Vibratory stimulation of expiratory muscles inhibits respiration

Induced vibration of the respiratory muscles alters the breathing pattern but controversy persists concerning the most effective site to vibrate, the relative merits of sustained versus phase-locked vibration, and the neural mechanisms involved. In this study we applied a sustained vibratory stimulus to the expiratory intercostal or the external oblique abdominal muscle and compared its effects on tidal volume, inspiratory duration and mean inspiratory flow. Our objective was to deduce from the changes in these respiratory variables whether the sensory input evoked by the vibratory stimulation modulated the central inspiratory drive, the termination of inspiration, or both. Subjects rebreathed from a spirometer which initially contained pure oxygen without a CO2 absorber. Either unilateral or bilateral sustained vibration (100 Hz, 2 mm amplitude) was applied to the 7th or 8th intercostal space anterior to the midaxillary line where the intercostal muscle is a single layer and functions in expiration. Bilateral chest wall vibration suppressed the tidal volume in 7 of 9 subjects. In 4 of 9 subjects, the inspiratory duration was shortened, reflecting premature termination of inspiration. In 5 of 9 subjects, the mean inspiratory flow was reduced, reflecting depression of the central inspiratory drive. Sustained abdominal vibration suppressed tidal volume and reduced inspiratory duration in 4 of 5 subjects. Thus, sustained vibration of either of the expiratory muscles resulted in a reduction in tidal volume. We concluded that sensory input initiated by sustained vibration of abdominal muscles acts centrally to suppress the level of activity in inspiratory neurons.