Central chemosensitivity affects respiratory muscle responses to laryngeal stimulation in the piglet

Stimulation of laryngeal afferent fibers triggers reflex apnea, which can be very long in the neonatal period. The purpose of this study was twofold: 1) to determine the effect of superior laryngeal nerve (SLN) stimulation on expiratory as well as inspiratory muscle activity and 2) to compare the respiratory response to SLN stimulation under conditions that would appear to alter the level of central chemosensitivity in anesthetized spontaneously breathing piglets. In protocol 1, we measured electromyogram (EMG) responses from the diaphragm (DIA), triangularis sterni, and transversus abdominis muscles to graded SLN stimulation; in protocol 2, we compared the DIA response to different levels of SLN stimulation during normocapnia, hypercapnia, and cooling of the ventrolateral medullary surface (VMS). SLN stimulation performed during normocapnia caused significantly greater inhibition of both expiratory muscles (triangularis sterni and transversus abdominis) than of the DIA. During hypercapnia, laryngeal stimulation-induced inhibition of all three muscles was significantly diminished, and the degree of inhibition for the three muscles became equivalent. Inhibition of the DIA EMG was significantly greater with SLN stimulation during VMS cooling than with a VMS temperature of 38 degrees C. We conclude that the magnitude of respiratory inhibition induced by laryngeal stimulation during early postnatal life is inversely related to the level of central chemical input and speculate that functional deficiency of structures located in the VMS may contribute to potentially life-threatening apnea in infants.