (0.104)223 (0.080)0.0040.8
S. pneumoniae, N(%)325 (0.062)173 (0.072)152 (0.055)0.0110.7
B. pertussis, N(%)2 (0.00)1 (0.00)1 (0.00)NS0.0
varicella, N(%)0 (0.00)0 (0.00)0 (0.00)NSN/A
rubeola, N(%)0 (0.00)0 (0.00)0 (0.00)NSN/A

Decomposition of Additive Time Series - Summarized Results
OutcomeAdjusted Odds Ratio (95% CI)P Value
All-cause pneumonia
Q1 (Jan-Mar) vs Q3 (Jul-Sep)1.16 (1.12, 1.20)<0.001
Q2 (Apr-Jun) vs Q3 (Jul-Sep)1.03 (0.99, 1.07)0.096
Q4 (Oct-Dec) vs Q3 (Jul-Sep)1.13 (1.09, 1.17)<0.001
Q1 (Jan-Mar) vs Q3 (Jul-Sep)4.15 (2.56, 6.74)<0.001
Q2 (Apr-Jun) vs Q3 (Jul-Sep)0.80 (0.42, 1.53)0.501
Q4 (Oct-Dec) vs Q3 (Jul-Sep)2.63 (1.56, 4.42)<0.001

Decomposition of Additive Time Series - Summarized Results (cont)
OutcomeAdjusted Odds Ratio (95% CI)P Value
H. influenzae type B
Q1 (Jan-Mar) vs Q3 (Jul-Sep)1.41 (1.08, 1.83)0.011
Q2 (Apr-Jun) vs Q3 (Jul-Sep)1.17 (0.89, 1.54)0.264
Q4 (Oct-Dec) vs Q3 (Jul-Sep)1.22 (0.92, 1.61)0.172
S. pneumoniae
Q1 (Jan-Mar) vs Q3 (Jul-Sep)1.48 (1.05, 2.09)0.026
Q2 (Apr-Jun) vs Q3 (Jul-Sep)1.42 (0.99, 2.02)0.050
Q4 (Oct-Dec) vs Q3 (Jul-Sep)1.79 (1.27, 1.52)<0.001

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