Inhomogeneous activation of the parasternal intercostals during
breathing.
Troyer, Andr[acute]e De, and Alexandre Legrand.
Laboratory of Cardiorespiratory physiology, Brussels School of
Medicine; and Chest Service, Erasme University Hospital, 1070
Brussels, Belgium.
APStracts 2:0078A, 1995.
Recent computations of the mechanical advantage of the canine
intercostal muscles have suggested that the inspiratory advantage of
the parasternal intercostals is not uniform. In the present studies,
we have initially tested this hypothesis. Using a caliper and markers
implanted in the costal cartilages, we have thus measured, in four
supine paralyzed dogs, the length of the medial, middle, and lateral
parasternal fibers at FRC and after a one-liter mechanical inflation.
With inflation, the medial fibers always shortened more than the
middle fibers (-9.8 +/- 0.8 vs. -6.0 +/- 0.8 %; P<0.001), while the
lateral fibers remained virtually constant in length (-0.2 +/- 0.8
%). This gradient of mechanical advantage agreed well with the
gradient of orientation of the muscle fibers. Therefore, we have also
recorded the electromyograms of the medial, middle, and lateral
parasternal bundles during spontaneous breathing in nine anesthetized
animals (20 interspaces); each activity was expressed as a percentage
of the activity recorded during tetanic, supramaximal stimulation of
the internal intercostal nerve (maximal activity). The medial bundle
was invariably more active than the middle bundle during resting
breathing (57.3 +/- 3.3 vs. 25.5 +/- 3.4 % max.; P<0.001), and in ten
interspaces, medial activity consistently preceded middle activity at
the onset of inspiration. These differences persisted during
hypercapnia, during inspiratory resistive loading, as well as after
phrenicotomy. Activity was never recorded from the lateral bundle.
The fact that the parasternal intercostals in the dog have a
mediolateral gradient of mechanical advantage and show a mediolateral
gradient of activation during breathing supports the idea that the
pattern of respiratory muscle activation minimizes the work of
breathing.
Received 13 October 1994; accepted in final form 17 February
1995.
APS Manuscript Number A1057-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 March 1995.