This study investigates the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on autonomic cardiovascular regulation during exercise and recovery in individuals with chronic spinal cord injury (SCI). Participants undergo two experimental conditions (active taVNS and sham stimulation) in a randomized crossover design while performing a standardized exercise protocol. Heart rate variability (HRV) is used as a non-invasive biomarker to assess autonomic nervous system dynamics across different phases (baseline, exercise, and recovery). The aim is to characterize physiological responses to neuromodulation and explore whether taVNS modulates autonomic adaptability in this population. This is a mechanistic physiological study designed to improve the understanding of autonomic regulation in SCI and to explore potential biomarkers of response to neuromodulation.
Spinal cord injury (SCI) is associated with significant impairments in autonomic nervous system function, particularly affecting cardiovascular regulation. These alterations reduce physiological adaptability to internal and external stressors and may contribute to increased cardiovascular risk. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation technique that has been proposed as a potential tool to influence autonomic function. However, its effects on dynamic autonomic regulation during physiological stress, such as exercise, remain insufficiently characterized in individuals with SCI. The present study is a randomized, controlled, crossover experimental protocol designed to investigate the acute effects of taVNS on autonomic cardiovascular dynamics during a structured exercise and recovery paradigm. Participants with chronic SCI are exposed to two conditions: active taVNS and sham stimulation, applied in a randomized order. Each experimental session includes three phases: baseline (rest), exercise (submaximal effort), and recovery. Continuous electrocardiographic recordings are obtained to derive heart rate variability (HRV) metrics. Both linear (time and frequency domain) and non-linear analyses are performed to characterize autonomic regulation and system dynamics. The primary objective is to evaluate whether taVNS modulates autonomic responsiveness and adaptability across different physiological states. Secondary objectives include the exploration of HRV-derived biomarkers that may reflect autonomic complexity and the identification of response patterns to neuromodulation. This study is not designed to evaluate clinical efficacy or therapeutic outcomes but to provide mechanistic insight into autonomic regulation and neuromodulation effects in SCI. The results may contribute to the development of personalized approaches in neurorehabilitation and autonomic monitoring.
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
BASIC_SCIENCE
Masking
SINGLE
Enrollment
35
Non-invasive electrical stimulation applied to the auricular branch of the vagus nerve using transcutaneous electrodes during the experimental protocol.
Sham stimulation delivered with identical device setup but without effective activation of the vagus nerve.
University of the Balearic Islands
Palma de Mallorca, Balearic Islands, Spain
DFA α2
Detrended fluctuation analysis (α2) derived from RR intervals to assess long-term fractal scaling properties of heart rate variability. RR intervals are recorded continuously and analyzed offline. Data are processed using 5-minute overlapping windows. The value reported at each specific time point represents the calculation for the preceding 5-minute window to characterize autonomic regulation across the session.
Time frame: Baseline (last 5 minutes of rest), 10, 15, 20, 25, 30, 35, and 40 minutes.
RMSSD (ms)
Root mean square of successive differences (RMSSD) of RR intervals to assess short-term heart rate variability and parasympathetic activity. RR intervals are recorded continuously and analyzed offline. Data are processed using 5-minute overlapping windows. The value reported at each time point represents the calculation for the preceding 5-minute window..
Time frame: Baseline (last 5 minutes of rest), 10, 15, 20, 25, 30, 35, and 40 minutes.
HF power (ms²)
High-frequency (HF) power (0.15-0.40 Hz) derived from RR intervals to assess parasympathetic modulation. Spectral analysis is performed using standard frequency-domain methods. Data are processed using 5-minute overlapping windows. Each reported value corresponds to the preceding 5-minute window at the specified time point.
Time frame: Baseline (last 5 minutes of rest), 10, 15, 20, 25, 30, 35, and 40 minutes.
LF power (ms²)
Low-frequency (LF) power (0.04-0.15 Hz) derived from RR intervals to assess combined autonomic modulation. Frequency-domain analysis is performed after preprocessing of RR interval data. Data are processed using 5-minute overlapping windows. Each reported value corresponds to the preceding 5-minute window at the specified time point.
Time frame: Baseline (last 5 minutes of rest), 10, 15, 20, 25, 30, 35, and 40 minutes.
SD1 (ms)
SD1 derived from the Poincaré plot of RR intervals, representing short-term beat-to-beat variability. RR intervals are recorded continuously and analyzed offline. Data are processed using 5-minute overlapping windows. Each reported value corresponds to the preceding 5-minute window at the specified time point.
Time frame: Baseline (last 5 minutes of rest), 10, 15, 20, 25, 30, 35, and 40 minutes.
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