As vice president and engineer in a small electronics firm, my partner and I worked for several years with the inventor, a well known psychologist, on the design and construction of the
cardiac vagal tone monitor. This device calculates and displays as a digital value the relationship between cardiac arrhythmia and breathing rate, known as "vagal tone".
As we breathe in and out, the heart rate varies slightly to varying degrees depending on various factors which have been studied. Its value as a research device was investigated in many fields including SIDS in infants, neurotoxin level identification in crop dusters, polygraphy, hypnosis and other areas.
A lot of academic publications appeared world-wide as we built about 50 of these units. Here is one article I happened to find which
this professor has co-authored which relates to hypnosis.
There is no shortage of papers on the relationship of EEG to various factors such as hypnotic analgesia.