Researchers say although interference is rare, people with cardiac devices should be cautious when they are near smartphones.

If you have an implanted or wearable cardiac device, you should probably avoid being too close to a smartphone, just to be safe.

According to research presented at a joint meeting of the European Heart Rhythm Association and the European Society of Cardiology and Cardiostim, smartphones and cardiac devices don’t always mix.

Dr. Carsten Lennerz, study lead author and cardiology resident in the Clinic for Heart and Circulatory Diseases at Munich’s German Heart Centre, said that pacemakers in rare instances can mistakenly detect electromagnetic interference from mobile phones.

The devices can then interpret the interference as a cardiac signal, causing them to briefly stop working. This can sometimes result in the patient fainting.

The results are worse for implantable cardioverter defibrillators (ICDs). Researchers say the signal from smartphones can mimic ventricular tachyarrhythmia, causing the device to deliver a painful shock. This can be life-threatening.

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Device manufacturers and regulatory agencies such as the U.S. Food and Drug Administration (FDA) already recommend keeping cardiac devices at least 15 to 20 centimeters away from mobile phones. That advice is based on studies performed mostly in pacemakers about a decade ago.

Since then, smartphone and mobile standards have changed. Newer cardiac devices have come on the market in the past decade as well. These include ICDs, cardiac resynchronisation therapy (CRT) devices, and MRI compatible devices.

Lennerz’s study looked at whether or not the original recommendations were still relevant considering the new equipment on the market.

In the study, 308 patients were exposed to the electromagnetic field in three popular phones: the Samsung Galaxy 3, Nokia Lumia, and HTC One XL. The phones were placed on the skin directly above 147 pacemakers and 161 ICDs, including 65 CRTs.

Next, the mobile phones were connected to a radio communication tester to emulate a mobile network station. The researchers placed calls, let the phones ring, spoke on them, and disconnected calls.

Those functions were performed at the maximum transmission power. They were also conducted at 50 hertz, a frequency known to influence cardiac implantable electronic devices. Patients then had electrocardiograms.

Lennerz noted that earlier studies showed the most vulnerable phases of a phone action were ringing and connecting to the network — not talking.

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More than 3,400 tests were performed. One out of the 308 patients was affected by electromagnetic interference caused by a phone. In that case, the patient’s ICD misdetected electromagnetic waves from the Nokia and HTC smartphones.

“Interference between smartphones and cardiac devices is uncommon but can occur so the current recommendations on keeping a safe distance should be upheld,” Lennerz said in a statement.

Lennerz added that the affected device in the study was MRI compatible. This shows that those devices are also susceptible, he said.

Christof Kolb, a professor and also an author of the study, said there is the possibility of interference between cardiac devices and smartphones when they are too close.

Kolb recommends that patients with cardiac devices should never place their phone over the device, such as in a coat’s breast pocket. When talking, he said patients should hold their phone to the ear opposite of their device. The researchers also say that patients should limit exposure to high-voltage power lines.

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Dr. Marie-Noelle Langan, the medical director of operations for electrophysiology at the Icahn School of Medicine at Mount Sinai in New York City, said patients should follow the recommendations, but they should not be alarmed.

“The researchers showed that the potential impact of these devices is quite small, especially for those only talking on cell phones,” she said.

Langan agreed that patients should not leave their phones in a pocket near the device.

Additionally, using technology to monitor devices daily at home can be helpful. That technology can warn the doctor if any interruptions are recorded. In using a monitoring device, patients can better assess their environment to avoid recurrences.