The Role of Airway Secretions in COPD—Clinical Applications

Abstract

It has been established that mucus hypersecretion and decreased mucus clearance contribute to the morbidity of chronic obstructive pulmonary disease (COPD). Indeed, the classic definition of chronic bronchitis relies on determining the frequency and duration of sputum expectoration. Despite the well recognized importance of this symptom, there are few therapies routinely used to decrease the sputum production or to improve clearance. There are fewer well conducted clinical trials of existing medications and this has led many regulatory agencies, notably the Food and Drug Administration (FDA), to refuse to register these medications or approve their sale. Similarly, airway clearance devices and chest physical therapy have not been well studied in COPD. Carefully conducted studies of interventions to improve airway clearance, similar to those done in cystic fibrosis (CF), may help us to identify effective therapies and possibly novel diagnostic tests for the management of COPD.

Key words: :

• Mucolytics
• Expectorants
• Surfactant
• Mucociliary clearance
• Cough
• Mucokinetic medications
• Physical therapy

Introduction

The diagnosis of chronic bronchitis as a form of COPD is clinically and epidemiologically defined by complaints of daily sputum expectoration for at least 3 months a year for 2 consecutive years [1]. Despite this explicit recognition of the importance of sputum in COPD, there is relatively less known and fewer effective therapies for this important symptom.

Chronic airway inflammation is associated with mucousa gland and goblet cell hypertrophy [2], increased mucus production [3&4], decreased mucus clearance [5-7], and changes in sputum properties [3][8]. Impaired mucociliary transport leads to retained secretions in the airways and increased susceptibility to infection [9]. Accumulation of mucus can increase the risk of destructive, inflammatory and neoplastic lung disease by prolonging the contact time between inhaled materials and the airway mucosa [10]. Sputum retention also reduces expiratory airflow and thoracic gas trapping reduces the efficiency of the muscles of respiration further compromising lung function [11]. Impaired lung function is strongly related to pulmonary morbidity and mortality and chronic mucus hypersecretion has been shown to be an independent risk factor for death from obstructive lung disease [12].

Airway mucus clearance depends upon the physical properties of the mucous gel as well as interactions between mucus, airflow, and cilia. In this manuscript, we review mucus secretion and clearance as well as the properties of secretions in COPD. This subject is also thoroughly reviewed in a recent book edited by Rubin and Tamaoki [13].

Mucus in Smokers Without Chronic Bronchitis

Smoking produces chronic airflow obstruction as a result of injury throughout the respiratory tract. The earliest effect of smoking is chronic airway inflammation. Pulmonary fibrosis and emphysema can be produced in beagle dogs by direct inhalation of cigarette smoke over a relatively short period of time (2–7 cigarettes daily for 2–4 months). Microscopic examination of these lungs showed no evidence of bronchitis, bronchiolitis, or physical obstruction to the terminal airways despite the early development of fibrosis and emphysema.

A low-grade inflammatory reaction is sustained in the lungs of smokers. Neutrophils are associated with the centrilobular emphysema of cigarette smoking. The capillary bed of the lungs concentrates neutrophils approximately 100-fold producing a large pool of marginated cells. The presence of cigarette smoke increases the local concentration of neutrophils, and suggests that the lesions that characterize emphysema may be a result of the destruction of lung tissue by neutrophils that remain within pulmonary microvessels. Duration of smoking and pack-years smoking history are the best predictors of pulmonary dysfunction.

Acute exposure to irritants causes the secretion of watery, easily cleared mucus [14]. Similarly, asymptomatic smokers produce low viscosity mucus that is transported 30% faster than normal mucus on the ciliated frog palate [15]. In vivo mucociliary clearance is not increased in the light smoker, possibly because of ciliary damage [16]. Peripheral zone (i.e., predominantly non-cartilaginous conducting airways) radioaerosol clearance in smokers is similar to that in non-smokers but inner zone clearance (predominantly cartilaginous epithelia with submucosal glands) is significantly slower in smokers than in non-smokers suggesting that smoking initially affects the ciliated epithelium and disrupts ciliary clearance mechanisms [5][17].

Mucus in Chronic Bronchitis

There appears to be a genetic basis to the development of chronic lung disease in smokers. This is likely to be due, in part, to genetic variations in airway defense mechanisms. A clear example of this is observed in persons with homozygous alpha l-antitrypsin deficiency who have impaired pulmonary antiprotease enzyme defects. A large group of patients with this disorder was followed longitudinally and the mean age of onset of dyspnea in smokers (32 years) was significantly lower than that in non-smokers (51 years). Mean age at death excluding that from liver disease, was 48 years in smokers and 67 in non-smokers. In non-smokers, mean FEV₁ was 77% of predicted, but in smokers it was only 38% of predicted. In smokers, the decline in FEV₁ correlated with the extent of cigarette smoking in pack-years. The mean decrease in FEV₁ in non-smokers was abnormally high (80 ml/year), but significantly less than the mean decline of 317 ml/year in smokers [18].

