There is a quickly accumulating large literature, (a) on the effects of hyperventilation (hypercapnia), (b) yoga breathing (lit bibliography review summary by Lamb 2006 at Intern Assoc of Yoga Therapists), (c) hundreds of EEG or MRI studies now on various forms of yoga and meditation, and (d) specific studies on Grof holotropic breathwork, which arose out of rebirthing (see the breathwork website online for research articles). Medical studies focused on hyperventilation are sometimes critical of breathwork techniques out of fear of side-effects. This is out-dated and not very relevant. Holotropic breathwork specifically has list of contraindications, which are few. A study last year of psychiatric inpatients, 1100 cases studied, with no adverse side-effects and positive outcomes (Eyerman 2013).
A few articles from my archives. I've done holotropic breathwork and have facilitated individual and group neo-shamanic 'ecstatic body postures', and reflecting on this I once thought about writing up a paper on the different blood pressure and heart rate reactions to breathwork, trance, shock, etc. I may post these notes for such a paper on my website, as I have so many writing projects I may not get to this one. You might find it of interest (attached).