The new research was led by Daniel J. Conklin, Ph.D., an assistant professor at the University of Louisville in Kentucky.
Prof. Conklin and his colleagues set out to examine the effect of two aerosols commonly found in electronic cigarettes (e-cigarettes): propylene glycol and vegetable glycerin.
Although more and more people turn to e-cigarettes because of their perceived diminished health risks when compared with traditional cigarettes, the Centers for Disease Control and Prevention (CDC) warn that aerosols often contain cancer-causing chemicals and heavy metals, among other things.
The CDC also admit that the exact "health effects and potentially harmful doses of heated and aerosolized constituents of e-cigarette liquids [...] are not completely understood."
The new research — which will be presented at the American Heart Association's (AHA) Scientific Sessions 2017, held in Anaheim, CA — brings us one step closer to understanding these health effects, with a focus on their impact on the cardiovascular system.
Studying e-cigarettes' effects in mice
Prof. Conklin and team set out to examine the "acute electrocardiogram (ECG) effects of inhalation exposure" to the aerosols in so-called electronic nicotine delivery systems (ENDS).
ENDS include e-cigarettes and any other product that produces "an aerosolized mixture containing flavored liquids and nicotine that is inhaled by the user."
The researchers also wanted to see how these effects would fare compared with mainstream cigarette smoke.
To this end, they exposed healthy male mice to ENDS for 9 minutes, a period of time that qualifies as acute exposure. The mice had ECG transmitters implanted in them.
Additionally, the mice were exposed to smoke from traditional cigarettes with and without nicotine, as well as to ENDS constituents. The ECG measurements were compared with "time-matched, filtered-air controls."
E-cigarette aerosols slow down heart rate
The researchers found that being exposed to both ENDS aerosols and mainstream cigarette smoke quickly slowed down the rodents' heart rate.
In other words, they induced bradycardia. This is a condition that can sometimes cause problems, especially in older individuals. Doctors say that although not always an issue, bradycardia is something that people should get checked out if they have accompanying symptoms.
The researchers also found that exposure to the two aerosols extended the heart's electrical cycle.
Similar to mainstream cigarette smoke, the authors conclude, "ENDS aerosols strongly affect cardiovascular function in mice."
"Heated humectants generate aldehydes," they add. Aldehydes are toxic and known to contribute to cardiovascular and pulmonary disease.
Acrolein, acetaldehyde, and formaldehyde were also released when the two aerosols were heated. Of these, only acrolein seemed to cause a slow heartbeat in the mice.
Finally, the study also revealed that, before their heart rate dropped, the mice also exhibited an increase in blood pressure.
The evidence "suggests that the use of ENDS may increase risks of arrhythmia and overall [cardiovascular disease]," conclude the researchers.
Relevance for human use
In their policy statement on the toxicity of e-cigarettes, the AHA write, "Although animal models have obvious limitations, [...] these models could be useful in assessing the pharmacokinetic, pharmacodynamic, and toxicokinetic properties of e-cigarette exposures."
"The pathophysiological outcomes and biomarkers, identified in animal studies," they add, "should also be evaluated in controlled human exposure studies to develop validated concordance between animal and human data."
It is worth mentioning that e-cigarette smoking is becoming more and more prevalent, particularly among young people. In fact, between 2013 and 2014, the number of young adults who said that they had used e-cigarettes at least once in their lives doubled.
In 2016, over 11 percent of high schoolers and more than 4 percent of middle school students admitted to having used e-cigarettes.