This cross-sectional study aims to investigate the relationship between variations in muscle thickness and subcutaneous tissue, measured by ultrasound, and fluid compartments, evaluated using bioelectrical impedance analysis (BIA), in critically ill patients in the intensive care unit (ICU). Critically ill patients frequently experience muscle wasting and tissue edema due to a combination of inflammation, immobilization, and aggressive fluid resuscitation protocols designed to counteract hemodynamic instability.

Ultrasound is widely used to monitor muscle thickness because it is fast, non-invasive, and repeatable. However, muscle thickness measurements during the first days of ICU admission may be influenced by fluid overload, which causes edema and might lead to an overestimation of actual muscle mass. Bioelectrical impedance analysis (BIA) is a complementary tool that assesses both intra- and extracellular fluid compartments. This study aims to correlate daily fluid balance with changes in muscle thickness and subcutaneous tissue measured by ultrasound, and to determine if BIA can accurately reflect fluid status and potentially identify edema in these patients.

Furthermore, at ICU discharge, patients' muscle strength will be assessed using both the MRC-sum score (0-60) and handgrip strength (using a Jamar dynamometer), to investigate whether the presence of edema at discharge correlates with muscle strength deficits. Data collection will occur daily, tracking fluid balance, clinical parameters, and body weight, alongside ultrasound and BIA measurements in a standardized position. The study will help clarify the interactions between fluid management, muscle mass changes, and clinical outcomes in critically ill patients, providing valuable insight into early rehabilitation strategies.

Muscle wasting and edema are common complications in critically ill patients, particularly those requiring intensive care. The rapid decrease in muscle mass and strength, known as ICU-acquired weakness, affects both peripheral and respiratory muscles, reducing patients' functional capacity in both the short and long term. Early interventions like physical therapy and mobilization have been shown to improve patient outcomes, but there is a need for accurate monitoring of muscle mass and fluid balance during the ICU stay to guide treatment.

This study leverages two complementary measurement techniques: ultrasound and bioelectrical impedance analysis (BIA). Ultrasound allows for the rapid and non-invasive assessment of muscle thickness and subcutaneous tissues. Previous studies have demonstrated that patients in the ICU can lose around 2% of their rectus femoris muscle thickness per day during the first week of their ICU stay. Ultrasound can also assess the structural changes in muscle tissue, such as alterations in echogenicity, that occur due to the inflammatory state of critically ill patients. However, patients undergoing aggressive fluid resuscitation to counteract shock and hypotension may experience fluid overload, which can distort early ultrasound measurements by causing tissue edema.

Bioelectrical impedance analysis (BIA), on the other hand, is a widely used method for evaluating body composition, particularly in terms of fat-free mass, fat mass, and fluid compartments. In the ICU, BIA is highly sensitive to changes in fluid balance, providing estimates of extracellular and intracellular water. This makes BIA particularly useful for tracking fluid shifts and edema development over time in critically ill patients.

The primary objective of this study is to explore the correlation between daily fluid balance and the changes in muscle thickness and subcutaneous tissue, as measured by ultrasound, and to assess the utility of BIA in evaluating fluid status and estimating the presence of edema. Specifically, the study will address two main research questions: (1) Is there a correlation between fluid balance and muscle thickness or subcutaneous tissue variation during the ICU stay? (2) Can BIA accurately reflect positive fluid balance and help estimate the extent of tissue edema?

The study will also include an exploratory analysis at ICU discharge to determine whether there is a link between the presence of edema and patients' muscle strength. Muscle strength will be assessed using both the MRC-sum score (0-60) and handgrip strength with a Jamar dynamometer. Previous studies have shown that ICU-acquired weakness is associated with poorer long-term outcomes, and this study will explore whether edema contributes to this weakness.

Patients will be recruited according to predefined inclusion and exclusion criteria. Upon obtaining consent from the patient or their legal representative, baseline clinical data (age, sex, BMI, reason for ICU admission, comorbidities, and severity scores such as APACHE-2 and SOFA) will be collected. Daily data will include vital signs, ventilatory settings, fluid balance, body weight, and medication details. Ultrasound and BIA measurements will be taken daily, with the patient positioned in a standardized way (supine position with a 30° incline). The ultrasound measurements will focus on the rectus femoris, tibialis anterior, and biceps brachii muscles, along with key subcutaneous tissues prone to fluid accumulation. BIA measurements will estimate total body water, extracellular water (ECW), intracellular water (ICW), muscle mass, and fat mass.

At ICU discharge, handgrip strength and MRC-sum score measurements will be performed to assess functional recovery and muscle strength. This study will provide valuable insights into how fluid management and edema contribute to muscle wasting and weakness in critically ill patients, informing future therapeutic strategies.

Inclusion Criteria:

• Adults aged over 18 years.
• Patients present and admitted to the ICU at Epicura Hornu Hospital between October 14, 2024, and November 15, 2024 (to be updated).
• Informed consent to participate in the study has been signed by the patient or their legal representative.

Exclusion Criteria :

• Patients in post-operative or other surveillance with an expected ICU stay of less than 48 hours.
• Patients for whom a decision to withdraw therapy has been made prior to inclusion.
• Presence of skin conditions (e.g., wounds or ulcers) that hinder ultrasound measurements or the application of skin electrodes.
• Pregnancy.
• Presence of an implanted pacemaker or defibrillator.

The study population will consist of critically ill adult patients aged over 18 years, admitted to the Intensive Care Unit (ICU) at the Centre Hospitalier Epicura Hornu between October 14, 2024, and November 15, 2024 (to be updated). Participants will include individuals who have provided informed consent to participate in the study or whose legal representatives have signed on their behalf. The cohort will represent a diverse range of medical conditions requiring intensive care, and all participants will be monitored for muscle thickness variation and fluid balance throughout their ICU stay.