Am. J. Respir. Crit. Care Med., Vol 155, No. 3, Mar 1997, 1085-1089.
Actions of the inspiratory intercostal muscles in flail chest
M Cappello and A De Troyer
Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Belgium.
We have previously shown in dogs that the ribs in flail chest move
paradoxically inward during inspiration but continue to move cranially. We
have also shown that flail elicits, probably via an increased activation of
the muscle spindles, a threefold to fourfold increase in external
intercostal inspiratory EMG activity without inducing any changes in
parasternal intercostal activity. Therefore, the present studies were
undertaken to test the hypothesis that the persistent cranial motion of the
fractured ribs resulted primarily from the action of the external
intercostals. A flail was induced in seven supine anesthetized animals by
fracturing both dorsally and ventrally ribs 3 to 6 on the right side of the
chest, after which the external intercostal muscles in interspaces 1 to 7
were severed. Severing the external intercostals caused a small increase in
the inspiratory inward displacement of the fractured ribs, from 2.76 +/-
0.31 to 3.25 +/- 0.38 mm (p < 0.05), but it did not affect the
parasternal intercostal EMG activity or the cranial rib displacement
(before, 3.61 +/- 1.03 mm; after, 3.22 +/- 1.43 mm; NS). However, when the
parasternal intercostals in interspaces 1 to 7 were also denervated, the
inspiratory inward displacement of the ribs increased markedly to 5.95 +/-
0.48 mm (p < 0.01), and their inspiratory cranial displacement was
reversed into a 1.05 +/- 0.58 mm inspiratory caudal displacement (p <
0.01). We conclude, therefore, that in dogs with flail chest the
respiratory displacements of the ribs are still primarily determined,
besides pleural pressure, by the action of the parasternal intercostals.
These observations also suggest that in anesthetized dogs, spindle-induced
excitation of the external intercostals has little impact on the mechanical
behavior of the ribs.
