Clinical review: Timing of renal replacement therapy

The primary goal of renal replacement therapy (RRT) is to compensate for, in part, the loss of renal function and associated sequelae. These include the accumulation of nitrogenous waste products, uraemic toxins, electrolyte disturbances, metabolic acidosis and volume overload. Organ support is much beloved by intensivists and, to an extent, defines us, but despite the introduction of convective therapies, RRT has changed little in the past 50 years. Furthermore, the use of current extracorporeal circuits does not compensate for other endocrinological and metabolic functions of the kidney.The cause of the acute kidney injury (AKI) necessitating RRT is also relevant. The unconditional acceptance of terms such as AKI must be considered together with the underlying aetiology in order to understand the basic pathological processes associated with kidney injury. Without an underlying cause, AKI tells us nothing save an observed disturbance in conventionally measured 'markers' of function coupled with reduced urine production. Clearly, the outcome from AKI in a young patient secondary to an interstitial nephritis is very different from that of an elderly diabetic developing AKI following systemic infection from a ruptured viscus. It may be that the aetiology of the underlying condition is also of great import with regard to timing of treatment. In many ways this highlights the differences between single organ 'AKI' and 'multiorgan AKI' in that timing of RRT on a renal unit may differ significantly from our patients on the ICU in terms of both dose delivered and duration of treatment.In ICU patients AKI is often encountered at an early stage before traditional measures of renal function are deranged. Therefore, symptoms may not be as pronounced compared to a patient developing renal failure prior to ICU admission. Furthermore, AKI may be regarded as a systemic disease, rather than organ failure in isolation characterised by a systemic inflammatory response with concomitant