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 l/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 pre-dilution 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.