1.1 Background
Chronic obstructive pulmonary disease (COPD) is a major health problem across the world and its medical, societal and economic impacts continue to grow. COPD is characterized by chronic obstruction of expiratory flow affecting peripheral airways, associated with chronic bronchitis (mucus hypersecretion with goblet cell and submucosal gland hyperplasia) and emphysema (destruction of airway parenchyma), together with fibrosis and tissue damage, and inflammation of the small airways1. The beneficial effects of smoking cessation in slowing the decline in lung function and the progression of disease have been clearly established. Whether other factors such as mucus hypersecretion, respiratory infections and airway hyper-reactivity contribute to disease progression independent of cigarette smoking is still being debated2.
Patients with COPD are often distressed by mucus hypersecretion which is one of the hallmarks of the disease but little is known about the impact of current therapies on mucus hypersecretion due to the logistical challenges of objectively quantifying mucus in clinical studies 3. Mucus hypersecretion occurs as a direct result of airway inflammation. Neutrophils that die by necrosis disgorge proteases and reactive oxygen species into the lumen. It is concluded that neutrophil death via necrosis leads to the high concentrations of free neutrophil elastase and reactive oxygen species in the sputum of patients with airway neutrophilia and mucus hypersecretion. Inflammatory cells (neutrophils), molecules (neutrophil elastase and reactive oxygen species), signaling pathways (EGFR), and cellular processes (neutrophil necrosis) contribute to mucus hypersecretion in COPD4.
Mucus hypersecretion leads to increased work of breathing, reduced exercise tolerance, deteriorating lung function and increases dyspnoea and cough. Often the mucus hypersecretion also interferes with nocturnal sleep and has a further negative impact on health related quality of life. Combined mucus hypersecretion, reduced clearance, and impairment of the lung defence mechanisms explain why COPD patients even with stable condition, carry potential respiratory pathogens in significant concentration, paving the way for infection and acute exacerbations of COPD5;6. In the natural history of COPD chronic mucus hypersecretion is thought to be linked to the accelerated decline of forced expiratory volume in one second (FEV1)5 7 .
Although mucoactive drugs (which improve the ease of mucus clearance) may be effective in mucus hypersecretion, the uncertainty surrounding their effectiveness, due primarily to the relative lack of evidence from randomised controlled trials and an uncertainty from epidemiological studies linking mucus hypersecretion with mortality8-11, have led to their poor diffusion and adoption in international guidelines 7. However, in combination with newer therapeutic strategies currently in research and development phase targeting airway inflammation and thus reducing mucus hypersecretion, there is a renewed interest in re-evaluating mucus clearance strategies in improving HRQL, exacerbations and morbidity associated with COPD 12;13.
Airway clearance forms a vital part the management of patients with CF14, Bronchiectasis and neuro-muscular diseases15 and has a positive impact on lung function, infection rate and mortality16;17. Individually tailored chest physiotherapy18 with forced expiratory manoeuvres is often considered the gold standard in airway clearance19, although self-administered techniques may be equally effective20 and preferred by patients17;21. Self-administered airway clearance techniques using devices which create a turbulent airflow thus encourage shearing of mucus from bronchial wall, have shown promise in improving HRQL and function in COPD patients22.
High frequency chest wall oscillation (HFCWO)23 is known to reduce respiratory rate but improve ventilation in patients with COPD24 and CF25. Experimental results suggest that shearing at the air-mucus interface could be a significant factor in the enhanced tracheal mucus clearance during HFCWO26. Unlike in CF patients who often have greater support from family members and show high levels of motivation and thus compliance in adhering to self-administered airway clearance techniques, COPD patients have low motivation and poor compliance. Thus self-administered techniques for mucosal clearance may be less effective in clinical settings away from dedicated research programs. Thus the HCFWO may offer a more passive modality of airway clearance with consequently higher compliance, if found to be equally effective and well-tolerated in this group of patients. If HFCWO is shown to improve HRQL, symptoms and exercise tolerance then it may have a significant role in reducing exacerbations and hospital admissions as well as allowing effective management in the community setting. The cost benefit of such improvements will need to be explored in larger studies in the future.
