Clinical examination must define the site of obstruction and evaluate the severity of the situation. The stridor is an inspiratory sound and can be biphasic in case of associated tracheal obstruction. When associated with wheezing, lower airway obstruction has to be considered.
Signs can vary from progressive respiratory symptoms to sudden cardio-respiratory arrest.
Several severity scores have been proposed, especially for croup , but were judged to be less useful than clinical evaluation by an experienced physician. Non-invasive capnometry was used in 85 infants admitted with respiratory distress to an emergency department. It was well tolerated and values of expired CO2 were correlated with capillary PCO2. Recently, it has been demonstrated that severity of UAO can be evaluated by respiratory inductance plethysmography and pulsus paradoxus measurement. These sophisticated methods are of little benefit in emergency departments but can be used for clinical research.