Noninvasive vagus nerve stimulation (VNS) devices promise stress-relief through increased parasympathetic activity and a more relaxed mental state, associated with heightened alpha wave electroencephalogram (EEG) activity. The cervical vagus nerve (CVN) contains five to six times more Aβ-fibers than the auricular branch of the vagus nerve (ABVN). In addition, the CVN contains afferent and efferent fibers, whereas the ABVN is a purely afferent nerve. We hypothesized that hemodynamic, autonomic, and cerebral responses to cervical VNS are more pronounced than those to auricular VNS. Young healthy adults were randomized into a time control (n = 10), a bilateral cervical VNS (n = 9, 41 Hz, 100 μs, <40 mA), and two unilateral auricular VNS (10 Hz, 300 μs, 2-3 mA) groups with electrodes at the cymba conchae (n = 7) or tragus (n = 9) of the right ear. Systolic blood pressure (SBP) decreased only during cervical VNS (baseline: 134 ± 15 mmHg vs. VNS: 120 ± 16 mmHg, means ± SD, n = 9, P < 0.05), which was associated with decreased low-frequency (LF) SBP variability (baseline: 17.1 ± 9.1 mmHg2 vs. VNS: 7.8 ± 9.1 mmHg2, means ± SD, n = 9, P < 0.05). Cervical but not auricular VNS was followed by increased alpha wave activity (baseline: 6.2 ± 1.8 μV vs. VNS: 7.7 ± 1.7 μV, means ± SD, n = 9, P < 0.05) recorded from the FP1 EEG electrode position. In conclusion, the hemodynamic, autonomic, and EEG responses to acute bilateral cervical VNS are more pronounced than those of unilateral auricular VNS, potentially, because the CVN contains five to six times more Aβ-fibers than the ABVN. Reduced SBP and LF SBP variability together with increased EEG alpha wave activity may indicate a more relaxed mental state during cervical VNS.NEW & NOTEWORTHY Acute application of noninvasive bilateral transcutaneous cervical vagus nerve stimulation caused a reduction in systolic blood pressure, associated with a decrease in low-frequency systolic blood pressure variability. Furthermore, cervical vagus nerve stimulation increased alpha wave EEG activity. These responses may indicate a more relaxed mental state during cervical VNS.
Keywords: autonomic nervous system; blood pressure variability; electroencephalogram; heart rate variability; neuromodulation.