Beyond known abnormalities of pulmonary antiproteases there appears to be a genetic basis for the development of COPD in smokers. Evidence from cross-sectional studies supports the concept of the healthy smoker as an individual who takes up the habit because his or her lungs are relatively resistant to the effects of smoking. Data show that young persons who smoke have better lung function initially than those who remain non-smokers. Relatively poor lung function appears to discourage young people (especially males) from becoming regular tobacco smokers. This also means that pulmonary function prediction equations based on so-called normal populations of non-smokers might underestimate normal lung function, and the adverse effect of smoking on lung function may be even greater than that estimated from cross-sectional studies [18&19].

When the mucociliary system is overwhelmed, cough and sputum expectoration becomes vital for airway hygiene. Most chronic smokers produce a rigid mucus (i.e., having greater viscoelasticity) with more sialic acid and fewer fucose residues [20]. This reduces mucociliary clearance while preserving cough transportability of secretions [21]. Anatomic disruption, ciliary impairment, and rigid secretions all contribute to poor mucociliary clearance in patients with chronic bronchitis. Plasma extravasation and mucosal edema deliver chemical mediators into the airway mucosa and lumen and can cause epithelial cell sloughing adding to the debris within the mucus layer [22]. Plasma exudation and fibrin formation can change the viscoelasticity of the mucus, impair the surfactant properties of the airway lining material, promote inflammatory cell recruitment, and cause small airway obstruction and gas trapping [23]. Although there is plasma exudation in severe COPD, one thing that distinguishes asthma from COPD is that there is much less exudation, edema, and fibrin deposition in COPD. There can be overlap in the phenotypes and all of these signs are associated with the degree of inflammation.

These changes in mucus clearance lead to ventilation-perfusion mismatch, impaired gas exchange, pulmonary hyperinflation, and inspiratory loading of the respiratory muscles leading to fatigue and ineffective cough. It has been postulated that impaired mucus clearance in chronic smokers can lead to prolonged contact of irritants with the airway epithelium and so promote cellular metaplasia and cancer [24].

Airway Secretions and Gel-Forming Airway Mucins

Airway mucus forms a protective barrier between the respiratory tract epithelium and the environment. Mucus is composed mainly of water and ions with approximately 5% of the content due to proteins secreted by airway cells and lipids [25-27]. In health, the mucin glycoproteins are the major macromolecular component of the mucous gel. The mucins are responsible for the protective and clearance properties of the mucus [28-30].

There are two major classes of mucins: the secreted and the membrane-tethered mucins [31&32]. Several secreted mucins (MUC2, MUC5AC, MUC5B, and MUC6), have genes that are clustered on chromosome 11p15 and contain domains with significant homology to the von Willebrand factor D domains that are sites for oligomerization [33]. In sputum, MUC5AC and MUC5B are the major oligomeric mucins, whereas MUC2 is almost absent [34]. MUC5AC appears to be produced primarily by the goblet cells in the tracheobronchial surface epithelium, whereas MUC5B is secreted primarily by the submucosal glands [35]. The membrane-tethered mucins, MUC1, MUC3, MUC4, MUC12, and MUC13, contain a transmembrane and a short cytoplasmic domain [33]. At least 8 mucin apoproteins (MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC7, and MUC8) are expressed by the tracheobronchial epithelium [33][36].

Mucus and Mucin in Normal and Diseased Airways

It has long been recognized by clinicians that airway mucus obstruction is a major cause of morbidity and mortality in chronic inflammatory airway diseases such as COPD, asthma, and CF. In a retrospective data analysis of The Copenhagen City Heart Study, 9,435 subjects with COPD were followed for more than 10 years. It was shown that chronic mucus hypersecretion is significantly associated with both an excess FEV₁ decline and an increased risk of hospitalization because of COPD. The authors concluded that the more sputum that is expectorated, the greater is the mucus burden and thus the risk of being admitted into hospital, the more frequent is the rate of exacerbations, and the greater is the risk of dying because of COPD [37&38].

Sputum production and associated airway mucus obstruction plays a major role in the pathogenesis of severe COPD and asthma [39]. CF airway pathology is marked by mucus obstructing the conducting airways and an intense neutrophilic inflammation [40&41]. COPD exacerbations are associated with goblet cell hyperplasia [42], mucus hypersecretion [43] and “mucus plugging” (Figure 1) in the airway lumen [44].

