A systematic evaluation of bioelectrical impedance measurement after hemodialysis session.

BACKGROUND: There is still no definitive indication about the ideal point of time to perform bioimpedance analysis (BIA) in hemodialysis patients. Furthermore, the interpretation of data in this regard is difficult because there is still no comprehensive information about the fluctuations in BIA variables occurring in these subjects. The aim of this study was to assess BIA changes occurring in hemodialysis and specifically in the dry-weight state.
METHODS: We studied 27 anuric patients (20 males and 7 females; age 56.1 +/- 13.7 years) on chronic hemodialysis. Single-frequency BIA (R, resistance; Xc, reactance; and PhA, phase angle) was performed (1) before and at the end of hemodialysis (dialysis period); (2) 15, 30, 60, 90, and 120 minutes after hemodialysis (postdialysis period); and (3) 24, 48, and 68 hours after hemodialysis (interdialysis period).
RESULTS: Body weight decreased by 2.8 +/- 0.8 kg during hemodialysis, was unchanged during the postdialysis period, and progressively rose during the interdialysis period. At the same time, BIA variables significantly increased during hemodialysis (R, 453 +/- 74 and 542 +/- 98 ohm; Xc, 38 +/- 10 and 53 +/- 16 ohm; P < 0.05), remained stable over the 120-minute period after treatment (R, 538 +/- 94, 539 +/- 95, 538 +/- 94, 541 +/- 95, and 544 +/- 95 ohm; and Xc, 53 +/- 15, 53 +/- 15, 51 +/- 16, 52 +/- 16, and 52 +/- 16 ohm; NS), and subsequently declined [R, 471 +/- 79 (P= <0.05 vs. postdialysis), 449 +/- 71 (P= <0.05 vs. postdialysis), 424 +/- 68 (P= <0.05 vs. postdialysis) ohm; Xc, 42 +/- 13 (P= <0.05 vs. postdialysis), 37 +/- 10 (P= <0.05 vs. postdialysis), 34 +/- 13 (P= <0.05 vs. postdialysis) ohm]. The stability of BIA measures during postdialysis was confirmed by the constant relationship found between R/height and Xc/height. Also PhA increased after dialysis (4.8 +/- 1.1 degrees vs. 5.7 +/- 1.3 degrees, P < 0.05), was unchanged during the following 120 minutes and decreased in the interdialysis period (5.1 +/- 1.3 degrees, 4.8 +/- 1.0 degrees, and 4.5 +/- 1.1 degrees, P < 0.05). At the end of hemodialysis and during the postdialysis period total body water (TBW) estimated from BIA was similar on average to TBW calculated using Watson formulas (37.2 +/- 6.3 L vs. 36.2 +/- 5.7 L, NS). On the contrary, when patients were hyperhydrated BIA significantly overestimated the Watson's values.
CONCLUSION: In hemodialysis patients BIA variables fluctuate to a considerable extent (with the highest values immediately after hemodialysis), but remain constant and highly reproducible over the 120 minutes after the end of hemodialysis, that is, in a dry-weight state. Thus, taking into consideration that the point in time chosen for performing BIA is crucial to properly assess body composition, BIA can be appropriately performed at anytime during the postdialysis period, provided that hydration status does not change due to food or drink consumption.