High ventilation breathwork (HVB) is increasingly used in psychotherapeutic and contemplative contexts to induce altered states of consciousness (ASC), yet its neurobiological mechanisms remain poorly understood. Thirty experienced HVB practitioners (mean age 44.97 ± 11.80 years) underwent perfusion and resting-state fMRI to assess acute cerebrovascular and post-acute network changes. Cerebral blood flow (CBF) was measured using arterial spin labelling during baseline and the interval of maximal subjective effects. Acute psychotropic effects were rated using an online Likert scale, ASC intensity was assessed immediately after breathwork with the 5-Dimensional Altered States of Consciousness (5D-ASC) scale. HVB induced a 45.5% reduction in global grey matter cerebral blood flow (CBF; 29.81 ± 7.58 vs. 16.24 ± 4.35 ml/100 g/min; t(29) = -10.69, p < 0.001), accompanied by increased heart and respiratory rate. Default mode network (DMN) functional connectivity (FC) did not change significantly at the group level. In further exploratory analyses, greater decreases in DMN FC (pre - post breathwork) predicted higher 5D-ASC rating (β = -4.88, p < 0.05) in analyses using global signal regression, though this association was not significant without global signal regression (p = 0.75). CBF reductions were not associated with ΔDMN-FC or 5D-ASC ratings. HVB produces marked global cerebral hypoperfusion without systematic DMN FC changes, suggesting that hypoperfusion and network dynamics may represent partially dissociable mechanisms underlying breathwork-induced ASC. As a controllable, non-pharmacological means of inducing ASC, HVB may hold promise as a psychotherapeutic and psychedelic-adjacent intervention.
