Effect of reversible upper respiratory bypass on the electroretinogram in sheep

Ten Rambouillet ewes (2 groups of 5 each) were surgically implanted with a reversible tracheal bypass cannula constructed 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 was adjusted. In 1 group, an electroretinogram recorded an increased amplitude and decreased duration of the b wave during bypassed breathing. When the ewes were allowed to breathe simultaneously through the tracheal opening and nostrils, the b-wave amplitude was decreased, compared with the amplitude during normal nasal breathing alone. Several recordings for individual breathing phases were consistent for each animal. In the 2nd group, arterial blood gas analysis showed no differences in the partial pressures (PCO2 and PO2) during both breathing phases. Seemingly, functional elimination of the upper respiratory cooling of venous blood influences the heat exchange in the ophthalmic plexus and cavernous sinus, resulting in relatively warmer blood reaching the eye.