Ventilatory afterdischarge and central respiratory drive interactions in the awake goat

We have previously established the existence of ventilatory afterdischarge (VAD) in the awake goat by means of an isolated perfused carotid body (CB) technique. In the present series of experiments we used this animal preparation to examine the effects of systemic (central nervous system) hypoxia, mild hypercapnia, and hypocapnia on the manifestation of VAD in ventilatory variables and respiratory muscle electromyogram activity after hypoxic stimulation of the isolated CB. With systemic isocapnic normoxia, inspired minute ventilation remains above control for 30-40 s (time constant = 16.8 s) after termination of CB hypoxia; however, with systemic hypocapnia, VAD is short (time constant = 5.5 s) and hypoventilation is common after removal of CB stimulation. During mild systemic hypercapnia, VAD is prolonged (time constant = 39.9 s). However, systemic (central nervous system) hypoxia did not decrease VAD (time constant = 17.0 s). These results indicate that the manifestation of VAD is more sensitive to the level of arterial PCO2 and central chemoreceptor activity than it is to the state of central oxygenation. Inspiratory and expiratory muscle electromyogram activities qualitatively tracked ventilation during CB stimulation and during the VAD period in all conditions.