Effect of upper airway negative pressure on respiratory timing.

The effects of upper airway negative pressure on respiratory timing and respiratory muscle activity were investigated in 13 urethane-pentobarbital anesthetized adult rabbits. Diaphragm and upper airway muscle EMGs were recorded with fine wire electrodes. The upper airway was converted into a closed system and negative pressure changes were made at will with a syringe attached to a laryngeal cannula. Both inspiratory and expiratory durations (Ti and Te) were prolonged during the negative pressure trials. Maximal prolongation occurred on the first experimental breath for Te and on second breath for Ti. Decreased effects were seen during maintained negative pressures. Peak diaphragm EMG and average slope of diaphragm EMG decreased during these trials. Diaphragmatic apnea (Te greater than or equal to 5 sec) occurred in 15% of trials. In some of these trials apnea lasted as long as the negative pressure stimulus whereas in others spontaneous breathing resumed after a period of apnea. Phasic upper airway muscle activity occurred during diaphragmatic apnea in most of these trials. The superior laryngeal nerve section markedly reduced the effects of negative pressure, indicating that its afferents primarily mediate this response. Our results suggest that upper airway negative pressure acts centrally on both inspiratory and expiratory timing as well as on the motor output of thoracic and upper airway respiratory muscles.