Figure 193

In contrast to intermittent techniques, until recently, the terminology for continuous renal replacement therapy (CRRT) techniques has been subject to individual interpretation. Recognizing this lack of standardization an international group of experts have proposed standardized terms for these therapies [5]. The basic premise in the development of these terms is to link the nomenclature to the operational characteristics of the different techniques. In general all these techniques use highly permeable synthetic membranes and differ in the driving force for solute removal. When arteriovenous (AV) circuits are used, the mean arterial pressure provides the pumping mechanism. Alternatively, external pumps generally utilize a venovenous (VV) circuit and permit better control of blood flow rates. The letters AV or VV in the terminology serve to identify the driving force in the technique. Solute removal in these techniques is achieved by convection, diffusion, or a combination of these two. Convective techniques include ultrafiltration (UF) and hemofiltra-tion (H) and depend on solute removal by solvent drag [6].

Diffusion-based techniques similar to intermittent hemodialysis (HD) are based on the principle of a solute gradient between the blood and the dialysate. If both diffusion and convection are used in the same technique the process is termed hemodiafiltration (HDF). In this instance, both dialysate and a replacement solution are used, and small and middle molecules can both be removed easily. The letters UF, H, HD, and HDF identify the operational characteristics in the terminology. Based on these principles, the terminology for these techniques is easier to understand. As shown in Figure 19-1 the letter C in all the terms describes the continuous nature of the methods, the next two letters [AV or VV] depict the driving force and the remaining letters [UF, H, HD, HDF] represent the operational characteristics. The only exception to this is the acronym SCUF (slow continuous ultrafiltration), which remains as a reminder of the initiation of these therapies as simple techniques harnessing the power of AV circuits. (Modified from Mehta [7]; with permission.)

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