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$P{\text{CO}}_2$ (${\text{Pa}}_{{CO}_2}$ ) 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 ${\text{Pa}}_{{CO}_2}$ without changing mechanical hyperventilation. The apneic threshold was measured by hyperventilating rats mechanically with O₂ until apnea had occurred and then raising ${\text{Pa}}_{{CO}_2}$ at constant hyperventilation until breathing reappeared. The mean${\text{Pa}}_{{CO}_2}$ 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 ${\text{Pa}}_{{CO}_2}$ level by 18 Torr.