Reflex influences from the extrathoracic trachea during airway occlusion

We have performed experiments in 26 dogs anesthetized with pentobarbital and fitted with an endotracheal tube. The inflatable cuff of this tube was positioned either at the level of the cricoid cartilage or at the thoracic inlet. In this latter situation the extrathoracic trachea (E.T.T.) is not subjected to any change in transmural pressure both during breathing and airway occlusion. We have compared the inspiratory output in term of the integrated phrenic discharge during airway occlusion at FRC with the tracheal tube positioned at either one of the two levels. In most of the experiments (16 out of 26) the inspiratory output during airway occlusion is significantly greater (157%) when the E.T.T. is not by-passed and this difference disappears after bilateral vagotomy. We interpret these results by the asymmetrical response of the tracheal stretch receptors to positive and negative transmural pressure (Pt); most of these receptors are active at FRC and decrease their activity at low negative Pt, as that attained in the first occluded breath. These results seem to suggest that the reflex influences from the extrathoracic tracheal receptors on the inspiratory output are similar to those originating from the intrathoracic airway stretch receptors.