Dwinell, M. R., P. L. Janssen, J. Pizarro, and G. E. Bisgard. Effects of carotid body hypocapnia during ventilatory acclimatization to hypoxia. J. Appl. Physiol. 82(1): 118-124, 1997.Hypoxic ventilatory sensitivity is increased during ventilatory acclimatization to hypoxia (VAH) in awake goats, resulting in a time-dependent increase in expired ventilation (E). The objectives of this study were to determine whether the increased carotid body (CB) hypoxic sensitivity is dependent on the level of CB CO₂ and whether the CB CO₂ gain is changed during VAH. Studies were carried out in adult goats with CB blood gases controlled by an extracorporeal circuit while systemic (central nervous system) blood gases were regulated independently by the level of inhaled gases. Acute E responses to CB hypoxia (CB PO₂ 40 Torr) and CB hypercapnia (CB PCO₂ 50 and 60 Torr) were measured while systemic normoxia and isocapnia were maintained. CB PO₂ was then lowered to 40 Torr for 4 h while the systemic blood gases were kept normoxic and normocapnic. During the 4-h CB hypoxia, E increased in a time-dependent manner. Thirty minutes after return to normoxia, the ventilatory response to CB hypoxia was significantly increased compared with the initial response. The slope of the CB CO₂ response was also elevated after VAH. An additional group of goats (n = 7) was studied with a similar protocol, except that CB PCO₂ was lowered throughout the 4-h hypoxic exposure to prevent reflex hyperventilation. CB PCO₂ was progressively lowered throughout the 4-h CB hypoxic period to maintain E at the control level. After the 4-h CB hypoxic exposure, the ventilatory response to hypoxia was also significantly elevated. However, the slope of the CB CO₂ response was not elevated after the 4-h hypoxic exposure. These results suggest that CB sensitivity to both O₂ and CO₂ is increased after 4 h of CB hypoxia with systemic isocapnia. The increase in CB hypoxic sensitivity is not dependent on the level of CB CO₂ maintained during the 4-h hypoxic period.