Mechanism of active expiration in tetraplegic subjects.

Traumatic tetraplegia produces paralysis of all the well-recognized muscles of expiration. Yet, tetraplegic subjects usually have a small expiratory reserve volume on spirographic examination. To understand the mechanism that enables these patients to empty their lungs actively, we studied the pattern of chest-wall motion during voluntary expiration. We found negligible changes in abdominal dimension, but all subjects had a marked and reproducible decrease in the dimension of the upper rib cage. Electrical measurements established that the subjects had active use of the clavicular portion of the pectoralis major, and changing the orientation of these muscle fibers by maintaining the shoulders in abduction reduced their expiratory reserve volume by about 60 percent (P less than 0.001). We therefore conclude that the clavicular portion of the pectoralis major plays a crucial part in the mechanism of active expiration in tetraplegic subjects. Training of this muscle bundle could, by increasing its strength and endurance, improve the effectiveness of coughing in such subjects and perhaps diminish the prevalence of bronchopulmonary infections.