Stridor is a coarse, high pitched sound generated by anatomical abnormalities or obstruction of the upper airway. It is not a disease in itself. It can be inspiratory (most common), expiratory, or biphasic. Inspiratory stridor arises from obstruction of the extra-thoracic airway (larynx, pharynx, upper trachea). Expiratory stridor arises from obstruction of the intrathoracic airway (lower trachea and and bronchi). Biphasic stridor implies glottic involvement.
Inspiratory stridor is more common in children as they have relatively narrowed and flexible airways that are more predisposed to collapse. If we consider a gas at rest, it will exert pressure in all directions equally. However, during inspiration, air is drawn into the airways and the force the gas exerts in the direction it is moving parallel to the airways is greater than the pressure exerted on the walls of the airway. As this latter pressure falls, the airway can collapse and be obstructed, causing stridor. Because children’s airways are narrower, there are also more likely to develop stridor secondary to upper airway obstruction from inflammation, oedema, or foreign bodies.
Expiratory stridor is more complicated. Upon inspiration, the expansion of the chest and resultant negative intrathoracic pressure causes the intrathoracic portion of the trachea to be drawn open. Conversely, the extrathoracic, upper portion of the trachea experiences an external positive pressure and its diameter reduces. Conversely, during expiration, the extrathoracic trachea increases in diameter and the intrathoracic trachea narrows as the intrathoracic pressure increases. In a healthy individual, the trachea remains sufficiently patent during inspiration and expiration such that stridor does not occur. However, when the intrathoracic trachea narrows beyond normal limits, especially in the presence of secretions or oedema, the airway obstructs and stridor will be heard on expiration rather than inspiration.