Respiratory and sympathetic activity during recovery from hypoxic depression and gasping in cats

In peripherally chemodenervated, vagotomized, chloralose-anesthetized cats, hypoxia can produce central cardiorespiratory depression or excitation depending on severity. We monitored phrenic and cervical sympathetic neurograms during either hypoxic depression or gasping and 30 min of subsequent isocapnic reoxygenation to determine whether the response of these outputs during hypoxia predicts their activity during recovery. Three levels of hypoxic response were produced in cats: 1) reduction of phrenic neurogram amplitude (PNA) by 30% [fractional inspired O2 (FIO2) = 14-18%)]; 2) production of phrenic apnea (FIO2 = 9-10%); and 3) hypoxic gasping (FIO2 = 6-8%). Recovery from the milder levels of hypoxia was characterized by transient (< 10 min) depression of PNA and inspiratory synchronous sympathetic activity. Respiratory frequency was unaffected or only transiently depressed. Tonic sympathetic activity was unaffected. During reoxygenation after gasping, both PNA and inspiratory synchronous sympathetic act...