Wheezing: Lower Airway Phenomenon
Wheezing is a lower airway sound, not an upper airway sound. Wheezes are continuous adventitious lung sounds produced in the bronchi and bronchioles of the lower respiratory tract, with a dominant frequency of 400 Hz or more 1.
Anatomic Origin and Mechanism
Wheezes originate from the lower airways (trachea, bronchi, and bronchioles) through airway wall flutter and oscillation induced by critical airflow velocity through narrowed or collapsible airways 1.
The mechanism involves fluttering of airway walls and fluid together when airflow reaches a critical velocity in partially obstructed lower airways 1.
The pitch of wheezing depends on the mass and elasticity of the airway walls and flow velocity, not upper airway structures 1.
Clinical Context and Diagnostic Implications
Lower airway pathology such as asthma, bronchiolitis, bronchomalacia, tracheomalacia, and bacterial bronchitis are the primary causes of wheezing 2.
The American Thoracic Society guidelines specifically address wheezing in the context of lower airway evaluation, recommending flexible fiberoptic bronchoscopy to identify lower airway bacterial infection and anatomic abnormalities like bronchomalacia in persistent cases 2.
Approximately 33% of infants with persistent wheezing have identifiable lower airway anatomic abnormalities including tracheomalacia, bronchomalacia, or tracheobronchomalacia 2.
40-60% of infants with recurrent or persistent wheezing have positive bronchoalveolar lavage cultures indicating lower airway bacterial infection 2.
Important Clinical Distinction
Upper airway obstruction produces stridor, not wheezing 2, 3.
Vocal cord dysfunction and exercise-induced laryngomalacia produce respiratory sounds that are often mischaracterized as wheezing but actually represent upper airway obstruction 3.
This distinction is critical because misattributing upper airway sounds as wheezing leads to inappropriate asthma diagnosis and treatment 3.
Relationship to Airway Obstruction
Wheezing correlates with the degree of lower airway obstruction, particularly when measured as the proportion of respiratory cycle occupied by wheeze (tw/ttot) 1.
The presence of wheezing indicates lower airway narrowing from bronchospasm, inflammation, secretions, or structural abnormalities affecting the bronchial tree 1.
Beta-agonists may adversely affect airway dynamics in children with lower airway malacia, despite the presence of wheezing 2, 4.