119 Cardiac surgery in Influenza-Like-Illness season 6 Introduction Pulmonary complications, such as pneumonia, pulmonary edema, and acute respiratory distress syndrome (ARDS), are currently reported as the main cause of mortality after cardiac surgery1. Pneumonia is common, occurring in approximately 6% of cardiac surgery patients and contributing significantly to morbidity and mortality2. Cardiogenic pulmonary edema is one of the leading causes of prolonged invasive mechanical ventilation (IMV) after cardiac surgery1. One of the most severe pulmonary complications is ARDS, a life-threatening pulmonary inflammatory response. Mortality of ARDS after cardiac surgery varies among studies, from 15% to 80%, with ARDS mortality in the general population estimated at less than 40%3,4. There are many risk factors for the development of pulmonary complications after cardiac surgery, not all of which can be easily influenced1. Examples are genetics, age, smoking status, comorbidities, and use and duration of cardiopulmonary bypass1. Another risk factor is the season in which cardiac surgery is performed. Earlier research found a higher prevalence of ARDS and a longer duration of IMV among patients who underwent cardiac surgery during the influenza-like illness (ILI) season compared with a period with lower incidence of ILI5. In the Northern hemisphere, the ILI season typically is a period between October and April, lasting 15 weeks on average but with differences between the years6. A possible explanation for these results could be that an asymptomatic or presymptomatic influenza, or other respiratory viral infection, is a risk factor for pulmonary complications after cardiac surgery5. Many common respiratory viral infections can be asymptomatic—estimates are that more than 75% of influenza infections are asymptomatic7. Other, noninfluenza respiratory viruses, such as respiratory syncytial virus and adenovirus, are the cause of a substantial part of ILI and could also be risk factors8. Respiratory viral infections can lead to lung inflammation and epithelial damage9. The damaged airways increase the risk of more severe postoperative pulmonary complications, such as pneumonia or ARDS9. A cytokine response to viral respiratory infection can already be seen in the presymptomatic phase, and this inflammatory response could prime the lungs for more severe pulmonary problems, with cardiac surgery acting as a second hit10 Another study has found higher mortality after cardiac surgery in the winter months11. If an asymptomatic or presymptomatic respiratory viral infection is a risk factor for pulmonary complications after cardiac surgery, testing for a viral infection or (influenza) vaccination before elective surgery could reduce the number of these complications. We hypothesized that undergoing cardiac surgery during ILI season increases the probability of the development of postoperative pulmonary complications, such as pneumonia or ARDS, prolonging the duration of mechanical ventilation and increasing mortality. The aim of this study was to investigate the association between ILI season
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