How to know if the respiratory failure is caused by an upper or lower airway disease?
Distinguishing upper from lower respiratory disease is a difficult but important part of clinical evaluation. Many children have disease processes that involve the upper and lower respiratory tracts simultaneously, for example, bronchiolitis caused by respiratory syncytial virus. In general, respiratory sounds reflecting upper airway inflammation are most prominent during inspiration, when negative intraluminal pressure causes upper airway narrowing and turbulent airflow in the trachea. Conversely, intrathoracic (lower airway) processes produce wheezing and other sounds of airway obstruction primarily during expiration, when positive intrapleural pressure compresses intrathoracic airways. If breath sounds are more obstructed on inspiration, the upper airway is probably involved. Wheezing or other sounds more prominent during expiration suggests lower airway disease.