Is your smartwatch trying to kill you? No, of course not. But if you have a pacemaker, you’d be wise to exercise a bit of caution, according to a new study from the University of Utah.

Specifically, the researchers call for more study into the effect of those that use bioimpedance to analyse body composition. This is when two electrodes on a device pass a tiny electrical current through the body, typically to measure body fat, muscle and water percentages, but sometimes for things like stress.

The current is tiny, for obvious reasons, and therefore completely harmless in most cases. But for those with pacemakers, it could spell trouble, as CIEDs (Cardiac-implantable Electronic Devices) can interpret the electrical signal as coming from the heart, not a wearable. In other words, the pacemaker may assume that the heart is operating in the normal beats-per-minute range and thus down tools.

“If the pacemaker gets confused by interference, it could stop working during the duration that it is confused. If that interference is for a prolonged time, the patient could pass out or worse,” said co-author, Benjamin Steinberg, an associate professor of medicine.

That “worse” doesn’t sound like something you want to test. And for those with implantable cardioverter-defibrillators, it could be nasty too. Interference from a bioimpedance reading could make the defibrillator give the wearer an unnecessary and unpleasant electric shock.

While the study specifically highlighted the Samsung Galaxy Watch 4, Fitbit smart scales and Moodmetric smart rings, other devices use bioimpedance, including Withings devices and Samsung’s newer Galaxy Watch 5.

Even though this isn’t an area that’s had much study devoted to it, certain manufacturers aren’t taking any chances. The Samsung support page for measuring body composition has a note saying “do not measure if you have an implanted cardiac pacemaker, defibrillator, or other electronic medical devices are inside of your body” as well as warning pregnant users to stay away.

Withings equally warns off those with pacemakers. “You should not use the Body+ if you wear a pacemaker or other internal medical device,” the on-site FAQ says, with guidance on how to turn off the feature. 

It’s not an immediate risk to patients, the researchers say, but it is a reason to study the implications further. “We need to test across a broader cohort of devices and possibly in patients with these devices,” Steinberg said. Pacemaker users, form an orderly queue.