Figure 1 Lung histology obtained from a patient who had died during a COPD exacerbation [pictures from Ref. [44]]. Left: The lumen of a terminal bronchiole showing mucus admixed with scattered inflammatory cells and desquamated epithelial cells. Focally, the mucosal layer shows basal cell hyperplasia and (arrow) mucosal lining by mucin-producing cells (hematoxylin-eosin, original × 400). Right: A respiratory bronchiole with luminal accumulation of mucus admixed with desquamated cells and inflammatory cells. The alveolar septa show vascular congestion and increased numbers of acute inflammatory cells (arrows) (pentachrome, original × 400).

Using tools such as human airway epithelial culture systems, specific anti-MUC antibodies, and in vivo models of chronic inflammatory airway diseases, there is a growing body of evidence that mucin glycoproteins play a critical role in goblet cell hyperplasia, mucus hypersecretion, and airway obstruction.

It is possibly that in disease, alterations in the mucin biosynthetic process may produce mucins with altered macromolecular structure. Sheehan et al. [45] have shown that the mucus from an individual who died of asthma was composed of MUC5B mucins with an extremely larger size and unusual network morphology. Because mucin composition, chain length, and organization can have profound effects on mucus rheology it is important to establish what factors affect mucin polymerization structure.

Mucin in the Normal Airway

There are few published data regarding the mucin protein composition of airway secretions in health or disease [34][46]. In the normal airway MUC5AC protein is the predominant gel-forming mucin [35&36], with a molecular weight of 14–16 MDa [28][47]. In cervical and tracheobronchial secretions, MUC5AC has lower sedimentation rates compared to MUC5B which means that MUC5AC is a smaller, less oligomerized mucin than MUC5B [35][48].

Stored intracellular mucins have a broad distribution of molecular mass (2–15 MDa) [49]. However, the processes of mucin biosynthesis, assembly, and secretion are not known. Analogous to the multi-subunit glycoprotein von Willebrand factor assembly, it can be speculated that mucins are slowly polymerized within storage granules. There is evidence that while in these granules the molecules can further be oligomerized to form large linear complexes with a relative molecular mass in excess of 10–40 MDa [49&50].

Mucin in Chronic Bronchitis

Mucins from COPD sputum have a molecular mass of 10–30 MDa similar to mucins obtained from normal subjects and patients with CF [28][47]. Thornton found that there is relatively less MUC5AC in sputum from patients with COPD than in normal mucus [36]. This might be related to the increased intracellular airway epithelial mucin stores in bronchial biopsies from cigarette smokers [51].

Examination of peripheral lung sections from smokers with COPD (n = 9) and age-matched controls including smokers (n = 11) and lifelong non-smokers with normal lung function (n = 6), revealed that the expression of MUC5AC was significantly higher in the bronchiolar epithelium of patients with COPD. It was found that within the bronchiolar lumen MUC5B was significantly more frequently identified in COPD patients [52].

DNA and Mucin Polymer Network

The rheology and surface properties of sputum are thought to influence cough and mucociliary clearability [8][53&54]. These properties are thought to depend on the concentration, molecular weight, and polymer structure of the macromolecular biopolymers present in the sputum. Sputum contains several species of biopolymers including high molecular weight DNA [55], mucin [56], and filamentous actin [57]. Co-polymers of DNA and F-actin in CF sputum have been visualized using fluorescent microscopy [57]. Further studies visualized and measured the length of polymers using laser scanning confocal microscopy, but mucin was not included in these analyses [58].

The Bueche theory of polymers states that longer polymer chains increase chain entanglements, increasing viscosity by a power of 3.4 to the length [59]. Which polymers contribute most to viscoelasticity however, is unclear, and may change with the underlying disease and severity of disease. Further, it is unclear how therapy affects the polymer network. Peptide mucolytics degrade DNA and actin filaments and leave the mucus network intact. Dornase alfa (Pulmozyme, Genentech, South San Francisco, CA) is approved for the treatment of CF and this medication reduces sputum viscosity [60], but there is significant variability in clinical response [61-63]. In patients with COPD, a small trial of aerosolized dornase alfa administered during acute exacerbations did not result in improved pulmonary function but appeared to reduce mortality starting several months after the study medication had been stopped [64]. However, a larger study did not support these earlier results [65]. Similarly, a double blind placebo-controlled study of nebulized dornase alfa administered for 14 days to adults with non-CF bronchiectasis showed no significant change in pulmonary function, quality of life (QOL), or sputum transportability but after incubation of sputum with dornase a fall in transportability was observed [66].

Therapy of Abnormal Secretions in COPD—Medications

Expectorants and Hydration

Expectorants are medications that are thought to promote secretion clearance by increasing hydration but there are few data demonstrating efficacy. Expectorants do not alter ciliary beat frequency or mucociliary clearance. Oral expectorants are thought to increase airway mucus secretion by acting on the gastric mucosa to stimulate the Vagus nerve, but this has not been well documented. Although guaifenesin (glycerol guaiacolate) appears to be a gastric stimulant, it has little, if any, mucoactive action in vitro [67&68].

