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Summary

Acute Respiratory Distress Syndrome (ARDS) is characterized by refractary hypoxemia due to a variable grade of alveolar collapse because of the gravitational gradient (sponge lung) and/or the alveolar occupation (alveolar floody). Current challenge is minimizing ventilator-induced lung injury (VILI) while providing a reasonable gas exchange.

Gas exchange and hemodynamics impact of mechanical ventilation (MV) depend on proportion of collapsed lung versus occupied alveoli which coexist in the same alveolar environment. When alveolar floody dominates, the lung has minimal recruitability and hemodynamic impact of insufflation is usually significant. When alveolar collapse is the most important phenomena we will have best recruitability and the hemodynamic impact of MV will be less. 

Some authors have shown good results with the use of maximum lung recruitment maneuvers to recruit lungs, even when alveolar occupation is predominant. However, it is a mistake to consider all injured lung tissue as potentially recruitable. High level of PEEP and recruitment maneuvers despite substantially improve oxygenation have shown no reduction in mortality.  

Reduce lung stress and strain using an appropriate ventilatory setting is achieved with very low tidal volumes (4 ml/kg PBW) and moderate/high level of PEEP, according to best respiratory system compliance. Currently, a study concluded that the driving pressure should be less than 16 cmH2O, because higher levels increase risk of death. These characteristics enforce to use an individual ventilatory approach.

Recently, it has been reported that VILI develops proportionally to external energy applied by the ventilator to the respiratory system; this concept is named mechanical power (MP). It considers the different variables related with the ventilator settings as cause of VILI: tidal volume (V_T), driving pressure, respiratory rate, inspiratory flow, resistance and PEEP (equation of motion). Furthermore, we have extrapulmonary variables such as: perfusion, pH, gas tension and temperaturewhich can also influence.