Volume status in dialysis patients at extreme body habitus: a prospective observational study

Bioimpedance and absolute blood volume measurement are objective assessments of volume status. However, uncertainties remain in their application at extreme body habitus. We aimed to compare blood volume and bioimpedance-based volume status at extreme body habitus and explore normalisation of blood volume in this context to support incorporation of these tools into fluid management decisions.

We defined two common extreme volume-status phenotypes in haemodialysis: obese and bioimpedance-dry (n = 29) and lean and bioimpedance-wet (n = 29). Control subjects were haemodialysis patients who were haemodynamically stable with normal volume status (n = 20). We compared post-dialysis blood volume at clinical "target weight", measured by dialysate dilution, in the obese/dry and lean/wet groups with controls. We explored normalisation of blood volume (n = 98) by body mass, lean tissue mass and an anthropometric-based estimate of blood volume determined by Lemmens' formula.

Post-dialysis blood volume normalized for body mass (specific blood volume) was lower in the obese/dry (57.9 mL/kg; P = 0.02) and higher in the lean/wet group (84.5 mL/kg; P = 0.001) than in controls (68.0 mL/kg). These differences were not present for lean tissue mass-normalised or Lemmens' formula-normalised blood volume. Normalising blood volume to body mass results in differences by sex and body mass, whereas normalising to Lemmens blood volume, which accounts for fat mass variation, does not.

Setting target weights to achieve fixed reference bioimpedance or blood volume values may not be appropriate at extreme body habitus, reinforcing the need for individualised approaches. Normalising blood volume to body mass may exacerbate established measurement bias observed with bioimpedance.