Does Increasing Dialyzer Blood Flow Always Improve Dialysis Efficiency?

As dialyzer blood flow is increased during hemodialysis, diminishing increments in clearance are inevitable. In addition, as clearance increases, diminishing increases in solute removal from the patient are inevitable. The causes of these equally self-defeating and additive effects are the fundamental self-limitation of the dialysis itself due to first-order kinetics, membrane-limited diffusion within the dialyzer, and disequilibrium within the patient. Access recirculation is a specialized cause of solute disequilibrium that is separately measurable and preventable. Cardiopulmonary recirculation (CPR) is a predictable form of solute disequilibrium that is found in all patients with peripheral arteriovenous shunts and is absent during vein-to-vein dialysis. Other forms of blood flow-dependent disequilibrium probably also play a role in diminishing the efficiency of hemodialysis. Sequestration of urea in muscle during hemodialysis is suggested by reduction in the magnitude of rebound when patients exercise (and increase muscle blood flow) during hemodialysis. This discussion is not intended to discourage attempts to increase solute removal by increasing blood flow, but rather to place this maneuver in a proper perspective. Other maneuvers such as increasing dialysis frequency may be more effective as a means of improving dialysis efficiency.