Chest Wall Influence on Respiratory System Mechanics in Morbidly Obese Patients

The goal of this study is to describe the influence of the chest wall on the respiratory system mechanics. Investigators want to describe how extreme obesity and Intra-Abdominal Hypertension (IAH) affect normal respiratory system behavior. The effects of increasing and decreasing positive end-expiratory pressure (PEEP) on respiratory system mechanics, lung volumes and gas exchange will be evaluated, both during controlled and assisted mechanical ventilation. Investigators will record and compare lung volumes, airway and transpulmonary pressure, gas exchange and hemodynamic changes caused by variations of PEEP. Patients will be studied, first, during the acute phase of respiratory failure, when requiring intubation and controlled mechanical ventilation. Patients will then again be evaluated during weaning from the ventilator to assess the influence of PEEP in assisted ventilation prior to extubation. Investigators believe that assessment of the transpulmonary pressure and lung volumes is essential to correctly evaluate respiratory system function in patients in which the relationship between the lung and chest wall is altered. Improper mechanical ventilation leads to lung damage. High ventilatory volume/pressure are associated with lung overdistension, while low volume/pressure leads to lung collapse and cyclic opening and closing of alveoli. All of these mechanisms have been associated with ventilator induced lung injury and poorer outcomes. Adequate PEEP and transpulmonary pressure are fundamental in preventing this vicious cycle.