Dehydration could potentially increase the tenacity of secretions by increasing adhesivity. The more secretions adhere or “stick” to the epithelium, the more difficult they will be to cough up. Thus theoretically, if there is a way to rehydrate the surface of dry secretions this would be of benefit. However, dehydration of secretions is rarely a clinically significant problem except in patients with severe systemic dehydration or those who are intubated and ventilated with inadequately humidified gases. Excessive hydration can lead to mucosal edema and impaired mucociliary clearance. Moderate hydration in patients with chronic bronchitis has not been shown to have a significant affect on sputum volume or ease of expectoration [69].

Iodopropylidene glycerol is an expectorant that seems to increase tracheobronchial clearance in patients with chronic bronchitis who are expectorating sputum daily [70] and to improve the quality of life in patients with chronic bronchitis [71]. However, a controlled study of 28 patients with stable chronic bronchitis showed no significant changes in pulmonary function, gas trapping, symptom scores, or sputum properties that could be attributed to therapy with this medication [72].

Hypertonic saline inhalation may increase mucociliary clearance by an increase in secretion output and ciliary beat frequency [73]. The inhalation of hypertonic saline (generally 4–5 ml of 5–10% saline by nebulization) has been used for sputum induction to evaluate for the presence of neoplastic cells, tuberculosis, or Pneumocystis. It also may be useful for studying the contribution of luminal neutrophils and eosinophils in COPD [74]. Short-term studies suggest that the inhalation of 6% saline can increase expectoration volume in patients with CF [75] and COPD [76]. All patients received aerosolized albuterol before inhaling the saline and there were no significant problems with bronchospasm reported. Because hypertonic saline is inexpensive and readily available, this could be a useful therapy to enhance secretion clearance. There is some concern that high concentrations of salt can inactivate airway beta defensins and so promote chronic airway colonization and infection [77]. Furthermore it was shown that an adverse lung function response to hypertonic saline is common in patients with moderate to severe COPD, which involves both bronchoconstriction and lung hyperinflation, and could be mediated, at least partially, through activation of mast cells [78]. Long-term studies will be needed to determine the efficacy and safety of this therapy but hypertonic saline or mannitol has the potential to improve sputum clearance in chronic lung diseases [79&80].

Mucolytics

Mucolytic agents disrupt the polymeric structures in secretions. Classic mucolytics with free thiol groups sever mucin polymers through hydrolysis of disulfide bonds. The peptide mucolytics exert their effect on pathologic polymers of neutrophil-derived DNA and F-actin.

Classic Mucolytics

Acetylcysteine is the most commonly used proprietary mucolytics agent. Acetylcysteine breaks disulfide bonds in mucin polymers to decrease mucus viscosity. Although acetylcysteine can be instilled directly into airways under bronchoscopic visualization to relieve airway plugging, because the aerosol has a pH of 2.2, it can cause airway irritation and bronchospasm.

Acetylcysteine can decrease mucus viscosity in vitro in human tracheobronchial secretions [81] and nasal secretions [82]. However, because oral acetylcysteine is rapidly inactivated and does not appear in airway secretions, it has no mucolytic affect whatsoever in vivo. Published evidence suggests that oral acetylcysteine may improve pulmonary function in selected patients with chronic suppurative lung disease including COPD [83-90]. Thus any clinical benefit observed is likely due to antioxidant properties. The daily usage of acetylcysteine reduces the risk of re-hospitalization for acute exacerbations of COPD by approximately 30% [91] but it does not modify the outcome in acute exacerbations of COPD [92]. Although there are no data that support the use of carbocysteine, Mesna, or similar agents as mucoactive medications, there are few well-controlled studies [93-102].

Peptide Mucolytics

The only peptide mucolytic agent approved for use in the United States is dornase alfa for the treatment of CF lung disease. Aerosolized dornase alfa reduces the viscosity and adhesiveness of infected sputum in vitro [60] and modestly improves FEV₁ in patients with CF [103-106]. The beneficial in vitro effect on rheological and transport properties has been reported in the purulent sputum of chronic bronchitis [107]. However, in patients with chronic bronchitis, dornase alfa does not appear to improve pulmonary function [64] or reduce morbidity [65].

Actin is the most prevalent cellular protein in the body, playing a vital role in maintaining the structural integrity of cells. Under proper conditions, actin polymerizes to form filamentous actin (F-actin). Extracellular F-actin probably contributes to the viscoelasticity of expectorated CF sputum although this has not been definitively demonstrated [108]. In vitro studies suggest that F-actin depolymerizing agents used in conjunction with dornase alfa may result in a greater reduction in sputum viscoelasticity and cohesivity than either used alone [109].

