Heart rate variability (HRV) is a key biomarker of autonomic function, linked to morbidity and mortality across various diseases. Transcutaneous auricular vagus nerve stimulation (taVNS) is a promising therapeutic method, but its effects on HRV and the influence of specific stimulation parameters remain unclear. This study investigated whether the acute effects of taVNS on HRV depend on combinations of stimulation frequency and pulse width.
Hypothesis
Increasing the pulse duration from 100 µs to 500 µs and the stimulation frequency from 10 Hz to 25 Hz will enhance parasympathetic modulatory activity.
Methods
Seventy-eight healthy adults participated in seven randomized sessions, each testing one of six active taVNS protocols or an inactive sham condition applied to the cymba conchae of the left ear. Active protocols varied by frequency (10 Hz or 25 Hz) and pulse width (100 μs, 250 μs, or 500 μs). Sessions included 15 minutes of baseline, 15 minutes of taVNS or sham condition, and 10 minutes of recovery. HRV was calculated using the standard deviation of NN intervals (SDNN) and the root mean square of successive differences (RMSSD) from continuous ECG recordings.
Results
Primary analysis revealed that the 10 Hz/250 μs, 10 Hz/500 μs, and 25 Hz/100 μs protocols significantly increased SDNN time series compared to the sham condition. Exploratory analysis revealed SDNN increases during the second 5 minutes of stimulation with the 10 Hz/500 μs protocol and during the first 5 minutes of recovery with the 10 Hz/250 μs and 25 Hz/100 μs protocols. No significant changes in RMSSD were found for any protocol. Secondary analysis revealed that taVNS can potentially influence SDNN indirectly via change of heart period.
Conclusion
TaVNS is safe in healthy adults, and specific frequency and pulse width combinations can acutely and selectively enhance overall HRV, as reflected in SDNN, but do not affect vagally mediated HRV, as reflected by RMSSD. The effects of taVNS on SDNN are partly mediated through heart rate. To optimize taVNS parameters for maximal physiological and clinical outcomes, further studies are needed.
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