PEEP Levels Selected by PEEP Titration and PEEP Levels Routinely Used in Post-operative Cardiac Patients With Hypoxemic Respiratory Failure

The purpose of this study is to: * Compare PEEP level selected by individualized PEEP titration by electrical impedance tomography and PEEP level routinely used in post-operative cardiac patients with Hypoxemic Respiratory Failure; * Evaluate the agreement between the results of a rapid titration (total procedure duration = 5 min) versus an already validated slow titration (total procedure duration = 40 min) of the same patient, sequentially. Specifically, degree of collapse and degree of distention in each PEEP level, estimated by EIT; * Compare hemodynamics during the two maneuvers of PEEP titration; * Evaluate the efficacy of the selected PEEP (minimum PEEP preventing lung collapse less than 5%) to maintain stable levels of the following variables: arterial oxygenation, respiratory system compliance, and degree of collapse by EIT; * Compare these results (evolution of the three variables, along 4 hours) with the control strategy (default strategy currently used in the institution) group.