Effect of a constant arterial CO2 tension on respiratory pattern in hear-stressed sheep.

Sheep were exposed in a climate chamber to a hot humid environment under two conditions: breathing atmospheric air and breathing air enriched with CO2. Measurements were made of rectal temperature (Tre), respiratory frequency (fR), mean arterial pressure (BP), heart rate (fH), and arterial O2 tension (PO2), CO2 tension (PCO2), and pH; depth of breathing was also estimated. Tre increased at a similar rate under both conditions. When breathing air, sheep exhibited rapid shallow panting followed by slower deeper (second phase) breathing during which they became severely hypocapnic and alkalotic. When the sheep breathed CO2-enriched air at a rate allowing normocapnia to be maintained, fR was lower and the depth of breathing greater, but the sequence of changes in respiratory activity was similar to that with air breathing. BP, fH, and PO2 each increased to a similar extent under both conditions. It is concluded that the normal biphasic panting response to severe heat stress in sheep is largely independent of arterial PCO2.