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ALL / Respiratory Medicine

Flow volume loop

Flow volume loop

A normal flow volume loop is often described as a 'triangle on top of a semi circle'
  • A flow–volume loop plots airflow (y-axis) against lung volume (x-axis) during forced inspiration and expiration.
  • Flow volume loops are the most suitable way of differentiating between fixed and variable upper airway obstruction
  • Non-invasive screening tool for suspected airway obstruction.
  • Helps localise (extrathoracic vs intrathoracic) and characterise (fixed vs variable) the obstruction.
  • Guides need for further imaging (CT neck/chest) or endoscopic assessment (bronchoscopy).
  • Useful in pre-operative assessment (e.g. thyroid surgery) to detect subclinical tracheal narrowing.
  • Can be repeated to monitor progression or response to treatment.

Example flow-volume loop patterns

Fixed upper airway obstructionVariable extrathoracic obstructionVariable intrathoracic obstruction
ExamplesTracheal stenosis, large goitre, tracheal tumour, fixed external compression.Vocal cord dysfunction, laryngeal tumour, goitre pressing on trachea at neck level.Tracheomalacia, intrathoracic goitre, proximal tracheal tumour inside the chest.
AppearanceFlattened inspiratory and expiratory limbs of the loop (both phases equally affected).

“Box-shaped” or plateau loop.		Inspiratory limb flattened (reduced inspiratory flow).

Expiratory limb usually normal.		Expiratory limb flattened (reduced expiratory flow).

Inspiratory limb usually preserved.
InterpretationObstruction is constant, independent of respiratory cycle.During inspiration, negative intrathoracic pressure narrows the extrathoracic airway, reducing flow.During expiration, positive intrathoracic pressure narrows the intrathoracic airway, reducing flow.
+PassMedicine Notes
A normal flow volume loop is often described as a 'triangle on top of a semi circle'
  • A flow–volume loop plots airflow (y-axis) against lung volume (x-axis) during forced inspiration and expiration.
  • Flow volume loops are the most suitable way of differentiating between fixed and variable upper airway obstruction
  • Non-invasive screening tool for suspected airway obstruction.
  • Helps localise (extrathoracic vs intrathoracic) and characterise (fixed vs variable) the obstruction.
  • Guides need for further imaging (CT neck/chest) or endoscopic assessment (bronchoscopy).
  • Useful in pre-operative assessment (e.g. thyroid surgery) to detect subclinical tracheal narrowing.
  • Can be repeated to monitor progression or response to treatment.
_ Fixed upper airway obstruction _ Variable extrathoracic obstruction _ Variable intrathoracic obstruction| Examples _ Tracheal stenosis, large goitre, tracheal tumour, fixed external compression. _ Vocal cord dysfunction, laryngeal tumour, goitre pressing on trachea at neck level. _ Tracheomalacia, intrathoracic goitre, proximal tracheal tumour inside the chest.| Appearance _ Flattened inspiratory and expiratory limbs of the loop (both phases equally affected).

“Box-shaped” or plateau loop. _ Inspiratory limb flattened (reduced inspiratory flow).

Expiratory limb usually normal. _ Expiratory limb flattened (reduced expiratory flow).

Inspiratory limb usually preserved.| Interpretation _ Obstruction is constant, independent of respiratory cycle. _ During inspiration, negative intrathoracic pressure narrows the extrathoracic airway, reducing flow. _ During expiration, positive intrathoracic pressure narrows the intrathoracic airway, reducing flow.|

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