Comparison of vector and conventional bioelectrical impedance analysis in the optimal dry weight prescription in hemodialysis

Background: Dry weight prescription is commonly based on symptoms induced b y inappropriate fluid removal by hemodialysis (HD). Aim of this study was t o compare the assessment of volume status by conventional bioelectrical imp edance analysis (BIA) and the resistance-reactance (RXc) graph method in HD patients achieving their target dry weight determined on clinical criteria . Methods: We studied 39 HD patients (23 males and 16 females, mean age 52 +/- 17 years, dialytic age 41.2 +/- 37 months). Dry weight, prescribed acco rding to the standard clinical criteria, was constantly achieved in the las t 3 months. Patients symptom-free over the last 3 months were defined as as ymptomatic. Patients with either muscular cramps or hypotensive episodes we re defined as symptomatic. Thirty-three healthy volunteers (11 males, 22 fe males, mean age 50 +/- 11 years) constituted the control group. Standard, s ingle frequency (50 kHz), tetrapolar, BIA measurements were obtained in con trols, and in patients before, every 60 min, and 30 min after one HD sessio n. Total body water (TBW), and extracellular water (ECW) were calculated us ing conventional BIA regression equations. In both groups, tissue hydration was also assessed by the RXc graph method. Results: On the basis of 95% to lerance interval (mean +/- 2 SD) for the ECW (%) calculated in healthy subj ects (ECW = 35-44%), HD patients were divided into 3 groups according to th eir post-HD ECW: 72% normohydrated with ECW 35-44%, 10% overhydrated with E CW >44%, and 18% underhydrated with ECW <35%. Patients were also classified into 3 categories according to the RXc graph method: 38% normohydrated wit h vectors within the reference 75% tolerance ellipse, 0% overhydrated with short vectors below the lower pole of the 75% tolerance ellipse, and 62% un derhydrated with long vectors above the upper pole of the 75% tolerance ell ipse. The progressive removal of body fluid during HD treatment was associa ted with a progressive increase in both impedance vector components, R and Xc. Eleven of thirty-nine patients (28%) were symptomatic during HD treatme nt in the last 3 months. The majority of these (73%) were classified as nor mohydrated according to ECW estimates, while 9 and 18% were classified as o ver- and underhydrated, respectively. This frequency distribution was signi ficantly different from that obtained with the RXc graph method (chi(2) = 6 .9, p = 0.03) where the majority (73%) were classified as underhydrated, wh ile 0 and 27% were classified as over- and normohydrated, respectively. The frequency distribution of the 28 asymptomatic patients also significantly differed between conventional BIA and RXc graph hydration categories (chi(2 ) = 10.8, P = 0.005), since 11, 71 and 18% vs. 0, 43 and 57% of patients we re classified as over-, normo-, and underhydrated, respectively. Conclusion s: The classification of volume status based on conventional BIA was insens itive to either clinical situation (presence or absence of symptoms). in co ntrast, the classification based on the RXc graph was consistent with the c linical course in symptomatic patients (73% dehydrated, and 27% normohydrat ed), while it did not reflect the clinical course in asymptomatic patients, 57% of whom were classified as (already) underhydrated. A longitudinal stu dy will establish the clinical usefulness of RXc graph indications in asymp tomatic patients. Copyright (C) 2000 S. Karger AG, Basel.