Body-water compartments measured by bio-electrical impedance spectroscopy in patients with chronic obstructive pulmonary disease

It was previously demonstrated that single frequency bio-electrical impedance (BIA) measurement at 50 kHz is a useful method to assess total body water (TBW) in patients with chronic obstructive pulmonary disease (COPD). In the present study it was examined whether bio-electrical impedance spectroscopy (BIS) could predict extracellular water (ECW) and improve the prediction of TBW in these patients. TBW and ECW (corrected bromide space) were measured by deuterium and bromide dilution. In 37 COPD patients prediction equations were obtained using BIS (5-500 kHz) measurements, and these were cross validated in a second group of 40 COPD patients. All patients were in a clinically stable condition. TBW predicted by BIS was not significantly different from actual TBW and demonstrated a comparable standard error of estimate (SEE) as found previously in healthy subjects (male correlation coefficient: r = 0.88, SEE: 2.3 L, female r = 0.85, SEE: 2.9 L). Predicted ECW using BIS-measurements was not significantly different from measured ECW (male r = 0.75, SEE: 1.4 L, female r = 0.73, SEE: 1.2 L), but the error in the prediction was relatively large and the correlation between predicted and actual ECW relatively low compared to most studies in healthy subjects. Predicted TBW using BIS was comparable to actual TBW, but presented no improvement of the prediction of TBW using BIA at 50 kHz and a patient specific regression equation. The error of the prediction of ECW by BIS limits the ability to predict fluid shifts in individual patients with clinically stable COPD.