FUNCTION OF THE ISOLATED PACED DIAPHRAGM AND THE CERVICAL ACCESSORY MUSCLES IN C1-QUADRIPLEGICS

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Abstract
Function studies of the electrically paced diaphragm and of the sternomastoid and trapezius muscles acting alone were conducted in 3 young C1 quadriplegic men. Phrenic pacing allows the examination of the function of the diaphragm acting alone, keeping the degree of phrenic activation constant while examining its mechanical responses to other variables. Changes in anteroposterior and lateral diameters of both rib cage and abdomen were measured with magnetometers; changes in pressure and volume were recorded together with thoracoabdominal motion as indices of inspiratory muscle effectiveness as lung volume and body position were changed. Isolated paced diaphragmatic contraction caused abdominal diameters and the lateral diameter of the lower rib cage to increase. The anteroposterior and lateral diameters of the upper rib cage decreased. Isolated contraction of the trapezii and sternocleidomastoids caused decreases in all diameters, except for the anteroposterior diameter of the rib cage which increased. Tidal volume resulting from paced diaphragmatic contraction was maximal in the supine position, decreasing with both head-up and head-down tilting. Changes in airway occlusion pressure and transdiaphragmatic pressure caused by paced diaphragmatic contraction both decreased as lung volume increased, possibly due to progressive shortening of diaphragmatic fibers. Airway occlusion pressure decreased as a function of lung volume when the neck muscles contracted alone. At high angles of head-up tilting, the isolated paced diaphragm did not drive the respiratory system along its passive (relaxation) characteristic, as it does in upright normal subjects breathing quietly. The diaphragm drove the rib cage somewhat more effectively in the upright than in the supine posture. In erect normal subjects, quiet inspiration apparently is not accomplished by the diaphragm alone but rather by coordinated activity of the diaphragm, rib cage inspiratory muscles, and possibly the abdominal muscles as well.