An exploratory prospective observational study of muscle cross-sectional area changes and energy balance in critically ill children: avoiding negative energy balance is necessary but not sufficient to maintain muscle mass in critically ill children

Abstract Rationale Muscle loss in critically ill children is likely multifactorial and linked to illness severity, immobility, and nutrition. Muscle ultrasound (US) is increasingly used to assess muscle changes, but studies using inaccurate energy expenditure equations have not consistently linked nutrition and muscle mass changes. Objectives To evaluate longitudinal change in muscle cross-sectional area (CSA) in critically ill children and explore its relationship with energy balance. Methods This nested prospective observational study enrolled intubated children (1month-18 years) eligible for indirect calorimetry measurements. Indirect calorimetry measurements were paired with two US measurements of the biceps brachii and rectus femoris. Percent change in muscle CSA and energy balance (%difference between energy expenditure and caloric delivery/energy expenditure) were calculated. Mann-Whitney tests compared energy balance between groups with and without muscle loss. Spearman correlation assessed the relationship between daily energy balance and muscle CSA. Results Energy expenditure and muscle CSA were measured in 23 patients (median age 6.2 years; 61% males). The rectus CSA decreased by a median (Q1, Q3) of 1.5% (-2.93, 0.83) per day, with greater losses in patients with negative energy balance ( p = 0.03). Patients with energy balance ≤ -5% had a median change in rectus CSA of -18.7% (-22.9, -8.1). Daily change in rectus CSA and energy balance showed a moderate correlation ( r = 0.59, p = 0.01). The rate of change in the biceps CSA was 0.28% (-4.67, 1.65) per day with no correlation with energy balance. Conclusions Muscle loss persists beyond the first week of critical illness and may be associated with energy deficits. Precision in nutrition and further study of muscle changes are needed to mitigate ICU-acquired morbidity.