The question of e-cigarette safety burns on.
There is little doubt that these devices deliver fewer carcinogens to the user, but, because they often contain nicotine, conversations regarding their safety are ongoing.
On the one hand, e-cigarettes offer a relatively safe option for nicotine-addicted individuals. On the other hand, they are seen by some as a new route to addiction with health concerns of their own.
Research published this week in the Journal of the American Heart Association investigates the potential heart health implications of non-smokers using nicotine-based e-cigarettes.
Lead study author Dr. Holly Middlekauff, of the University of California, Los Angeles, says, "While e-cigarettes typically deliver fewer carcinogens than are found in the tar of tobacco cigarette smoke, they also usually deliver nicotine."
E-cigarettes and the sympathetic response
Nicotine is not a carcinogen, but it is still a drug. It is a sympathomimetic, which is a compound that mimics the sympathetic nervous system, increasing adreline levels in circulation and raising heart rate and blood pressure. These are physiological changes associated with the "fight or flight" response.
It is this activation of the sympathetic nervous system and the influx of adrenaline that worries some medical researchers. These types of actions are, over the long-term, linked with increased cardiovascular risk.
Cardiac sympathetic nerve activity can be measured noninvasively using a heart monitor to detect heart rate variability (HRV), which is the variability in the duration between heartbeats. This measure can be used as a predictor of cardiovascular disease; lower HRV increases risk.
This rise in cardiac sympathetic nerve activity and its associated rise in circulating adrenaline, combined with a lack of long-term data on e-cigarettes, creates concern as to their overall safety.
In other research, Dr. Middlekauff's team showed that chronic e-cigarette use contributed to increased resting cardiac sympathetic nerve activity.
The current study was designed to find out whether this effect could be seen in acute, or short-term, use of e-cigarettes, and whether it is due to nicotine or other ingredients present in the devices - such as propylene glycol and vegetable glycerin.
E-cigarettes, nicotine, and cardiac risk
In total, 33 healthy volunteers - none of whom smoke cigarettes or e-cigarettes - were involved in the study. Each participant, on separate days, smoked an e-cigarette with nicotine, one without nicotine, and a sham (empty) e-cigarette.
According to the study authors, this is the first study of its kind to separate the nicotine from the non-nicotine components of e-cigarettes in this way.
For each individual, HRV was measured. A blood sample was also taken to assess oxidative stress by measuring levels of an enzyme called plasma paraoxonase.
After analysis, the team found that HRV was significantly altered when individuals used the nicotine e-cigarette but not in the non-nicotine and sham conditions. However, they saw no significant differences in markers of oxidative stress.
Dr. Middlekauff explains how the findings add to the body of evidence against nicotine as a safe drug. "While it's reassuring that the non-nicotine components do not have an obvious effect on adrenaline levels to the heart," she says, "these findings challenge the concept that inhaled nicotine is benign or safe."
She says, "Our study showed that acute electronic cigarette use with nicotine increases cardiac adrenaline levels. And, it's in the same pattern that is associated with increased cardiac risk in patients who have known cardiac disease, and even in patients without known cardiac disease."
Limitations and future research
The study has limitations; it included only a small number of participants and studied just one of the thousands of e-cigarette fluids. For this reason, the authors are keen to extend their findings.
"I think that just seeing this pattern at all is very concerning and it would hopefully discourage non-smokers from taking up electronic cigarettes."
Dr. Holly Middlekauff
In the future, the researchers plan to continue their studies - the team would like to investigate this effect in habitual e-cigarette smokers and take a more in-depth look at the potential role of oxidative stress.
The current findings are likely to intensify an already intense debate. The take-home message is that e-cigarettes are less likely to cause cancer, but they are not without their own dangers.