Chemoreflex drive of ventilation in the awake miniature pig

Awake, resting miniature pigs trained to breathe through a respiratory mask were studied at neutral ambient temperature first intact, then after bilateral chronic carotid body denervation. Resting pulmonary ventilation (V), tidal volume (VT) and ventilatory period (T), and O2 and CO2 partial pressures in the end-tidal gas (PETO2, PETCO2) were measured cycle-by-cycle in steady state at levels of PETO2 ranging from 35 to 300 Torr, and in the course of NaCN injections and transient pure-O2 inhalation)30-sec O2-test). Steady state toxic CO2 ventilatory responses curves were determined. Intact normoxic animals hyperventilated in response to NaCN injections, while carotid denervated ones did not. Intact animals responded to O2-tests with a fall of V accounting for about 40% of the control minute volume in normoxia PETO2 = 95 Torr), and 90% in hypoxia (PETO2 = 50 Torr equivalent to an altitude exposure at 4000 m). Denervated miniature pigs exhibited either no ventilatory changes, or hyperventilated in response to O2-tests. From V vs. PETCO2 relationships, hypercapnia and hypoxia interacted positively before carotid denervation; after denervation, the normoxic curve was shifted towards higher PCO2 values and its slope decreased. It is concluded that in awake miniature pigs the hypoxic chemoreflex drive from the carotid bodies controls about 40% of the resting minute volume near sea level, and 90% in hypoxic conditions equivalent to an altitude exposure to 4000 m. Bilateral carotid body denervation totally suppressed arterial chemosensitivity: there is thus no functional evidence for the existence of aortic chemoreceptors in miniature pigs.