To: Association between rectus femoris cross-sectional area and diaphragmatic excursion with weaning of tracheostomized patients in the intensive care unit

We read an interesting prospective, single-center, observational cohort study on the relationship between the cross-sectional diameter of the rectus femoris muscle, the degree of diaphragmatic excursion, and the outcome of weaning 81 critically ill patients by Vieira et al. (1) Successfully weaning critically ill patients from mechanical ventilation has been found to be associated with a larger cross-sectional area of the rectus femoris and diaphragmatic excursion. (1) The study is compelling but has limitations that should be discussed. The first limitation of the study is that the cross-sectional area of the rectus femoris muscle depends on several nonstandardized factors. The ultrasound measurement of the cross-sectional area of the rectus femoris depends on age, sex, caloric intake, diet, local arterial perfusion, physical condition of the patient before admission to the intensive care unit, innervation of the muscle, previous illness, comorbidities, and current medication. Therefore, few homogeneous cohorts can be generated, which makes the results unreliable. A second limitation of the study is that diaphragmatic deflection can also depend on multiple factors, such as previous lung or bronchial diseases, diseases of the central nervous system or the peripheral nervous system (PNS), status of the neuromuscular junction, premorbid physical activity (training condition), muscle function, and current medications.