Heat waves may double or triple the risk of the irregular heart rhythm atrial fibrillation, particularly for people whose hearts are not healthy to start with, a new study suggests.

Researchers tracked more than 2,000 people across the U.S., who were chosen for the study because they have implanted cardiac devices that continuously monitor their heart activity. Most of them had obesity, and all had weak heart muscles that struggled to pump enough blood to the body.

When outdoor temperatures reached 102.2 degrees Fahrenheit (39 Celsius), the odds of experiencing an episode of atrial fibrillation were 2.66 times higher than with outdoor temperatures between 41 F and 46.4 F (5 C to 8 C).

That rose to a 2.87 times higher chance of experiencing AFib at 104 F, and 3.09 times higher at 105.8 F.

AFib episodes occurred less often between 12 a.m. and 7 a.m. than during standard working hours (8 a.m. to 5 p.m.) and more often on weekdays than weekends, researchers, who will present the data at the AHA meeting, also found.

"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,” AHA spokesperson Sanjay Rajagopalan, MD, of Case Western Reserve University in Cleveland, who was not involved in the study, said in a statement.

“Susceptible individuals living in areas at risk for markedly increased temperatures should take note of these findings and ensure that they take appropriate precautions to stay cool and hydrated,” he added.