During the course of chronic airflow obstruction the effect of the pathological process and compensatory mechanisms that take place in the lungs to limit these effects express themselves by easily indentifiable clinical signs. The limitations of expiratory flow is responsible for the prolongation of the duration of maximal expiration: Pursed lipped breathing is probably the method used by certain patients to limit airway collapse; the increase in the residual volume and functional residual capacity results in distortion of the thorax and a change in the configuration of the inspiratory muscles, reducing their capacity to generate pressures (Hoover's sign: respiratory pulse, hypertrophy of the accessory respiratory muscles, thoraco-abdominal asynchrony); the considerable increase in the inspiratory thoracic depression accounts for the inspiratory descent of the trachea and the sub-sternal "tug". Finally the ventilatory pattern is different, ventilation being rapid and superficial, probably in order to adapt to the constraints imposed by the pathological process.