Preserving central blood volume: Changes in body fluid compartments duringhemodialysis

The understanding of fluid changes during hemodialysis (HD) is essential fo r reducing complications as well as efficacy of the procedure. Bioimpedance spectroscopy provides a noninvasive method of measuring total body water ( TBW), the distribution of intra (ICF) and extracellular (ECF) fluids, and t heir changes during HD. Segmental bioimpedance may be used to measure the s ame fluid shifts but from different body segments; the technique has previo usly been shown to compare well with whole body measures. It is possible th at fluid shifts occur differently in different body compartments during HD. Based on previous hemodynamic studies we postulated that during HD ultrafi ltration (UF) the body attempts to preserve its central blood volume (cardi opulmonary circulation plus great vessels), and thus fluid shifts would be greater from the periphery than from central compartments. To test this hyp othesis, segmental bioimpedance (Xitron Technologies, San Diego, CA) was pe rformed on 11 subjects undergoing HD where ECF and ICF values were obtained from the legs, arms and trunk before and after a period of UF. Blood volum e change (Delta BV%) was also followed using an on-line optical hematocrit (Hct) sensor (Crit-Line monitor, In-Line Diagnostics, UT) where Delta BV% = Delta .BV% (1 - Hct(1)/Hct(0)) x 100 (Hct(0) = baseline Hct; Hct(1) = post ultrafiltration Hct). The UF of 2.0 L +/- 0.79 L (M +/- SD) over 75 minutes was associated with a Delta BV% of -9.43% +/- 3.6% (M +/- SD), a significa nt (Student's paired t-test) reduction in total body (TB) ECF (p < 0.02), a weak correlation in reduction in TBW (p = 0.09) but not in TB ICF. The ECF reductions from the trunk, legs, and arms were all significant (minimum p < 0.02); no ICF changes from these compartments were significant. The amoun t of ECF reduction was greater from the legs (0.7 L +/- 0.6 L) than the arm s (0.12 L +/- 0.08 L) and trunk (0.2 L +/- 0.2 L) (all M +/- SD). Multiple regression analysis showed that TB ECF changes correlated strongly with leg (r = 0.94, p < 0.001) and arm (r = 0.72, p = 0.002) ECF changes but not wi th trunk changes. Delta BV% correlated weakly with leg (r = 0.45, p = 0.08) and arm (r = 0.42, p = 0.10) ECF changes but not with the trunk. As the De lta BV% represents the net volume change between OF and plasma water refill ing, this indicates that plasma water is being removed more from the periph eral compartments than from the trunk. These data suggest that plasma refil ling during HD to preserve central blood volume is more dynamic from the le g ECF than from elsewhere and may, in turn, explain the frequent occurrence of leg cramps during and after hemodialysis.