A 2-year-old boy is brought to the Emergency Department with a 1-day history of a barking cough and hoarse voice following several days of coryzal symptoms. Overnight his parents noticed a harsh breathing noise when he became upset, which settled when he calmed.

On examination he is alert and sitting comfortably on his mother’s lap. His voice is hoarse and he has a barking cough. When agitated he develops a high-pitched inspiratory noise, but this disappears when he settles.

Observations:

Temperature 37.8 °C

Heart rate 110 bpm

Respiratory rate 26/min

Oxygen saturation 98% on room air

The clinical picture is consistent with viral croup.

Stridor is a clinical feature rather than a diagnosis, and its timing within the respiratory cycle may suggest the anatomical level of airway obstruction.

Which of the following statements regarding stridor is NOT correct?