20/03/2016
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 same direction as water. This process requires the use of replacement fluid to prevent iatrogenic acidosis and electrolyte depletion as well as excessive fluid removal. The solutes in the removed filtrate are in the same concentration as those in the plasma, and solute concentration in the remaining plasma is diluted with substitution fluid. Combining diffusive and convective clearance with hemodiafiltration allows improved clearance of both small and large molecular weight substances. Using this method, blood urea nitrogen (BUN) clearances in the range of 23–30 mL/min can be achieved, even in hypotensive patients (McDonald and Mehta 1990).
The choice of modality is dependent on several factors including availability, cost, physician expertise, hemo-dynamic stability, and the primary purpose of the procedure (fluid removal vs solute clearance). There is currently only limited information comparing diffusive with convective blood purification techniques; results with CRRT techniques should be compared with those obtainable with IHD, which remains the gold standard therapy.
The most commonly applied modalities are continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), and continuous venovenous hemodiafiltration (CVVHDF). Arteriovenous (AV) modes of CRRT have been used in the past, whereby dialysis access was obtained through the femoral artery and the femoral vein. This type of CRRT used the patient’s own cardiac output to drive blood through the dialysis circuit. AV forms of CRRT have fallen out of favor in recent years due to the high access complication rate and the development of external circuit pumps.
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