During inspiration, the airway collapses and air flow becomes turbulent, producing the sounds we hear as inspiratory stridor. During expiration, the intraluminal pressure exceeds that of surrounding tissue, dilating the airway and improving airflow. For intrathoracic obstruction, the situation is reversed. During inspiration, the expansion of the chest wall decreases the intrapleural pressure compared to the intraluminal pressure. The trachea dilates and the obstruction is reduced.
Careful auscultation can therefore help differentiate between upper and lower airway obstruction.
Upper airway obstruction can be defined as subglottic, glottic, or supraglottic. Subglottic obstruction Subglottic obstruction is frequently associated with previous manipulation or trauma to the airway, including intubation or tracheotomy. Prolonged intubation increases the risk of subsequent subglottic stenosis. Subglottic stenosis can also occur in certain autoimmune disorders such as Wegener's granulomatosis. Mediastinal and cervical tumors such as adenoid cystic carcinoma can directly compress the subglottic lumen.