Table 4 Methodologies for assessment of skeletal muscle in ICU [64,65,66]

CT image analysis at the abdominal (L3) area

• Muscle CSA (cm²)

• Muscle quality (density) (Hounsfield units)

• Specialised software can be used to measure muscle area and density using a CT slice at L3

• Quantification of muscle CSA at L3 is highly correlated to whole body muscle (using scans performed for clinical purposes)

• Provides specific and precise results

• Published cut-off values to identify patients with lower than normal muscularity

• Limited for use in patients who have had a CT at L3 area

• Specialist training and time required for analysis

Bioimpedance analysis (multi-frequency or spectroscopy)

• Fat-free mass (kg)

• Phase angle (50 kHz)

• Involves application of a weak current at differing frequencies, through electrodes placed on the hands and feet

• Total body water, percentage body fat, and fat-free mass are estimated via regression equations (with assumed constants for estimating intra- and extracellular water)

• Raw data such as phase angle (which is independent of weight and related to cellular health) may be a predictor of outcome in critically ill populations

• Easy and quick to use

• Safe (no radiation involved)

• Fat-free mass estimates are not likely to be reliable in critically ill patients who experience significant fluid shifts

• Positioning (separation in limbs) and electrode placement may be challenging in some ICU patients

Ultrasound

• Muscle thickness (cm)

• Muscle CSA (cm²)

• Muscle quality (echogenicity)

• Muscle thickness and CSA can be measured at different sites (i.e. quadriceps, upper arm)

• Muscle quality can also be assessed using specialist software

• Readily available in most ICUs

• Easy, safe, and quick to use

• No consensus on the ideal sites to predict whole body muscle or to monitor changes over time

• Role of oedema on measurements is unclear

• No widely accepted cut-points to identify patients with low muscularity