Mucokinetic Agents

Effective cough clearance requires high airflow velocity to detach the sputum from the epithelium and to mobilize secretions so that they can be expectorated. Mucokinetic agents improve the cough clearance of mucus by increasing airflow or by altering the sputum-epithelium interface by reducing mucus adhesion. A substance that decreases adhesion is called an abhesive.

Medications Increasing Airflow

Beta agonist bronchodilators can improve airway clearance by reducing gas trapping and increasing airflow dependent clearance. Although the beta agonists increase ciliary beat frequency, data regarding their effectiveness in promoting mucociliary transport are conflicting [110-113]. Their role is limited in assisting sputum clearance, especially when there has been considerable ciliary damage. Xanthines also increase ciliary beat frequency but these medications have not been consistently demonstrated to improve mucus clearance [114]. The clearest indication for the use of bronchodilators as mucokinetic agents is when their use produces improvement in airflow documented by spirometry.

Abhesives (Lubricants)

Surfactant can reduce sputum adhesivity and increase the efficiency of energy transfer from the cilia to the mucous layer. Several investigators have demonstrated a decrease in the amount of bronchial surfactant [115] and abnormal sputum phospholipid composition [116&117] in patients with chronic bronchitis.

Sputum tenacity, the product of adhesivity and cohesivity, has the greatest influence on the cough clearability of sputum [53]. Decreasing tenacity with surfactant effectively increases the cough transportability of secretions [118]. We showed that 14 days of aerosolized surfactant (607.5 mg DPPC/day) increased in vitro sputum transportability, improved FEV₁ and FVC by more than 10 percent, and deceased trapped thoracic gas (RV/TLC ratio) by more than 6% in patients with stable chronic bronchitis. This effect persisted for at least a week after treatment was completed [119].

Bromhexine stimulates mucus secretion in the canine trachea, and it was speculated that this was a preferential effect on submucosal glands [120]. It has been used in a number of investigations and oral administration results in an increase in expectoration of sputum in chronic bronchitis [121]. In a multicenter placebo-controlled trial of 237 patients with COPD, oral bromhexine significantly improved sputum “quality” and ease of expectoration [122]. There were also significant improvements in cough, dyspnea, physician's overall assessment and in lung function.

Ambroxol is related to bromhexine and is thought to stimulate surfactant secretion. This medication has been used for many years in Europe for the management of chronic bronchitis but has never been approved in the United States or Canada. Results of clinical studies with this agent have been conflicting—with some demonstrating clinical benefit [123-126] while others have not [127].

Some of the expectorant activity of the classic mucolytics may be attributed to abhesive action. Although decreasing the viscosity of a mucous plug might actually reduce sputum cough clearability by decreasing the height of the mucous layer, if a mucolytic decreases mechanical impedance at the epithelial surface (i.e., frictional adhesive forces), it is possible to “unstick” secretions from the underlying ciliated epithelium making airflow dependent clearance more efficient.

Mucoregulatory Agents

Mucoregulatory agents inhibit mucus secretion or mucus production. These agents have been most thoroughly studied for the therapy of bronchorrhea, CF, and diffuse panbronchiolitis (DPB).

Cholinergic nerves are the dominant neural stimulant to mucin secretion in the airways [128]. Mucin secretion is mediated via muscarinic M3 receptors on the secretory cells, with water secretion mediated via M1 receptors [129&130]. M2 receptors are auto-inhibitory and regulate the magnitude of cholinergic mucin output.

Anticholinergic medications reduce the volume of stimulated secretions without changing their viscoelasticity [112][131-133]. It is unlikely that they decrease constitutive secretion in the lower respiratory tract [134-137]. Ipratropium bromide and oxitropium bromide may reduce the volume of secretions in patients with chronic bronchitis when given chronically without changing mucociliary transport or mucus viscoelasticity [138&139].

Tiotropium bromide is a newer long-acting anticholinergic with selectivity for M1 and M3 receptors [140]. It should have greater efficacy than first generation anticholinergics due to reduced inhibition of contra-regulatory M2 receptors, although its effects on secretion have not been reported.

Many inflammatory mediators are potent secretagogues [137][141-143] and chronic inflammation leads to mucous gland hyperplasia. Anti-inflammatory agents have been used as mucoregulatory agents for many years [144]. There is experimental evidence that steroids are effective in reducing the volume of secretions [145]. There are no published data on the use of inhaled steroids as mucoregulatory agents but it is possible that this mode of delivery would also be effective in decreasing mucus hypersecretion.

