Apneic threshold for CO2 in the anesthetized rat:: fundamental properties under steady-state conditions

Experiments were performed to measure the apneic threshold for CO2 and its fundamental properties in anesthetized rats under steady-state conditions. Breathing was detected from diaphragmatic electromyogram activity. Mechanical hyperventilation resulted in apnea once arterial PCO2 (Pa-CO2) had fallen far enough. Apnea was not a reflex response to lung inflation because it did not occur immediately, was not prevented by vagotomy, and was reversed by raising Pa-CO2 without changing mechanical hyperventilation. The apneic threshold was measured by hyperventilating rats mechanically with O-2 until apnea had occurred and then raising Pa-CO2 at constant hyperventilation until breathing reappeared. The mean Pa-CO2 level of the apneic threshold in 42 rats was 32.8 +/- 0.4 Torr. The level of the threshold did not depend on the volume at which the lungs were inflated. The level of the threshold, under steady-state conditions, was the same when approached from hypocapnia as from eupnea. The level of the threshold could be raised by 9 Torr by chronic elevation of the eupneic Pa-CO2 level by 18 Torr.