Effect of lung volume on the respiratory action of the canine
sternomastoid.
Muza, Stephen R., Gerard J. Criner, Steven G. Kelsen.
Departments of Medicine and Physiology, Temple University School of
Medicine, Philadelphia, PA 19140 and U.S. Army Research Institute of
Environmental Medicine, Natick, MA 01760
APStracts 2:0475A, 1995.
We tested the hypothesis that since the resting length of the canine
sternomastoid (SM) muscles is relatively insensitive to lung volume
change, the SM may maintain its inspiratory force generation
regardless of lung volume. The relationships between SM pre- and
postcontraction in situ fiber lengths and SM-produced inspiratory
pressure generation (Pes) and rib cage displacements were examined in
adult, supine, anesthetized dogs at RV, FRC and TLC. SM muscle
contraction was produced by isolated bilateral supramaximal
electrical stimulation during hyperventilation induced apnea. In all
animals, SM contraction produced negative intrathoracic pressure
changes (i.e., an inspiratory action). Passively increasing lung
volume from RV to TLC decreased (P < 0.01) the SM-produced Pes
by -66 +/- 4 %, but had relatively small effect on SM in situ pre-
and post-contraction fiber length (<3%). Whereas, SM contraction
at RV produced a cranial displacement of the sternum and increased
the upper rib cage cross-sectional area, passively elevating lung
volume diminished the SM-produced expansion of the upper rib cage.
Hyperinflation did not increase the impedance of the sternum to
cranial displacement during SM contraction, suggesting that
hyperinflation caused a dissociation between the mechanical action of
the sternum and the upper rib cage. These results suggest that
mechanical dissociation of the ribs and sternum may diminish the
contribution of the SM to inspiratory volume generation when
breathing from elevated end-expiratory lung volumes.
Received 19 January 1995; accepted in final form 5 October 1995.
APS Manuscript Number A67-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95