Smoking just one e-cigarette might expose users to enough nicotine to trigger an adrenaline surge in the heart that can contribute to high blood pressure and other health problems, a small experiment suggests.

For the experiment, researchers asked 33 healthy nonsmokers to come to a lab on different days to try an e-cigarette with nicotine, a nicotine-free alternative, and a sham device that wasn’t a real e-cigarette. Each time, participants wore a heart rhythm monitor to assess variability in the time between heartbeats, an indicator of increased adrenaline.

People experienced increased adrenaline only when they smoked e-cigarettes with nicotine, and not with the nicotine-free alternatives or sham devices.

Smoking traditional cigarettes has long been known to increase adrenaline levels in the heart, and the current findings suggest that nicotine in e-cigarette emissions does this too, said senior study author Dr. Holly Middlekauff, of the David Geffen School of Medicine at the University of California, Los Angeles.

“Inhaled nicotine delivered by e-cigarettes, traditional cigarettes and other tobacco products (e.g. hookah) rapidly enters the bloodstream and the brain and stimulates nerves that carry adrenaline and the adrenal gland to release adrenaline,” Middlekauff said by email.

“Perpetually increased cardiac adrenaline levels detected by abnormal heart rate variability are associated with increased risk for heart attack and sudden death,” Middlekauff added.

Big U.S. tobacco companies are all developing e-cigarettes. The battery-powered gadgets feature a glowing tip and a heating element that turns liquid nicotine and flavorings into a cloud of vapor that users inhale.

Some previous research suggests vapor from e-cigarettes may be less toxic than traditional cigarette smoke, but the electronic alternatives still release chemicals that aren’t normally in the air and the long-term health effects of the ingredients and flavorings in e-cigarettes are unclear.

The current study doesn’t offer a complete picture of e-cigarette safety or address the potential for health problems to emerge after long-term use.

But it does reinforce concerns about the heart risks of nicotine, the researchers note in the Journal of the American Heart Association.

Epinephrine, commonly called adrenaline, is a hormone released when something triggers a so-called fight or flight response in the body’s sympathetic nervous system. Sudden fear or anger can lead to a sudden release of adrenaline that may increase the heart rate and blood pressure.

The experiment also looked at what’s known as oxidative stress, which increases the risk for hardening of the arteries and heart attack. In blood tests, researchers didn’t find changes in markers for oxidative stress after people used e-cigarettes with and without nicotine.

One limitation of the study, beyond its small size, is that it only looked at a small number of markers for oxidative stress.

In addition, because the experiment only looked at a single time smoking e-cigarettes, it doesn’t offer insight into the long-term health effects of these devices or suggest how the number of e-cigarettes smoked during one occasion or over a single day might impact the heart.

Even so, it makes sense that e-cigarettes and traditional cigarettes could have a similar impact on the heart when they expose users to similar amounts of nicotine, said Riccardo Polosa, director of the Center for Smoking Prevention and Treatment at University of Catania in Italy.

“Providing that the e-cigarettes deliver enough nicotine, the physiological responses of the human body to vaping are no different from those of smoking traditional cigarettes,” Polosa, who wasn’t involved in the study, said by email.

However, it’s also possible that e-cigarettes may still be less harmful than traditional cigarettes because they have lower concentrations of cancer-causing chemicals, Polosa added.

“Moreover, patients who reduce or quit smoking by switching to e-cigarettes may improve their cardiovascular, respiratory and metabolic outcomes in the long term,” Polosa said. “Former smokers using and smokers intending to use e-cigarettes should receive correct information about the residual risks and potential benefits of these products.”