The ARDSNet trial compared tidal volumes of 6 mL/kg versus 12 mL/kg in ARDS patients. The low-volume group developed higher CO₂ levels (permissive hypercapnia). Why was this accepted rather than corrected by using larger breaths?
AElevated CO₂ is beneficial in ARDS because it suppresses the inflammatory response
BThe volutrauma caused by larger tidal volumes to normalize CO₂ costs more in lung injury than the elevated CO₂ is worth
CThe ventilators used in the trial could not regulate CO₂ levels precisely enough
DElevated CO₂ improves oxygen delivery by shifting the hemoglobin dissociation curve rightward
The ARDSNet insight was that the mortality cost of volutrauma from larger tidal volumes (22% higher mortality in the 12 mL/kg group) outweighs the cost of mild hypercapnia. 'Permissive hypercapnia' is a deliberate trade-off: accept elevated CO₂ to protect the lung from stretch injury. Option D describes the Bohr effect, which is real but is not why hypercapnia was permitted; option A is incorrect — elevated CO₂ does not play a beneficial anti-inflammatory role in this context.
Question 2 Multiple Choice
A mechanically ventilated ARDS patient has a plateau pressure of 27 cmH₂O — apparently safely below the 30 cmH₂O threshold. Why might significant volutrauma still be occurring?
APlateau pressure is unreliable in ARDS because airways are collapsed and do not transmit pressure accurately
BThe ARDS lung is heterogeneous — tidal volume distributes almost entirely into the small fraction of open alveoli, which suffer enormous stretch even at acceptable global airway pressures
CVolutrauma requires pressures above 30 cmH₂O; below that threshold it cannot occur regardless of volume
DBiotrauma elevates pressure readings, making them appear lower than the actual injurious pressure
This is the 'baby lung' concept: consolidated and atelectatic regions are functionally unavailable, so a tidal volume calculated for a normal-sized lung channels almost entirely into a much smaller volume of recruitable alveoli. Each of those alveoli is stretched far beyond safe limits even though global airway pressure appears controlled. This is why driving pressure (plateau pressure minus PEEP) is a better surrogate for alveolar strain than plateau pressure alone — it normalizes to the actual compliant lung volume.
Question 3 True / False
Positive end-expiratory pressure (PEEP) can be both protective against ventilator-associated lung injury and a cause of it, depending on how much is applied.
TTrue
FFalse
Answer: True
PEEP prevents atelectotrauma by maintaining recruited alveoli open at end-expiration, eliminating the repetitive collapse-re-expansion cycle and its shear forces. But excessive PEEP overdistends already-open alveoli in adjacent regions, causing volutrauma — and can increase right ventricular afterload enough to compromise cardiac output. Optimal PEEP is patient-specific, requiring titration that balances recruitment against overdistension in each patient's unique lung architecture.
Question 4 True / False
Barotrauma — injury from high airway pressure — is the primary mechanism of ventilator-associated lung injury, making pressure monitoring more important than volume monitoring.
TTrue
FFalse
Answer: False
Volutrauma — injury from alveolar wall stretch due to excessive volume change — is now understood to be more important than barotrauma alone. Classic experiments showed that lung injury correlates better with tidal volume than with peak pressure: high-pressure, low-volume ventilation causes less injury than low-pressure, high-volume ventilation. Plateau pressure is useful as a surrogate because it approximates distending pressure, but the actual injurious force is alveolar stretch, not pressure per se. This is why volume limits (6 mL/kg ideal body weight) are the foundation of lung-protective ventilation.
Question 5 Short Answer
Why does the heterogeneous nature of ARDS — with mixed areas of consolidated, atelectatic, and still-normal lung — make weight-based tidal volume calculations potentially misleading?
Think about your answer, then reveal below.
Model answer: A tidal volume calculated for the patient's full body weight actually distributes only into the small, open fraction of recruitable lung. This concentrated volume causes massive overdistension in the remaining open alveoli, even when global airway pressures look acceptable. The 'baby lung' concept captures this: the functional lung receiving ventilation may be only 20–30% of normal size, so a volume that seems modest by weight creates enormous regional strain in the open alveoli.
This is why driving pressure (ΔP = plateau pressure − PEEP) is a better correlate of alveolar strain and mortality than tidal volume per kilogram alone — it normalizes volume to the actual compliant lung volume. Emerging tools like electrical impedance tomography can directly visualize regional ventilation distribution, enabling truly individualized lung protection.