[Compensatory reactions of the rabbit respiratory system to hypoxic stimulation in hyperbaric conditions]

Two series of experiments were performed with awake rabbits to study ventilatory reactions of the respiratory system to hypoxic stimulus. Two types of tests included alternating exposure to hypoxic mixture with 14%, 12%, and 10% of O2 (for 10 min) and rebreathing (12% O2 + 4.5% CO2) with concurrent sampling of arterial blood for gas analysis. The experiments were performed under the atmospheric pressure and in the nitrogen-oxygen air at 0.6, 1.0, 1.3, 1.5, 2.0, and 3.0 MPa. Both of the tests disclosed a distinct reduction of pulmonary ventilation gain in hyperbaric conditions due to inhalation of a hypoxic mixture. In the experiment with alternating exposure to hypoxic mixtures the gain reduction was gradual and correlated with a pressure rise. During rebreathing, hypoxemia appeared as early as the first minutes of experiment. PaCO2 grew by the end of exposure and abrupt inhibition of the ventilatory reaction occurred even at 0.6 MPa. Sharp degrees of hypoxemia during rebreathing seemed to suppress the respiratory center to the extent at which the reaction to hypoxia became weaker rather than stronger and was manifested by a sudden attenuation. These results suggest that hyperbaric conditions reduce the ventilatory reaction to hypoxia and balancing potentials of the respiratory system, and impair the respiration controls.