Measurement of intercompartmental fluid shifts during haemodialysis in children

Seven children (age range 12-19 years, post-dialysis weights 23-43 kg) were studied during 20 haemodialysis sessions. Impedance between wrist and ankl e (on the non-fistula side) was recorded using the Xitron 4000B analyser. A 2 ml sample of blood was taken for total protein and haematocrit from the arterial line at the start of dialysis. At approximately 20 minute interval s during dialysis, the time and volume of ultrafiltrate removed were record ed, and a simultaneous measurement of whole body impedance made over 25 log arithmically spaced frequencies in the range 5-500 kHz. A 2 ml sample of bl ood was also taken, from which serum protein and haemodialysis were calcula ted. Hypotensive episodes occured during four haemodialysis sessions. The p ercentage change in extracellular fluid (ECF) volume was calculated, at eac h sample time for each session, using the impedance measurements and ultraf iltration measurements (denoted DeltaV(i) and DeltaU respectively). Changes in the intravascular volume were estimated using measurements of haematocr it and serum protein (and denoted DeltaV(h) and Delta (p) respectively). Least-squares regression gave DeltaV(i) = 3.77 DeltaV(h), 1.33 DeltaV(p) an d 0.39 DeltaU, and r(2) = 0.72, 0.94 and 0.95 respectively (p < 0.0001 in e ach case) for the 16 dialysis sessions without hypotensive episodes. Simila r analysis of four dialysis sessions with hypotensive episodes gave similar relationships with col-relation coefficients 0.64, 0.97 and 0.94. These re lationships may not be accounted for by the anthropometric terms alone in t he impedance equations. Impedance measurements also detected the addition o f 300 mi isotonic saline given at the onset of each of the four hypovolaemi c episodes. The regression equations support the following hypothesis: during haemodial ysis, ultrafiltrate is removed from the intravascular volume but is repleni shed by fluid from the interstitial volume. The reduction in ECF volume mea sured by impedance (where the ECF comprises the intravascular and interstit ial volumes) DeltaV(i) is therefore greater than a DeltaV(h) and DeltaV(p), which only measure intravascular volume, but less than DeltaU since the EC F is replenished by fluid item the interstitial space. That a DeltaV(h) is greater than DeltaV(p) may be due to protein loss during dialysis. The resu lts suggest that whole body impedance measurements reflect changing body wa ter distribution during dialysis in children.