Effects of Cold Stimulation on Cardiac-Vagal Activation in Healthy Participants: Randomized Controlled Trial

Abstract

Background: The experience of psychological stress has not yet been adequately tackled with digital technology by catering to healthy individuals who wish to reduce their acute stress levels. For the design of digitally mediated solutions, physiological mechanisms need to be investigated that have the potential to induce relaxation with the help of technology. Research has shown that physiological mechanisms embodied in the face and neck regions are effective for diminishing stress-related symptoms. Our study expands on these areas with the design for a wearable in mind. As this study charts new territory in research, it also is a first evaluation of the viability for a wearables concept to reduce stress.

Objective: The objectives of this study were to assess whether (1) heart rate variability would increase and (2) heart rate would decrease during cold stimulation using a thermode device compared with a (nonstimulated) control condition. We expected effects in particular in the neck and cheek regions and less in the forearm area.

Methods: The study was a fully randomized, within-participant design. Volunteer participants were seated in a laboratory chair and tested with cold stimulation on the right side of the body. A thermode was placed on the neck, cheek, and forearm. We recorded and subsequently analyzed participants' electrocardiogram. The cold stimulation was applied in 16-second intervals over 4 trials per testing location. The control condition proceeded exactly like the cold condition, except we manipulated the temperature variable to remain at the baseline temperature. We measured heart rate as interbeat intervals in milliseconds and analyzed root mean square of successive differences to index heart rate variability. We analyzed data using a repeated-measures ANOVA (analysis of variance) approach with 2 repeated-measures factors: body location (neck, cheek, forearm) and condition (cold, control).

Results: Data analysis of 61 participants (after exclusion of outliers) showed a main effect and an interaction effect for body location and for condition, for both heart rate and heart rate variability. The results demonstrate a pattern of cardiovascular reactivity to cold stimulation, suggesting an increase in cardiac-vagal activation. The effect was significant for cold stimulation in the lateral neck area.

Conclusions: The results confirmed our main hypothesis that cold stimulation at the lateral neck region would result in higher heart rate variability and lower heart rate than in the control condition. This sets the stage for further investigations of stress reduction potential in the neck region by developing a wearable prototype that can be used for cold application. Future studies should include a stress condition, test for a range of temperatures and durations, and collect self-report data on perceived stress levels to advance findings.

Keywords: acute stress; cold stimulation; diving reflex; heart rate reduction; lateral neck region; psychological stress; stress reduction; technology for stress relief; thermode-based stimulation; vagus nerve stimulation; wearable electronic devices.

Figure 1

Schematic breakdown of the experiment: each session consisted of four 16-second trials at one body location and 16 seconds at baseline temperature after each trial. The duration of baseline and recovery periods was 3 minutes, and the rest period (pause) between experimental blocks was 5 minutes.

Figure 2

Thermode head used for stimulus application during the sessions. Cold is transmitted only via the 3×3-cm surface (dark red) of the head.

Figure 3

Schematic depiction of the thermode’s temperature change during stimulus application.

Figure 4

Timeline for the randomized controlled trial. The duration of the experimental blocks and the pause was calculated at 20-25 minutes to compensate for the time of thermode placement on the different body areas and for restabilization of the electrocardiographic signal.

Figure 5

Normalized root mean square of successive differences (rMSSD) of all body locations for cold stimulus and control conditions (n=61). The error bars depict within-participant standard error following Cousineau-Morey corrections [81].

Figure 6

Mean interbeat intervals (IBIs) of all body locations for cold stimulus and control conditions (n=61). The error bars depict within-participant standard error following Cousineau-Morey corrections [81].