Physiology of Airway Mucus Clearance

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Physiology of Airway Mucus Clearance

Bruce K Rubin MEngr MD FAARC

Introduction

Role of Mucus in Mucociliary Clearance

Mucus Properties

Sputum

Cilia

Airway Surfactant

Cough Clearance

Role of Inflammation

Chest Physical Therapy

Summary

Respiratory tract secretions consist of mucus, surfactant, and

periciliary fluid. The airway surface fluid is present as a bilayer,

with

a superficial gel or mucous layer and a layer of periciliary fluid

interposed between the mucous layer and the epithelium. A thin layer

of

surfactant separates the mucous and periciliary fluid layers. The

mucous

layer extends from the intermediate airway to the upper airway and is

approximately 2-10 micro gmm thick in the trachea. Airway mucus is the

secretory product of the goblet cells and the submucosal glands. It

is a

nonhomogeneous, adhesive, viscoelastic gel composed of water,

carbohydrates, proteins, and lipids. In health, the mucous gel is

primarily composed of a 3-dimensional tangled polymer network of

mucous

glycoproteins or mucin. Mucin macromolecules are 70-80% carbohydrate,

20%

protein, and 1-2% sulfate bound to oligosaccharide side chains. The

protein backbones of mucins are encoded by mucin genes (MUC genes), at

least 8 of which are expressed in the respiratory tract, although

MUC5AC

and MUC5B are the 2 principal gel-forming mucins secreted in the

airway.

Mucus is transported from the lower respiratory tract into the

pharynx by

air flow and mucociliary clearance. Expectorated sputum is composed of

lower respiratory tract secretions along with nasopharyngeal and

oropharyngeal secretions, cellular debris, and microorganisms.

Disruption

of normal secretion or mucociliary clearance impairs pulmonary

function

and lung defense and increases risk of infection. When there is

extensive

ciliary damage and mucus hypersecretion, airflow-dependent mucus

clearance such as cough becomes critically important for airway

hygiene.

Key words: mucus, sputum, cough, cilia, mucociliary clearance,

surfactant, submucosal glands, goblet cells, cystic fibrosis, chronic

bronchitis, asthma. [Respir Care 2001:47(7):761-768]

Introduction

Mucus secretion and clearance are extremely important for airway

integrity and pulmonary defense. It has been estimated that mucus

secretion volume is between 10 and 100 mL per day in health. Airway

mucus

is a viscoelastic gel containing water, carbohydrates, proteins, and

lipids. It is the secretory product of the mucous cells (the goblet

cells

of the airway surface epithelium and the submucosal glands). Mucus is

transported from the lower respiratory tract into the pharynx by air

flow

and mucociliary clearance. In human large airways, and in many larger

species of mammal, the capacity to secrete mucus in response to a

stimulus seems to lie principally in the glands. However, at rest,

goblet

cells may contribute a greater fraction to the total mucus volume,

considering the contribution of distal airways, where surface mucous

cells are found in the absence of submucosal glands.

Mucus consists of a superficial gel or mucous layer and a liquid or

periciliary fluid layer that bathes the epithelial cilia. These 2

layers

are separated by a thin layer of surfactant (Fig. 1). In health, the

mucous layer is about 2-5 micro gmm thick in the trachea, and it

extends

from the bronchioles to the upper airway. The periciliary fluid layer

lies between the cell surface and the mucous layer at a depth that is

just less than the height of a fully extended cilium. Mucus protects

the

epithelium from foreign material and from fluid loss. The depth and

composition of mucus depends on secretion from airway glands, goblet

cell

discharge, and active ion transport across surface epithelium.

Sputum consists of lower respiratory tract secretions, nasopharyngeal

and

oropharyngeal material (including saliva), microorganisms, and cells.

When there is mucous hypersecretion and impaired clearance, abnormal

respiratory secretions can impair pulmonary function, reduce lung

defenses, and increase the risk of infection and possibly neoplasia.

The collection of normal mucus for analysis requires sampling from

endotracheal tubes, bronchial aspirates from healthy animals or

persons,

or secreted material from animal trachea or human bronchial explants.

Commonly the study of airway secretions consists of examining

expectorated sputum, but that material would give only limited insight

into the properties of native, uninfected mucus.

Positioning Versus Postural Drainage

B Fink MSc RRT FAARC

Introduction

Gravity

Posture and Turning

Postural Drainage

Practice to Evidence

Postural Drainage Procedure

External Manipulation of the Thorax

Contraindications for Postural Drainage

Hazards/Complications

Role of Exercise

Summary

For the past 70 years positioning and postural drainage have played an

important role in increasing lung volumes, perfusion, oxygenation and

mobilization of secretions. While gravity is not a primary mechanism

for

normal secretion clearance, it plays a major role in depth and

pattern of

ventilation, perfusion, and lymphatic drainage. Changing patient

position, or turning patients on a regular basis, is a powerful tool

in

maintaining lung health in a broad range of patients. In contrast,

postural drainage requires considerable investment of time, and has

been

shown to have limited benefit in most patients. Postural drainage has

been shown to improve mobilization of secretions in patients with

cystic

fibrosis as well as patients who produce, and have difficulty

clearing,

large quantities of sputum. The benefits of postural drainage appear

technique-dependent, requiring sufficient drainage time (3-15 min) for

each position drained. The evidence does not support the use of

vibration

and percussion independent of active postural drainage. Exercise

offers

benefit in secretion clearance, which increases when combined with a

program of postural drainage. In conclusion, routine turning,

mobilization and exercise is important to maintain lung health in all

patients, while postural drainage, properly applied, has been shown to

improve secretion clearance in a relatively narrow range of patients

with

cystic fibrosis and excessive sputum production. Key Words: postural

drainage, secretion clearance, chest physical therapy, cystic

fibrosis.

[Respir Care 2001:47(7):769-777]

Introduction

Since the 1930s, clinicians have used gravity (by turning the

patient) to

increase lung volumes and oxygenation and to help mobilize secretions

(via postural drainage). Low lung volume, ineffective cough,

ventilation/perfusion mismatch, and thick secretions are commonly

associated with pulmonary complications. Though postural drainage has

become synonymous with secretion clearance in patients who have large

volumes of secretions, there is a greater body of evidence supporting

the

therapeutic implications of mobilization and patient positioning for a

broader patient population. This became evident to the team charged to

develop the American Association for Respiratory Care's Clinical

Practice

Guideline on Postural Drainage. Consequently, that document expanded

its

scope beyond postural drainage, to include the therapeutic impact of

positioning. In this paper I explore the practice of and the rationale

and evidence for positioning and postural drainage in secretion

clearance.