Response of the canine inspiratory intercostal muscles to chest wall vibration

High-frequency mechanical vibration of the rib cage reduces dyspnea, but th e effect of this procedure on the respiratory muscles is largely unknown. I n the present studies, we have initially assessed the electrical and mechan ical response to vibration (40 Hz) of the canine parasternal and external i ntercostal muscles (third interspace) during hyperventilation-induced apnea . When the vibrator was applied to the segment investigated, prominent exte rnal intercostal activity was recorded in the seven animals studied, wherea s low-amplitude parasternal intercostal activity was recorded in only four animals. Similarly, when the vibrator was applied to more rostral and more caudal interspaces, activity was; recorded commonly from the external inter costal but only occasionally from the parasternal. The two muscles, however , showed similar changes in length. We next examined the response to vibrat ion of the muscles in seven spontaneously breathing animals, Vibrating the rib cage during inspiration (in-phase) had no effect on parasternal interco stal inspiratory activity but induced a marked increase in neural drive to the external intercostals. For the animal group, peak external intercostal activity during the control, nonvibrated breaths averaged (mean +/- SE) 43. 1 +/- 3.7% of the activity recorded during the vibrated breaths (p < 0.001) . External intercostal activity during vibration also occurred earlier at t he onset of inspiration and commonly carried on after the cessation of para sternal intercostal activity. Yet tidal volume was unchanged. Vibrating the rib rage during expiration (out-of-phase) did not elicit ably parasternal or external intercostal activity in six animals, These observations thus in dicate that the external intercostals, with their larger spindle density, a re much more sensitive to chest wall vibration than the parasternal interco stals. They also suggest that the impact of this procedure on the mechanica l behavior of the respiratory system is relatively small.