Critical Care - Renal Replacement Therapy

Renal replacement therapy, or RRT, is a treatment that replaces certain functions of the kidneys, such as fluid and electrolyte balance, acid-base regulation, and excretion of nitrogenous waste products and toxins.

RRT is indicated in conditions where kidney function is impaired or absent, including acute kidney injury and end-stage renal disease, but it can also be used to treat other temporary serious conditions, like electrolyte imbalances, fluid overload, poisoning, or drug overdose.

RRT can be done continuously over many hours or days like in continuous renal replacement therapy, or CRRT, or intermittently, like in hemodialysis or peritoneal dialysis, also known as PD.

Okay, so to perform RENAL REPLACEMENT THERAPY, an access site is necessary to gain entry to the body. In hemodialysis, access to the bloodstream will vary depending on the type of therapy and whether RENAL REPLACEMENT THERAPY is needed temporarily or on a long-term basis.

Starting with temporary RENAL REPLACEMENT THERAPY, vascular access can be obtained through a percutaneous catheter within a large central vein such as the internal jugular vein or the femoral vein. This temporary vascular access can be used immediately after placement, so it’s often used in conditions that require emergency RENAL REPLACEMENT THERAPY.

For long-term hemodialysis access, an arteriovenous, or AV, fistula can be created. During this process, an artery and a vein are surgically connected, most commonly joining the radial artery and cephalic vein in the arm. This causes the vein to become “arterialized,” meaning it enlarges and develops thicker walls so it can withstand higher pressure and flow of blood; and it allows for the insertion of a large-bore needle to obtain arterial blood. This “maturing” process takes several weeks.

Another type of access is an AV graft, which also involves connecting an artery and vein, but is done using a synthetic tube. The graft will also take weeks to mature before it’s ready for use.

The final type of RENAL REPLACEMENT THERAPY access is for PERITONEAL DIALYSIS, which is obtained by surgically inserting a catheter directly into the peritoneal cavity.

Now, there are two main mechanisms that make RENAL REPLACEMENT THERAPY possible. First, diffusion is the process where nitrogenous waste products and other solutes are removed from the body as they move from a greater concentration, across a semipermeable membrane, to a lesser concentration, until the two solutions have the same concentration of solutes. On the other hand, ultrafiltration primarily involves fluid and certain low molecular weight particles, like electrolytes, moving across a semipermeable membrane using an osmotic gradient.

Okay, now, let’s review the types of RENAL REPLACEMENT THERAPY to understand diffusion and ultrafiltration in action. Hemodialysis is performed using a dialysis machine in acute care settings, outpatient centers, or in the home.

During therapy, the patient’s blood travels from their access site through one tubule and enters the dialyzer, sometimes called a hemofilter, which is the blood-filtering component of the machine that essentially acts as an artificial kidney. The dialyzer has two compartments, separated by a semipermeable membrane. The patient’s blood enters on one side and the other side contains the dialysate, which is a type of solution that creates the pressure gradient that makes diffusion and ultrafiltration happen.

As blood moves through the dialyzer, cellular components and proteins stay within the blood, while excess fluid, electrolytes, and nitrogenous wastes like urea and creatinine move into the dialysate solution on the other side of the semipermeable membrane. Once the blood passes through the dialyzer, a second tubule returns the filtered blood from the dialyzer back to the patient, and the used dialysate is discarded.

Now, fluids and electrolytes typically shift somewhat quickly during hemodialysis, which usually takes 3 to 4 hours. So, patients who are hemodynamically unstable may not be able to tolerate intermittent hemodialysis. Instead, these patients will undergo CRRT, which slows the process of fluid and solute removal, so the shifts in blood composition occur more gradually, usually over 24 hours.

Another type of dialysis is PERITONEAL DIALYSIS, which uses the peritoneal membrane, which surrounds the abdominal cavity and lines the abdominal organs, as the semipermeable membrane through which diffusion and ultrafiltration occurs. During PERITONEAL DIALYSIS, dialysate is infused by gravity through a catheter that’s placed into the abdominal cavity. The dialysate dwells in the peritoneal space for about 4 to 6 hours, depending on the patient’s individual needs.