Pannu and Gibney often been based on a concern that dialysis itself may delay recovery of renal function. These fears have been largely dispelled by a recent study by Schiffl et al (2002), which compared outcomes in patients with ARF treated with daily vs alternate day dialysis for patients with ARF. Daily dialysis was not associated with a delay in renal recovery or an adverse effect on patient outcome. What types of renal replacement therapy are available for use in the intensive care unit? The available modalities of renal replacement therapy include: • peritoneal dialysis (PD) intermittent hemodialysis (IHD) continuous renal replacement therapies (CRRT). Peritoneal dialysis Peritoneal dialysis uses the peritoneum as a natural semi- permeable membrane for diffusive removal of solutes. It is a very effective treatment modality in patients with chronic renal failure, and patient outcomes are at least equivalent to those treated with hemodialysis (Held et al 1994; Murphy et al 2000). Peritoneal dialysis is also valuable in pediatric critical care where vascular access is challenging and peritoneal surface area is relatively larger than in adults (Williams et al 2002). In adult patients, acute peritoneal dialysis is not widely used. The use of peritoneal dialysis is limited by both logistical and practical considerations. Acute peritoneal dialysis requires surgical insertion of a peritoneal dialysis catheter, requiring the additional involvement of a surgical team. Acute PD is frequently complicated by catheter leakage and malfunction. In addition, the use of PD is limited by low solute clearance in hypercatabolic patients, potential pulmonary restriction due to expansion of the peritoneal cavity, and its contraindication in postoperative patients who require abdominal surgery or surgical drains (Phu et al 2002). A study comparing PD with CRRT in critically ill septic patients with ARF showed more rapid correction of acidosis, solute clearance, and significantly improved survival with CRRT (Phu 2002). Intermittent hemodialysis Hemodialysis is a process of solute clearance based on diffusion across the membrane driven by a concentration gradient between the blood and dialysate. The total amount of solute transported per unit of time (clearance), depends on the molecular weight of the molecule, membrane characteristics (dialysance), dialysate flow, and blood flow. In general, intermittent dialysis is prescribed for 3–6 h per treatment. Chronic hemodialysis patients are treated three times per week, the adequate dose for IHD in patients with ARF has not yet been determined. Slow efficiency daily dialysis (SLEDD) or EDD (extended daily dialysis) is a variant of IHD where the duration of dialysis is extended to between 8 and 12 h, the blood flow is lowered, fluid removal is more gradual, and solute clearance slower. SLEDD is associated with less hemodynamic instability than IHD and provides excellent solute control (Kihara et al 1994). This modality may have several advantages over continuous renal replacement therapies with respect to cost and improved patient mobility, however the two therapies have never been compared directly in a clinical trial. Continuous renal replacement therapy (CRRT) has become increasingly popular for the management of ARF in the critically ill, and is now used to the exclusion of IHD in Australia (Mehta and Letteri 1999; Silvester et al 2001; Mendelssohn and Hyman 2002). Continuous renal replacement therapies (CRRT) CRRT is any renal replacement therapy that is intended to be applied for 24 h per day in an ICU. The term CRRT describes a variety of blood purification techniques, which may differ significantly according to the mechanism of solute transport, the type of membrane, the presence or absence of dialysate solution, and the type of vascular access. CRRT provides slower solute clearance per unit time as compared with intermittent therapies but over 24 h may even exceed clearances with IHD. Solute removal with CRRT is achieved either by convection (hemofiltration), diffusion (hemodialysis), or a combination of both these methods (hemodiafiltration). Hemodialysis most efficiently removes small molecular weight substances such as urea, creatinine, and potassium. Middle and larger molecular weight substances are more efficiently removed using hemofiltration as compared with dialysis. During hemofiltration, hydrostatic pressure causes the filtration of plasma across a semi-permeable membrane. Solutes are dragged across the membrane along with the plasma resulting in convective transport of solutes in the 142 Therapeutics and Clinical Risk Management 2005:1(2)