Abstract Body (Do not enter title and authors here): Background
Heat exposure is associated with an increase in cardiovascular mortality, particularly among older adults. Climate change has led to an increase in days with hot temperatures. The burden of hospitalizations associated with heat exposure is not well known.

Methods
Data on all hospitalizations among Medicare beneficiaries 65 years of age and older in summer months (May through September) for 2016 to 2019 were obtained from Medicare Provider Analysis and Review (MEDPAR) files. Total daily cardiovascular and all-cause hospitalizations in each US county in the contiguous United States (US) were determined and daily county-level maximum heat index levels were obtained from the gridMET dataset. Counties with <100 elderly residents were excluded. The association between daily heat index and hospitalizations was quantified using the distributive lag non-linear framework, with a generalized linear regression model – assuming a quasi-Poisson distribution. The model accounted for the lagged association, for up to 10 days, for daily heat index as well as covariates (ozone, particulate matter, and precipitation levels). Estimates from this model were used to determine the number and fraction of hospitalizations attributable to high temperatures on hot days. Hot days were defined as those with a heat index level in the 90^th percentile based on a county-specific historical period (1979-2015).

Results:
A total of 2085 counties (out of 3108) were included. The median threshold for hot days was 90.2°F (IQR 84.8°F to 95.5°F) [32.3°C (IQR 29.3°C to 35.3°C)]. Between 2016 and 2019, hot days were associated with 3044.0 (95% CI 2641.5 to 3380.9) or 0.067% (95% CI 0.058% to 0.074%) of all cardiovascular hospitalizations. Hot days were associated with 42132.2 (95% CI 41159.9 to 43082.1) or 0.22% (95% 0.21% to 0.225%) of all hospitalizations among Medicare beneficiaries.

Conclusions:
Between 2016 and 2019 days with high heat index levels were associated with higher levels of cardiovascular and all-cause hospitalizations among elderly Medicare beneficiaries across the contiguous US. As the number of hot days continues to increase due to climate change, the burden of heat related hospitalizations is likely to continue to increase.