Journal of Applied Physiology, Vol 75, Issue 3 1247-1255, Copyright © 1993 by American Physiological Society
Breathing route and resistive loading: influence on ventilatory response to hypoxia in conscious dogs
K. Y. Cao, C. E. Sullivan, M. Berthon-Jones and C. W. Zwillich
David Read Laboratory, Department of Medicine, University of Sydney, New South Wales, Australia.
Unlike normal humans, the tracheostomized conscious dog does not show
ventilatory adaptation in response to sustained isocapnic hypoxia. To
determine whether this phenomenon is a result of the breathing route or the
relatively low airflow resistance of tracheostomy breathing, we evaluated
the ventilatory response to sustained isocapnic hypoxia (20 min; arterial
oxyhemoglobin saturation = 80%) in five awake dogs during nasal-oral (mask)
breathing, tracheal breathing, and tracheal breathing with added matched
resistance of upper airway breathing. Mask breathing, like unloaded
tracheal breathing, was associated with a consistent level of
hyperventilation during the entire hypoxic exposure period. However, mask
breathing was always less (P < 0.05) than that found during unloaded
tracheal breathing. Loaded tracheal breathing during hypoxia resulted in
initial hyperventilation similar to that of unloaded tracheal breathing
followed by a "roll off" to a lower minute ventilation similar to that of
mask breathing. Our findings demonstrate that ventilatory adaptation is
only present during loaded tracheal breathing in dogs and suggest that the
breathing route and upper airway resistive loading may play roles in
ventilatory adaptation.
