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The effect of inhaled carbon dioxide and hypoxia mixtures on the electrical and mechanical activities of respiratory muscles in dogs.

The effects of hypercapnia and hypoxia on the regulation of breathing was studied in six mongrel dogs during pentobarbital anaesthesia with special reference to their action on the respiratory muscles. Carbon dioxide (5.5% in air) inhalation produced an increase of minute volume due to augmentation of respiratory frequency and tidal volume. The number of electrical impulses per breath remained unaffected despite the increase tidal volume. Therefore the mechanical ventilation increased more than the electrical output to the respiratory muscles indicating that the efficiency of the muscles increased. The tidal volume increased more than the pressure impulse (mechanical work by the respiratory muscles) suggesting that the airway conductance was increased during hypercapnia. During hypoxic stimulus (11% oxygen in nitrogen) the increase of minute volume was due to increase in respiratory frequency. The ratio of tidal volume to the number of electrical impulses in the respiratory muscles remained unaltered. During combined carbon dioxide and hypoxia stimulus the responses were similar to those of carbon dioxide stimulus alone. However, the increase of minute volume and the increase in the ratio of tidal volume and pressure impulse to the number of electrical impulses in the respiratory muscles were more pronounced. These findings suggest that carbon dioxide affects the regulation of breathing above the respiratory muscles and in the muscles themselves. Hypoxic stimulus seems not to have any direct effect on the respiratory muscles in dogs during pentobarbital anaesthesia.