Point-of-Care Ultrasound as a Prognostic Tool in Critically Ill Patients: Insights Beyond Core Muscle Mass.

Background: Muscle wasting is a major concern in ICUs, contributing to morbidity, mortality, and prolonged rehabilitation. While CT-derived L3 Skeletal Muscle Index (L3SMI) assesses core muscle mass, it may not capture peripheral muscle atrophy or fluid-based changes. Point-of-care ultrasound (POCUS) offers a rapid, non-invasive alternative. This study evaluated the prognostic value of POCUS-based muscle measurements compared with L3SMI in predicting mortality, frailty, and functional outcomes. Methods: In this prospective study, 50 critically ill adults meeting Sepsis-3 criteria or requiring respiratory/vasopressor support underwent POCUS assessments of biceps brachii, rectus femoris, and vastus intermedius thickness at days 1, 7, and 14 post-ICU admission. Twenty-eight patients also had CT scans within seven days for L3SMI calculation. The primary outcome was 90-day mortality; secondary outcomes included in-hospital and 30-day mortality, Clinical Frailty Score, and Zubrod/ECOG performance status. Muscle measurements were analyzed both raw and indexed to body surface area, with predictive performance assessed via correlation and ROC analysis. Results: Day 1 biceps brachii thickness strongly predicted in-hospital mortality (AUC 0.84; sensitivity 1.0, specificity 0.67) and retained predictive value for 30-day and 90-day mortality. Vastus intermedius thickness on day 1 was moderately predictive (AUC 0.79). At later time points, larger vastus intermedius measurements correlated negatively with ICU- and ventilator-free days, suggesting edema-related pseudohypertrophy. L3SMI did not significantly correlate with ultrasound-based muscle measurements or clinical outcomes. POCUS-derived peripheral muscle indexing was associated with frailty indices, highlighting its role in capturing meaningful functional deficits. Conclusion: POCUS-based muscle assessments, particularly of the biceps brachii and vastus intermedius, provide valuable prognostic insights beyond conventional L3SMI. While L3SMI remains a core muscle measure, fluid shifts and localized muscle wasting in critical illness may be better captured by ultrasound.