Arterial blood gas composition, consciousness and death in rabbits

Conscious rabbits were exposed to environments with low oxygen and/or high carbon dioxide. The electroencephalogram and arterial blood pressure were recorded. Arterial blood samples were taken sequentially. Rabbits became 'unconscious' at an arterial blood PO2 of 25 +/- 1 mm Hg and PCO2 of 99 +/- 27 mm Hg, when simultaneously the environmental oxygen was decreased and carbon dioxide increased. Brain death occurred at a PO2 of 23 +/- 1 mm Hg and PCO2 of 113 +/- 32 mm Hg. Cerebral perfusion was still adequate. In rabbits, the margin between 'unconsciousness' and 'death' is narrow so far as PO2 is concerned. If these results apply generally, animals culled with muscle relaxants may perceive stress involved until shortly before death.