Ventilation and acid-base balance in awake dogs exposed to heat and CO2

Some awake quiet dogs pant at cool ambient temperature (Ta) and some do not pant even when acutely exposed to heat. The purpose of the study was to determine whether this puzzling variability in respiratory behavior diminished during prolonged heat. The contributions of thermal and CO2 drives to respiratory adaptations were also examined. Five awake dogs acclimated to 20 degrees C were studied before and 2 and 48 h following exposure to 30-31 degrees C. Rectal temperature did not change; the important thermal stimulus, even at 48 h, appeared to be the increase in peripheral temperature. Variability between nonpanting and panting persisted over 48 h. On the average, ventilation (VE) doubled during heat, largely due to increased dead space ventilation. Nonpanting dogs at cool Ta decreased the threshold of the ventilatory response to CO2. A panting dog at cool Ta changed its slope of the ventilatory response from negative to positive. During hypercapnia in acute heat, ventilatory pattern changed so that frequency increased and tidal volume decreased for a given VE. By 48 h of heat, the ventilatory response to CO2 returned to control in only two dogs, but the ventilatory pattern during hypercapnia returned to control in four dogs. Since thermal stimuli remained unchanged at 48 h, adaptations of respiratory control may have been related to progressive adjustments of strong ions and acid-base balance.