Aggressive zero balance ultrafiltration on CPB in patients with renal failure may cause cerebral edema: a theoretical analysis.

The objective of this study was to determine the brain volume changes that occur secondary to hemofiltration during cardiopulmonary bypass in patients with renal failure. We hypothesized that in patients with elevated urea levels, quick aggressive hemofiltration could be associated with cerebral edema. We constructed a simple two-compartment model similar to the urea kinetic model developed by Depner. Intracellular urea exit was assumed to be minimal based on known urea redistribution times. Calculations were based on a 70-kg patient, with an intracellular volume of 25 L, extracellular volume of 15 L, and a preoperative urea of 40 mmol/L filtered to a post-procedure urea of 6 mmol/L. Analysis showed that a standard size 1500-mL human brain filtered from a preoperative urea of 40 to 6 mmol/L over a short period will expand by 59 mL secondary to the osmotic disequilibrium secondary to hemofiltration (p < .05). The higher the preoperative urea, the larger the fluid shift. This figure does not include the cerebral edema component that is known to arise secondary to cardiopulmonary bypass. Significant cerebral edema theoretically occurs secondary to hemofiltration during cardiopulmonary bypass. More detailed mathematical urea kinetic analysis and clinical correlation are needed.