Assisted ventilation represents, nowadays, the preferred ventilation mode in clinical practice.It has been shown that assisted ventilation modes improve ventilation/perfusion matching, descrease risk of Ventilator induced lung injury and muscle atrophy and have less influence on haemodynamic function.

However, PSV (Pressure Support Ventilation) is not free from complications: it may worsen or cause lung injuries by increasing alveolar and intrathoracic negative pressure and by loosing control on Tidal Volume (Vt). Indeed, it has been demonstrated that Vt is the main factor related to VILI.

It has been shown that lower Vt and higher PEEP can improve clinical outcome only if associated with a simultaneous reduction in Driving Pressure. Increase in Driving Pressure resulted strongly associated with negative outcomes, especially if higher than 15 cm H2O.

PSV is currently the most used assisted ventilation mode. NAVA (Neurally Adjusted Ventilatory Assist) is a ventilation mode in which the diaphragmatic electrical activity (EAdi) is used as a trigger to start a mechanical breath, applying positive pressure during patient's inspiration. Diaphragmatic electrical activity (EAdi) can be detected by a particular nasogastric tube (EAdi catheter). EAdi is the currently available signal closest to the neural breathing centers, which can estimate the patient's respiratory drive, if phrenic nerves are not damaged. It has been demonstrated that NAVA ventilation can reduce the incidence of patient-ventilator asynchronies, because the delivery of the support and the cycling between inspiration and expiration are completely controlled by the patient.

However, although PSV and NAVA have been widely compared in many investigations, up to now there are no studies about driving pressure variation during these two modalities of mechanical assisted ventilation. The aim of this study is to measure changes in driving pressure at different levels of ventilatory assistance in PSV and NAVA ventilation modes.

Secondary end points are respiratory mechanics indices and patient/ventilator related asynchrony evaluation and comparison.

Any patient who is already on an assisted mode of ventilation and displays triggering efforts will be enrolled in the study and will be submitted to 3 ventilation trials, in PSV and NAVA ventilation modes; each trial will last 20 minutes. Every trial will be performed in a randomized order, based on random computer generated sequences.

During the first trial, PSV will be set in order to obtain a Vt between 6 and 8 ml/kg;this support level will be defined as PSV100. Subsequently, the corresponding NAVA level (NAVA 100) will be determined using a dedicated ventilator function (NAVA Preview) which is able to estimate NAVA level in order to deliver an equivalent inspiratory peak pressure (Paw peak) compared to that obtained during PSV mode. Afterwards, pressure support level of assistance of PSV100 and NAVA100 will be firstly increased (PSV150 and NAVA150) during the second trial and then decreased during the third trial (PSV50 and NAVA150) by 50% from basal value.

During the study period, PEEP and FiO2 will be kept equal to the values in use before patient enrollment. End-inspiration and end-expiration pauses will be performed at the end of each trial by pressing the dedicated button on the ventilator control panel. Airway pressure and flow will be recorded.

Patients, as usual, clinical practice, will be sedated at different levels and this could compromise their content of consciousness.

At the beginning of each trial, an endotracheal tube suction will be done. Last 5 minutes of each trial will be recorded and stored in a computer for subsequent statistical analysis. Respiratory mechanical indices (airway pressure, tidal volume, flow) and electric diaphragmatic activity will be recorded by a dedicated software called NAVA- tracker. At the end of each trial an arterial blood gas analysis (ABGs) will be performed to evaluate PaCO2, PH and blood oxygenation (PaO2). at the end of each trial, an ultrasound evaluation of diaphragm will be performed.

Inclusion Criteria:

• Age >18 years
• Every patients undergoing partial assisted mechanical ventilation

Exclusion Criteria:

• Gastro-esophageal surgery in the previous 12 months;
• Gastro-esophageal bleeding in the previous 30 days;
• Esophageal varices history;
• Maxillo-facial surgery or trauma;
• Haemodinamic instability despite adequate fluid infusion (i.e. need for continuous infusion epinephrine or vasopressin or dopamine at a dose greater than 5 mcg/kg/min to obtain systolic pressure > 90 mmHg);
• Body temperature > 38° C during the study screening;
• Coagulation disorders (INR > 1.5, aPTT > 44 sec);
• Vt < 8 ml/kg with minimum inspiratory effort of 8 cmH2O;
• Inclusion in other research protocols