Ultrasound-derived quadricep muscle layer thickness in the intensive care unit: A scoping review of techniques and reported outcomes

Survivors of critical illness experience significant physical weakness, primarily attributed to a rapid depletion of skeletal muscle during an intensive care unit (ICU) admission equivalent to 2% muscle loss per day (1). Furthermore, patients who enter the ICU with pre-existing low muscle mass are at a significantly higher risk for poor outcomes, including mortality (2, 3) and frailty (4). Despite the association between low muscle mass and adverse outcomes, current clinical practice rarely quantifies muscle mass at the bedside, rather relying on simple methods such as body weight and body mass index that assess net body size as opposed to body composition. Ultrasound (US) has emerged as a promising, non-invasive tool for assessing muscle mass at the bedside. Specifically, quadriceps muscle layer thickness (QMLT), which measures the combined thickness of the M. vastus intermedius and M. rectus femoris muscles, has been identified as a reliable and accurate method of measuring muscle mass (5) and has been the most widely utilised in critical care research. In a scoping review published in 2024 by Lima et al. (6), 107 studies were identified where either skeletal muscle mass quantity or quality using US in the ICU setting were evaluated, with 51 studies reporting the use of QMLT to measure muscle mass. For most studies (70.3%) significant skeletal muscle mass depletion during hospitalisation was reported; however, the methods of measuring muscle mass via US varied considerably across the studies making comparisons difficult. Even though US-derived muscle mass measurements are commonly used to measure and evaluate changes in muscle health in critical care research, the translation of these methods have not been widely adopted in clinical practice. Although not limited to the ICU setting, a 2022 systematic review where the use of US to measure muscle mass and quality to predict clinical outcomes in a mixed patient population was investigated, it was found that 76% of studies (28/37) reported a significant association between reduced muscle mass or quality and worsening clinical or functional outcomes (7). These outcomes included prolonged mechanical ventilation, higher mortality rates, longer length of hospital stay, increased readmission rates, impaired physical function, a greater incidence of ICU-acquired weakness and a higher likelihood of requiring long-term care post-discharge.(7) Whilst associations between low US-derived muscle mass and adverse clinical and functional outcomes have been demonstrated, there is a paucity of data exploring cut-points of baseline muscle mass or the degree of muscle depletion during ICU admission that can predict patient outcomes. This scoping review will build on previous work (1, 6-9) to identify studies that have used US to measure QMLT during the ICU admission, report on the US techniques used, and the clinical and patient-centred outcomes reported. The findings of this scoping review will inform future work by identifying studies eligible for inclusion in an individual patient data meta-analysis aimed at developing cut-points to predict clinical and patient-centred outcomes.