the introduction of new MV modalities has shown promising results in reducing the incidence of weaning failure, mainly due to a more physiologic approach which allows respiratory muscle preservation. Among them, the Neurally Adjust Ventilatory Assist (NAVA) seemed to be associated with lower incidence of weaning failure and subsequent duration of mechanical ventilation, compared to standard modalities like the Pressure Support Ventilation (PSV) . Moreover, NAVA allows the evaluation of the diaphragm electrical activity (EAdi), an index of diaphragmatic neural respiratory drive. However, no study has compared TFic values during PSV and NAVA modalities in patients with difficult weaning from MV admitted in ICU.

Respiratory muscles dysfunction is one of the main causes leading to failure of weaning from mechanical ventilation (MV) in critically ill patients. Muscular activity is important during the inspiratory phase of ventilation, which is mainly due to diaphragm and external intercostal muscles. Diaphragm dysfunction is frequent among critically ill patients and ranges from 33 to 95% . Although several indexes have been investigated to help clinician in identifying its disturbance, their clinical value is controversial . External intercostal muscles dysfunction epidemiology is unknown because of lack of non-invasive tool to investigate such condition. However, the echography assessment of intercostal muscles thickening fraction (TFic) has shown promising results in this field as a non-invasive tool to assess inspiratory muscle function and predict weaning failure .

On the other hand, the introduction of new MV modalities has shown promising results in reducing the incidence of weaning failure, mainly due to a more physiologic approach which allows respiratory muscle preservation. Among them, the Neurally Adjust Ventilatory Assist (NAVA) seemed to be associated with lower incidence of weaning failure and subsequent duration of mechanical ventilation, compared to standard modalities like the Pressure Support Ventilation (PSV) [. Moreover, NAVA allows the evaluation of the diaphragm electrical activity (EAdi), an index of diaphragmatic neural respiratory drive . A recent study suggests that EAdi and derived parameters, may be helpful to predict weaning failure in chronic obstructive pulmonary disease (COPD) patients .

However, no study has compared TFic values during PSV and NAVA modalities in patients with difficult weaning from MV admitted in ICU.

the aim of this study will be the comparison of TFic values during PSV and NAVA modalities in patients with difficult weaning from MV admitted in ICU and the assessment of the inspiratory effort parameters and the occurrence of respiratory asynchrony in patients with difficult weaning from MV admitted in ICU.](streamdown:incomplete-link)

Inclusion Criteria:

• patients who have failed at least one weaning attempt
• mechanical ventilation for at least 24 hours

Exclusion Criteria:

• Pregnancy
• Obesity (Body Mass Index > 35 kg/m2)
• Contraindication to the insert of a nasogastric tube
• Neuromuscular diseases

Critically ill patients on mechanical ventilation (MV) for at least 24 hours, who have failed at least one weaning attempt, hospitalized at the Intensive Care Unit of Fondazione Policlinico Universitario A. Gemelli IRCCS.

We will assess the availability of data based on the results