Effects of hypercapnia on Breuer-Hering threshold for inspiratory termination

Lateral parabrachial nucleus mediates shortening of expiration and increase of inspiratory drive during hypercapnia

We have previously shown that unilateral or bilateral lesions of the lateral parabrachial nucleus (LPBN) in anesthetized, vagotomized rats markedly and selectively attenuate the shortening of expiratory duration (T(E)) during hypoxia without appreciably affecting all other hypoxic response components. Here, we report that unilateral LPBN lesion by kainic acid in the same group of animals not only abolished normal T(E)-shortening during central chemoreceptors activation by hyperoxic hypercapnia, but led to paradoxical T(E)-prolongation and corresponding decrease of respiratory frequency. Furthermore, LPBN lesion significantly attenuated the increase in phrenic activity during hyperoxic hypercapnia, without appreciably affecting the corresponding shortening of inspiratory duration (T(I)). These findings provide the first evidence indicating that central chemoafferent inputs are organized in parallel and segregated pathways that separately modulate inspiratory drive, T(I), and T(E) in conjunction with similar parallel and segregated central processing of peripheral chemoafferent inputs reported previously [Young, D.L., Eldridge, F.L., Poon, C.S., 2003. Integration-differentiation and gating of carotid afferent traffic that shapes the respiratory pattern. J. Appl. Physiol. 94, 1213-1229].

Dynamic properties of respiratory timing following cocaine administration

We assessed the timing and amplitude characteristics of diaphragmatic muscle activity following administration of intravenous cocaine HCl (10 mg/kg) to awake, unrestrained cats. Cocaine produced a pronounced tachypnea which was interrupted by deep inspiratory efforts coincident with tonic-clonic movements over the first 10 min following cocaine administration. Following that period, diaphragmatic cycle rates slowly increased for up to 1 h and were interrupted occasionally by longer inspiratory efforts which were not associated with other overt motor activities. As respiratory rate increased, breath-to-breath variability decreased, and the incidence of deep inspiratory efforts decreased. As total cycle time decreased, the ratio of inspiratory time to expiratory time remained the same between precocaine and early, intermediate and late intoxication periods. The amplitude of diaphragmatic EMG activity increased with the extreme tachypnea. A number of neural mechanisms may mediate the changes in diaphragmatic muscle activity, including hyperthermia and alteration of rostral brain influences on brainstem timing mechanisms.

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.