This is a single-center prospective observational study that aimed to describe the trajectory of change in body composition among critically ill patients who were able to function independently prior to admission. Ultrasound measurement of the quadriceps muscle and bioelectrical impedance analysis will be conducted at baseline, day 7, day 14 and before ICU discharge. The relationship between the change of body composition and clinical outcomes, activities of daily living and quality of life at 6-month post ICU admission will be investigated. Further, the association between nutritional (energy and protein) intake and change in body composition will also be investigated.

Malnutrition is associated with poor clinical outcomes. Nutritional status is closely linked with body lean mass, specifically the skeletal muscle. Several tool s had been suggested to determine nutrition risk and status for critically ill patients such as The Nutrition Risk in Critically Ill (NUTRIC) and subjective global assessment (SGA). However, they are surrogate (NUTRIC) or subjective (SGA) measures of nutritional status. Ideally, clinicians should measure muscle directly to determine the patient nutritional status. Several imaging techniques allow direct and accurate measurement of muscle mass, these include magnetic resonance imaging (MRI) and computed-tomography (CT). However, the measurement of MRI and CT involves the transfer of patients out of the intensive care unit or expose the patient to radiation (CT) and therefore it is not justifiable to conduct MRI or CT for the sole purpose of measuring body composition. Ultrasound and bioelectrical impedance analysis (BIA) are non-invasive and can be conducted at the bedside and therefore are promising techniques in assessing patients' nutritional status. Ultrasound measurement of quadriceps thickness and rectus femoris cross-sectional area can measure muscle directly and are widely used in the research setting. BIA phase angle is a direct measure of cell integrity and is an independent predictor of mortality in the ICU.

This study aimed to describe the trajectory of change in body composition and investigate the relationship of the changes with clinical outcomes and activities of daily living and quality of life at 6-month post ICU admission. Further, the association between nutritional (energy and protein) intake and change in body composition will also be investigated to determine if the ultrasound or BIA measurements of body composition can help to assess the response to nutritional intervention.

Inclusion Criteria:

• ≥ 18 years old
• Mechanically ventilated* within 48 hours of ICU admission
• Expected to stay in the ICU ≥96 hours
• CT imaging is done within 72 hours prior and after ICU admission (only applicable for CT analysis)

Exclusion Criteria:

• Pregnant
• Expected death or discussion to withdraw life-sustaining treatments within this hospitalization
• Patients in the hospital (either in the ward or ICU) ≥5 days in the past 2 weeks before enrolment
• Not ambulating independently prior to illness that led to ICU admission (use of gait aid permitted). Also exclude if unable to walk 50 feet (15 meters) prior to hospitalization
• Pre-existing (chronic or acute presentation) primary systemic neuromuscular disease (e.g. Guillain Barre)
• Patient on pacemaker
• Bilateral lower limb deep vein thrombosis
• Presence of iliopsoas abscess or hematoma (only applicable for CT analysis)

Adult mechanically ventilated critically ill patients who are expected to stay at least 96 hours in the intensive care unit.