Effects of intrapulmonary CO2 and airway pressure on phrenic activity and pulmonary stretch receptor discharge in dogs

Abstract

Intrapulmonary CO2 and stretch sensitivity were studied in anesthetized, thoracotomized dogs in which both lungs were independently ventilated. The left pulmonary artery was occluded so that changes in left (Test) lung CO2 did not alter systemic arterial PCO2. Adequate gas exchange was maintained in the right lung, which was vagally denervated. In one series of experiments, neural activity in the C5 root of the phrenic nerve was integrated to assess ventilatory drive at various levels of Test lung CO2 and airway pressure both during cyclic ventilation and static lung inflation. In a second series, single unit activity in vagal afferents from pulmonary stretch receptors (PSR) was recorded. Both phrenic activity and PSR discharge were strongly affected by changes in airway pressure but not by changes in Test lung CO2 between 2 and 7%. However, when test lung CO2 was decreased below 2%, phrenic activity decreased and PSR activity increased. These changes were invariably accompanied by an increase in peak airway pressure during cyclic ventilation suggesting that lung mechanics had been altered. The results indicate that, in this preparation, the effects of lung stretch on ventilatory drive spans a wide range whereas intrapulmonary CO2 exerts an effect only at very low levels. It appears that the reflex effects of both stimuli can be accounted by an effect on PSR activity.