Non‐Invasive Auricular Vagus Nerve Stimulation Decreases Heart Rate Variability Independent of Caloric Load

FIGURE 1. Schematic summary of the experiment. (A) The timeline of a session includes the phases of baseline (30 min, no stimulation) followed by stimulation (30 min) when either taVNS or sham was applied. Participants received a high‐caloric load to challenge the autonomic system, while we collected the phase after the caloric load (30 min) with concurrent stimulation. (B) The electrode locations at the cymba conchae for the transcutaneous auricular vagus nerve stimulation (taVNS, red) and the ear lobe for sham stimulation (blue) are displayed. (C) Illustration of the stimulation protocol with a biphasic impulse frequency of 25 Hz, the pulse width of 250 μs with alternating intervals of 30 s stimulation on and 30 s stimulation off. The stimulation intensity (mA) was adjusted for each participant until they felt a “mild pricking”.

FIGURE 2. Stimulation decreases and caloric load increases heart rate (HR). (A) During stimulation, HR decreases across both sides (left vs. right) and both stimulation conditions (taVNS in red vs. sham in blue), indicated by the p‐values below the heading. We observed no significant differences between stimulation conditions on each side, indicated by the bootstrapped distributions of changes relative to baseline. (B) After the caloric load, HR increases across both sides (left and right) and stimulation conditions (taVNS and sham), as indicated by the p‐values below the heading. We observed no significant differences between stimulation conditions on each side, indicated by the bootstrapped distributions of changes relative to baseline. The dots depict each participant's change in average HR for taVNS (in red) and sham (in blue) compared to the baseline.

FIGURE 3. taVNS decreases HRV as indexed by SDRR, RMSSD, and HF‐HRV. (A) Across both stimulation conditions (taVNS vs. sham), SDRR, RMSSD, and LF/HF ratio are altered across both sides (left vs. right) before the caloric load, indicated by the p‐values above each plot. taVNS (red distribution) decreased SDRR, RMSSD, and LF/HF ratio side‐specific compared to sham (blue distribution) before the challenge, indicated by the bootstrapped distributions of the delta to the baseline separated for each side. Statistics for the taVNS‐induced effects across both sides are displayed below the panel. (B) The caloric load alters SDRR, HF‐HRV, and LF/HF ratio, except for RMSSD, across both sides (left vs. right) and stimulation conditions (taVNS vs. sham), indicated by the p‐values above each plot. taVNS decreased SDRR, RMSSD, and HF‐HRV after the caloric load compared sham, as indicated by the bootstrapped distributions.

FIGURE 4. No differences between the sides of vagus nerve stimulation in heart rate and heart rate variability (SDRR, RMSSD, HF‐HRV, LF/HF ratio). (A, B) For cardiovascular indices, dots depict the participant's average score for SDRR, RMSSD, HF‐HRV, LF/HF ratio, and HR (taVNS—sham of their change from baseline), and the group distribution shows no significant difference for the side‐specific comparison (left vs. right) for either the stimulation or caloric load phases. (C, D) TaVNS‐induced changes (red) in SDRR, RMSSD, HF‐HRV, LF/HF ratio, and HR are more strongly correlated across sides compared to sham (blue), indicated by the stars at the correlation lines for the data of each participant. (E) Higher correlation coefficients for cardiovascular indices between sides of stimulation after taVNS compared to sham.