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Flow Volume Loops

Axes Representation

• Y-axis: Represents the flow rate.
  • Exhalation: Above the x-axis.
  • Inhalation: Below the x-axis.
• X-axis: Represents lung volume.
  • Volume decreases from left to right.
  • Starts at total lung capacity (left) and ends at residual volume (right).

Loop Direction

• The flow volume loop begins on the left of the x-axis and follows a clockwise direction.

Peak Expiratory Flow Rate (PEFR)

• Normal PEFR:
  • Men: Averages between 440 and 740 L/min.
  • Women: Averages between 340 and 530 L/min.
• PEFR is located at the highest point on a flow volume loop.

Normal

COPD

• PEFR usually decreases slightly so that the initial expiratory flow is not affected significantly. Instead of the almost linear decrease in expiratory flow, there is a “scooping” of the loop soon after the PEFR. This scooping represents the decreased amount of flow secondary to difficulty in expelling the volume of gases left in the distal airways.
• In severe COPD, the PEFR is affected more drastically. The expiratory flow does not come close to the flow of a normal subject.

Restrictive

Intra Vs Extra Thoracic Obstruction

Intrathoracic obstruction affects expiratory flow

Extrathoracic obstruction affects inspiratory flow

Summary

Links

• Lung function testing
• Respiratory physiology and Thoracic anaesthesia

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References:

1. Raj, T. D. (2017). Data interpretation in anesthesia.. https://doi.org/10.1007/978-3-319-55862-2
2. Ntima, N. and Lumb, A. B. (2019). Physiology and conduct of pulmonary function tests. BJA Education, 19(6), 198-204. https://doi.org/10.1016/j.bjae.2019.02.002

Summaries:
ICU-OP_Flow-volume-loops

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© 2025 Francois Uys. All Rights Reserved.

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