**2. What are the types of renal replacement techniques?**

#### **2.1 What is renal replacement therapy?**

Renal replacement therapy (RRT) is for patients with severe kidney injury. Hemodialysis is a method that uses a dialysis machine to filter the patient's blood and remove waste products and any volume surplus. The machine uses catheters that fit into the venous system, arteriovenous fistula (AVF) or arteriovenous grafts (AVGs) that will drain the blood and replace it after it is cycled through.

Blood flows through the hemofilter and comes to a semipermeable membrane, using the process of diffusion to separate waste and volume from the patient's blood. The smaller contents such as water and electrolytes can be filtered through and larger molecules such as cells and proteins are maintained in the blood. The semipermeable membrane has different pore sizes that can determine what is able to move through, 10–100 angstroms that will allow smaller molecules <5000 Da.

For diffusion to occur a change in concentration must pull certain toxins and water from the blood out of the circulation. A dialysate fluid is used around the semipermeable membrane to provide the concentration difference. Dialysate fluid usually uses bicarbonate as a buffer such that bicarbonate can be added to the blood; it can have low levels of potassium so that potassium from the blood can be pulled out. Dialysate runs countercurrent to the plasma to maximize the solute difference on both sides.

Multiple modalities of RRT are available, categorized as intermittent hemodialysis (IHD) and continuous renal replacement therapy (CRRT). IHD can be used in patients who are more hemodynamically stable. It can also be used for patients on lower dosing and stable requirement of pressors in ICU at the discretion of nephrologist and critical care physician. They get HD at different intervals depending on need and renal function. CRRT or continuous dialysis is usually 24 hours a day and is used in an ICU setting. There are also hybrid therapies such as sustained low efficiency dialysis (SLED) and extended duration dialysis (EDD). Hybrid therapies are used infrequently, though, this tends to be ICU and institution specific.

CRRT machines can do hemodialysis as described above with diffusive clearance. They can also use hemofiltration which is a convective clearance. The fluids are removed with hydrostatic pressure so that all the toxins and electrolyte abnormalities are removed from the plasma. This can remove small, medium, or large solutes. The greater the fluid movement the more "solute drag" is had, where solutes are moved out of the plasma because of the force of fluid movement. Replacement fluid is then added back to the plasma. The fluid is physiological and contains electrolytes and proteins that are closer to what the body needs or should have. The replacement fluid will have normal concentrations of potassium, bicarbonate, etc. The replacement fluid can be added before the filtration which would dilute the plasma and cause less solute clearance. However, it can prevent clotting and preserve the filter. When added after filtration it is called post-dilution. This can allow for more solute clearance but a higher chance of clotting. A mix of both will allow for balancing the negative consequences. This added fluid will be pulled out through ultrafiltration.

Ultrafiltration works by having fluid cross a semi-permeable membrane. This is done in response to a pressure gradient that can be osmotic, oncotic, or hydrostatic. There can be positive pressure in the plasma pushing fluid out or negative pressure in the dialysate drawing fluid from the plasma. This creates a transmembrane pressure that can control how much fluid is being drawn from the blood.

There are multiple types of CRRT modalities depending on the needs of the patient. These include continuous venovenous hemofiltration (CVVH), continuous venovenous hemodialysis (CVVHD), and continuous venovenous hemodiafiltration (CVVHDF).

**CVVH** uses hydrostatic pressure with the concept of convection that removes solutes and dialysate fluid is not used. The fluid pulled from the plasma is high, about 20–25 ml/kg/h so there is significant volume depletion. This must be replaced according to the goal that is desired for maintaining even fluid balance or a net negative balance. The addition of fluid dilutes the elevated concentrations of solutes such as urea or creatinine. Predilution also allows urea to be moved out of RBCs into the plasma so once it goes through the filter it can be removed easier.

**CVVHD** removes solute by diffusion, in which dialysate is used. This is how hemodialysis is done generally and the dialysate fluid runs countercurrent at a rate of 1–2 l/h. The ultrafiltration in this setting is based on desired fluid removal that is desired; no IV fluid replacement is needed.

**CVVHDF** combines these two modalities. It uses replacement of fluid and dialysate to pull solutes as well as dilute the plasma. The ultrafiltration volume is variable and replacement fluid is used to maintain volume status.

**SLED**, or **sustained low-efficiency daily dialysis**, is a term to describe prolonged intermittent kidney replacement therapy (PIRKT). The indication is AKI requiring dialysis and they are patients who are too hemodynamically unstable to tolerate standard IHD. SLED is an alternative to CRRT. CRRT, blood pressures are more stable compared to standard intermittent RRT, because the rate of solute and fluid is slower. Mortality rates are comparable with other forms of RRT, including CRRT. PIKRT should be performed at least three times per week to provide adequate dialysis dose. The time per session ranges from 6 to 18 hours. But typically, is about 8 hours per session. Truly, the length of the session depends on the need of the patient and the hemodynamic stability. A systemic review by Aldahbi et al. published in 2021 [2] found no advantage of using CRRT over SLED in hemodynamically unstable AKI patients.

**SCUF (slow continuous ultrafiltration)** is a RRT technique that removes excess fluid and solutes from the blood in a gradual and continuous manner. This method operates at a slower rate, minimizing abrupt shifts in fluid and electrolyte balance and reducing the risk of hemodynamic instability in critically ill patients. Unlike CRRT, SCUF primarily focuses on fluid removal rather than solute clearance, making it a suitable choice for patients with fluid overload but stable solute levels. However, it may not be as effective in managing severe electrolyte imbalances or uremic toxins as other dialysis modalities.

**Peritoneal dialysis (PD)** is another modality that can be used. This involves using the patient's peritoneal membrane, the thin membrane that lines the abdominal cavity, as a filter to remove waste products and excess fluid from the body. A catheter with two tubes is placed surgically into the patient's abdomen; one tube is used for inserting the fluid and the other is used for draining the used solution. The solution contains a specific concentration of electrolytes and dextrose that pulls waste products and excess fluids. It remains in the cavity for a certain amount of time, known as the dwell time. This allows the peritoneal membrane to act as a semi-permeable

*Renal Replacement Therapy in Intensive Care Unit DOI: http://dx.doi.org/10.5772/intechopen.112139*



**Table 2.**

*RRT types along with their respective advantages and disadvantages [2, 3].*

membrane. After the dwell time, the dialysis solution that now has waste products and excess fluid is drained out by gravity using the cycler machine. This process of fluid going in, dwelling, and draining is repeated multiple times during the day or overnight depending on the patient's requirements. PD has certain advantages, namely: it does not require anticoagulation, it is better tolerated and is cheaper than using an expensive dialysis machine and dialysis nurse or tech. However, it's not always feasible to emergently place a PD catheter when emergency dialysis is required. It's preferred that at least 2 weeks be allowed before using the PD catheter after it is inserted. Because the solute and volume clearance are slow, it is not a desirable choice for life-threatening hyperkalemia or pulmonary edema. Patients who have had abdominal surgery or peritoneal scarring cannot use the peritoneum as a dialytic membrane (**Table 2**).
