Aim of mechanical ventilation is to improve gas exchange and to unload the respiratory muscles delivering a form of mechanical support to the ventilation. At the same time, it is essential that the support is individually-tailored to avoid the development of muscular atrophy, a process called "ventilatory-induced diaphragm dysfunction"

Aim of the present study is that the continuous ultrasonographic assesment of diaphragm function, as obtained by the device under investigation (DiaMon, Respinor AS, Oslo, Norway) is related to the degree of effort of inspiratory muscles, as measured by gold-standard indices such as esophageal and gastric pressure measurement.

A secondary aim is that the data assessed by the device are related to a standard ultrasonographic examination performed by expert operators.

In particular, we will enroll a population of critically ill patients undergoing mechanical ventilation in assisted mode, and we will perform a decremental pressure support trial, with the following aims:

1. to evaluate the performance of a continuous and automated device for the monitoring of diaphragm contractile activity, as compared to standard mechanical indices of respiratory effort such as the pressure-time product (PTP)
2. to evaluate the performance of a continuous and automated device for the monitoring of diaphragm contractile activity, as compared to the ultrasonographic assesment of muscle function performed by an expert operator.

Inclusion Criteria:

• Acute respiratory failure which necessitated endotracheal intubation and mechanical ventilation, with ventilation in pressure-support mode.
• Positive end-expiratory pressure (PEEP) lower than 10 cmH2O
• Pressure support level between 4 and 10 cmH2O
• Ratio between PaO2 and FiO2 >200 mmHg
• Resolution of the cause of respiratory failure

Exclusion Criteria:

• Diagnosis of Chronic Obstructive Pulmonary Disease (COPD)
• Pregnancy
• Circulatory failure