UCLA

Over the last decade, e-cigarette (EC) use has increased rapidly, garnering widespread attention, particularly in teens and young adults. In addition, with its perceived assumption of safety bolstered by controversial findings indicating ECs toxicants to be found at lower levels than in tobacco cigarettes (TCs), ECs have spurred much debate in public health about its role in contributing to adverse health effects. However, while multiple studies support a causal link between tobacco cigarette (TC) use and a decline in health, there is a paucity of data with EC use in particular with cardiovascular (CV) health. Furthermore, previous studies have primarily focused on CV vital signs (i.e.: heart rate and blood pressure) using varying nicotine concentrations, flavors and EC designs thereby contributing to EC polemics. Since no empirical data are available for the increased risk of CV disease from EC exposure, we must examine CV health from a different perspective. Indeed, one human study assessing heart rate variability (HRV) found increased sympathetic activation (cardiac autonomic dysregulation) in “chronic” EC users, a biomarker known to indicate reduced cardiovascular health from TC exposure. However, EC-mediated alteration in heart rate variability has not been studied in an acute setting. In addition, no animal models exist with respect to HRV analysis and EC use that take into consideration human vaping topography like puff duration, inter-puff interval, and the episodic nature of EC use. Therefore, we aimed to investigate the role acute EC exposures play in changing heart rate variability using an in-vivo exposure system in which real-life conditions could be reflected.

Telemetry devices were implanted in the abdomen of six eight-week-old C57BL/6 mice to monitor electrocardiographic activity continuously. Mice underwent a 1-hour acclimation phase in exposure chambers followed by a 1-hour exposure to air (control). E-cigarette exposures consisted of two 15-minute sessions (4-sec puff/26-sec air) to a BluPLUS+ device containing 2.4% nicotine with Classic Tobacco flavoring. To validate previous studies using the same exposure system, a 15-minute period of aerosolized phosphate-buffered saline (PBS) was added as a secondary control. For each exposure episode, a 45-minute post-exposure event followed. Ponemah v6.20 software was used for heart rate variability analysis of the time and frequency domain. Additionally, real-time particle number and mass concentrations as well as size distributions were monitored with a scanning mobility particle sizer and an aerodynamic particle sizer for particles within the 7.37 – 289 nm and 0.5 – 19.8 �m size ranges, respectively. In addition to real-time measurement, a personal cascade impactor was used to measure particulate matter with an aerodynamic diameter of 2.5 or below (PM2.5) mass concentration during a 15-minute exposure period following the gravimetric method.

TSI Data Merge Software Module (TSI Inc., Shoreview, MN), which converts electronic mobility diameter measured by the SMPS to aerodynamic diameter was used to merge SMPS data with APS resulting in composite size distributions. Aerosol data was then plotted using normalized concentrations (dN/dlogDp) to correct for differences between instruments and provide a best fit for single source aerosols. Frequency domain parameters were log-transformed and a linear mixed-effects model with a robust estimator was used to determine the relationship between EC events vs. air and PBS and frequency parameters were log-transformed. R (Version R3.6.3) was utilized to test for statistical significance at α = 0.05.

Size distributions and temporal profiles of particle number and mass concentration analysis revealed the ability for the EC aerosol exposure system to effectively deliver EC aerosol in an acute manner. Additionally, the detection of a bimodal distribution of ultrafine and fine particles support previous research and may indicate these particles were generated from pyrolysis, metals and volatile organic compounds. Furthermore, high levels of PM2.5 levels (8072 μg/m3) were detected during a 15-minute exposure period. With respect to HRV, the standard deviation of normal sinus beats (SDNN) and the root mean square of successive differences between normal heartbeats (RMSSD) were analyzed to measure non-physiological changes of autonomic variability. EC exposures significantly decreased SDNN and RMSSD by 5.00 (CI95% [-8.28, -1.23]) and 9.34 (CI95% [-15.01, -3.67]) units as compared to air exposures, suggesting reductions in total and short-term autonomic variability. In contrast, the proportion of normal consecutive heartbeats differing by six, (pNN6) was elevated (14.72, CI95% [11.96, 17.47]). However, further studies are needed to corroborate the reliability of this measure.

Frequency domain parameters were also analyzed to determine physiological changes in HRV. Such parameters included in this study were low frequency (LF), an indicator suggestive of sympathetic activity, high frequency (HF), a marker for sympathetic tone, and the ratio of the LF to HF (LF/HF), a parameter reflecting sympathovagal balance. Significant increases in log transformed low frequency (LF) by 1.33 units (CI95% [1.14, 1.52]),) were also observed. Results for the log (HF) parameter (0.51, CI95% [0.28, 0.73]) also indicated the presence of the parasympathetic branch during the EC exposures, but the magnitude was less than that of LF. Even so, log (LF/HF) by 0.11 units during EC exposure in comparison to combined air suggest parasympathetic predominance. However, this measure may not be a true indicator of the balance between the sympathetic and parasympathetic branches. Furthermore, when EC exposures were compared to PBS, differences were more striking in almost all HRV parameters. Moreover, when looking at individual EC exposures, larger differences were observed during the second EC exposure episode than the first which may be important for chronic exposure assessment. In short, this study demonstrates that acute ECs have the potential to induce changes to HRV which may have implications in autonomic dysregulation.