Apnea-genic reflexes in the upper airway

We have utilized a tracheostomized, unanesthetized, sleeping dog model to study the reflex effects of constant, oscillating or graded pressures in the isolated UA. Constant square-wave negative pressures were applied by suction of the UA during eupnea. Oscillating pressures (30Hz) were applied via a piston pump during eupnea and tracheal occlusions. Graded UA pressures were achieved by opening a fenestration in the tracheostomy tube, which exposed the UA to the negative inspiratory intrathoracic pressures generated against tracheal occlusions. When applied during EXPIRATION, square-wave pressures sufficient to collapse the UA (<-8 cmH₂O), or oscillating pressures (±2 to ±5 cmH₂O) in the patent UA inhibited inspiratory drive and prolonged expiratory time. When applied during INSPIRATION, transient inhibition or "fractionation" of the diaphragm EMG was observed during application of (i) square-wave pulses <-20 cmH₂O, with UA collapse, (ii) graded inspiratory negative pressures <-8 cmH₂O, with spontaneous UA collapse, and (iii) oscillating pressures between ±2 and ±5 cmH₂O, with a patent UA. Inspiration continued immediately following these "fractionations" and inspiratory time was prolonged. Premature termination of inspiration was also seen when square-wave pulses were applied during late inspiration and when oscillating pressures were applied during inspiration. These findings demonstrate the presence of powerful apnea-genic reflexes in the UA of the sleeping dog. UA collapse and distortion appear necessary for these reflexes to be elicited with constant or graded pressures. Oscillating pressure-sensitive receptors appear to be highly sensitive as these apneic reflexes are elicited by small pressure changes in the patent UA.