Response of the canine internal intercostal muscles to chest wall vibration

Although high-frequency mechanical vibration of the rib cage reduces dyspne a, its effects on the respiratory muscles are largely unknown. We have prev iously shown that in anesthetized dogs, vibrating the rib cage during inspi ration elicits a marked increase in the inspiratory electromyographic (EMC) activity recorded from the external intercostal muscles but does not affec t tidal volume (Vr). In the present studies, we have tested the hypothesis that the maintenance of VT results from the concomitant contraction of the internal interosseous (expiratory) intercostals. When the rib cage was vibr ated (40 Hz) during hyperventilation-induced apnea, a prominent activity wa s recorded from the external intercostals but no activity was recorded from the internal intercostals, including when the muscles were lengthened by p assive inflation. The internal intercostals remained also silent when vibra tion was applied during spontaneous inspiration, and the phasic expiratory EMG activity recorded from them was unaltered when vibration was applied du ring expiration. Thus, the internal interosseous intercostals in dogs are m uch less sensitive to vibration than the external intercostals, and they do not interfere with the action of these latter during rib cage vibration. T his lack of sensitivity might be the result of a reflex inhibition of the m uscle spindle afferents by afferents from external intercostal muscle spind les.