The use of certain electronic cigarettes may increase the risk of cardiac sympathetic dominance, electrical dysfunction, and arrhythmias, according to preclinical research published by Kucera et al in Nicotine & Tobacco Research. In the study, researchers exposed electrocardiogram-telemetered C57BL/6J mice to filtered air or electronic cigarette aerosols from propylene glycol and vegetable glycerin solvents without nicotine (vehicle) or with 1%, 2.5%, and 5% concentrations of nicotine for three 9-minute sessions per increasing concentration, with the goal of determining the cardiovascular effects of different types of nicotine-containing electronic cigarettes. The researchers included three types of nicotine in the study during the exposure and washout phases: racemic, free base, and salt. Compared to mice exposed to filtered air or vehicle, those exposed to ≥ 2.5% concentrations of racemic nicotine–containing electronic cigarettes experienced reduced heart rates and increased heart rate variability, demonstrating parasympathetic dominance. Further, compared with both control groups, those exposed to 5% concentrations of nicotine salt–containing electronic cigarettes experienced elevated heart rates and decreased heart rate variability during the washout phases, demonstrating sympathetic dominance and increased ventricular premature beat frequency. In mice pretreated with atenolol, electronic cigarette–induced elevations in heart rate and reductions in heart rate variability were abolished—representing the ability of beta-1 adrenergic receptor stimulation to mediate electronic cigarette–evoked sympathetic dominance. The researchers concluded that if they are able to validate their findings in human studies, regulatory measures should subject nicotine salt manufacturers to implement minimum pH standards or acid additive limits to mitigate the adverse cardiovascular effects of using electronic cigarettes.