Properties of Airway Mucus

Water can bind to mucous glycoprotein (MGP) macromolecules and influence viscosity. Viscosity can be increased by dehydration of the mucus, as can adhesion of mucus-epithelium. In purulent sputum, the correlation between viscosity and dry weight of solids is poor and may explain why mucous glycoprotein content is a poor index of viscoelasticity in chronic bronchitis, bronchiectasis, and cystic fibrosis. Mucus s viscoelasticity increases with acidic pH (3), causing reduced mucociliary clearance. Its viscoelasticity is also dependent on the content of low molecular weight electrolytes. These properties reflect the polyelectrolyte nature of mucins (4-6). 

Another measure of elasticity is the spinnability (Spinnbarkeit,filance — the thread-forming ability of mucus under the influence of large-amplitude elastic deformation. Spinnability has been correlated positively with mucociliaiy clear- 

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Role of Mucus s Viscoelasticity in Mucociliary and Cough Ciearance 

Mucociliary clearance can be measured by visualizing the movement of particles, such as Teflon disks, tantalum powder, or charcoal powder, placed on the airway during bronchoscopy. An inert, radiolabeled tracer can also be placed on the 

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Lee MM, Schiireh S, Roth SH, Jiang X, Cheng S, Bjamason S, Green FHY. Effect of acid aerosol exposure on the surface properties of airway mucus. Exp Lung Res 1995 21 835 851. 

Lee MM, Green FHY, Bjamasson S, Zhang Y, Schilrch S. Surface properties of airway mucus exposed to acid aerosol. In Liepsch D, ed. Biofluid Mechanics. Dus-seldorf VDI Verlag, 1994 133 144. 

The major weakness is the requirement of nonreactivity of gases in the mucous layer. Very weakly reactive gases may be treated as nonreactive. However, the uptake of ozone, which is known to decompose in water and is expected to react rapidly with biopolymers and other organic molecules in the mucus layer, is probably underestimated in the upper airways and overestimated in the terminal airways of their model. Thus, their model represents a worst-case estimate of dosage of ozone to the terminal airways, which are unprotected by mucous. Too little is known of the chemical and physical properties of the mucus layer, and there is great uncertainty in the values of the diffusivity of ozone or other gases to be used in the liquid phase of gas uptake models. 

Mucociliary clearance is an organized, complex process which is highly dependent upon the composition and depth of the epithelial lining fluid and the viscoelastic properties of the mucus. In many airways diseases there is hypersecretion of mucus. This may cause an overloading of the ciliary transport process, resulting in... 

The relation between the viscosity and elasticity of the secretions is one of the determining factors in transport velocity. If the gel phase is in practice the only one really transported, the sol phase creates a low-resistance milieu where the cilia can beat, an environment that is essential for transport in the direction of the upper airways. One of the most important rheological properties of mucus is viscosity. Viscosity is resistance to flow and represents the capacity of a material to absorb energy while it moves. Elasticity is the capacity to store the energy used to move or deform material. The ratio between viscosity and elasticity appears to be an important determinant of the transport rate (6,10). Mucus transport by ciliary beating is influenced by the viscoelastic and surface properties of the mucus. Theoretical models suggest that a decrease in the ratio of viscosity to elasticity can result in an increase in mucociliary transport (13). 

It is widely held that mucus should possess specific rheological properties for clearance from the airway to occur. Any alteration in mucus rheology that compromises clearance can predispose the individual to airway disease and infection. Some agents capable of altering mucus rheology are listed in Table 9.2. In addition, the state of mucus hydration (or mucin concentration) will affect the properties of the gel. In... 

Hence, mucocihary transport is a complex interaction of ciliary beat and elastoviscous properties of the overlaying mucus and its (dis)continuity in the various airway generations. Mucociliary action is an important function of the airway walls, and the determination of this function is essential to assess the functional and structural integrity of the conducting airways. To study this function, suitably labeled particles are deposited in the airways and mucus transport is determined by external measurement of the label. 

Foster et al. (32) also measured mucociliary clearance in the main bronchi after inhalation of radiolabeled particles, using a gamma-camera. For healthy nonsmokers, a linear velocity of 2.4 0.5 mm/min was measured in the bronchi, compared with 5.5 0.4 mm/min in the trachea. Furthermore, they (32) found a correlation between tracheal and bronchial mucous velocities. There is no direct information available about mucous velocities in more distal airways. From the foregoing observation, it can be assumed that linear mucous velocity will decrease in more peripheral airways. By making assumptions about the properties of mucus during its transport from distal airways towards the trachea, mucous velocity has been modeled by several authors (33-36). From these calculations the mucus-transport velocity in terminal bronchioli could be more than three orders of magnitude lower than in the U achea. 

The surfactant film at the air-mucus interface is probably important for the rheological properties of the extracellular layer by stabilizing the aqueous layer mechanically and by reducing the evaporation of water (54,93). These are well-known functions of surfactant films if spread on top of a water surfaee (94). Fluid balance in the airway lining layer may also be influenced by the recently demonstrated effeet of surfactant in stimulating chloride secretion by airway epithelial cells (95). 

Salathe M, Guldimann P, Conner GE, Wanner A. Hydrogen peroxide-scavenging properties of sheep airway mucus. Am J Respir Crit Care Med 1995 151 1542-1550. 

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