Early Detection of Muscle Weakness and Functional Limitations in the Critically Ill: A Retrospective Evaluation of Bioimpedance Spectroscopy

Background The potential for bioimpedance spectroscopy (BIS) to identify muscle weakness and functional limitations in critical illness is unknown; this study aimed to determine association of BIS with strength/function and differences between 3 intensive care units (ICUs). Methods A retrospective post hoc analysis of BIS, strength, and functional data from adults who required >= 48 hours of mechanical ventilation was conducted. Measures of body composition included the proportion (%) of total body water (TBW), fat mass (FM), and fat-free mass (FFM). The Medical Research Council sum score (MRC-ss) and Physical Function in ICU Test-Scored (PFIT-s) were used for strength and functional assessments. Nonparametric cross-sectional analyses were done at enrollment (<= 48 hours of admission: site-A, site-C) and awakening from sedation (site-A, site-B). Raw impedance variables including 50-kHz phase angle (PA) and impedance ratio (IR) were available from site-A and site-B. Results Participants were 135 adults (site-A n=59, site-B n=33, site-C n=44), with a median (interquartile range) age of 59 (50-69) years. At enrollment, TBW%, FM%, and FFM% were similar between site-A and site-C (P>.05); pooled data were not associated with MRC-ss at awakening or MRC-ss/PFIT-s at ICU discharge. At awakening, there was less TBW%, less FFM%, and greater FM% at site-B vs site-A (P <=.001) but no associations with MRC-ss/PFIT-s when using pooled data. Trends with pooled data of a lower PA and higher IR being associated with awakening MRC-ss were confirmed within site-B (PA rho=0.70, P <=.001; IR rho=-0.79, P <=.001). Conclusion Site-by-site data suggest that raw impedance variables might be useful for screening weakness and poor function.