Abstract Body (Do not enter title and authors here): Background: Electronic Cigarettes (e-cig) are popular among young adults raising questions about the cardiovascular health impacts. In research settings, acute e-cig use leads to transient heart rate (HR) elevation. We evaluated how e-cig use intensity relates to HR during daily living using a text-messaging program partnered with continuous HR monitoring. Methods: We enrolled healthy young people (age 18-24) sole e-cig users and non-user controls in a text-messaging based program that assessed self-reported e-cig use intensity (vape sessions and puff count) over 5 two-hour time windows spanning awake period over 2 days. HR was collected using Fitbit and aligned to the text-messaging windows. We compared information collected about use intensity (vape sessions and puff number) to HR measures in two models: linear regression associations at a per person level over the total wake time and a repeated measures analysis based on individual time windows accounting for clustering by participant using compound symmetry. Results: In the 78 participants, mean age 21±2 years was similar in e-cig users (N=59, 47% women, 39% Asian, 7% Black, 9% Hispanic) and non-users (N=19, 61% women, 28% Asian, 22% Black, 6% Hispanic). At baseline e-cig users reported high intensity of use: 26±6 days/month, 16±20 vape sessions/day, 6±5 puffs/session. Over the 2-day monitoring period, total vape sessions reported was 23±26 and total puff 77±68 measured per participant and 2.4±4 vape sessions and 8.1±13.3 puffs per 2 hour window. In the per participant analysis, both higher number of vape sessions and puff counts were associated with higher average HR (β=0.08±0.03, P=0.01 and β=0.03±0.01, P=0.003) but not maximum or minimum HR. In the repeated-measures per window analysis, vape sessions and puff counts were associated with higher average HR (β=0.89±0.14, P<0.0001 and β=0.29±0.05, P<0.0001) as well as maximum HR (β=1.1±0.18, P<0.0001 and β=0.33±0.05, P<0.0001) and minimum HR (β=0.70±0.13, P<0.0001 and β=0.20±0.04, P<0.0001), providing temporal evidence linking variability in vaping intensity measures and HR measures. Conclusion: Our findings using an innovative mobile health platform suggest that the intensity of e-cig use is associated with higher HR during daily living in young people. Further investigation is needed to link short term hemodynamic impact of e-cig use to long-term health impacts.