121 Cardiac surgery in Influenza-Like-Illness season 6 these numbers consist of people presenting with ILI to their general practitioner, the absolute number of cases in the general population is likely to be higher. Primary and Secondary Outcomes The primary outcome was duration of mechanical ventilation (hours). Secondary outcomes were differences in the in-hospital mortality in median to fraction of inspired oxygen (P/F) ratio on day 1, day 3, and day 7 after surgery in the ILI season vs baseline season. The P/F ratios were not available after discharge from the ICU. An analysis including the intermediate season was performed. Only the first episode of mechanical ventilation was studied. Statistical Analyses Median and interquartile range were used to describe continuous variables, and differences between seasons were compared using the Mann-Whitney U test for two groups and the Kruskal- Wallis test for more than two groups. The χ2 test was used for categoric variables. A P value of less than .05 was considered to indicate statistical significance. Multiple comparisons were adjusted using the Bonferroni method. Records with missing data were excluded from the analysis. Statistical analysis was performed using R 4.0.3 statistical software (R Foundation for Statistical Computing). Clinically relevant variables were included as predictors in the multivariable analysis: ILI season; year of surgery; age; sex; body mass index; Acute Physiology and Chronic Health Evaluation IV (APACHE IV) predicted mortality14; type of surgery; Charlson comorbidity index; Age, Creatinine, and Ejection Fraction II risk score; hospital type; and ICU occupancy rate. Because congestive heart failure is part of the Charlson comorbidity index, New York Heart Association class IV was not included separately. For the duration of mechanical ventilation, a Cox regression analysis was done, with extubation as the event of interest. Higher rates of extubation necessarily imply shorter duration of ventilation, albeit this duration is not the explicit outcome in a Cox model. The proportional hazards assumption for ILI season was checked by visual inspection and including an interaction term for ILI season and duration of mechanical ventilation. The duration of mechanical ventilation was censured at the time of death for patients who died in-hospital, if they died while receiving IMV. Because the severity of influenza seasons differs per year, we compared each ILI season with the baseline season in the same year. To do so, terms for the interaction between the year in which surgery was performed (eg, 2014-2015) and the season in which surgery was performed (baseline/ILI) were included in the Cox regression model. This way, for every ILI season, the effect on our primary endpoint (duration of mechanical ventilation, in hours) could be calculated, compared with baseline season of that same year.
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