Sex Differences in the Effects of High Temperature on Cardiovascular Disease Hospitalizations in Montreal

Background: Climate change has increased temperatures in Canada. Although heat-related mortality is well documented, less is known about its impact on cardiovascular disease (CVD) morbidity, particularly hospitalization patterns. This study examines the association between ambient temperature and CVD hospitalizations in Montreal, focusing on sex-stratified vulnerability. Methods: A time-series analysis was conducted in Montreal, Quebec, examining CVD hospitalizations during the warm season (May 1-September 30) from 2018-2022. Ambient temperature was represented by daily mean temperature. Analyses were stratified by sex and heat vulnerability index (HVI) levels. The HVI integrates multiple vulnerability dimensions (exposure index, sensitivity index, and coping capacity index); higher HVI scores indicate greater vulnerability to extreme heat impacts. Daily mean temperature (lag, 0-7 days) and CVD hospitalizations were analyzed using distributed lag nonlinear models, adjusting for possible confounders. Results: Among 262,740 CVD patient hospitalizations (mean age 76.4 years; 49.6% female), the relative risk (RR) for CVD hospitalization at the 99th percentile of temperature (28.1°C), as compared to the identified minimum morbidity temperature (MMT; 23°C) for the total population was 1.08 (95% confidence interval [CI]: 1.05-1.11). Although overall heat-related risk was similar between sexes, vulnerability components revealed differences. Female participants exhibited significant cardiovascular risk in areas with poor adaptive resources based on coping capacity index (RR = 1.13, 95% CI: 1.04-1.23), whereas male patients did not (RR = 1.03, 95% CI: 0.96-1.12), suggesting greater susceptibility when community-level resources are limited. Conclusions: Female participants demonstrated greater vulnerability in areas lacking adaptive resources, indicating that sex-specific heat-protection strategies should prioritize enhancement of community-level cooling infrastructure and support systems.