Ventilatory strategies in ARDS

The real triggers of Ventilator Induced Lung Injury (VILI) results from un-physiological lung stress (transpulmonary pressure) and strain (inflated volume to functional residual capacity ratio). The average lung stress and strain relationship is defined by specific lung elastance . At the moment VILI prevention is based on the recognition of the “harmful” thresholds” for airway pressure and tidal volume referenced to ideal body weight (i.e. 30 cm H2O airway plateau pressure and 6 mL/Kg tidal volume referenced to ideal body weight). However we found that lung stress and strain were not predictable from plateau airway pressure and VT per IBW, in fact for a given applied VT, the lung strain variability was remarkable and transpulmonary pressure cannot be predicted from the airway pressure due to the great variability of the ratio of lung elastance to the total respiratory system elastance. Recent CT-data have shown that lung injury severity assessed by non-aerated tissue, is widely distributed in ARDS population (5 to 70% of total lung weight). Moreover the lung recruitability is strictly associated with the severity of lung injury: greater is the amount of non-aerated tissue at 5 cmH2O PEEP, greater is the amount of non-aerated tissue regaining aeration at 45 cmH2O airway pressure The assessment of lung recruitability seems also a prerequisite for a rational PEEP setting: patients with higher lung recruitability should have higher-PEEP than patients with lower lung recruitability