Ventilator-Associated Lung Injury

Core Idea

Mechanical ventilation, while life-saving, can paradoxically injure the lungs through barotrauma (high pressures rupturing alveoli), volutrauma (overdistension from large tidal volumes), atelectotrauma (repetitive opening/closing of alveoli), and biotrauma (mechanical stress triggering inflammatory mediator release). Ventilator-associated lung injury is a spectrum from subclinical inflammation to overt pulmonary edema and gas trapping. Lung-protective ventilation strategies (low tidal volume 6-8 mL/kg, PEEP to prevent collapse, plateau pressure <30 cmH2O) reduce VALI incidence.

How It's Best Learned

Study the mechanisms of each type of VALI. Understand why PEEP is both preventive (reopens collapsed alveoli) and harmful (overdistension if excessive). Compare lung-protective ventilation strategies and their physiologic basis.

Common Misconceptions

PEEP is not always harmful; appropriate PEEP prevents atelectotrauma by maintaining alveolar recruitment. Ventilator-induced lung injury can occur even with controlled, 'protected' ventilation if underlying disease is severe. The balance between ventilation and avoiding injury is dynamic and requires frequent reassessment.

Explainer

Mechanical ventilation saves lives by taking over the work of breathing when a patient cannot maintain adequate gas exchange. But the mechanics of artificial ventilation differ fundamentally from physiologic breathing, and those differences carry real risks. Normally, inhalation is driven by diaphragmatic contraction creating negative intrathoracic pressure — the lung is pulled open from outside. Positive-pressure mechanical ventilation pushes air in from above, stressing the lung from inside. You know from your study of ARDS pathophysiology that the diseased lung is not uniformly stiff but a heterogeneous patchwork of consolidated, atelectatic, and still-normal regions. A tidal volume that seems reasonable by weight-based calculation gets channeled almost entirely into the small fraction of open lung, producing enormous regional overdistension in those units even while global airway pressures look acceptable.

The four mechanisms of VALI each represent a different way that ventilatory physics can damage cells. Barotrauma is the most visible: high peak airway pressures rupture alveoli, forcing air into the pleural space (pneumothorax), mediastinum (pneumomediastinum), or subcutaneous tissue. Volutrauma is subtler and arguably more dangerous: it is the *volume change* — the stretch of alveolar walls — not peak pressure per se, that tears epithelial and endothelial cells. A highly non-compliant ARDS lung may transmit high pressures to a tiny volume of recruitable alveoli, each suffering enormous stretch while plateau pressures measured at the airway opening appear controlled. Atelectotrauma is produced by repetitive collapse and re-expansion of unstable alveoli: each breath cycle requires breaking the surface tension of a collapsed alveolus, generating shear forces at the liquid-air interface that mechanically injure surfactant-depleted epithelium. This is why PEEP (positive end-expiratory pressure) is protective — by maintaining a baseline airway pressure that keeps recruited alveoli open at end-expiration, PEEP prevents the repetitive collapse-re-expansion cycle. Biotrauma is the systemic consequence: stretch-activated epithelial and endothelial cells release pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) that enter the bloodstream and contribute to multi-organ dysfunction syndrome — connecting ventilator settings directly to distant organ failure.

Lung-protective ventilation emerged from the landmark ARDSNet trial published in 2000, which showed that tidal volumes of 6 mL/kg ideal body weight reduced ARDS mortality by 22% compared to 12 mL/kg — despite the low-tidal-volume group having higher CO₂ levels (permissive hypercapnia). The key insight was that maintaining "normal" CO₂ by using larger breaths costs more in volutrauma than the benefit is worth. The lung-protective bundle — low tidal volume, plateau pressure <30 cmH₂O, sufficient PEEP to prevent atelectotrauma — has become standard of care.

The therapeutic tension within this framework is that optimal PEEP is not a fixed number. Too little PEEP allows atelectotrauma by permitting repeated alveolar collapse. Too much PEEP overdistends already-open alveoli in adjacent regions, causing its own volutrauma and compromising cardiac output by increasing right ventricular afterload. Finding the optimal PEEP requires titrating to the individual patient's lung mechanics — using the pressure-volume curve inflection point, driving pressure (plateau pressure minus PEEP as a surrogate for strain), or emerging tools like electrical impedance tomography that can visualize regional ventilation distribution in real time. This is the frontier of individualized lung-protective care: moving from population-level protocols to patient-specific ventilator settings that balance recruitment against overdistension in each patient's unique lung architecture.