Modalities of Continuous Renal Replacement Therapy: Technical and Clinical Considerations

THE CLINICAL APPLICATION OF CRRT—CURRENT STATUS Modalities of Continuous Renal Replacement Therapy: Technical and Clinical Considerations Jorge Cerda´* and Claudio Ronco† *Division of Nephrology, Albany Medical College and Capital District Renal Physicians, Albany, New York, and †Department of Nephrology, Ospedale San Bartolo, Vicenza, Italy ABSTRACT Continuous renal replacement therapies (CRRT) are con- hemodynamic stability, metabolic control to almost normal tinuous forms of renal functional replacement used to parameters, and removal of large-size toxins and cytokines. manage acute kidney injury (AKI) in the critically ill Moreover, CRRT allows better long-term clearance of patient. Depurative mechanisms include convection, diffu- small and middle molecules than other dialysis modalities. sion, and membrane adsorption utilizing high-flux highly This article focuses on the different modalities of CRRT permeable biocompatible dialysis membranes. The simulta- and reviews both the basic concepts and the newest neous infusion of replacement fluid permits fluid removal approaches to the management of the critically ill patient without intravascular volume contraction and better with AKI. Most hospital-acquired acute kidney injury (AKI) CRRT modalities require the use of high-flux, highly occurs in the intensive care unit (ICU) and is associated permeable biocompatible dialysis membranes. with elevated morbidity and mortality (1,2). Continuous Most systems are designed with a desire to ‘‘keep renal replacement therapies (CRRT) provide extracor- things simple’’ (Table 1); such need for simplicity is poreal treatment of these hemodynamically unstable, essential when these techniques are to be implemented critically ill patients, who are often hypercatabolic and by critical care personnel with variable degrees of dia- fluid overloaded. A recent international survey showed lytic expertise and nephrological support. Unfortu- that 80% of patients with AKI in the ICU are currently nately, the various system designs are often daunting to treated with continuous therapies, 17% with intermit- the uninitiated and inhibit a more widespread use of tent therapies, and 3% with peritoneal dialysis or slow their most powerful modality variations. continuous ultrafiltration (3). These novel techniques of When a new renal replacement therapy (RRT) is renal substitution therapy have permitted a conceptual started, practitioners must simultaneously decide on shift from renal ‘‘replacement’’ to renal ‘‘support’’ thera- dialysis modality, membrane biocompatibility, dialyzer pies (4), whereby the strategies to treat AKI have performance, and dialysis delivery (Table 2). CRRT become an integral part of overall critically ill patient has made possible the delivery of RRT to hemody- management, with ‘‘renal’’ and ‘‘nonrenal’’ applications namically unstable patients. In practice, hemodynamic such as sepsis and acute respiratory distress syndrome stability of the critically ill patient is the main deter- (ARDS). minant of the choice of dialysis modality (5) (Table 3). In their simplest definition, CRRTs are continuous In addition to the patient’s hemodynamic stability, forms of renal functional replacement used to manage the choice between the various RRTs rests on solute AKI in the critically ill patient. CRRT techniques com- clearance goals, volume control, and anticoagulation monly use three types of depurative mechanisms: con- (Table 4). vection, diffusion, and membrane adsorption. The This article will focus on the different modalities of simultaneous infusion of replacement fluid permits fluid CRRT and emphasize the basic concepts and the newest removal without intravascular volume contraction, met- approaches to this technology and its application in the abolic control to almost normal parameters, and ICU. The discussion will center on the use of diffusive removal of large size toxins and cytokines. By and large, and convective mechanisms, and briefly address the use of membrane adsorption as an additional valuable method of large molecule removal. Previous reviews Address correspondence to: Jorge Cerda´ , MD, FACP, FASN, (6,7) have discussed the fundamental operational Capital District Renal Physicians, 62 Hackett Boulevard, characteristics of CRRT and will not be reiterated here. Albany, NY 12209-1718, or e-mail: [email protected]. More recently, the acute dialysis quality initiative Seminars in Dialysis—Vol 22, No 2 (March–April) 2009 pp. 114–122 (ADQI) published a consensus on fluid (8) and volume DOI: 10.1111/j.1525-139X.2008.00549.x management in AKI (9) relevant to the present ª 2009 Wiley Periodicals, Inc. discussion. 