Studies of diaphragm function have been greatly facilitated by the ready accessibility of the cervical phrenic rootlets enabling controlled diaphragmatic activation by phrenic nerve stimulation. By means of phrenic stimulation, both the mechanical and intrinsic contractile properties of the diaphragm can be assessed. Use of intercostal muscle stimulation for this purpose has been hampered by the lack of an accurate and reproducible means of producing a coordinated contraction of this muscle group. Consequently, much less is known about the mechanical function of the intercostal muscles and mechanisms by which these muscles are synchronously activated. In preliminary experiments, we have shown that activation of the intercostal muscles by spinal cord stimulation (post-phrenicotomy) results in large inspired volumes (approaching 35-40% of inspiratory capacity), suggesting that coordinated electrical activation of these inspiratory muscles can be achieved. In the proposed studies, to be performed in anesthetized dogs, we intend to use the spinal cord stimulation technique as a method of assessing intercostal muscle function. We plan to evaluate (a) the mechanisms whereby intercostal muscles are activated to produce a coordinated displacement of the rib cage during inspiration, (b) the intrinsic contractile properties of intercostal muscles including their force-length, force-frequency, twitch, velocity of shortening and endurance characteristics, and (c) the mechanical interaction between diaphragm and intercostal muscle contraction in force generation and volume displacement. The results of these studies should provide insights into the mechanical behavior and physiologic characteristics of intercostal muscle contraction as compared to the better studied diaphragm.
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