Which fluid space is affected by ultrafiltration during hemodiafiltration?

Ultrafiltration (UF) is a common procedure performed during almost all dialysis sessions. During UF, several liters of fluid are removed; however, what proportion of this fluid is removed from which fluid space could not be clinically measured easily until now; we designed this study to evaluate the fluid spaces most affected by UF. This is a prospective cohort study of 40 prevalent chronic hemodialysis patients receiving thrice weekly hemodiafiltration (HDF). We measured the patients' fluid spaces using a whole-body bioimpedance apparatus to evaluate the changes of fluid spaces before and immediately after the HDF sessions. We recorded the data on fluid spaces, UF volume, and blood pressures. The cohort consisted of 40 prevalent HDF patients, aged 60.0 ± 5.2 years (37.5% men; 27.5% people with diabetes), and body weight 71.03 ± 15.48 kg. Achieved UF was 2.38 ± 0.98 L on HDF (measured fluid overload: 2.35 ± 1.44 L). The extracellular fluid (EC) volume decreased from 16.84 ± 3.52 to 14.89 ± 3.06 L (P < 0.0001) and intracellular fluid (IC) volume from 16.88 ± 4.40 to 16.55 ± 4.48 L (P = 0.45). Although urea volume of distribution remained effectively unchanged (31.38 ± 7.28 vs. 30.70 ± 7.32 L; P = 0.45), the degree of EC volume overload decreased from 13.60% ± 7.30% to 3.83% ± 8.32% (P < 0.0001). The mean arterial pressure also decreased from 122.95 ± 19.02 to 108.50 ± 13.91 mmHg (P < 0.0001). We conclude that source of net fluid loss by ultrafiltration is almost exclusively the EC fluid space. The intracellular fluid space is not significantly affected immediately after HDF.