Indomethacin is an anti-inflammatory agent that has been effectively administered by aerosol for the treatment of mucus hypersecretion associated with DPB [146]. One of the most interesting discoveries is that the 14-member macrolide (e.g., erythromycin, clarithromycin, roxithromycin) and 15-member azalide (azithromycin) antibiotics are able to reduce mucin output from cells in vitro [147]. This action is unrelated to the antibacterial activity but may be due in part, to an immunomodulatory action. The effect of clarithromycin (100 mg twice daily) on the volume, viscoelasticity (elastic modulus and dynamic viscosity), and the percent solid composition of sputum samples was shown in a double-blind, placebo-controlled, 8-week trial of 31 patients with chronic bronchitis, bronchiectasis, or DPB [148]. Treatment with clarithromycin reduced the volume of sputum expectorated by patients from 51 g/day to 24 g/day (p < 0.001). Therapy with clarithromycin increased the percent solid composition and the elastic modulus (p < 0.05) but did not alter the dynamic viscosity of the sputum. Based on these data, the investigators suggested that clarithromycin reduces both mucus secretion and water in the airways. Clarithromycin was also shown to significantly reduce the volume of mucus secretion and improve the physical properties of nasal mucus such as viscoelasticity, cohesion, hydration, and ciliary transportability in 10 patients with purulent rhinitis [149]. A randomized, double-blind, placebo-controlled study of the effect of roxithromycin on airway responsiveness in 25 children with bronchiectasis showed a significant decrease in sputum purulence following treatment with roxithromycin for 12 weeks [150]. The mechanism by which macrolides inhibit mucus secretion is unclear but possibly involves the inhibition of pro-inflammatory cytokines [151&152]. It is interesting to note that these effects appear to be unrelated to the drug's antimicrobial effects and that the 16-member macrolide antibiotics have no significant immunomodulatory activity. These interesting effects are reviewed in a recently published book [13].

Therapy of Abnormal Secretions in COPD—Chest Physical Therapy and Devices

Chest physical therapy (CPT) includes the application of directed cough, forced expiratory techniques, postural drainage, chest percussion, clapping, vibration, high frequency oscillation, and breathing exercises. CPT has been poorly studied in chronic bronchitis and there is little evidence of its efficacy [153-156]. Although many studies showed the effectiveness of CPT for the therapy of CF [157-159] there are few data supporting its use in COPD. Mucus transport by expiratory airflow (including cough) is the primary transport mechanism in patients with pulmonary diseases when mucociliary transport is damaged [160]. Airflow or cough transport is dependent on several factors:

Airflow Velocity

At high airflow velocity there is a characteristic interaction between gas and fluid called annular flow and this increases mucus transport such that airflow velocity is directly related to mucus transport.

Thickness of the Mucus Layer

The minimal flow velocity at which mucus transport by gas liquid interaction arises is dependent on the thickness of the mucus layer [161]. As the thickness of the mucus layer increases less airflow velocity is needed for cough transport. A corollary to this is that excessive thinning of secretions by a mucolytic may paradoxically decrease the effectiveness of cough.

Peak Expiratory Flow

The peak expiratory flow (PEF) reached during a forced expiration is important in that a decreased PEF minimizes the effectiveness of forced expiration. The rapid application of a high expiratory (peak) flow may also decrease the viscosity of mucus. This phenomenon of frequency dependent shear stress reduction of viscosity in pseudoplastic materials is known as thixotropy [8].

High PEF can facilitate detachment of adherent airway secretions from the epithelial surface. The mucociliary and mucus-epithelial interaction is most pronounced at the interface between the two surfaces. Detachment can be affected either by airflow forces or by application of physical therapy techniques such as chest percussion; analogous to pounding on a ketchup bottle [156]. Once a critical airflow is reached (detachment velocity) there will be marked augmentation in mucus clearance from the airway as well as improvement in ciliary efficacy.

Cough is thought to be most effective at clearing secretions from the cartilagenous central airways [155]. CPT may also trigger cough receptors. CPT combined with vigorous, directed cough is effective in clearing the airways of patients with retained secretions [162]. Evidence from studies using inhaled radioaerosol techniques show that cough alone and cough combined with chest physical therapy were equivalent in promoting central airway mucus clearance whereas combined techniques were better for accelerating clearance from the small airways [163].

High expiratory airflow (mucus shearing forces) depends on the generation of large positive intrapleural pressures best achieved at high lung volumes. High expiratory flow can be achieved by a forced expiration and this appears to be most efficacious in the patient with optimally treated airflow obstruction. Dynamic compression of the airways takes place during a forced expiration upwards from the airway equal pressure point. The location and the magnitude of the compression can be varied by expiration force (pleural pressure) and lung volume (elastic recoil pressure) [164]. For these reasons breathing exercises are often combined with CPT.

Addition of percussion to conventional physiotherapy does not improve sputum yield or mucociliary clearance in most studies except possibly for patients with CF. Postural (gravity assisted) drainage, as distinct from chest percussion, adds little to the effectiveness of chest percussion and increases the risk of aspiration from increased gastroesophageal reflux [165&166].

