Extreme heat triggers first-time acute myocardial infarction: Evidence from a case-crossover study in Tianjin, China.

Objective: To assess whether extreme high temperatures (EHT) and heat waves (HW) are associated with acute myocardial infarction (AMI) risk, and to determine whether these effects differ between first-time and recurrent events. Methods: We performed a time-stratified case-crossover study of 7573 AMI admissions in Tianjin, China (2021-2023). Meteorological exposures were derived from ground-based monitoring and satellite products. Conditional logistic regression with distributed lag nonlinear models quantified immediate and delayed associations between temperature and AMI, stratified by event type (first-time vs recurrent). Effect modification was examined by age, sex, and ST-segment elevation. Results: Both EHT and HW were associated with increased AMI risk, with effects concentrated among first-time events. The strongest single-day associations for first-time AMI were observed 4-5 days after exposure (peaking at lag 4 for EHT_90TH: OR = 1.1594; 95 % CI: 1.0622-1.2655), while cumulative risks peaked across lag windows of 4-6 days (peaking at lag 0-6 for EHT_90TH: OR = 1.1564; 95 % CI: 1.0558-1.2665). Among first-time AMI, higher risks were seen in men, individuals <65 years, and those with non-ST-segment elevation myocardial infarction. Conclusions: Short-term exposure to extreme heat significantly increases the risk of first-time AMI, but not recurrent events. Risk persists for several days after exposure, particularly among younger adults, men, and NSTEMI patients. These findings highlight the need for targeted heat-health messaging, early recognition of symptoms, and preventive strategies tailored to individuals at risk of a first AMI in the era of climate change.