114 MODALITIES OF CONTINUOUS RENAL REPLACEMENT THERAPY 115 TABLE 1. Characteristics of the ‘‘ideal’’ treatment modality not available (6,10,11). Their advantages include their of AKI in the ICU ease of setup and operation and low extracorporeal Characteristic volume. Unfortunately, AV systems require arterial cannulation and expose the patient to the risk of main Preserves homeostasis arterial damage, atheroembolism, or distal limb ische- Does not increase co-morbidity mia. Furthermore, AV systems cannot control blood Does not worsen patient’s underlying condition Is inexpensive flow adequately, as they are dependent on the patency Is simple to manage of the vessel and the patient’s hemodynamics, thus Is not burdensome to the ICU staff making dose and ultrafiltration (UF) management very From Lameire et al. (61) difficult. AKI, acute kidney injury; ICU, intensive care unit. Conversely, veno-venous (VV) systems offer a decreased risk of vascular damage, maintain blood flow TABLE 2. Considerations in renal replacement therapy for AKI independent of mean arterial pressure, and achieve greater blood flows regulated safely by blood pumps, Consideration Components Varieties thus obtaining higher clearances. For these reasons, VV Dialysis modality Intermittent hemodialysis Daily, every is preferred to AV modalities, and AV is not recom- other day, mended except in emergent situations where other alter- SLED natives are not accessible, such as unavailability of Continuous renal AV, VV appropriate blood pumps or venous access (6). Multiple replacement therapies Peritoneal dialysis VV access options are offered, as discussed elsewhere in Dialysis Membrane characteristics this issue (see section on Vascular Access). biocompatibility Dialyzer Efficiency performance Flux Dialysis delivery Timing of initiation Early, late Choice of CRRT Modality Intensity of dialysis Prescription versus The different modalities of CRRT (Fig. 1) are defined delivery by the main mechanism with which clearance is Adequacy of dialysis Dialysis dose achieved: simple diffusion (continuous veno-venous he- AKI, acute kidney injury; SLED, sustained low efficiency daily modialysis, CVVHD), convection (continuous veno- dialysis; AV, arterio–venous; VV, veno-venous. venous hemofiltration, CVVH), or a combination of both (continuous veno-venous hemodiafiltration, TABLE 3. Indications for specific renal replacement therapies CVVHDF). See Table 5. Therapeutic goal Hemodynamics Preferred therapy Many intensivists and nephrologists prefer to use CVVH in the belief that pure convection will remove a Fluid removal Stable Intermittent greater number of larger molecules than diffusion-based isolated UF CVVHD. Others argue that CVVHD is easier and, given Unstable Slow continuous UF Urea clearance Stable Intermittent the lack of comparative evidence, prefer this mode. Still hemodialysis a third school favors CVVHDF on the basis that with- Unstable CRRT out evidence, providing both modes are safe (12). The Convection: CAVH, choice of CRRT modality rests on the available equip- CVVH Diffusion: CAVHD, ment (membranes, pump systems) and appropriate CVVHD fluids and cost and conceptual considerations. Both: CAVHDF, The main feature of convective treatments is the use CVVHDF of high-flux membranes, characterized by high perme- Severe Stable ⁄ Intermittent hemodialysis ability for water as well as low- and middle-molecular hyperkalemia Unstable Severe metabolic Stable Intermittent hemodialysis weight solutes (in the range of 1000–12,000 Daltons) and acidosis Unstable CRRT high ‘‘biocompatibility.’’ It is a widespread opinion that Severe Stable ⁄ CRRT convective treatments like high-flux hemodialysis, hemo- hyperphosphoremia Unstable diafiltration, and hemofiltration offer a clinical advan- Brain edema Unstable CRRT tage over standard dialysis when considering From Murray et al. (62) physiological outcomes. The crucial point is that up until UF, ultrafiltration; CRRT, continuous renal replacement thera- now, studies have not been able to demonstrate supe- pies; CAVH, continuous arterio–venous hemofiltration; CVVH, continuous veno-venous hemofiltration; CVVHD, continuous riority of these techniques on morbidity, mortality, and veno-venous hemodialysis; CVVHDF, continuous veno-venous quality of life (6,13–18). hemodiafiltration. Comparison of CRRT with Other Renal Arterio–Venous (AV) or Veno-Venous (VV) Replacement Modalities Blood Circuits Continuous renal replacement therapies techniques Arterio–venous (AV) systems are not currently used offer better long-term clearance of small and middle except in emergent situations such as earthquakes or molecules than IHD or SLED. Liao et al. (19) modeled other environments where appropriate machinery is a comparison between IHD, SLED, and CVVH and 116 Cerda´ and Ronco TABLE 4. Advantages and disadvantages of various renal replacement modalities Use in hemodynamically Solute

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