Airway clearance techniques in cystic fibrosis.

Whilst the trials quoted in the Cochrane reviews [5–7] provide various degrees of quality of evidence, the key information that is missing from these studies is an understanding of the physiological principles by which these different techniques work. Physiologically, the techniques need to do the following. 1) Increase absolute peak expiratory flow (PEF) to move secretions towards the oropharynx. 2) Use two-phase gas– liquid flow, both in closed and open airways. In the latter, mucus transport can be achieved by expiratory airflow during forced expiration, as well as tidal breathing. The peak expiratory flow/peak inspiratory flow ratio (PEF/PIF) needs to be .1.1 to achieve this [8, 9] and the frequency of oscillation needs to be 3–17 Hz, with the ideal frequency being around 13 Hz. 3) Decrease mucus visco-elasticity in the airway, and hence improve mucus transport [10]. 4) Elicite spontaneous coughs by mechanical stimulation of the airways to remove mucus from the trachea, inner and intermediate regions of the lungs [11, 12]. 5) And finally, to increase expectorated mucus volume [13]. However, what all of these are dependent upon is the mechanical properties of the lungs of CF patients, which may deteriorate with disease progression [14, 15]. This might mean alternative approaches have to be adopted.