Effect of change in fluid distribution in segments in hemodialysis patients at different ultrafiltration rates on accuracy of whole body bioimpedance measurement.

This study explored divergence (error) between ultrafiltration volumes (UFV) and intradialytic changes in extracellular volume (ΔECV) in hemodialysis (HD) patients measured by whole body (wBIS) and sum of segmental bioimpedance spectroscopy (sBIS). The primary aim of the study was to evaluate the effect of different ultrafiltration rates (UFR) on error of estimation of ΔECV by changes in their distribution in body segments (arm, trunk, and leg). Forty-four HD patients (26 men, age 63.5 ± 14.3 yr) were studied twice in the same week following high and low UFR treatments. ΔECV and distributions (segmental ΔECV/Σsegmental ΔECV, %) in arm, trunk, and leg were measured. ΔECV by wBIS underestimated UFV (0.58 ± 0.43 in high vs. 0.36 ± 0.5 liters at low UFR; P < 0.001, respectively); however, using sBIS no significant difference between UFV and ΔECV was present. Divergence using wBIS but not sBIS correlated positively with UFR. ΔECV distribution in trunk and leg at high UFR (44.1 ± 8.3, 47.2 ± 8.5, %) differed significantly (P < 0.01) from low UFR (36 ± 15.7, 53.8 ± 14.7) respectively, but in arm did not differ between UFR. Primary sources of whole body resistance are arms and legs. Due to different cross-sectional areas between trunk and limbs, wBIS is insensitive to detection of changes in trunk volume. At higher UFR, plasma water was rapidly and largely removed from the trunk but with only a small change in whole body resistance. As a result, accuracy of estimation of ECV by wBIS is further decreased by high UFR, while sBIS remains accurate using separate measurements of segmental volumes.