Mechanical ventilation (MV) is associated with adverse outcomes in ventilated patients, and impact of MV-induced diaphragm changes are still unclear.

The objective of this prospective observational study is to assess muscle thickness and strength, specifically in limb muscles such as the quadriceps, among critically ill patients who undergo extended mechanical ventilation during their Intensive Care Unit (ICU) stay.

The primary inquiries this study seeks to address are:

• Is there an association between muscle thickness and strength in the limbs of critically ill patients undergoing prolonged MV?
• How much thickness and strength variation can be expected in respiratory and limb muscles in critically ill patients undergoing prolonged MV?

The methodology for measuring the thickness of the diaphragm and quadriceps involves the use of bedside ultrasound. In terms of diaphragmatic assessment, to ensure consistency over multiple timepoints, the areas of the lower limbs evaluated (Quadriceps thickness, Qthick and Cross section area, CSA) will be marked on the skin using a dermatographic pen during the peripheral assessments.

Muscle strength in the limbs will be assessed utilizing the Medical Research Council (MRC) Score, specifically in patients who are awake and cooperative (RASS 0 +/- 1). Additionally, a dynamometer will be employed for the precise measurement of quadriceps strength, serving as the gold standard.

The timeline for data collection during the observational period is structured as follows:

T1: 48-72 hours from the start of invasive MV after intubation T2: At the initial trial in pressure support mode. T3: Immediately prior to extubation. T4: Before discharge from the Intensive Care Unit.

Inclusion Criteria:

• Age ≥ 18 years;
• Intubated patients on mechanical ventilation presumably for 48 hours;
• Informed consent

Exclusion Criteria:

• Contraindication (absolute or relative) to assessment: acute bleeding, multiple fractures or trauma, spinal instability;
• Hemodynamic instability during assessment;
• ICU admission due to thoracic surgery, including lung transplant patients;
• History of preexisting neuromuscular diseases;
• History of preexisting functional impairment;
• Pheripheral Neural disease;
• Cervical spine injury;

Patients admitted to the Intensive Care Unit (ICU) who are expected to need mechanical ventilation for at least 48 hours following ARDS.