Retinal vein diameter during reversible upper respiratory bypass

A reversible tracheal cannula was implanted in 14 Rambouillet sheep so that the sheep could breathe normally through the nostrils when the 'flow-through' insert was in place or through the tracheal opening when the 'bypass' insert of the cannula was in place. The outer diameter of the superotemporal retinal vein increased significantly in size during nasal bypass breathing compared to normal nasal breathing. Anteroposterior histological sections of the retina and optic disc revealed disc elevation and marked venous engorgement during nasal bypass breathing. These results suggest that a possible increase in the cerebrospinal fluid pressure during nasal bypass breathing must have caused a corresponding increase in optic nerve tissue pressure resulting in axoplasmic flow stasis and swelling of the optic nerve axons in the prelaminar area of the optic disc. The swollen axons apparently caused the compression of the thin-walled retinal veins resulting in venous engorgement.