Relaxation of tracheomotor tone by PEEP: influence of hypocapnia and acidemia.

The effect of PEEP on airway smooth muscle tone is difficult to assess using standard lung resistance and compliance techniques. In this study we isolated the extrathoracic trachea from lower airway pressure by performing a low cervical tracheostomy in dogs. We then measured the pressure (PTE) within a water-filled cuff of an endotracheal tube which was placed in the isolated extra-thoracic tracheal segment above the tracheostomy as a measure of tracheomotor tone. Sudden application of 10 cm H2O PEEP in normocapneic animals (PaCO2 = 5.6 +/- 0.2 kPa) caused an immediate dilation in this extrathoracic tracheal segment (PTE decreased from 5.7 +/- 0.3 kPa to 2.4 +/- 0.4 kPa). This decrease in tracheomotor tone was transient, returning to control level by four minutes in spite of 10 cm H2O PEEP maintained on the lower airway. With zero end-expiratory pressure the respiratory rate was increased from 9 to 19 breaths per minute and PaCO2 reduced to 3.9 +/- 0.2 kPa. No detectable tracheomotor dilation was observed after the application of 10 cm H2O PEEP to the lower airway. When exogenous CO2 was added to the inspired gas mixture at a respiratory rate of 19/min, the PaCO2 increased from 4.2 +/- 0.2 kPa to 6.7 +/- 0.4 kPa and a tracheomotor dilation in response to PEEP was again detectable. Finally, a 0.1 N infusion of HCl was infused into hypocapneic animals (PaCO2 = 3.7 +/- 0.3 kPa; pH = 7.47 +/- 0.02). After 30-70 minutes, pH decreased to 7.26 +/- 0.02 and PaCO2 remained 3.5 +/- 0.3 kPa.(ABSTRACT TRUNCATED AT 250 WORDS)