Distinction between induction and maintenance dosing in continuous renal replacement therapy

Abstract

In their excellent review of dosing continuous renal replacement therapy (CRRT), Dr Prowle and colleagues concluded that patients should be prescribed 20 to 25 ml/kg/h [1]. However, by averaging CRRT dose over time, studies in this area obfuscate the benefits of appropriately higher dose therapy early in the course of illness, potentially misguiding clinicians into blindly adopting a 'one-size-fits-all' approach and consequently prescribing inadequate doses in life-threatening emergencies. To take a crude example, it would be inappropriate to prescribe 20 ml/kg/h CRRT in a patient with serum potassium 9 mmol/L. Rather, the highest possible dose of CRRT should be initially prescribed to maximize solute clearance. This depends on the maximum circuit flow permitted by the access catheter, which in turn determines the maximum dose, assuming that the countercurrent flow to blood flow ratio should be <0.3 with diffusive CRRT, or a filtration fraction with convective therapy <0.2 [2]. As the potassium level falls, the dose can be lowered to more conventional levels. Parallels could be drawn to general anaesthesia, where induction and maintenance are two distinct phases with different requirements. CRRT prescription could similarly be conceptualized as 'induction', where life-threatening abnormalities are corrected quickly with high-dose therapy, then 'maintenance' where solute clearance is achieved with more temperate doses (for example, 20 to 25 ml/kg/h) to avoid complications such as hypophosphataemia. It seems unlikely that this issue will be the subject of prospective research. Yet the principle that faster correction of life-threatening abnormalities leads to better patient outcomes seems both practical and intuitive.