Whether it’s a horror, thriller, or action movie you’re watching, chances are that you’re on the edge of your seat — or under the covers — pretty frequently. These intense movies have you wondering impatiently what’s going to happen next, as you yell at the girl in the movie to stop what she’s doing and “run!” These movies tend to be wildly entertaining, but that suspense that gets your heart going may not be best for everyone, according to a new study, which found that people who have weakened hearts may be at risk of complications.

The study was small, however, it points to a potential risk factor when assessing people with weakened hearts — emotional and mental stress. The researchers, from the University College London at Kings College London, found that people who had weakened hearts were more likely to experience rising blood pressure, quicker breathing, and changing rhythms in their heartbeats.

“This is the first time that the effects have been directly   measured and although the results varied from person to person, we consistently saw changes in the cardiac muscle,” said Dr. Ben Hanson, of UCL’s Mechanical Engineering, in a press release. “If someone already has a weakened heart, or if they experience a much more extreme stress, the effect could be much more destabilizing and dangerous.”

For their study, the researchers looked at 19 patients’ hearts undergoing cardiac catheterization treatment, a procedure used to diagnose and treat heart conditions. They placed electrodes on the patients’ hearts and showed them all a clip from the film Vertical Limit, which, if you haven’t seen it, is about a rescue mission atop the world’s second highest, but most dangerous mountain, K2. Then, they looked at how the patients’ hearts responded.

“Film clips are considered to be among the most powerful stimuli to elicit affective responses in the laboratory setting, and have several advantages including their dynamic nature, as sustained effect, and the combination of visual and auditory inputs,” study author Peter Taggart said in the release. Indeed, each patient showed changes in breathing, as well as heartbeat and blood pressure.

A follow-up experiment then tested the patients to see if they could replicate their breathing patterns from the first experiment, and whether the patterns would cause changes in blood pressure and heart rate. They didn’t. The researchers attributed this to the emotional experience coupled with watching the clip from Vertical Limit.

It shouldn’t come as a surprise to most people. Watching thrilling movies, such as horror, are akin to psychological rollercoasters, as the neurotransmitter dopamine is released in the body. The hormone is released during rewarding — such as when a character gets out of a tight situation alive — or sinful experiences, such as during sex, alcohol consumption, or when using drugs like heroin or cocaine.

The researchers called the findings exciting, and said that they will help them understand the mechanisms involved between emotional stress and the heart. In the meantime, it might incline moviegoers to know that just like flashing lights may affect those with epilepsy and seizures, action movies may affect those with heart issues.

Source: Taggart P, Child N, Hanson B, et al. Effect of Mental Challenge Induced by Movie Clips on Action Potential Duration in Normal Human Subjects Independent of Heart Rate. Circulation: Arrhythmia and Electrophysiology. 2014.