The optimal ratio of extracellular water to total body water (ECW/TBW) determined by bioelectrical impedance analysis (BIA) for setting dry weight in hemodialysis patients

Abstract

Recently, we have reported that bioelectrical impedance analysis（BIA）using a body composition analyzer （InBody S20）is useful for assessing the body fluid components of hemodialysis patients. The ratio of extracellular water（ECW）to total body water（TBW）appears to be a useful marker for estimating clinical dry weight（Cl-DW). The present study investigated the optimal ECW／TBW ratio for setting DW under various clinical conditions such as diabetes, heart failure, hypoalbuminemia, dialysis-related hypotension, obesity, long-term dialysis, and non-oliguric HD patients. The subjects comprised 57 patients who were undergoing maintenance dialysis. In addition to routine blood test, various body fluid components using an InBody S20 were measured before and after dialysis. Furthermore, we measured the cardiothoracic ratio（CTR）on chest Xp, and human atrial natriuretic peptide（hANP), brain natriuretic peptide（BNP), the inferior vena cava（IVC）diameter and the left ventricular ejection fraction（EF）on cardiac ultrasonography after dialysis. The ECW／TBW ratio significantly decreased after dialysis under all clinical conditions（p＜0.0001). The post-dialysis ECW／TBW ratio was negatively correlated with the serum albumin concentration（r＝−0.720, p＜0.0001),and it was higher in diabetic patients than in non-diabetic patients（p＜0.0001). Thus, we divided the patients into 4 groups based on the presence or absence of diabetes（DM）and hypoalbuminemia（hypo Alb). The post-dialysis ECW／TBW ratio was significantly higher in Group II（DM（−), hypo Alb（＋）；0.397±0.013, p＜0.001）and Group III（DM（＋), hypo Alb（−）；0.398±0.011, p＜0.001), compared to that in Group I（DM（−), hypo Alb（−）；0.384±0.005). The highest value was obtained from Group IV（DM(＋), hypo Alb(＋）；0.404±0.012, p＜0.0001 vs. Group I). There was a strong positive correlation between BIA-DW, which is equal to the BW when the ECW／TBW ratio is 0.380, and Cl-DW（r＝0.996, p＜0.0001). These values were almost identical in Group I. On the contrary, Cl-DW was approximately 0.5〜0.8 kggreater than BIA-DW in Group IIand Group III, and approximately 1 kg greater in Group IV. The post-dialysis ECW／TBW ratio did not correlate with blood pressure, heart function, duration of dialysis or urinary volume. In conclusion, the post-dialysis ECW／TBW ratio determined by BIA may be a useful marker for predicting DW in hemodialysis patients with various clinical backgrounds.