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Exposure to extreme heat may double or triple the risk of irregular heart rhythms in people with implanted defibrillators, new research suggests.

Outdoor temperatures that reach or exceed 100 degrees Fahrenheit nearly tripled the number of episodes of an irregular heart rhythm known as atrial fibrillation, or AFib, in people who had electronic devices implanted in their chests to help monitor and regulate their heartbeats, the study found. will be presented Nov. 18 at the Ãå±±ÊÓƵ's Scientific Sessions in Chicago. They are considered preliminary until full results are published in a peer-reviewed journal.

"AFib is becoming more prevalent both in the U.S. and globally, with its incidence rising at an alarming rate," the study's lead researcher, Dr. Barrak Alahmad, said in a . Alahmad is a research fellow in environmental health at the Harvard T.H. Chan School of Public Health in Boston.

Study co-investigator Dr. Theofanie Mela, a cardiac electrophysiologist at Massachusetts General Hospital in Boston, said in the release that understanding the physiological processes underlying the study's findings would help researchers "concentrate on preventing conditions that trigger AFib to minimize the burden of arrhythmias. In the meantime, we recommend patients avoid extreme temperatures and use air conditioners so that they do not expose their bodies to the severe stress of extremely high heat."

More than 12 million people may be living with AFib by 2030, according to . Heart disease and other medical conditions are known to increase the risk of developing AFib, but how heat-induced stress may affect the condition has not been well studied.

In the new study, researchers analyzed health data for 2,313 adults in more than 100 U.S. cities who had an implantable cardioverter defibrillator (ICD) or cardiac resynchronization therapy defibrillator (CRT-D) implanted between 2016 and 2023. Participants were 71 years old on average when the devices were implanted and were predominantly men. Most participants had a condition called cardiomyopathy, which occurs when the heart cannot pump enough blood to the body. Their average body mass index also met the criteria for obesity.

Researchers matched the day of each patient's first AFib episode recorded on their devices to temperatures provided by local weather stations in the cities where the devices were implanted. Overall, the analysis found temperatures between 41 and 46.4 degrees Fahrenheit had the lowest odds of leading to an AFib episode, while temperatures that hit more than 102 degrees had the highest odds. At 102.2 degrees, the odds of having an AFib episode were 2.66 higher. They were 2.87 times higher at 104 degrees and more than three times higher if temperatures hit 105.8.

Compared to standard working hours (8 a.m. to 5 p.m.), fewer episodes of AFib occurred between midnight and 7 a.m. Episodes also were more common on weekdays than on weekends.

Dr. Sanjay Rajagopalan said in the release that the study may be the first to link ICD detection of AFib with temperature spikes. Rajagopalan, who chaired the on building heart-healthy and sustainable cities, noted that heat stress has been linked to an increase in stroke, heart attack and cardiovascular death. He was not involved in the new research.

"These findings reiterate other recent research about the link between outdoor temperature and cardiovascular health," said Rajagopalan, chief of cardiovascular medicine at University Hospitals Harrington Heart and Vascular Institute and a professor at Case Western Reserve University, both in Cleveland. "Given the rising prevalence of AFib in the general population due to increasing age and the prevalence of obesity, we now may also have to contend with rising temperatures."

Rajagopalan said susceptible individuals in areas at risk for extreme temperatures should take note of these findings and take appropriate precautions to stay cool and hydrated.

Unfortunately, he said, the people most at risk "may be those in lower socioeconomic groups who may not have access to air conditioning and other means of sheltering during warm days."

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