Aim of this study is to better understand pathophysiology of the alteration of respiratory mechanics and cardiovascular function in obese volunteer subjects. The investigators plan to test this hypothesis with a physiological, interventional study conducted on volunteers by using Electrical Impedance Tomography in a group of patients and magnetic resonance imaging (MRI) in another group.

Obese subjects are prone to develop respiratory insufficiency when requiring mechanical ventilation. Atelectasis is the primary responsible for respiratory insufficiency and impossibility to wean obese patients from respiratory support. The investigators do believe that the respiratory system derangements observed in the previous study during the critical illness are already present, although in lower severity, in the obese patients in their basal condition.

This study will help to understand the standard cardiac and respiratory function of an obese non critically ill subject to better target the therapies during the management of the critical illness to reestablish the homeostasis of the system:

The investigator's hypotheses are:

• To demonstrate if morbidly obese patients show atelectasis at spontaneous breathing in the supine position and whether the increase in lung volume following PEEP titration is due to alveolar recruitment rather than overdistention.
• To measure regional variations in ventilation/perfusion coupling at different ventilator settings
• To investigate the role of diaphragm position in the development/treatment of respiratory insufficiency due to increased pleural pressure
• To test if reopening of lung atelectasis through the application o a recruitment maneuver and titrated PEEP level would lead to an improvement in right heart function.
• To assess pulmonary circulation at different levels of PEEP.

Inclusion Criteria:

• Have a photo identification document
• BMI ≥ 35 kg/m2
• Waist circumference > 88 cm (for women)
• Waist circumference > 102 cm (for men)

Exclusion Criteria:

• Psychiatric disturbances such as anxiety, depression, schizophrenia requiring pharmacological treatment or hospitalization in the last year
• Subjects with any known condition, including claustrophobia or pain, which limits their ability to lie in the MR scanner for the duration of the research study
• Known presence of esophageal varices
• Recent esophageal trauma or surgery
• Known Coagulopathy
• History of pneumothorax
• Pregnancy
• Diabetes
• Presence of prosthesis incompatible with MR
• Thoracic diameter grater than 70 cm
• Resting heart rate (HR) < 50 or > 120 bpm and/or systolic blood pressure < 90 or > 160 mmHg and/or peripheral oxygen saturation (SpO2) < 88%
• Currently enrolled in another research study