Glottic closure and high flows are not essential for productive cough.

For about 150 years, there has been emphasis on the importance of glottic closure and high flows in coughing. During voluntary coughing in normal subjects, the main expiratory flow usually begins as thoracic and abdominal pressures are rising. However, sometimes it starts at peak pressure whereas at other times flow and pressure rise together. Plateau flow in single coughs remains relatively constant in the face of large changes in pressure. In a group of coughs following a single inspiration, the fall in pressure is less during plateau flow. Consequently, for the same oral flows, it is likely that the mean linear velocity in the intrathoracic airways will be greater than in a single cough. Recordings have also been made of early morning involuntary coughing in patients with obstructive airways disease. Flow, volume and sound were recorded and sputum expectoration was noted. Sputum expectoration usually occurs after groups of coughs following single inspirations. Hence the movement of the equal pressure point peripherally may be important. These patients have very low flows and yet their coughs are productive. Sound often occurs during the whole of the low flow period immediately before the main expulsive phase. This could only occur if the glottis was open. Thus productive cough can occur without glottic closure and with low airflows.