COUGHING AND HUFFING
Coughing is a forced expiration against a closed glottis causing a rise in intrathoracic pressure. As the glottis opens, there is a difference in pressure between the smallest airways and the upper trachea, causing a rapid flow of air. This rapid flow, combined with the narrowing of the airways, increases the force of the air, which dislodges mucus and foreign particles into the pharynx. Since a high intrathoracic pressure diminishes the return of blood to the heart, a prolonged bout of coughing may cause a fall in the cardiac output and the patient may faint (cough syncope). Patients must be taught to avoid this.
A forced expiratory manoeuvre (cough or huff) produces compression and narrowing within the airways from a point dependent on lung volume. The compression point is adjacent to the equal pressure point, where the pressure within the airways is equal to the pleural pressure. At high-lung volume this point lies in the trachea and main bronchi, and secretions in this area can normally be cleared by coughing or huffing at high-lung volume. At mid-lung volume these points are in the lobar and segmental bronchi. If the lung volume further decreases, the point at which this compression takes place moves a little further down the bronchial tree towards the alveoli. Throughout this forced expiratory manoeuvre there is a vibratory movement in the posterior wall of the trachea and upper bronchi.
This concept of the equal pressure point was described by Mead et al (1967). A cough or huff is only effective from the point downstream (nearer the mouth) of the compression point. Mead advocated a series of coughs without intervening inspirations to clear progressively deeper portions of the lungs. Many patients find this can be exhausting and, clinically, it seems that a single continuous huff down to the same lung volume is as effective and less exhausting. To produce a huff, a forced expiratory effort is made but the glottis remains open and the intrathoracic pressure does not rise to such high levels as with a cough. Nevertheless, the pressure generated again produces compression and narrowing of the trachea and bronchi and secretions are moved up the bronchial tree.
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