This study showed that in elderly ICU patients, low PM CSA at ICU admission was associated with prolonged mechanical ventilation, a higher SOFA score during the ICU stay and lower mortality.
In previous studies[28, 29] on skeletal muscle mass in the ICU using CT as an assessment tool, lumbar muscle at the level of T3 had been the main concern. In contrast, this study focused on the thoracic skeletal muscles. According to a previous study, up to 65% of all ICU chest x-rays (CXR) had unexpected or abnormal findings, many of which affected management. The problem is further compounded as CT scanning is being used in critically ill patients who may have multiple medical problems that may not be easily discriminated by the CXR.[30, 31] Without any additional radiation exposure, CSA analysis through existing chest CT scans provides an objective index of future prognosis in elderly ICU patients.
Previous studies[16, 28] included a younger patient population, whereas this study focused on the elderly medical patient population. As the skeletal muscle loss increases with aging and the proportion of elderly patients in the ICU is increasing,  it is important to accurately stratify risk of sarcopenia in elderly ICU patients. In addition, previous studies[16, 32] focused mainly on mortality or ventilator-/ICU-free days, whereas this study focused on mortality, longitudinal changes in organ dysfunction, namely the SOFA score, and prolonged ventilation. As skeletal muscle mass is related with the short- and long-term outcomes in various diseases,[33, 34] it would be meaningful to focus on longitudinal changes in the prolonged mechanical ventilation and SOFA score.
The most appropriate treatment for elderly ICU patients may not necessarily mean maximal treatment, and in patients without improvement of their clinical situation, the therapeutic intensity level may no longer be in accordance with patients’ chances of long-term survival with acceptable quality of life, at which time a clinical decision might need to be made. In the algorithm on the decision-making process for caring of critically ill old patients purposed by Bertrand et al., it is recommended to make a reassessment on the patient condition and arrange a meeting with the family at day 2 or 3 on ICU admission. Adding thoracic skeletal muscle CSA at the time of ICU admission into consideration in deciding therapeutic intensity in elderly ICU patients may help in making medical decisions.
In this study, CSAs of the thoracic skeletal muscles were related with outcomes. We believe that the thoracic skeletal muscles reflect the general health status and nutritional status and are markers of frailty before ICU admission. Skeletal muscles are important in regulating the immune function, glucose disposal, cytokine signaling, and protein synthesis. Skeletal muscle loss is associated with an increased risk of developing nosocomial infections and falls in the hospital.  There are also studies that skeletal muscle loss is associated with depression. It is possible that the factors described above contributed to these findings.
In abdominal CT, the muscle area at the L3 vertebra level, divided by the patient height2, is accepted as a surrogate marker of loss of skeletal muscle quantity and cachexia. However, in the thoracic level, the method of assessing skeletal muscle loss is not established for each study, and the muscle areas measured for each study were different. A study by Ariel et al. assessed pectoralis muscle CSA, a study by Florian et al. assessed thoracic skeletal muscles at the level of T5, a study by Lee et al. assessed diaphragm thickness, and a study by Fuseya et al. used erector spinae muscle CSA at the level of T12. In this study, we analyzed the CSA of the pectoralis. More studies are needed to clarify which muscles among the thoracic muscles most reflect sarcopenia well.
Our study has limitations. First, the nature of this study was retrospective. Second, the populations were from a single center, consisted only of Asian population, and involved no replication cohort. Thus, our results may not be fully generalizable to other populations. However, validity of the reported prognostic factors in ICU patients, such as CCI and the SOFA score, was confirmed in this study population, which supported the present findings. Third, the physical activity levels of all subjects were not directly evaluated. We could not include data regarding physical function testing. However, based on other reports,[6, 7, 12] we suspected that skeletal muscles of the chest may reflect both physical activity and physiological parameters. Further analysis is needed to verify these assumptions.
In conclusion, low CSA of the pectoralis muscle obtained from a single-slice axial chest CT image was associated with prolonged mechanical ventilation, a higher SOFA score during ICU stay and higher mortality. Taking into consideration the CSA of the thoracic skeletal muscle at the time of ICU admission while deciding therapeutic intensity in elderly ICU patients may help in making medical decisions.