Both reflex and voluntary coughs are initiated by an inspiration of variable depth (124, 125, 130, 228). When this preparatory inspiration exceeds the base-line tidal volume, the increased inspiratory effort is accompanied by exaggerated abduction of the vocal cords. The next phase, absent if the subject has a tracheostomy, is one of compression. The glottis is tightly closed, and expir­atory muscles contract, typically raising intrapulmonary pressure quickly to levels that decrease lung volume appreciably by compression (125, 228). The duration of the compression phase is ~200 ms in humans. It is terminated abruptly by an explosive abduction of the cords, which releases the com­pressed gas both by expansion and under continued force exerted by the expiratory muscles. The coughing maneuver results in extensive dynamic compression of intrathoracic airways, which, coupled with the high rate of airflow, results in very high linear velocities of air movement through the narrowed segments. These high airstream velocities account for the effec­tiveness of cough in clearing debris from the airways.

-Bartlett D, Jr. Respiratory functions of the larynx. Physiological reviews 1989;69:33-57.

124 LEITH, D. E. Cough. In: Respiratory Defense Mecha­nisms, edited by J. D. Brain, D. F. Proctor, and L. M. Reid. New York: Dekker, 1977, vol. 5, pt. II, chapt. 15, p. 545. [Lung Biol. Health Dis. Ser.]

125 LEITH, D. E., J. P. BUTLER, S. L. SNEDDON, and J. D. BRAIN. Cough. In: Handbook of Physiology. The Respira­tory System. Mechanics of Breathing. Bethesda, MD: Am. Physiol. Soc., 1986, sect. 3, vol. Ill, pt. 1, chapt. 20, p. 315-336.

130 MACKLEM, P. T. Physiology of cough. Ann. Otol. Rhinol. Laryngol. 83: 761-768,1974.

228 VON LEDEN, H., AND N. ISSHIKI. An analysis of cough at the level of the larynx. Arch. Oto-Laryngol. 81:616-625, 1965.