During airway obstruction no gases pass. There is no oxygenation and no ventilation. Patients may maintain adequate oxygenation by absorbing oxygen from the reserves in the lung (functional residual capacity: FRC).

The PO₂ falls progressively during airway obstruction.

Due to the shape of the oxygen-haemoglobin dissociation curve (OHDC), oxygen desaturation may be delayed if the initial PO₂ is high (Graph 1, Zone A).

Once a saturation of 85 % is reached, further desaturation to life-threatening levels is rapid (Graph 1, Zone B).

Desaturation from 85 % to 50 % rarely takes longer than 30-60 seconds.