Dialysis Modalities For Acute Renal Failure

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Intermittent therapies

Continuous therapies

Hemodialysis (HD)

Peritoneal (CAPD, CCPD)

Single-pass

Ultrafiltration (SCUF)

Sorbent-based

Hemofiltration (CAVH, CVVH)

Peritoneal (IPD)

Hemodialysis (CAVHD, CVVHD)

Hemofiltration (IHF)

Hemodiafiltration (CAVHDF, CVVHDF)

Ultrafiltration (UF)

Several methods of dialysis are available for renal replacement therapy. While most of these have been adapted from dialysis procedures developed for end-stage renal disease several variations are available specifically for ARF patients [1] .

Of the intermittent procedures, intermittent hemodialysis (IHD) is currently the standard form of therapy worldwide for treatment of ARF in both intensive care unit (ICU) and non-ICU settings. The vast majority of IHD is performed using single-pass systems with moderate blood flow rates (200 to 250 mL/min) and countercurrent dialysate flow rates of 500 mL/min. Although this method is very efficient, it is also associated with hemodynamic instability resulting from the large shifts of solutes and fluid over a short time. Sorbent system IHD that regenerates small volumes of dialysate with an in-line Sorbent cartridge have not been very popular; however, they are a useful adjunct if large amounts of water are not available or in disasters [2]. These systems depend on a sorbent cartridge with multiple layers of different chemicals to regenerate the dialysate. In addition to the advantage of needing a small amount of water (6 L for a typical run) that does not need to be pretreated, the unique characteristics of the regeneration process allow greater flexibility in custom tailoring the dialysate. In contrast to IHD, intermittent hemodiafiltration (IHF), which uses convective clearance for solute removal, has not been used extensively in the United States, mainly because of the high cost of the sterile replacement fluid [3]. Several modifications have been made in this therapy, including the provision of on-line preparation of sterile replacement solutions. Proponents of this modality claim a greater degree of hemodynamic stability and improved middle molecule clearance, which may have an impact on outcomes.

As a more continuous technique, peritoneal dialysis (PD) is an alternative for some patients. In ARF patients two forms of PD have been used. Most commonly, dialysate is infused and drained from the peritoneal cavity by gravity. More commonly a variation of the procedure for continuous ambulatory PD termed continuous equilibrated PD is utilized [4]. Dialysate is instilled and drained manually and continuously every 3 to six hours, and fluid removal is achieved by varying the concentration of dextrose in the solutions. Alternatively, the process can be automated with a cycling device programmed to deliver a predetermined volume of dialysate and drain the peritoneal cavity at fixed intervals. The cycler makes the process less labor intensive, but the utility of PD in treating ARF in the ICU is limited because of: 1) its impact on respiratory status owing to interference with diaphragmatic excursion; 2) technical difficulty of using it in patients with abdominal sepsis or after abdominal surgery; 3) relative inefficiency in removing waste products in "catabolic" patients; and 4) a high incidence of associated peritonitis. Several continuous renal replacement therapies (CRRT) have evolved that differ only in the access utilized (arteriovenous [non-pumped: SCUF, CAVH, CAVHD, CAVHDF] versus venovenous [pumped: CVVH, CVVHD, CVVHDF]), and, in the principal method of solute clearance (convection alone [UF and H], diffusion alone [hemodialyis (HD)], and combined convection and diffusion [hemodiafiltration (HDf)]).

CRRT techniques: SCUF

A-V SCUF

V-VSCUF

Mechanisms of function Treatment Pressure profile Membrane Reinfusion Diffusion Convection

SCUF

TMP=30mmHg

High-flux

CRRT techniques: CAVH - CVVH

Treatment

CAVH-CVVH

CVVH 1R

V

®

H-J V

|Uf

Qb =

50-200 mL/min

Qf =

CAVH-CVVH

Mechanisms of function Pressure profile Membrane Reinfusion Diffusion Convection

TMP=50mmHg

High-flux Yes Low High

FIGURE 19-2

Schematics of different CRRT techniques. A, Schematic representation of SCUF therapy. B, Schematic representation of continuous arteriovenous or venovenous hemofiltration (CAVH/CVVH) therapy.

(Continued on next page)

CRRT techniques: CAVHD - CVVHD CAVHD CVVHD

CRRT techniques: CAVHD - CVVHD CAVHD CVVHD

Dial. in Dial. Out

Qb = 50-100 mL/min Qf=1 -3 mL/min Qb = 50-100 mL/min Qf=1-5 mL/min Qd= 10-20 mL/min Qd=10-30 mL/min

Mechanisms of function Treatment Pressure profile Membrane Reinfusion Diffusion Convection

ÇAVHD-CyVHD

TMP=50mmHg

Low-flux

High

CRRT techniques: CAVHDF - CVVHDF

CRRT techniques: CAVHDF - CVVHDF

Treatment

CAVHDF-CVVHDF

Treatment

CAVHDF-CVVHDF

Qb = 100-200 Qd=20-40 mL/min Qf = 10-20 mL/min Mechanisms of function Pressure profile Membrane Reinfusion Diffusion Convection

TMP=50mmHg

High-flux

High

High

FIGURE 19-2 (Continued)

C, Schematic representation of continuous arteriovenous/ venovenous hemodialysis (CAVHD-CVVHD) therapy.

D, Schematic representation of continuous arteriovenous/ venovenous hemodiafiltration (CAVHDF/CVVHDF) therapy. A—artery; V—vein; Uf—ultrafiltrate; R—replacement fluid;

P—peristaltic pump; Qb—blood flow; Qf—ultrafiltration rate; TMP—transmembrane pressure; in—dilyzer inlet; out— dialyzer outlet; UFC—ultrafiltration control system; Dial— dialysate; Qd—dialysate flow rate. (From Bellomo et al. [5]; with permission.)

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