Muscle Ultrasound Changes and Physical Function of Critically Ill Children: A Comparison of Rectus Femoris Cross-Sectional Area and Quadriceps Thickness Measurements

Quadriceps thickness (QT) and rectus femoris cross-sectional area (RF(CSA)) are both used to evaluate muscle changes in critically ill children. However, their correlation and association with physical function has not been compared. OBJECTIVES: To compare QT with RF(CSA) changes, and their association with physical function in critically ill children. DESIGN, SETTING, AND PARTICIPANTS: Secondary analysis of a prospective cohort study of children 0–18 years old admitted to a tertiary mixed PICU between January 2015 and October 2018 with PICU stay greater than 48 hours and greater than or equal to one organ dysfunction. MAIN OUTCOMES AND MEASURES: Ultrasound QT and RF(CSA) were measured at PICU admission, PICU discharge, hospital discharge, and 6 months post-discharge. QT and RF(CSA) changes from baseline were compared with each other and with change in motor function, physical ability, and physical health-related quality of life (HRQOL). RESULTS: Two hundred thirty-seven images from 66 subjects were analyzed. RF(CSA) change was not significantly different from QT change at PICU (–8.07% [interquartile range (IQR), –17.11% to 4.80%] vs –4.55% [IQR, –14.32% to 4.35%]; p = 0.927) or hospital discharge (–5.62% [IQR, –15.00% to 9.42%] vs –8.81% [IQR, –18.67% to 2.39%]; p = 0.238) but was significantly greater than QT change at 6 months (32.7% [IQR, 5.74–109.76%] vs 9.66% [IQR, –8.17% to 25.70%]; p < 0.001). Motor function change at PICU discharge was significantly associated with RF(CSA) change (adjusted β coefficient, 0.02 [95% CI, 0.01–0.03]; p = 0.013) but not QT change (adjusted β coefficient, –0.01 [95% CI, –0.02 to 0.01]; p = 0.415). Similar results were observed for physical HRQOL changes at hospital discharge (adjusted β coefficient for RF(CSA) change, 0.51 [95% CI, 0.10–0.92]; p = 0.017 and adjusted β coefficient for QT change, –0.21 [–0.76 to 0.35]; p = 0.458). Physical ability was not significantly associated with RFCSA or QT changes at 6 months post-discharge. CONCLUSIONS AND RELEVANCE: Ultrasound derived RF(CSA) is associated with PICU motor function and hospital discharge physical HRQOL changes, unlike QT, and may be more useful for in-hospital muscle monitoring in critically ill children.