Raw Bioelectrical Impedance Analysis Variables Are Independent Predictors of Early All-Cause Mortality in Patients With COPD

Background: Bioelectrical impedance analysis (BIA) is a valuable method for estimating fat-free mass and fat mass in patients with COPD by using specific predictive equations. In addition, raw BIA variables such as high- to low-frequency impedance ratios (IRs) and phase angle, most likely as a result of providing information on muscle quality, have been related to disease severity and mortality in patients with several diseases but never in COPD. The aim of this study was to investigate the predictive role of raw BIA variables on 2-year survival in COPD. Methods: Impedance (Z) at 5-10-50-100-250 kHz and phase angle at 50 kHz were determined in 210 patients with COPD. Three IRs were calculated: Z at 50 kHz/Z at 5 kHz (50/5 IR), Z at 100 kHz/Z at 5 kHz (100/5 IR), and Z at 250 kHz/Z at 5 kHz (250/5 IR). Demographic, respiratory, and body composition data at baseline were recorded. All-cause mortality was assessed during 2 years of follow-up. Results: After the follow-up period, all-cause mortality was 13.8%. Statistically significant differences between nonsurvivors and survivors emerged in terms of age, weight, BMI, FEV1, inspiratory capacity, and modified Medical Research Council dyspnea score. With respect to nutritional variables, nonsurvivors had lower fat-free mass (P = .031), lower fat mass (P = .015), higher IRs (P < .001 for all the ratios), and lower phase angle (P < .001) compared with survivors. After adjustment for confounding factors, each unit increase of IRs and each unit decrease of phase angle were associated with a higher risk of death. Conclusions: IRs and phase angle, as raw BIA variables, are independent and powerful predictors of all-cause mortality in COPD and should be considered, together with inspiratory capacity and 6-min walk distance, as significant prognostic factors in the short- to middle-term.