The ventilatory response to hypoxia in the anesthetized rat

The hypoxic ventilatory response of the anesthetized rat was measured using a progressive hypoxia test whilst end-tidalP _{CO 2} was maintained at a constant level. The ventilatory response to hypoxia was expressed by the equation, \(\dot V_E = \dot V_O + A/(Pa_{O_2 } - C)\) ( \(\dot V_E\) , total ventilation in 1 BTPS ·min⁻¹;Pa _{O 2}, arterialP _{O 2} in mm Hg). The hypoxic ventilatory drive,A, averaged 4.1±2.5 l · min⁻¹ · mm Hg (mean±SD), from which a value of 252 l · min⁻¹ · mm Hg was calculated on the basis of appropriate allometric relationships, for a 70 kg body mass. This value is higher than those reported for the anesthetized dog and for human subjects. When end-tidalP _{O 2} was gradually decreased from hyperoxia to normoxia, a significant increased in \(\dot V_E\) due to an increase in breathing frequency was observed, suggesting that the ventilation of the rat is maintained by a considerable ‘hypoxic drive’ even in normoxia. Furthermore, hypoxic ventilatory depression occurred at a relatively higherPa _{O 2} level (45–60 mm Hg) than in other species. Thus, in the rat, the ventilation vs. end-tidalP _{O 2} curve is shifted to the right compared to other species. After section of the carotid sinus nerve, the hypoxic drive (A) was reduced to 11%, indicating that almost all the ventilatory drive of hypoxia was mediated by the carotid chemoreceptors.