Solute mass balance during isovolaemic high volume haemofiltration

Objective: To evaluate the effect of changing the amount of pre-dilution replacement fluid on the sieving coefficient (SC) and mass transfer of small solutes during isovolaemic high-volume haemofiltration (HVHF). Design and setting: Prospective interventional study in the intensive care unit of a tertiary university hospital. Patients: Eight patients with septic shock. Interventions: Isovolaemic HVHF (6 1/h of replacement fluid) was performed. The proportion of replacement fluid delivered in pre-filter was altered to progressively decrease it from 6 to 0 l/h. Samples were simultaneously taken from the "pre-filter", "post-filter" and ultrafiltrate (UF) sampling ports. Measurements and results: Sodium, potassium, chloride, total calcium, total magnesium, phosphate, total CO₂, urea, creatinine and glucose concentrations were measured in each sample. The sieving coefficients of chloride, total CO₂, phosphate, urea and glucose were higher than 1 in most pre-dilution states. The sieving coefficients of sodium, potassium, calcium, magnesium, total CO₂ and urea decreased significantly with decreasing pre-dilution fluid rate. The sieving coefficients of chloride and glucose increased with decreasing pre-dilution fluid rate. There was a significant mass gain of sodium and glucose under all pre-dilution conditions. Mass chloride gains decreased with decreasing pre-dilution rates and changed into chloride loss during 6 l/h of post-dilution. Decreasing predilution improved urea and creatinine mass removal. Conclusions: Small solute SC and mass transfer during isovolaemic HVHF are significantly affected by the proportion of replacement fluid administered pre-filter. Isovolaemic HVHF is neither isonatraemic nor isochloraemic.