Abstract Body (Do not enter title and authors here): Background: While previous studies have noted seasonal variation in acute cardiovascular conditions, such as higher myocardial infarction in the winter, there are limited data on the impact of seasonal variations in pulmonary embolism (PE) outcomes.

Methods: All adult (greater than or equal to 18 years) non-elective admissions with a primary diagnosis of PE with available data on the month of admission were identified using the National Inpatient Sample (2016-2020). We stratified the seasons into spring (March-May), summer (June-August), fall (September-November), and winter (December-February). Outcomes of interest included in-hospital mortality, total hospitalization costs, hospital length of stay, and discharge disposition.

Results: During 01/01/2016 to 12/31/2020, 903,254 PE admissions with were identified. Spring, summer, fall, and winter had 23.9%, 25.1%, 25.2%, and 25.5% admissions, respectively. Admissions during the four seasons were comparable on sex distribution. Spring had higher admissions of white race (spring 69.6%, summer 68.8%, fall 68.9%, winter 69.2%) whereas summer had highest black race admissions (spring 18.3%, summer 19.0%, fall 19.0%, winter 18.6%) and those in lowest socioeconomic quartile (spring 28.2%, summer 28.7%, fall 28.5%, winter 28.1%) (all p<0.05). Respiratory failure was the highest in winter (spring 26.2%, summer 25.7%, fall 26.4%, winter 26.8%), whereas renal failure (spring 13.4%, summer 13.7%, fall 14.2%, winter 13.9%) bleeding complications (spring 5.7%, summer 5.8%, fall 6.1%, winter 5.9%), were higher in fall (all p<0.05). Mechanical thrombectomy rates were highest in fall (spring 1.0%, summer 1.3%, fall 1.7%, winter 1.2%, p< 0.001), whereas systemic thrombolysis (spring 2.8%, summer 3.2%, fall 3.1%, winter 3.1%, p=0.02) was highest in the summer. Use of catheter directed therapies was comparable. There were no difference in-hospital mortality rates among seasons but winter had highest length of stay, lowest discharges to home, and fall had highest hospitalization costs.

Conclusion: In this five-year study duration, despite some differences in organ failure and complication rates, there was no significant seasonal variation in the in-hospital mortality from PE. Winter months had the highest in-hospital resource utilization.