Abstract Body (Do not enter title and authors here): Background: Atrial fibrillation (AF) is becoming more prevalent both in the US and globally, with its incidence rising at an alarming rate. While this rise can be partly explained by enhanced surveillance, an aging population, or increase in traditional risk factors, it is also prudent to start examining the role of emerging environmental risk factors like extreme heat driven by climate change.

Methods: Data from the Medtronic Product Surveillance Registry for all patients in the US who underwent a cardioverter-defibrillator (ICD) or cardiac resynchronization therapy defibrillator (CRT-D) device implant from 2016 to 2023. For each patient, we extracted the first arrhythmic episode of AF detected on the device. These episodes were matched with hourly ambient temperature recordings from the nearest station in their respective town/city. In a case-crossover design, we created multiple control hours within the same individual that matched the episode hour by time of day, day of the week, month, and year. Conditional logistic models were then pooled across all climate zones in the US to obtain the dose-response relationship between outdoor heat and AF episodes, allowing for non-linearity. In an additional analysis, we limited episodes to those lasting 30 minutes or longer.

Results: A total of 2,313 individuals with atrial arrhythmic events were analyzed. We found fewer AF episodes in the early morning hours (12am-7am) compared to normal working hours (8am-5pm) (p<0.001). We found fewer AF episodes occurring on Saturday and Sunday compared to weekdays (p<0.001). Pooled across all regions in the US, compared to locally optimal temperatures (those associated with least odds; between 5°C to 8°C), the odds ratios of AF events when outdoor temperatures were 39°C, 40°C, and 41°C were 2.66 (95% CI: 1.57 – 4.54), 2.87 (95% CI: 1.57 – 5.22), and 3.09 (95% CI: 1.58 – 6.01), respectively. Lagging temperature exposure did not improve the models. Restricting to episodes longer than 30 minutes showed similar results.

Conclusion: We observed a dose-response relationship between rising outdoor temperatures and increased AF events with doubling to tripling of odds when temperatures reach extreme hot levels. This study highlights the urgent need to consider emerging environmental factors, such as climate change-induced heat, in managing and preventing AF.