Once again good info coming from cousin Cfs/ Fms.
Read article please. Better yet peruse other articles and you will find lots of good info that I believe could also apply to POIS.
Note that this is NOT a suugestion you for try this drugs Just good info related to vagus and what doctors may use to affect it.
"A bevy of drugs, many of which have not been tried ME/CFS or FM, can improve parasympathetic nervous system functioning. They do so either by blocking aceytlcholine from being degraded (e.g. Mestinon), or by increasing acetylcholine release, or by inhibiting sympathetic nervous system activity.
Reversible parasympathomimetic drugs include Donepezil, Edrophonium, Neostigmine, Physostigmine, Pyridostigmine, Rivastigmine, Tacrine, Caffeine (non-competitive) and Huperzine A."
http://www.healthrising.org/blog/2016/06/17/mestinon-chronic-fatigue-vagus-nerve-stimulation-exercise/Bookmark site. Lots of gems here.