TONIC RESPIRATORY DRIVE IN THE ABSENCE OF RHYTHM GENERATION IN THE CONSCIOUS DOG

This study was designed to determine whether a chemoreceptor-mediated tonic respiratory drive exists below the apneic threshold. Expiratory (triangularis sterni) and inspiratory (diaphragm and parasternal intercostal) electromyographic activities were recorded in three awake relaxed dogs breathing through an endotracheal tube inserted into a permanent tracheostomy. The cervical vagus nerves were cold blocked to avoid the complicating effects of vagal inputs on respiratory activity. During hypocapnia produced by mechanical hyperventilation, expiratory muscle activity converted from rhythmic to tonic discharge when inspiratory muscle activity and spontaneous breathing movements were abolished. in hypocapnia, changes in arterial PCO2 (in hyperoxia) were produced by changing the ventilator rate for steady-state (>6 min) CO2 stimuli and by disconnecting the ventilator for transient CO2 stimuli. By use of either method, a CO2-mediated drive to the expiratory muscle was consistently observed during hypo capnic apnea. At a constant level of hypocapnia, inhalation of 5% O-2 consistently caused the onset of spontaneous breathing; the onset of phasic inspiratory activity was associated with reciprocal inhibition of the tonic expiratory activity. However, inhalation of 10 and 15% O-2 caused an inhibition of the tonic expiratory activity, even without the onset of breathing. These results suggest that the response threshold of the respiratory chemoreceptors is lower than the apneic threshold and that a chemoreceptor-mediated tonic respiratory drive persists during apnea.