Abstract Body (Do not enter title and authors here): Background
Extreme heat events have increased in frequency and intensity and are projected to continue increasing due to climate change. Extreme heat is associated with an increase in both all-cause and cardiovascular (CV) mortality. However, how extreme heat impacts CV disease related emergency department (ED) visits is not well studied.

Methods
Data on all ED visits among adults 20 years and older from 16 US states in summer months (May through September) for 2010 to 2018 were obtained from State ED all-payer claims databases. After determining the monthly number of ED visits in each county, the county-level monthly number of extreme heat days was determined. Extreme heat days were defined as any day with a maximum heat index ≥90°F (32.2°C) and in the 97.5^th percentile based on a county-specific historical period (1979-2007). CV ED visits were identified using ICD-10 diagnostic codes (I00-I99). A Poisson fixed effects regression model with county, year, and month fixed effects was estimated with monthly ED visits as the outcome and the monthly number of extreme heat days as the primary predictor of interest. Environmental, economic, and demographic variables were also included in the model. Excess ED visits were estimated as the difference between the number of visits, in each county, with the observed number of extreme heat days and the estimated number of visits if no extreme heat days had occurred.

Results
There were approximately 91 million all-cause and 1.5 million CV ED visits across 985 counties during summer months in 2010 to 2018. The population-weighted, total median number of extreme heat days over the study period was 79 (IQR 63-98). From 2010-2018, each additional extreme heat day per month was associated with a 0.21% (95% CI 0.02-0.41, p=0.03) increase in monthly CV ED visits and a 0.20% (95% CI 0.06-0.35, p=0.006) increase in monthly all-cause ED visits. The estimated number of extreme heat associated excess CV ED visits over the study period was 5658.4 (95% CI 525.0-10791.9). The estimated number of excess all-cause ED visits was 329055.4 (95% CI 95625.4-562485.4)

Conclusion
Extreme heat was associated with an increase in CV and all-cause ED visits in 16 states in the US between 2010 and 2018. As extreme heat events continue to increase, understanding how this will impact emergency departments and health systems across the country is important to be able to devise strategies to address the associated increase in utilization.