Effects of Electronic Cigarette Liquid Solvents Propylene Glycol and Vegetable Glycerin on User Nicotine Delivery, Heart Rate, Subjective Effects, and Puff Topography.

Participants took significantly longer puffs in the 2PG:98VG condition compared to the 55PG:45VG and 100PG conditions in bout 1 (p < 0.05).
In bout 2, subjects took significantly longer puffs in the 2PG:98VG condition relative to all other conditions. For all conditions, with the exception of the 100PG condition, participants took longer and larger average puffs in the second bout compared to the first bout (p < 0.05).
For all conditions combined, mean plasma nicotine concentration increased significantly (p-value not specified) from a baseline level of 2.60 ng/mL to 10.40 ng/mL after the first bout, and to 11.11 ng/mL after the second bout; the difference between bout 1 and bout 2 was not significant (p-value not specified).
When measurements were compared by treatment group, after the second bout, subject's mean plasma nicotine concentration was significantly higher in the 100PG condition (13.40 ng/mL) compared to the mean plasma concentrations in the 20PG:80VG (9.59 ng/mL) and 2PG:98VG (8.58 ng/mL) conditions (p <0.017). Furthermore, the subjects' total nicotine concentration was significantly greater in the 100PG condition (276.75 ng min/mL) compared to the 2PG:98VG condition (178.32 ng min/mL) in bout one and the total nicotine concentration was significantly greater in the 100PG condition (373.24 ng min/mL) compared to both the 2PG:98VG (257.75 ng min/mL) and 20PG:80VG conditions (251.93 ng min/mL) (p<0.05). Mean heart rate did not change significantly by treatment group.
For bout 1, mean AUC for the 100 PG condition was significantly greatly relative to the mean AUC for the 2PG:98VG condition (p < 0.05). For bout 2, mean AUC was significantly greater for the 100PG condition than the 2PG:98VG condition and the 20PG:80VG condition (all p < 0.05).
Mean heart rate increased significantly from 63.71 beats per minute at baseline, to 70.98 beats per minute after bout 1, and to 70.92 beats per minute after bout 2 (p<0.05).
Compared to baseline, scores for anxious, craving, difficulty concentrating, drowsy, and urge on the Hughes-Hatsukami withdrawal scale were significantly reduced after each e-cigarette puffing bout (p's<0.05). Factor one scores on the Tiffany Drobes QSU-Brief were significantly reduced after each bout as well (p's<0.05). Participants reported feeling significantly less awake in the 100PG condition relative to the 2PG:98VG condition after the first bout (p<0.017) and participants reported significantly lower scores for awake, calm, pleasant, and satisfying in the 100PG condition relative to the other conditions. Both the perception of harshness and throat hit were significantly stronger for the 100PG condition compared to the 2PG:98VG and 20PG:80VG conditions (p's<0.017). The authors concluded that "results from the present study revealed that the [e-cigarette] liquid containing PG as the only solvent delivered more nicotine, reduced mean puff duration and volume, and was less satisfying overall to participants than the two VG-based liquids" (p. 198). The authors proposed assigning a standard PG:VG ratio to the regulatory framework.

Spindle, T. R., Talih, S., Hiler, M. M., Karaoghlanian, N., Halquist, M. S., Breland, A. B., ... & Eissenberg, T. (2018). Effects of Electronic Cigarette Liquid Solvents Propylene Glycol and Vegetable Glycerin on User Nicotine Delivery, Heart Rate, Subjective Effects, and Puff Topography. Drug and Alcohol Dependence, 188, 193-199.