Expansion
Scooped expiratory limb in obstruction; flattening indicates large airway obstruction
Expansion
- Normal: a rapid rise to peak expiratory flow, then a straight fall
- Obstructive: a scooped or concave expiratory limb, with reduced flow at low volumes
- Restrictive: a normally shaped but narrow and tall loop, shifted towards lower volumes
Large airway obstruction produces flattening, and the pattern localises it:
- Fixed obstruction (tracheal stenosis, large goitre): both limbs flattened
- Variable extrathoracic (vocal cord palsy or dysfunction, laryngeal tumour): inspiratory limb flattened, since negative airway pressure sucks the lesion closed on inspiration
- Variable intrathoracic (tracheomalacia, bronchial tumour): expiratory limb flattened, since positive pleural pressure compresses the airway on expiration
The logic is that a lesion outside the thorax is worsened by inspiration, and one inside the thorax is worsened by expiration.