The physical sign common to all causes of upper respiratory tract (URT) obstruction is stridor.

Stridor is due to Venturi effects created by somewhat linear airflow through a variably collapsible tube, the airway. When one inhales, the relative pressure in the center of the tube becomes greater than that at its edges. This pressure differential leads to collapse of the airway walls. During expiration, the previously collapsed areas are reopened owing to the relative increase in air pressure during expiration. Forced expiration or expiration against a partially closed glottis may cause expiratory stridor even in patients with a normal airway. As one progresses from the supraglottic to the glottic and subglottic and finally the tracheal areas of the airway, there is an increase in physiologic support and therefore a decrease in the amount of collapse that occurs upon inspiration.

Supraglottic obstructions cause inspiratory stridor, with marked inspiratory and expiratory variation; obstructions at the glottic and subglottic areas commonly cause both inspiratory and expiratory stridor of lesser magnitude.1 Finally, obstructions at the level of the trachea and primary bronchi may be associated with inspiratory or expiratory stridor, although usually of a much lesser degree. Although the posterior walls of the trachea and bronchi are somewhat collapsible, good support is provided by the horseshoe-shaped cartilage within its walls. In premature infants and infants with tracheomalacia, these supports are not well formed and inspiratory and expiratory stridor may be impressive. Expiratory stridor, or wheeze, is common in distal airways, since intrathoracic pressure may become much greater than atmospheric pressure during expiration. The pressure differential creates high relative laminar flow through semicollapsible bronchi, resulting in wheezes. This information can be clinically useful when one is evaluating a wheezing child. Commonly, gentle pressure over the chest wall or midabdomen during expiration will increase the intrathoracic pressure and exacerbate wheezing. This maneuver may also assist in detecting inspiratory stridor because the child will follow this relative forced expiration with a much deeper inspiration than at rest. Patients with marked variation in the pattern of stridor should be considered to have a foreign body in the airway until proven otherwise. The quality of the pitch is not clinically useful for diagnosis. The answers to two questions, the age of the patient and the duration of symptoms, will narrow the differential diagnosis considerably, since stridor in an infant below 6 months of age has different causes that it does in older children ( Table

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