High frequency oscillation is also used to improve mucus transport. Oscillations can be applied at the mouth using a modified loudspeaker [167], or at the thorax using an inflatable vest [168&169]. Usually a frequency between 3–17 Hz is used. As a single procedure the effect on mucus transport seems to be frequency dependent [170]. A frequency of about 10–15 Hz, which is outside the range of the manual techniques, seems to have the best effect on mucus transport and there appears to be no benefit in frequency “cycling” as some manufacturers advocate. We generally use external high frequency oscillation at a set frequency of 12 Hz only. Lower or higher frequencies seem to be less effective perhaps because intrathoracic pressures induced by vibration and the vibration induced expiratory flows are frequency dependent. The direction and the amplitude of the induced flow are also important and are more effective in improving mucus transport with higher flows directed towards the mouth [170].

A number of authors have studied the short-term effects of independent methods to improve sputum mobilization. The active cycle of breathing (ACB), positive expiratory pressure (PEP) mask, Flutter breathing, and autogenic drainage all seek to avoid the problem of dynamic airway compression which may inhibit sputum mobilization. The ACB (previously called the forced expiratory technique) may cause less bronchial compression and collapse, and be more effective in clearing airway secretions in selected patients. The ACB appears to be more effective than cough alone and supervised directed coughing may be as effective as conventional postural drainage, vibration and/or percussion and coughing in patients with CF [171].

Flutter breathing entails the patient breathing through a small device shaped like a smoker's pipe. This contains a steel sphere that oscillates up and down as the patient exhales into a mouthpiece imposing frequency oscillations (6–20 Hz) on a positive expiratory pressure [172]. In a non-CF bronchiectasis study over one month the flutter device was as effective as ACB [173]. A short-term study showed that the flutter device may enhance the bronchodilator response in patients with stable COPD [174]. Although this device is inexpensive and theoretically attractive, there are no published studies demonstrating long-term pulmonary function or clinical benefits in COPD or CF and published short-term studies have relied on the measurement of expectorated sputum volume as the primary outcome variable [175]. Unfortunately, expectorated sputum volume does not correlate at all with clinical improvement.

PEP mask therapy has been best studied in patients with CF. Although several short term studies found no difference between patients with CF using this technique compared to patients on conventional chest percussion, [176&177] other generally longer studies have demonstrated improvement in pulmonary function of CF patients using this technique [178&179]. The technique requires repetitive breathing through a face mask and is combined with the FET. Inspiration is unobstructed but expiratory pressure is generated through a flow or threshold resistor. Different types of masks are available. Flow-dependent masks use a manometer to determine PEP. Patients using flow-dependent systems without a manometer will have greater variation in pressure. The PEP technique is easily applied and allows independent treatment by older patients.

In a 12 month study in subjects with moderately severe COPD the number of acute exacerbations was lower in the PEP group (n = 20) compared to the control group (n = 23). The PEP group had a small increase in FEV₁ (mean 62 ml) compared to a small decrease (mean 43 ml) for the control group [179].

Contraindications to Mucoactive Therapy

Severe airflow limitation reduces the ability to mobilize secretions by cough. There is a theoretical risk that if secretions are thinned or loosened, but the patient cannot clear them from the airway, they can lodge deeper into the airways causing greater obstruction. Although a recent study of patients with moderately severe CF lung disease (FVC < 40% predicted) receiving dornase alfa did not demonstrate a worsening of pulmonary function with therapy [180], any mucoactive therapy (except that which increases airflow or does not require active expectoration on the part of the patient) should be used with caution in patients with end-stage pulmonary disease or neuromuscular weakness.

Patients with acute mucus retention such as acute exacerbations of chronic bronchitis appear to be less responsive to mucoactive medications than stable patients. This may be due to decreased airflow caused both by the increase in infection and muscular weakness in association with the pulmonary exacerbation, further reducing airflow dependent clearance mechanisms.

In some patients with asthma and chronic bronchitis (especially those with hyperreactive airways, scanty sputum production, and small airways disease) wheezing and bronchoconstriction may follow the use of chest physical therapy. It is also possible that when mucus transport is increased by chest physical therapy and cough, mucociliary clearance may be slowed due to the airway compression during these procedures. Finally, patients with gastroesophageal reflux and inability to protect their airway are at risk from postural drainage and may be at risk from the use of mucus mobilizing or expectorant therapy, particularly when the medication increases the volume of secretions produced.

