This study finds that auricular vagus nerve stimulation (which I am currently studying in the clinic along with hrv) leads to improved hrv.  I haven’t found that consistently at all.  At least not yet…
Brain Stimul. 2014 Nov-Dec;7(6):871-7. doi: 10.1016/j.brs.2014.07.031. Epub 2014 Jul 16.

Non-invasive vagus nerve stimulation in healthy humans reduces sympathetic nerve activity.

Author information

  • 1School of Medicine, University of Leeds, Leeds LS2 9JT, UK.
  • 2School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK.
  • 3School of Biomedical Sciences, University of Leeds, Leeds LS2 9JT, UK. Electronic address: J.Deuchars@leeds.ac.uk.

Abstract

BACKGROUND:

Vagus nerve stimulation (VNS) is currently used to treat refractory epilepsy and is being investigated as a potential therapy for a range of conditions, including heart failure, tinnitus, obesity and Alzheimer’s disease. However, the invasive nature and expense limits the use of VNS in patient populations and hinders the exploration of the mechanisms involved.

OBJECTIVE:

We investigated a non-invasive method of VNS through electrical stimulation of the auricular branch of the vagus nerve distributed to the skin of the ear–transcutaneous VNS (tVNS) and measured the autonomic effects.

METHODS:

The effects of tVNS parameters on autonomic function in 48 healthy participants were investigated using heart rate variability (HRV) and microneurography. tVNS was performed using a transcutaneous electrical nerve stimulation (TENS) machine and modified surface electrodes. Participants visited the laboratory once and received either active (200 μs, 30 Hz; n = 34) or sham (n = 14) stimulation.

RESULTS:

Active tVNS significantly increased HRV in healthy participants (P = 0.026) indicating a shift in cardiac autonomic function toward parasympathetic predominance. Microneurographic recordings revealed a significant decrease in frequency (P = 0.0001) and incidence (P = 0.0002) of muscle sympathetic nerve activity during tVNS.

CONCLUSION:

tVNS can increase HRV and reduce sympathetic nerve outflow, which is desirable in conditions characterized by enhanced sympathetic nerve activity, such as heart failure. tVNS can therefore influence human physiology and provide a simple and inexpensive alternative to invasive VNS