Available modalities of RRT in AKI treatment
Which modalities of RRT are available for treating AKI patients?
The modalities that could be used in the treatment of patients with AKI include:
- a. Conventional intermittent hemodialysis (IHD)
- b. Various types of continuous renal replacement therapies (CRRT) such as:
- • Continuous venovenous hemofiltration (CVVH)
- • Continuous venovenous hemodialysis (CVVHD)
- • Continuous venovenous hemodiafiltration (CVVHDF)
- c. Prolonged intermittent renal replacement therapies (PIRRT) that combine aspects of both IHD and CRRT, such as slow low-efficiency dialysis (SLED), slow continuous ultrafiltration (SCUF), or extended daily diafiltration.
- d. Peritoneal dialysis (PD).
The removal of solutes can be achieved by convection (hemofiltration), diffusion (hemodialysis), or the combination of the two methods (hemodiafiltration). The amount of solute transported per unit of time (clearance) depends on the molecular weight of the solute, the characteristics of the membrane, and both the dialysate and blood flows. IHD has been used widely for the last four decades to treat end-stage kidney disease (ESKD) and AKI.
- • Diffusive clearance is more effective for small-molecular-weight solutes such as potassium, urea, and creatinine.
- • Solutes with higher molecular weight (between 500 and 60,000 Da)—so-called middle molecules—are better removed by convection, where hydrostatic pressure forces plasma across a membrane. To provide effective solute clearance, the volume of plasma that must be removed by ultrafiltration is greater than the volume that can be tolerated, so it is replaced partially or completely with a hemofiltration solution. The solution can be infused pre- or post-filter. Intermittent ultrafiltration—in contrast to intermittent hemodiafiltration—can be done with the same machines as IHD, but is used specifically for volume removal. Most nephrologists use isolated ultrafiltration as a method of rapid fluid removal when the major indication for renal replacement or support is pulmonary edema or refractory congestive cardiomyopathy.
Extended daily dialysis (EDD), or SLED, differs from IHD in that dialysate and blood flow are intentionally kept low, but the duration of the treatment is extended. These hybrid modalities can be performed at night for 8 to 12 hours, using intensive care unit (ICU) staff, thereby eliminating an interruption of therapy, reducing staff requirements, and avoiding scheduling conflicts. Studies comparing hybrid modalities to CRRT have revealed favorable hemodynamic tolerance in critically ill patients while achieving dialysis adequacy and ultrafiltration targets, since the fluid removal as well as the solute clearance is more gradual.
There are few studies that have compared PD with other RRT modalities for treating patients with AKI. A single-center randomized trial found that there were no differences in the mortality rate and the recovery of kidney function when PD was compared to daily IHD, although PD was associated with a shorter duration of need for dialysis. Another study, a meta-analysis of eight observational cohorts and four randomized, controlled trials, found no difference in mortality comparing PD to extracorporeal modalities of RRT.
Since RRT can be provided in various forms as shown above, one should consider the use of continuous and intermittent RRT as complementary therapies in AKI patients. CRRT should be used, rather than standard intermittent RRT, for hemodynamically unstable patients. The choice of a specific type of RRT should be based also on the availability of resources, the needs of the patient, and the expertise of the staff.
