Cross-sectional Study on the Variation of Muscle Thickness and Subcutaneous Tissues by Ultrasound and Bioelectrical Impedance in Association with the Fluid Balance of Patients in Intensive Care

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.