On a flow-volume loop, a patient shows a 'sawtooth' (oscillatory) pattern during tidal inspiration and expiration. This pattern is most characteristic of:

• A Variable extrathoracic obstruction (e.g., vocal cord paralysis)
• B Fixed large airway obstruction (e.g., tracheal stenosis)
• C Upper airway resistance from secretions or neuromuscular oscillation (e.g., Parkinson's disease) ✓
• D Small airway disease (emphysema) with flow limitation on expiration

Explanation

The sawtooth pattern on the flow-volume loop represents rhythmic fluctuations in flow during both inspiration and expiration, caused by intermittent upper airway vibrations or oscillations. This is characteristically seen in obstructive sleep apnoea and in Parkinson's disease (cogwheel rigidity affecting upper airway muscles), where rhythmic muscle oscillations interrupt smooth flow. Variable extrathoracic obstruction causes selective inspiratory flattening; fixed obstruction causes flattening of both limbs; small airway disease causes a 'scooped-out' expiratory curve.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.