122 Chapter 6 The Cox regression model results in a hazard ratio, expressing the hazard of stopping IMV in the ILI season divided by the hazard of stopping IMV in the baseline season. The proportional hazards assumption was tested using residual plots. For in-hospital mortality, a logistic regression analysis was performed. Finally, for the P/F ratios on day 1, 3, and 7, linear regression analyses were performed. Differences in APACHE IV predicted mortality reflect the severity of illness after surgery as it is measured at the start of ICU admission. However, following the hypothesis that a viral respiratory infection at the time of surgery leads to worse outcomes, it could be that the APACHE IV predicted mortality is influenced by a process that started earlier and is exacerbated by the surgery. Therefore, a sensitivity analysis excluding the APACHE IV predicted mortality as a predictor was performed. Ethical Approval Per Dutch legislation, approval from an Ethical Committee is not required for large register-based studies with routinely collected health care data, provided the results cannot be traced to an individual. However, hospitals consented to their data being used for this analysis. Results Baseline Characteristics Between January 1, 2014, and February 1, 2020, a total of 42,277 adult patients underwent elective cardiac surgery, 12,994 (30.7%) in the ILI season, 13,440 (31.8%) in the baseline season, and 15,843 (37.5%) in the intermediate season. There were no patients with missing data for the primary outcome and few with missing data for the secondary outcomes (ie, in-hospital mortality, 0%; P/F day 1, 4%; P/F day 3, 1.5%; and P/F day 7, 0.6%). During our study period, 130 weeks were defined as baseline season, 129 weeks were defined as intermediate season, and 114 weeks were in the ILI season. On average, 103 surgeries were performed each week during baseline season, 123 surgeries per week during intermediate season, and 114 surgeries each week during ILI season. Table 1 shows the demographics for patients who underwent surgery in baseline season compared with ILI season. Patients who underwent surgery in the ILI season had a slightly higher APACHE IV predicted mortality, and ICU occupancy rate was higher in ILI season compared with baseline season. Demographics including the intermediate season can be found in Supplemental Table 1.
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