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-tidal Pco2 was maintained at a constant level. The ventilatory response to hypoxia was expressed by the equation, VE = Vo + A/(Pao2-C)(VE, total ventilation in l BTPS . min-1; Pao2, arterial Po2 in mm Hg). The hypoxic ventilatory drive, A, averaged 4.1 +/- 2.5 1 . min-1 . mm Hg (mean +/- SD), from which a value of 252 1 . 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-tidal Po2 was gradually decreased from hyperoxia to normoxia, a significant increased in VE 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 higher Pao2 level (45-60 mm Hg) than in other species. Thus, in the rat, the ventilation vs. end-tidal Po2 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.