Renal replacement therapy

When the kidneys ultimately fail, RRT is needed. There are generally two forms of RRT utilized in the intensive care setting: hemodialysis and hemofiltration. In hemodialysis, blood is pumped through a semi-permeable filter which is bathed in a dialysate fluid. Electrolytes and fluid move down a concentration gradient into the dialysate fluid and it is removed, carrying off potassium, phosphate, urea, water, etc. Hemodialysis utilizes rapid blood flow rates over a 2-4 h duration and is performed on a daily or every other day basis. As such, it is felt to be associated with hemodynamic instability and large fluid shifts, which may not be tolerated in an unstable patient. That being said, slower forms of dialysis (sustained low-efficiency dialysis, SLED), which occurs over a longer time period, up to 12 h, is now being utilized in some centers. Peritoneal dialysis is generally not utilized in the ICU.

In hemofiltration, blood is pumped through highly permeable fibers with hydrostatic pressure driving water, urea, and electrolytes out to be collected as an 'ultrafiltrate.' There is no concentration gradient, so it is based on pressure. Generally, 1-61 of ultrafiltrate are generated each hour. At the same time that ultrafiltrate is being drawn off, fluid and electrolytes (in the amount the clinician determines) are added back to the now highly concentrated blood before its return to the body (Fig. 8.13). The blood flow for hemofiltra-tion is approximately 200 ml/h (slower than hemodialysis) and thus has the potential advantage of providing more hemodynamic stability. Proponents have also advocated that it provides better control over volume status and that it may help clear inflammatory mediators hastening the recovery of SIRS/sepsis. The disadvantages of hemofiltration is that it is a continuous 24 h process. This makes it work intensive and difficult for mobilizing a critically ill patient who may need studies, return to surgery, etc.

Blood from patient to CVVH circuit

Blood from patient to CVVH circuit

Figure 8.13. Schematic of CVVH. Blood is pumped from the patient through a filter while ultrafiltrate is removed and discarded. Replacement fluids are added to the concentrated blood before it is returned to the patient.

Filtered blood returned to patient

Figure 8.13. Schematic of CVVH. Blood is pumped from the patient through a filter while ultrafiltrate is removed and discarded. Replacement fluids are added to the concentrated blood before it is returned to the patient.

Hemofiltration was initially done by withdrawing blood from the arterial side and returning it to the venous side (continuos arterial venous hemofiltration - CAVH). This was fraught with complications such that continuous veno-venous hemofiltration (CVVH) is now the standard mechanism of filtration. A counter-current of dialysate fluid can also be run though the CVVH circuit to enhance electrolyte clearance. This is know as CVVH dialysis (CVVHD).

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