Biomedical Engineering Reference
In-Depth Information
of a high expiratory (peak) flow, may also decrease the viscosity
of mucus. This phenomenon of shear stress reduction of viscosity
in non-Newtonian fluids is known as thixotropy. A high peak
flow can facilitate detachment of adherent airway secretions from
the epithelial surface and can be facilitated by physiotherapy tech-
niques such as chest percussion (140). Once a critical airflow is
reached, termed detachment velocity, there is marked augmenta-
tion in mucus clearance with improvements in ciliary efficacy.
Cough is best at clearing secretions from central airways (139,141).
Chest physiotherapy may trigger cough receptors and, when combined with
physiotherapy, vigorous, directed cough is effective in clearing the airways
of patients with retained secretions (142). Inhaled radioaerosol studies show
that cough alone and cough combined with chest physiotherapy were
equivalent in promoting central airway mucus clearance whereas combined
techniques were better for accelerating clearance from small airways (143).
High expiratory airflow rates that develop mucus shearing forces
depend upon generation of large positive intrapleural pressures (144). Con-
sequently, reasons breathing exercises are often combined with chest phy-
siotherapy. Addition of percussion to conventional physiotherapy does
not improve sputum yield or mucociliary clearance in most studies, except
for patients with cystic fibrosis. Postural (gravity assisted) drainage, as dis-
tinct from chest percussion, adds little to the effectiveness of chest percus-
sion and increases the risk of aspiration from increased gastroesophageal
reflux (145,146). High frequency oscillation are also used in an attempt to
improve mucus transport, although there is little evidence for clinical effi-
cacy in COPD (147). The effectiveness, if any, of the technique is
frequency-dependent, with a frequency of about 10-15Hz, which is outside
the range of the manual techniques, being optimal for enhancing mucus
transport. Oscillations are applied either at the mouth using a modified
loudspeaker (147), or at the thorax using an inflatable vest (148). Other,
newer, techniques aimed at improving mucus clearance include the active
cycle of breathing, positive expiratory pressure (PEP) mask, 'flutter' breath-
ing, and autogenic drainage. All of these techniques seek to avoid the
problem of dynamic airway compression which may inhibit sputum mobili-
zation. Most have not been adequately examined in terms of their effective-
ness in patients with COPD.
VIII.
SUMMARY AND CONCLUSIONS
Airway mucus hypersecretion and the pathophysiological changes that
accompany it, for example goblet cell hyperplasia, are features of many
patients with COPD. The impact of airway hypersecretion on morbidity
and mortality is now more fully understood, albeit that it may be limited
