Specific Power and Risk of Ventilator-induced Lung Injury

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The lung in acute respiratory distress syndrome (ARDS) is a heterogeneous viscoelastic system, in which areas with different time constants coexist, causing tidal volume to be distributed unevenly within a lung which is anatomically and functionally reduced. The administration of a really high tidal...

The lung in acute respiratory distress syndrome (ARDS) is a heterogeneous viscoelastic system, in which areas with different time constants coexist, causing tidal volume to be distributed unevenly within a lung which is anatomically and functionally reduced. The administration of a really high tidal volume for this limited functional residual capacity prones to the overdistension of the best ventilated alveoli as well as to injury by cyclic opening and closure of the most unstable alveoli (tidal atelectrauma). In this sense, using low tidal volume and homogenizing the lung by means of the prone position have proved to be beneficial in ARDS. Tidal volume, driving pressure, inspiratory flow and respiratory rate have been identified as responsible for mechanical ventilation-induced lung injury (VILI). All these factors represent both the mechanical power (P = W/?t) and the injuring energy (W) which is repeatedly applied to a vulnerable lung parenchyma (5,6). The amount of mechanical power can be calculated through a derived 'power equation', increasing its applicability in clinical practice. Recently, it was shown that mechanical power is associated with worse outcomes in critically ill patients receiving mechanical ventilation for more than 48 hours. Despite being a promising idea that combines several variables related to VILI, the concept of mechanical power carries a number of limitations such as the fact that it does not define the amount of energy applied to vulnerable lung tissue (specific power). Objectives: To correlate the risk of VILI (stress, strain and atelectrauma) with specific power. Design: Patients with ARDS will be ventilated under the same conditions of tidal volume and plateau pressure. The final positive expiratory pressure (PEEP) will be adjusted to reach 30 cmH2O of PPlat. A CT scan will be performed on inspiration and expiration. Transpulmonary pressures will be measured and lung volumes will be calculated (volume analysis software, Toshiba, Japan). Stress was defined as TP at the end of inspiration, tension: tidal volume / final expiratory pulmonary volume (EELV) and atelectrauma as the difference of non-aerated lung tissue between both respiratory times in relation to their baseline condition. Specific power results from the relation between mechanical power and EELV.

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