Breathing frequency responses to pulmonary CO₂ in an isolated lobe of the canine lung

Recent studies have indicated that the breathing frequency responses to inspired CO₂ in part result from changes in pulmonary stretch receptor activity. Pulmonary CO₂ may alter frequency by direct inhibition of stretch receptor discharge, or secondarily, by changes in airway mechanics. The vascularly isolated left lower lobe (LLL) of the canine lung was used to determine the effect of hypocapnic airway constriction on the pulmonary CO₂ reflex. The upper and middle lobes of the left lung were removed and the right vagus nerve sectioned. Blood was recirculated through the LLL. Diaphragm electromyogram was used as an index of respiratory center activity and to trigger ventilation of the left lower lobe. Lobar hypocapnia increased peak airway pressure and reduced respiratory rate. However, infusion of isoproterenol or the use of a mechanical overflow system to block the airway pressure response prevented the frequency changes associated with CO₂. Although both the direct and mechanical effects of CO₂ on stretch receptors may contribute to the reflex, in the LLL preparation the mechanical effects predominate.