The nicotine in e-cigarette vapor may cause adrenaline levels to spike in the heart, potentially increasing risk of heart attack and sudden cardiac death, a new study reports.

Electronic cigarettes have been promoted as a healthier alternative to tobacco cigarettes because they deliver vastly lower levels of carcinogens, researchers say.

But laboratory studies show that e-cigarettes still could pose a threat to health because of the nicotine that the devices typically deliver, said lead researcher Dr. Holly Middlekauff. She's a cardiologist with the David Geffen School of Medicine at the University of California, Los Angeles.

Nicotine-containing e-cigarettes appear to promote a fight-or-flight response by the sympathetic nervous system, causing a release of adrenaline that increases heart rate and reduces the amount of time between heart beats, researchers found.

"The concern is that extended use of nicotine is going to expose you to long-term high adrenaline levels in the heart," Middlekauff said. "That has been shown to be a risk factor for heart attack."

Previous studies have linked e-cigarettes to abnormal heart rate variability, or the time interval between heart beats, Middlekauff said. However, researchers weren't sure whether this was caused by nicotine in e-cigarettes or other chemicals contained in the vapor.

To figure this out, Middlekauff and her team recruited 33 healthy adults who do not smoke to use either e-cigarettes or tobacco cigarettes.

Participants went to the lab three times, performing a regimented 60 puffs over 30 minutes. But the device they used changed each time -- a typical e-cigarette containing nicotine one time, a nicotine-free e-cigarette at another visit, and a "sham" e-cigarette containing no liquid another time.

"Only after using the e-cigarette with nicotine did we see this abnormal pattern associated with high adrenaline levels in the heart," Middlekauff said.

The nicotine-delivering e-cigarette created a significant 20 percent shift in heart rate variability and a 10 percent increase in heart rate, Middlekauff said.

According to Aruni Bhatnagar, an American Heart Association spokesman, that finding is "an indicator there might be some adverse effects with the use of e-cigarettes. Rapid and persistent increases in heart rate and blood pressure cannot be good for you in any scenario." Bhatnagar is a professor at the University of Louisville School of Medicine in Kentucky.

Gregory Conley, president of the American Vaping Association, disagreed with this assessment, arguing that the researchers overstated their findings.

"Past studies have shown that eating meals high in carbohydrates induces heart rate variability. Are we going to be warning people away from corn flakes?" Conley said. "This is weak, inconsequential science produced by researchers who appear desperate to generate headlines."

Based on the results, Middlekauff said, current tobacco cigarette smokers still would be better off switching to e-cigarettes. They would avoid the carcinogens produced by burning tobacco, even though they could face heart health effects from nicotine.

At the same time, there's now evidence that e-cigarettes could pose a health risk to people who have never smoked tobacco because of the nicotine they contain, Middlekauff added.

"If you don't smoke at all, I would strongly recommend against using electronic cigarettes, because they're not harmless," she said.

The study was published Sept. 20 in the Journal of the American Heart Association.


_{SOURCES: Holly Middlekauff, M.D., cardiologist, UCLA's David Geffen School of Medicine, Los Angeles; Aruni Bhatnagar, Ph.D., professor and director, Diabetes and Obesity Center, University of Louisville School of Medicine, Louisville, Ky.; Gregory Conley, president, American Vaping Association; Sept. 20, 2017, Journal of the American Heart Association, online}