Fluid Shifts and Muscle Loss in Critical Care: Accuracy of Ultrasound Versus Bioelectrical Impedance Analysis

Background: Critically ill patients experience rapid muscle wasting during their ICU stay. Ultrasound (US) and bioelectrical impedance analysis (BIA) are widely used to assess muscle mass; however, their accuracy may be affected by fluid balance alterations. This study aimed to compare the reliability of US and BIA in detecting muscle loss under varying fluid balance conditions in ICU patients. Methods: In this prospective observational study, adult ICU patients with an ICU stay of ≥7 days were evaluated on Days 1, 5, and 7. Muscle thickness was measured using US, and phase angle (PhA) using BIA. Cumulative fluid balance, C-reactive protein (CRP), and lactate levels were recorded. Patients were stratified according to cumulative fluid balance. Results: A total of 143 ICU patients were included in the final analysis. US demonstrated a progressive decrease in muscle thickness (−3.54% ± 10.90% from Day 1 to Day 5 and −7.56% ± 11.82% from Day 1 to Day 7 (both p < 0.0001)), whereas BIA showed no significant change in PhA. Positive fluid balance significantly reduced PhA compared with the negative balance group, p < 0.001, whereas no statistically significant effect on US measurements was detected. CRP > 200 mg/L was associated with greater US-detected muscle loss on Day 5, while lactate > 2.5 mmol/L was associated with lower PhA. Conclusions: Ultrasound reliably identified structural muscle wasting in critically ill patients, with no statistically significant effect of fluid balance detected in this cohort. Furthermore, ultrasound measurements were associated with inflammation-related muscle loss. In contrast, BIA was strongly influenced by hydration and perfusion status, limiting its ability to assess true muscle mass loss in the ICU setting.