AF events may soar up to threefold in extreme heat

High outdoor temperatures may predispose individuals to atrial fibrillation (AF), with the odds increasing two- to threefold at extremely high temperature levels, according to a study.

Analysis of hourly arrhythmic events among thousands of patients with passive rhythm monitoring through implanted devices showed a significant dose–response relationship between high outdoor temperatures and AF events.

With a preselected optimal temperature of 19 °C as the reference, the odds of AF events doubled at temperatures of 39 °C (odds ratio [OR], 2.41, 95 percent confidence interval [CI], 1.37–4.25), 40 °C (OR, 2.60, 95 percent CI, 1.38–4.89), and 41 °C (OR, 2.81, 95 percent CI, 1.40–5.63). [J Am Heart Assoc 2025;doi:10.1161/JAHA.124.040352]

At the maximum temperature of 42 °C, the OR for AF further increased to 3.03 (95 percent CI, 1.41–6.49).

Results were consistent across several analyses that used lagged temperature exposures, restricted to episodes longer than 30 min, removed humidity from the model, and stratified according to region and subregion.

No significant association was observed between high temperatures and ventricular tachycardia (VT)/ ventricular fibrillation (VF). Additionally, cold temperatures had no effect on the risk of atrial tachycardia (AT)/AF and VF/VT.

“Reports on the relationship between extreme temperatures and all arrhythmias have yielded mixed findings,” with more recent studies showing complex relationships, the investigators noted.

For example, a Taiwanese study identified “apparent” but not ambient temperatures as a contributing factor to increased ventricular tachyarrhythmias, particularly in susceptible patients. Additionally, a large study in North Carolina, US, indicated that “humidity,” rather than just hot temperatures, was a key factor in ventricular arrhythmias. [Front Med (Lausanne) 2020;7:624343; JACC Adv 2025;4:101463]

“Our analysis expands on these findings by including multiple locations across different climate zones in the US, showing that very extreme hot ambient temperatures may now increase arrhythmia. The temperature ranges observed in our analysis were unlikely to have been encountered in these earlier studies,” the investigators said.

“While our findings are still far from direct clinical implications, this multicity analysis … supports consideration for precautionary measures for at‐risk [individuals],” they added.

For the study, the investigators used data from the Product Surveillance Registry by Medtronic (2016–2023) in the US and looked at 3,079 individuals with implantable cardioverter‐defibrillator (ICD) or cardiac resynchronization therapy defibrillator devices. Overall, 2,536 AT/AF events and 1,348 VT/VF events were recorded. These events were matched to ambient temperatures from local weather stations.

“An incidental finding from this analysis was that some arrhythmic events occurred more frequently during working hours (9 am–4 pm) and weekdays,” which is consistent with a previous study pooling multiple cohorts of patients with ICDs showing higher occurrences of ventricular arrhythmia during daytime hours (8 am–10 pm) than at midnight (12–1 am), according to the investigators. “[This suggests] a potential circadian influence on arrhythmic events.” [JACC Clin Electrophysiol 2019;5:979-986]

They acknowledged that the study was limited by the inability to analyse individual demographic and comorbidity data due to logistical and data restrictions related to privacy concerns, reliance on outdoor temperatures from the nearest meteorology station to assign exposure, and dependence on device algorithms to detect arrhythmic events.

“Looking ahead, future studies should improve exposure assessment by integrating personalized wearable sensors, which offer a cost‐effective way to capture real‐time heat exposure. Second, expanding research to high‐risk regions, such as the Stroke Belt [in the US], will help identify population‐specific vulnerabilities. Third, mechanistic studies are needed to distinguish how extreme temperatures differentially impact VT/VF versus AT/AF,” the investigators said.

“Finally, interventional studies should explore strategies to mitigate climate‐related arrhythmic risk, informing targeted prevention and adaptation measures in a warming world,” they added.