Role of imaging in setting mechanical ventilation

Over the last two decades, the use of Computed Tomography (CT) has provided new insights on ARDS pathophysiology, demonstrating that ARDS does not homogeneously affect the lung parenchyma. Recent data have shown that lung injury severity, as assessed by the CT quantitative analysis of the gasless tissue, is widely distributed in ARDS population, accounting for 5 to 70% of the total lung weight. The lung recruitability is strictly associated with the severity of lung injury: greater is the amount of gasless tissue at 5 cmH2O positive end-expiratory pressure, greater is the amount of gasless tissue regaining aeration at 45 cmH2O airway pressure. These finding cast doubts about the current diagnosis of ARDS, suggesting that lung edema, as assessed by CT-scan, should be included into the definition. The lung CT findings may provide a firm rationale for tailoring tidal volume during mechanical ventilation. This should be ideally sized according to the portion of the lung open to ventilation, as assessed by CT-scan, rather than to the body weight. The assessment of lung recruitability by CT-scan seems a prerequisite for a rational setting of PEEP: higher-PEEP should be applied in patients with higher lung recruitability, lower-PEEP in patients with lower lung recruitability.