Zudin Puthucheary (London, UK)
US can track changes in muscle mass of critically ill pts
Rectus femoris cross-sectional area (RF CSA) validated against Fibre CSA (gold standard – muscle biopsy of vastus lateralis) and Ratio of Protein:DNA in myocyte (biochemical gold standard in muscle mass measurement; UNAFFECTED by hydration status)
RF CSA and Fibre CSA both underestimate muscle wasting compared to myocyte Protein:DNA
–>> muscle wasting visualised on Ultrasound is likely to also be an underestimate
US sensitively discerns muscle loss between pts with different severity of illness – significantly greater in pts with higher number of failed organs
US + Bx of muscle, repeated 10d later –> smaller, brighter RF = cellular infiltrate + myonecrosis (present in up to 40% of critically ill pts) — US is a non-invasive tool to detect changes in Muscle QUALITY and has been used in the paediatric population for over a decade
Limitations with using US to measure Muscle MASS – How to assess?
- RF CSA (historically closely linked to sporting performance) vs. muscle layer thickness (MLT)
- MLT underestimates muscle wasting compared to RF CSA
Changes in RF CSA are associated with changes in muscle strength (MRC score)
This change is not seen in MLT i.e. MLT is not associated with muscle function in critical illness
MLT also does not correlate with muscle mass (measured on CT)
–> MLT is not a good indicator of muscle mass in critically ill population
–> reliability, reproducibility, accuracy should not be assumed
