Blood and CSF acid-base changes, and rate of ventilatory acclimatization of awake dogs to 3,550 m.

Awake, intact dogs trained to wear a respiratory mask were studied in a hypobaric chamber at 140 m and at various stages of a 4-week exposure to 3,550 m. Resting ventilation, pulmonary gas exchanges, arterial blood gases and pH, acid-base status of the cisternal fluid (CSF) and ventilatory responses to transient O2 inhalation were measured. Attention is focussed on the time course of ventilatory acclimatization to altitude, characterized by hyperventilation with hypocapnia and a consequent increase of arterial Po2. (1) 75 percent of the increment in pulmonary ventilation due to hypoxia was achieved in 30 minutes; (2) the further increase, 25 percent of the total hyperventilation, was complete after 3 hr, with a corresponding Pco2 drop and pH increase in blood and CSF, and an increase in Pao2; (3) the secondary increase in ventilation, beyond the acute exposure period, was not related to return of [H+] in CSF towards control value; (4) the large transient decrease of ventilation following brief oxygen inhalation demonstrated a strong arterial chemoreflex drive in acclimatized animals. The extremely rapid ventilatory acclimatization to moderately high altitude in normal dogs appears to be mediated not by CSF hydrogen ion concentration but by a strong chemoreflex drive of ventilation.