During forced expiration at low lung volumes (below the equal pressure point), the dynamic airway compression mechanism is explained by the 'Starling resistor model.' At what point does flow become effort-independent, and what sets the flow limit?

• A When driving pressure equals critical closing pressure at the choke point; upstream transmural pressure determines maximum flow ✓
• B When alveolar pressure equals atmospheric pressure; lung compliance sets the maximum flow rate
• C When pleural pressure becomes positive; total airway resistance from the mouth to alveolus limits flow
• D When expiratory muscle force exceeds elastic recoil; residual volume determines the flow plateau

Explanation

In the Starling resistor (choke point) model of flow limitation, expiratory flow becomes effort-independent when the transmural pressure at the choke point reaches zero, causing dynamic collapse of the airway. Increasing expiratory effort raises both alveolar driving pressure and pleural pressure equally, so the transmural pressure at the choke point does not change — additional effort cannot increase flow. The maximum flow is determined by upstream recoil pressure (alveolar pressure minus choke-point critical closing pressure) and upstream resistance. This model explains why PEFR and effort-independent flows on MEFV curve are characteristic of airway disease severity.

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

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Written and medically reviewed by the StethoPrep medical team.