Skeletal Muscle Ultrasound in Critical Care: A Tool in Need of Translation

Abstract

With the emerging interest in documenting and understanding muscle atrophy and function in critically ill patients and survivors, ultrasonography has transformational potential for measurement of muscle quantity and quality. We discuss the importance of quantifying skeletal muscle in the intensive care unit setting. We also identify the merits and limitations of various modalities that are capable of accurately and precisely measuring muscularity. Ultrasound is emerging as a potentially powerful tool for skeletal muscle quantification; however, there are key challenges that need to be addressed in future work to ensure useful interpretation and comparability of results across diverse observational and interventional studies. Ultrasound presents several methodological challenges, and ultimately muscle quantification combined with metabolic, nutritional, and functional markers will allow optimal patient assessment and prognosis. Moving forward, we recommend that publications include greater detail on landmarking, repeated measures, identification of muscle that was not assessable, and reproducible protocols to more effectively compare results across different studies.

Keywords: critical illness; intensive care; muscle atrophy; muscle cross-sectional area; muscle thickness.

Figures

Figure 1.

Depiction of diverse factors that contribute to the progressive cycle of reduced protein synthesis and increased protein breakdown leading to muscle atrophy in critically ill patients.

Figure 2.

Several quantitative and qualitative features of skeletal muscle can be measured using ultrasound that are associated with clinical and functional outcomes. ICU = intensive care unit.

Figure 3.

Illustrations of diverse patient and scan characteristics obtained by ultrasonography. (A) Anatomical labels are identified in a scan depicting muscle layer thickness of a 23-year old non–intensive care unit female with a body mass index of 22.8 kg/m2. Muscle layer thickness would include rectus femoris and vastus intermedius depicted in red. (B) Example of cross-sectional area of a rectus femoris muscle highlighted in red. (C) Muscle layer thickness is shown for a non–intensive care unit individual who has a body mass index of 28.9 kg/m2 and body fat percentage of 45.6% (measured using dual-energy X-ray absorptiometry). The red line depicts muscle layer thickness. (D) Muscle layer thickness is imaged using a linear probe and depicted in red. (E) Muscle layer thickness is imaged using a curvilinear probe, depicted in red, in the same individual as in (D). B = bone; INT = interface between subcutaneous adipose tissue and rectus femoris; RF = rectus femoris; S = skin; SAT = subcutaneous adipose tissue; VI = vastus intermedius.