There is little evidence about the mechanical characteristics and muscular function in patients with Acute Respiratory Distress Syndrome (ARDS) at the time of weaning of the mechanical ventilation, as well as the behavior of the mechanical properties, breathing pattern, muscular effort, and gas exchange during a successful and failed spontaneous breathing trial.

The study of the respiratory system (RS) mechanics in patients with ARDS has been focused on the initial phase of ventilatory support, with few studies observing the weaning in mechanical ventilation.The RS in patients with ARDS is characterized by a greater or lesser degree of low functional residual capacity (FRC) in relation to normal.

At the time of performing a spontaneous breathing trial, these mechanical characteristics determine the need to develop a high transpulmonary pressure during each breath, with high work of breathing (WOB) to achieve adequate alveolar ventilation.

Inspiratory muscle weakness could alter the ability to generate the necessary transpulmonary pressure over time according to the loads imposed at the time of performing a spontaneous breathing trial, with the consequent development of fatigue and ventilatory failure.

This is why the investigators propose to evaluate the association between elastic WOB and the failure of the spontaneous breathing trial.

Inclusion Criteria:

• over 18 years old
• requiring mechanical ventilation for more than 12 hours
• moderate/ severe ARDS according to Berlin Definition
• able to make a Spontaneous Breathing Trial according to the institution protocol

Exclusion Criteria:

• pregnant patients
• history of neuromuscular disease
• patients in palliative care
• bronchopleural fistula
• contraindication of measurement of esophageal pressure

The investigators will meassure adult, ARDS, Intensive Care Unit patients that are able to perform a Spontaneous Breathing Trial.