120 Chapter 6 and a season with very low prevalence of ILI, and prolonged mechanical ventilation and in-hospital mortality in a Dutch cohort of adult cardiac surgery patients. Methods Study Design and Study Population In this observational cohort study, all consecutive patients who underwent elective cardiac surgery and were subsequently admitted to intensive care between January 1, 2014, and February 1, 2020, from the National Intensive Care Evaluation (NICE) database were included. The NICE database is a national database, and all 16 hospitals that perform cardiac surgery in the Netherlands upload data to the NICE database. Details were described previously12. From those 16 hospitals, three had a period of 2 years each in which they did not supply data on the duration of mechanical ventilation, and these periods were excluded from the analysis. Subjects less than 18 years of age were excluded. The different types of surgery included in this study were aortic valve surgery (including transcatheter aortic valve replacement); coronary artery bypass graft; coronary artery bypass graft and single valve (replacement or repair); coronary artery bypass graft and two valves (replacement or repair); congenital defect repair; mitral valve surgery; pericardiectomy; pulmonary valve surgery; tricuspid valve surgery; and other cardiovascular surgery (not otherwise specified). Details on data collection, calculation of Age, Creatinine, and Ejection Fraction II risk score, intensive care unit (ICU) occupancy rate, and data availability can be found in the Supplemental Material. Definition of Influenza Season The ILI season at the time of ICU admission was used as a proxy for risk of respiratory viral infection. The dates and duration of the national ILI season were determined using publicly available data from the Netherlands Institute for Health Services Research, which collects data from general practitioners13. The epidemic threshold is determined each ILI season. For the years used in this study, to and including the ILI season of 2018-2019, the epidemic threshold was set at 51 per 100,000 persons presenting with ILI for 2 consecutive weeks. From the ILI season of 2019-2020 onward, the epidemic threshold was set at an incidence 58 per 100,000 persons presenting with ILI. All patients who underwent surgery in the weeks in which the epidemic threshold was met were included in the ILI season group. Similar to previous studies5, a threshold of 25 or less per 100,000 persons presenting with ILI was set for the baseline season. Weeks in which the incidence was between 25 and the epidemic threshold were set as an intermediate season. The ILI season ended when the number of people presenting with ILI was lower than the epidemic threshold for 2 consecutive weeks. To have the biggest contrast in the number of respiratory viral infections between groups, ILI season and baseline season were compared. As
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