Respiratory diseases frequently involve the small airways, yet many respiratory therapists lack a real understanding of the concept of small airways disease – and with good reason. The definition of “small airways disease” varies based on one’s perspective.
A pulmonologist may consider small airways disease as a group of lung disorders involving the terminal airways assessed by airflow obstruction, while a radiologist might define it by evaluating high-resolution computed tomography scan changes. A pathologist would likely identify small airways disease based on histological specimens.
A number of diseases fall under the general umbrella of small airways disease. Small airways usually are defined as airways <2 mm in internal diameter absent of cartilage.
Because classifications of lung diseases change and overlap, classification of small airways disease has varied. One method to classify small airways diseases is to determine whether the disease is primary to the small airways or secondary to other lung disease, such as asthma or smoke exposure.
Research shows that in healthy subjects, small conducting airways are thin-walled and offer little resistance to airflow that is laminar. Resistance to airflow varies inversely to the fourth power of the airway radius. Therefore a 50 percent decrease in airway radius results in a 16-fold increase in resistance to airflow. This decrease in airway radius may result from increased amounts of mucus and inflammatory cells obstructing the airway lumen, increased thickness of the submucosa, fibrosis, or increased smooth muscle mass or contraction.
Understanding ‘bronchiolitis’
Bronchiolitis is a generic term clinically used to describe a variety of inflammatory conditions involving the small airways. The inflammatory conditions may be acute, chronic, or both acute and chronic:
Acute bronchiolitis is common among infants and young children and is recognized as the most common respiratory diagnosis during the first year of life.1 Its etiology is variable but generally infectious, and approximately 75 percent of cases are due to respiratory syncytial virus (RSV). Infants typically present with wheezing, tachypnea, and tachycardia. More severe cases will result in chest retraction, nasal flaring, and lung hyperinflation. A high-resolution chest CT may show branching linear opacities.
Chronic bronchiolitis describes a pattern in which bronchioles contain chronic inflammatory cells. Chronic bronchiolitis may be a feature of various diseases, and symptoms may overlap with acute bronchiolitis. Fibrosis may or may not be present in association with chronic bronchiolitis. However, the presence of lung fibrosis indicates some level of irreversibility in the disease process.
Tobacco-associated bronchiolitic disease (membranous bronchiolitis and respiratory bronchiolitis) are terms used to label the extensive and variably severe inflammatory and fibrotic changes caused by tobacco smoke. These changes are found in both the terminal and respiratory bronchioles. Patients are often asymptomatic, but pulmonary function evaluation may show obstructive change such as decreased expiratory flow rates. Other smoking-related lung changes include chronic bronchitis and emphysema.
Small airways in asthma, COPD
Airway inflammation and remodeling occur in both the large and small airways. Although small airways add little to airway resistance in healthy patients, they are the major site of airflow limitation in both asthma and COPD.
Additionally, distal lung inflammation is present in even those with mild asthma without persistent symptoms. The predominance of distal or small airway obstruction in asthma has resulted in the understanding that it is a disease of the entire respiratory tract. Reasonably then, RTs should consider the small airways as a target for medication delivery.
It is now accepted that in asthma, inflammatory cells, in particular eosinophils and T cells, increase in the small airways and the lung parenchyma. This finding may prove to be important because the total volume and combined surface area of the small airways are much greater than the combined volume and the surface area of the large airways. Peripheral airways also have been recognized as a predominant site of airflow obstruction in asthmatics.2
COPD is defined as an abnormal inflammatory response of the lung to noxious particles and gases, mainly cigarette smoke.3 Airways <2 mm in diameter, which represent <10 percent of total airway resistance in healthy subjects, contribute to the majority of the abnormally high resistance work in patients with COPD. This increased resistance may be related to a reduction in small airway numbers and/or narrowing of small airways.
