Extrathoracic cuff pressure reflects changes of intrathoracic large airway circumference

The pressure measured in water-filled cuffs of endotracheal tubes has been used to evaluate the effect of drugs and physiologic stimuli on large airway tone. In six mongrel dogs, extrathoracic cuff pressure was monitored while simultaneously monitoring intrathoracic tracheal circumference measured by an implanted mercury strain gauge. The airways of each animal were alternately dilated with intravenous aminophylline (10 mg/kg) and constricted with edrophonium (10 mg). In each animal a significant linear relationship was found between cuff pressure and intrathoracic tracheal circumference. The slopes of these linear relationships were consistent for each animal, but varied between animals. In a second group of 11 dogs, cuff pressure was measured at extrathoracic and intrathoracic tracheal sites, as well as in the left main stem bronchus. Similar directional changes in cuff pressure were found at each site. This suggests that a water-filled system monitoring cuff pressure in the extrathoracic trachea reflects the direction of circumference change in the intrathoracic trachea and major bronchi.