Ventilatory and waking responses to laryngeal stimulation in sleeping mature lambs

Ventilatory and waking responses to laryngeal stimulation were studied in six chronically tracheostomized lambs breathing through an endotracheal tube. A balloon catheter inserted in the rostral tracheal segment allowed application of distilled water onto the larynx. Apnea, the main primary ventilatory response, was shorter in wakefulness (7.9 sec) than in quiet (10.7 sec) or active sleep (10.6 sec, residual variance = 3.29), where the response to laryngeal stimulation was poorly reproducible. Arousal delay measured from the neck muscle EMG was longer in active (21.1 sec) than in quiet sleep (5.4 sec, residual variance = 9.05). Arousal preceded apnea termination more often in quiet than in active sleep. When breathing resumed a sigh frequently occurred following prolonged apneas. We conclude that laryngeal stimulation elicits apnea in all states in mature lambs. The termination of apnea is related to waking up in quiet but not in active sleep, where arousal is depressed. A comparison of our results with data from preterm and adult mammals strongly suggests that the ventilatory response to laryngeal stimulation during active sleep is unaffected by maturation.