This study aims to determine whether shear wave elastography (SWE), a non-invasive ultrasound technique that measures nerve stiffness, can help detect ICU-acquired weakness (ICU-AW) early in critically ill patients. Adults receiving mechanical ventilation in the ICU will undergo ultrasound examinations of the median and tibial nerves on Days 1, 4, and 7. The study will evaluate whether early changes in nerve elasticity can predict ICU-AW, which is usually diagnosed later using a muscle strength score that requires patient cooperation.

Intensive Care Unit-Acquired Weakness (ICU-AW) is a common and severe complication in critically ill patients, associated with poor long-term outcomes. The current gold standard for diagnosis, the Medical Research Council (MRC) score, requires patient cooperation and is often delayed.

Shear Wave Elastography (SWE) is an advanced ultrasound technique that quantitatively measures tissue stiffness. Pathophysiological changes in ICU-AW, such as axonal degeneration and edema in nerves, are expected to alter their biomechanical properties (elasticity). This study posits that these changes may be detectable early using SWE, preceding overt clinical weakness.

This study will prospectively enroll adult patients admitted to the ICU who are expected to require prolonged mechanical ventilation. Participants will undergo serial ultrasound examinations of the median and tibial nerves on days 1, 4, and 7 of their ICU stay. The examinations will include SWE to measure nerve elasticity (stiffness) and conventional B-mode ultrasound to measure nerve cross-sectional area (CSA).

On day 7, all consenting and cooperative patients will be assessed for ICU-AW using the MRC score. The primary objective is to determine the diagnostic accuracy of early nerve SWE measurements for predicting the diagnosis of ICU-AW on Day 7. Secondary objectives include describing the dynamic changes in nerve elasticity and size over the first week, and exploring the correlation between these ultrasound parameters and clinical severity scores and patient outcomes.

Inclusion Criteria:

1. Age ≥ 18 years;
2. Newly admitted to ICU with an expected ICU stay > 7 days;
3. Initiated on mechanical ventilation within 24 hours of admission, with an expected ventilation duration > 48 hours;
4. Patient or legally authorized representative provides written informed consent.

Exclusion Criteria:

1. Known pre-existing neuromuscular diseases (e.g., myasthenia gravis, Guillain-Barré syndrome, amyotrophic lateral sclerosis);
2. Pre-existing limb dysfunction that would interfere with muscle strength assessment (e.g., hemiplegia, paraplegia, severe arthritis);
3. Presence of acute trauma, surgery, skin breakdown, infection, or severe edema at the measurement sites (wrist, elbow, ankle) that would interfere with ultrasound probe placement;
4. Pregnant or lactating women;
5. Any other condition that the investigator believes would make the patient unsuitable for the study.

Critically ill adults on mechanical ventilation with an anticipated ICU stay greater than 7 days.