A patient with ARDS is placed on 100% inspired oxygen (FiO2 = 1.0), but their arterial PaO2 remains critically low, giving a P/F ratio of 120. Why does supplemental oxygen fail to correct the hypoxemia?
AHigh FiO2 worsens inflammation by producing oxygen radicals, directly lowering the PaO2
BThe diffusion barrier across the thickened alveolar membrane is too great for oxygen to cross
CBlood flows through fluid-filled, non-ventilated alveoli and returns unoxygenated regardless of FiO2
DARDS reduces respiratory rate, so total alveolar ventilation is insufficient
ARDS hypoxemia is primarily a shunt physiology: fluid-filled alveoli cannot ventilate, but their capillaries still carry blood. That blood returns to the left heart without picking up oxygen — a true shunt. Adding more oxygen to the ventilated alveoli cannot help blood that bypasses those alveoli entirely. Option B (diffusion impairment) would respond somewhat to high FiO2 by increasing the partial pressure gradient; it is the shunt that is truly refractory. Option D is incorrect: ARDS patients typically hyperventilate due to hypoxic drive.
Question 2 Multiple Choice
Why does low tidal volume ventilation (6 mL/kg) reduce mortality in ARDS, rather than larger tidal volumes that would better maintain PaO2?
ASmall tidal volumes reduce FiO2 requirements, limiting oxygen toxicity to the airway
BLarger tidal volumes cause volutrauma and barotrauma to the remaining aerated lung tissue, worsening injury
CLow tidal volumes allow the inflammatory response to resolve more quickly by reducing lung movement
DSmall tidal volumes prevent the fibroproliferative phase from being triggered
In ARDS, large portions of the lung are consolidated and non-aerated. The remaining aerated alveoli receive the full tidal volume, overstretching them — this volutrauma (and the resulting barotrauma) perpetuates inflammatory injury in the very tissue needed for gas exchange. Accepting lower PaO2 and higher CO2 (permissive hypercapnia) is the deliberate tradeoff: lung-protective ventilation prioritizes preventing secondary injury over maximizing oxygenation. Options A, C, and D are not the primary mechanisms established in the ARDSNet trial.
Question 3 True / False
In ARDS, the hypoxemia is primarily caused by intrapulmonary shunting — blood flowing past non-ventilated, fluid-filled alveoli.
TTrue
FFalse
Answer: True
This is the central hemodynamic abnormality in ARDS. Inflammatory barrier breakdown floods alveoli with protein-rich exudate. Perfusion of these non-ventilated alveoli constitutes a shunt (V/Q = 0 regions), returning deoxygenated blood to the arterial circulation. This is confirmed by the failure of high FiO2 to correct the hypoxemia — a hallmark of shunt physiology that distinguishes it from diffusion impairment or hypoventilation.
Question 4 True / False
Low tidal volume ventilation in ARDS improves oxygenation by recruiting collapsed alveoli through the application of positive pressure.
TTrue
FFalse
Answer: False
Low tidal volume ventilation does NOT directly improve oxygenation — in fact, it accepts lower PaO2 as a deliberate tradeoff. Its purpose is to prevent further lung injury (volutrauma/barotrauma) in the already-compromised tissue. Alveolar recruitment is a goal of PEEP (positive end-expiratory pressure), not reduced tidal volume. The mortality benefit of lung-protective ventilation comes from limiting secondary injury, not from better gas exchange.
Question 5 Short Answer
Why does ARDS produce hypoxemia that is 'refractory to supplemental oxygen,' and what does this tell us about the underlying mechanism?
Think about your answer, then reveal below.
Model answer: ARDS hypoxemia is refractory because the mechanism is true intrapulmonary shunting: a large fraction of cardiac output passes through alveoli that are completely flooded and non-ventilated. No amount of oxygen delivered to ventilated alveoli can oxygenate blood that bypasses them entirely. This contrasts with hypoventilation or mild V/Q mismatch, where increasing FiO2 raises alveolar PO2 and corrects hypoxemia. The refractoriness is the diagnostic signature of shunt physiology.
Understanding why oxygen-unresponsive hypoxemia indicates shunt is clinically critical. It directs treatment toward alveolar recruitment (PEEP), reducing inflammation, and protective ventilation — not simply increasing FiO2, which risks oxygen toxicity without benefit. The Berlin definition's PaO2/FiO2 threshold encodes this: even with maximum FiO2, the ratio remains low because FiO2 gains buy almost nothing against a true shunt.