A mechanically ventilated patient in the ICU develops an abrupt fall in SpO2. The lung compliance curve shows a leftward shift of the pressure-volume loop. Which pathophysiological mechanism accounts for these combined findings?

• A Surfactant depletion reducing alveolar surface tension and increasing lung stiffness ✓
• B Pneumothorax increasing transpulmonary pressure at a given tidal volume
• C Bronchospasm increasing airway resistance without changing compliance
• D Pulmonary embolism increasing dead space without altering compliance

Explanation

Surfactant depletion (as in ARDS or neonatal RDS) reduces surface tension at the air-liquid interface, causing alveolar collapse and a marked decrease in lung compliance. Graphically, the P-V loop shifts leftward and narrows, reflecting that higher pressures are needed for the same tidal volume. Pneumothorax increases transpulmonary pressure but would shift the loop differently; bronchospasm affects the flow-volume loop, not the static compliance curve; PE changes dead space but not compliance.

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

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