Nomenclature for renal replacement therapy in acute kidney injury: basic principles

Abstract

This article reports the conclusions of a consensus expert conference on the basic principles and nomenclature of renal replacement therapy (RRT) currently utilized to manage acute kidney injury (AKI). This multidisciplinary consensus conference discusses common definitions, components, techniques, and operations of the machines and platforms used to deliver extracorporeal therapies, utilizing a "machine-centric" rather than a "patient-centric" approach. We provide a detailed description of the performance characteristics of membranes, filters, transmembrane transport of solutes and fluid, flows, and methods of measurement of delivered treatment, focusing on continuous renal replacement therapies (CRRT) which are utilized in the management of critically ill patients with AKI. This is a consensus report on nomenclature harmonization for principles of extracorporeal renal replacement therapies. Devices and operations are classified and defined in detail to serve as guidelines for future use of terminology in papers and research.

Keywords: CRRT efficiency; CRRT membranes; CRRT modalities; Clearance; Convection; Diffusion; Dose; Terminology; Transmembrane pressure; Ultrafiltration.

Fig. 1

Schematic diagram of sieving coefficient profiles for low-flux (blue), high-flux (red) and high cut-off membranes (green)

Fig. 2

Practical example showing the different trends in efficiency (ml/kg/h, y axis) vs treatment time (h, x axis) during treatment with continuous renal replacement therapy (CRRT). Target efficiency (prescribed): “It is the amount of clearance prescribed for the specific patient in his/her specific clinical condition, and represents the amount of clearance that the doctor wants to achieve in that patient. Example: according to literature, the doctor decides that a dose of 35 ml/kg/h is the most adequate for his patient”. Target machine efficiency (set): “It is the amount of clearance that the physician wants to achieve in the machine. It is the only value that can be set in the machine. Example: taking into account the average downtime, the doctor sets the target machine dose to reach the target dose (prescribed). For example, to obtain a target dose (prescribed) of 35 ml/kg/h, the doctor sets flow rates and modalities to achieve a target machine dose of 40 ml/kg/h”. Current dose (estimated from treatment parameters): “It is the clearance at the present time, estimated considering the set flows in the extracorporeal circuit. During downtime, the current dose is zero. Example: based only on the instantaneous flow rates, the machine calculates the current dose at every moment of the treatment. A current dose of zero allows the user to recognize downtime”. Average dose (measured/calculated): “It is the clearance calculated for the current dose applied over the total time of treatment. Example: based on the total time of treatment and the current dose calculated at every moment, the machine displays the average dose. At a particular moment of the treatment, if the average dose equals 35 ml/kg/h (the target dose prescribed), the physician can assume that the patient is undertreated”. Projected dose (calculated/estimated): “It is the weighted-mean clearance that will theoretically be obtained at the end of the treatment. Example: based on the average dose obtained until a specific moment and the set target machine dose, the machine estimates the dose that theoretically will be obtained at the end of treatment session (24 h). At a particular moment during the treatment, if the projected dose is less than 35 ml/kg/h (target prescribed dose), the physician can assume that the patient will be undertreated at the end of the treatment”. Current effective delivered dose (measured): “It is the amount of clearance observed at every moment during treatment time. Unlike the current dose, it is based on blood concentrations. Example: the doctor now calculates actual blood clearance based on concentrations of solute markers. He often finds differences with the current dose (estimated from treatment parameters) because technical issues in the measurement of flow rates limit the accuracy of the estimation”. Average effective delivered dose (measured): “It is the clinically relevant amount of (measured) clearance delivered to the patient. It is calculated on the basis of the weighted-mean of the current effective delivered dose, over the total time of treatment until that specific moment”