Thoracic traction on the trachea: mechanisms and magnitude

Both inspiratory increases and tonic thoracic traction (pull of the thorax) on the trachea [Ttx(tr)] have been shown to improve patency of the upper airway. To evaluate the origins and magnitude of Ttx(tr), we studied 15 anesthetized tracheotomized dogs. We divided the midcervical trachea and attached the thoracic stub to a strain gauge. Ttx(tr), esophageal pressure, and carinal displacement were observed during various conditions. These included unobstructed and obstructed spontaneous breathing, mechanical ventilation at various levels of positive end-expiratory pressure, and progressive hypercapnic stimulation. Observations during spontaneous breathing were performed before and after vagotomy. We found that inspiratory increases in Ttx(tr) were substantial, averaging 81 +/- 8 g force and increasing to 174 +/- 22 g force at an end-expiratory CO2 concentration of 10%. Ttx(tr) did not result simply from the pull of mediastinal and pulmonary structures transmitted through the carina. Changes in intrathoracic pressure acted independently to either draw the trachea into or push the trachea out of the thorax. Thus Ttx(tr) could be explained as the sum of mediastinal traction and force generated by changes in intrathoracic pressure.