Tidal Hyperinflation during Low Tidal Volume Ventilation in Acute Respiratory Distress Syndrome

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Abstract
Rationale: Tidal volume and plateau pressure limitation decreases mortality in acute respiratory distress syndrome. Computed tomography demonstrated a small, normally aerated compartment on the top of poorly aerated and nonaerated compartments that may be hyperinflated by tidal inflation. Objectives: We hypothesized that despite tidal volume and plateau pressure limitation, patients with a larger nonaerated compartment are exposed to tidal hyperinflation of the normally aerated compartment. Measurements and Main Results: Pulmonary computed tomography at end-expiration and end-inspiration was obtained in 30 patients ventilated with a low tidal volume (6 ml/kg predicted body weight). Cluster analysis identified 20 patients in whom tidal inflation occurred largely in the normally aerated compartment (69.9 ± 6.9%; “more protected”), and 10 patients in whom tidal inflation occurred largely within the hyperinflated compartments (63.0 ± 12.7%; “less protected”). The nonaerated compartment was smaller and the normally aerated compartment was larger in the more protected patients than in the less protected patients (p = 0.01). Pulmonary cytokines were lower in the more protected patients than in the less protected patients (p < 0.05). Ventilator-free days were 7 ± 8 and 1 ± 2 d in the more protected and less protected patients, respectively (p = 0.01). Plateau pressure ranged between 25 and 26 cm H2O in the more protected patients and between 28 and 30 cm H2O in the less protected patients (p = 0.006). Conclusions: Limiting tidal volume to 6 ml/kg predicted body weight and plateau pressure to 30 cm H2O may not be sufficient in patients characterized by a larger nonaerated compartment.