Athletes with implantable cardioverter defibrillators may play competitive sports, according to four-year research that confirms an earlier short-term study.

An ICD is a battery-powered device placed under the skin that tracks the heart rate and delivers an electric shock when it detects an abnormal heart rhythm called an arrhythmia.

A 2015 American Heart Association scientific statement said athletes with ICDs may be considered for participating in competitive sports, based on a 2013 study in which researchers followed 372 athletes with ICDs from a multinational registry for two years.

In the new study, researchers followed 440 ICD recipients 10-60 years old from the same registry for four years. They included competitive athletes at national and international levels, high school and college athletes, and others who participated in vigorous sports such as running, basketball, soccer, tennis, volleyball, skiing and snowboarding.

During the study, 121 people received a total of 184 shocks. Among them, 7 percent received appropriate shocks while participating in competition or practice (identical to the 2013 study); 5 percent received shocks during other physical activities; and 6 percent received shocks while resting.

“Even though some people did receive shocks while they were participating in sports, no harm came to patients,” said Rachel Lampert, M.D., professor of internal medicine at the Yale School of Medicine in New Haven, Connecticut. “After a four-year follow-up, we still did not see any failures of the device to terminate an arrhythmia or injuries related to the arrhythmia or device in these patients.”

Vigorous sports may be less safe for athletes who have arrhythmogenic right ventricular cardiomyopathy, a rare inherited condition in which muscle tissue in the right ventricle dies and is replaced with scar tissue, which disrupts the heart’s electrical signals and causes arrhythmias.

“Patients with ARVC were more likely than others to experience life-threatening ventricular arrhythmias requiring shock during sports, and more likely to require multiple shocks to terminate their arrhythmias during physical activity,” Lampert said.  “While all the shocks were ultimately successful in this study, this raises concern that vigorous physical activity could lead to arrhythmias that would not be terminated by the device. Although the ICD did work eventually and got the ARVC patients out of the arrhythmias, we don’t like to see people requiring multiple shocks.”

Other athletes who have these devices and want to participate in sports should talk to their doctors, because the risk for many of them is lower than had been previously thought, Lampert said.

The new research is published in the American Heart Association journal Circulation.