Physiology of Cough

Abstract

The effectiveness of cough depends upon the linear velocity of the gas in the airways. Because velocity = flow/cross-sectional area, a high flow and a small cross-section are the ideal conditions for an effective cough. Pleural pressures become positive during cough and compress the large airways producing a marked reduction in cross-sectional area. At high lung volumes, expiratory flow rates are high so that the linear velocities in the trachea are approximately one-third of the speed of sound. The velocity falls in higher bronchial generations, both because the total cross-sectional area of each generation becomes progressively larger beyond the lobar bronchi, and because at high volumes the compressed segment of the airway only extends from the lobar bronchi to the thoracic outlet of the trachea. In normal lungs cough is effective in clearing secretions from these airways only. In chronic bronchitis and emphysema, expiratory flow rates are markedly reduced. Furthermore, in some cases the large airways are more easily compressed than normal. This results in a shorter segment of the airway being compressed. For both reasons, the efficiency of cough is markedly decreased leading to retention of secretions. In cystic and varicose bronchiectasis the problem is different. There is no flow through the bronchiectatic segments because they are blind sacs, and the efficiency of cough is independent of the velocity. To empty them of their secretions is analogous to squeezing toohpaste out of a tube. This is theoretically possible at low lung volumes when the compressed segment is longer.