Stridor, a harsh, grating respiratory sound often easily heard without a stethoscope, usually represents extrathoracic airflow obstruction. Although most often inspiratory, it can become biphasic in critical airway obstruction. The noise is caused by rapid, turbulent airflow through a narrowed airway. Poiseuille’s Law states that airway resistance is inversely proportional to the radius of the airway to the fourth power. Therefore, a small change in airway diameter can have a large effect on resistance. Hence, pediatric patients, particularly infants with smaller airways, are especially susceptible to changes in airway caliber from inflammation, secretions, or edema.

Acute-onset stridor is primarily caused by infection, inflammation, or foreign body aspiration. The evaluation of a patient with acute-onset stridor requires an initial assessment of the respiratory pattern and severity of airway obstruction to determine the need for urgent intervention. Oxyhemoglobin desaturation or hypoxemia indicates...

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