COPD Exacerbation and Biological Markers

Exacerbations are a common cause of morbidity [181&182] and mortality in COPD patients [183-185]. In the EU there are an average of 17,300 annual physician visits for COPD per 100,000 people. This accounts for 10.3 billion (13.5 billion) in healthcare costs. Despite medical treatment, approximately one third of patients discharged from the emergency department with acute exacerbations have recurrent symptoms within 14 days [186], and 17% relapse and require hospitalization [187]. Identification of patients at risk for relapse improves decisions about hospital admissions and follow-up [188-190]. Conventional end points for efficacy of bronchodilator, corticosteroid and antibiotic therapy in exacerbations include symptoms and bacteriological resolution at 2–4 weeks. These end points have been used to evaluate new drugs but may lack clinical relevance. Other end points, such as an exacerbation-free interval, resource use (hospitalization, clinic visits, medication use, lost work days, etc.), quality of life and sputum composition may be more suitable end points in this patient population [191-194].

Only a few studies describing changes in biological markers during exacerbations have been described. Many of the studies have focused on markers of inflammation. The heterogeneity of COPD exacerbations presents a problem in pursuing such biomarkers, as inflammation may not be common to all exacerbations [195&196]. Serum IL-6 levels have been shown to increase during exacerbations, leading to increased plasma fibrinogen levels [197&198]. It is likely, however, that IL-6 is highly correlated with viral infection and may therefore not represent a global marker [197]. Exacerbations have also been associated with increased urinary excretion of isoprostane F2a-III in hypoxemic patients [199], increased plasma levels of the acute-phase reactant C-reactive protein [200], serum eosinophilic cationic protein and myeloperoxidase [201]. Bronchial biopsies obtained from patients at the time of an exacerbation have shown marked airway eosinophilia with an associated increase in RANTES expression and a lesser increase in the number of neutrophils, T-lymphocytes and tumor necrosis factor alpha (TNFα) positive cells in the bronchial mucosa [202&203]. COPD patients have an increased number of CD8 + cells in sputum as compared with smokers without COPD (p = 0.0001) and control subjects (p = 0.001). CD8 + -IL4 cells are reduced both in COPD and in smokers without COPD compared to controls (p = 0.0001), while CD8 + -Interferon gamma (IFN gamma) cells are significantly reduced only in COPD (p = 0.001) as compared with controls. A significant (p = 0.02) relationship between the CD8 + -IL4/CD8 + -IFNgamma ratio and FEV1 (% of predicted) was found only in COPD patients. These findings suggest that an imbalance both in T-lymphocyte subpopulation (CD4/CD8) and in CD8 + cell subsets (Tc1/Tc2) characterizes the inflammatory responses of smokers with established COPD [204].

While this approach can provide information on tissue changes related to an exacerbation, the invasive nature of the technique limits its application, particularly during an exacerbation. Bronchoalveolar lavage (BAL) has proved a useful alternative providing insights into the inflammatory nature of exacerbations. Studies of BAL fluid have shown changes in cell populations consistent with a recruitment of polymorphonuclear leucocytes in to the airway lumen [205&206]. Observed increases in levels of granulocyte–macrophage colony-stimulating factor (GM-CSF) suggests a role for this cytokine in the inflammatory processes during exacerbations [205]. The assessment of induced sputum is a less invasive alternative. Induced sputum has increased IL-6 during exacerbations and higher IL-6 and IL-8 levels in patients with frequent exacerbations [197][207]. During stable COPD, the sputum concentration of inflammatory markers such as IL-8 and sICAM-1 seem to be reproducible and stable [208]. Exacerbations classified by sputum color seem to be much less reliable. “Purulent” exacerbations are associated with inflammation, increased neutrophilic inflammation (with myeloperoxidase providing the characteristic green color) and increased LTB4 concentrations [209]. Mucoid exacerbations are associated with less inflammation [196]. It has not been determined how gel-forming mucin secretion is associated with COPD exacerbations. It is possible that mucins can be useful markers since they are easily accessible and not strictly related to viral, bacterial or other etiological reasons for COPD exacerbation. While data from biomarker studies are limited, they have opened a new avenue of research towards finding accurate markers for COPD exacerbations and recovery.

Abbreviations
ACB:	=	active cycle of breathing
BAL:	=	bronchoalveolar lavage
CF:	=	cystic fibrosis
COPD:	=	chronic obstructive pulmonary disease
CPT:	=	chest physical therapy
DPB:	=	diffuse panbronchiolitis
DPPC:	=	dipalmitoyl phosphatidalcholine
FDA:	=	US Food and Drug Administration
GM-CSF:	=	granulocyte–macrophage colony-stimulating factor
IL:	=	interleukin
PEP:	=	positive expiratory pressure
PEF:	=	peak expiratory flow
QOL:	=	quality of life
TNFα:	=	tumor necrosis factor alpha

Notes

^aBy convention, the term mucus is used as a noun and mucous as an adjective or adverb. Sputum is expectorated airway secretions.