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Canadian Journal of Anesthesia, Vol 32, 12-19, Copyright © 1985 by Canadian Anesthesiologists' Society

Relaxation of Tracheomotor Tone by Peep: Influence of Hypocapnia and Acidemia

ROBERT J. BYRICK MD FRCP(C)1

1 Department of Anaesthesia, St. Michael's Hospital, University of Toronto

Address correspondence to: Dr. Robert J. Byrick, Dept. of Anaesthesia, St. Michael's Hospital, 30 Bond Street, Toronto, Ontario, M5B 1W8.

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. A detectable, but reduced tracheomotor response to 10 cm H2O PEEP was then observed. We conclude that:

1. sudden application of 10 cm H2O PEEP causes transient relaxation of tracheomotor tone (< 4 minutes).

2. this tracheomotor relaxation is diminished by hyperventilation with hypocapnia.

3. the reduced responsiveness to PEEP of tracheal smooth muscle during hyperventilation is partially returned by either adding CO2 to the inspired gas mixture or systemic acidification with 0.1 N HCl.

This study suggests that the tracheal dilation resulting from increased lung volume after PEEP is transient and is variable depending on such factors as systemic acid-base status, alveolar and arterial PaCO2, lung volume as well as the existing balance of autonomic influences on airway tone.

Key Words: RESPIRATORY SYSTEM: airway tone, PEEP






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Copyright © 1985 by the Canadian Anesthesiologists' Society.