Renal Replacement Therapy (RRT) is a treatment that takes over the function of kidneys when they are no longer able to clean the blood effectively. This life-sustaining intervention is used for individuals experiencing severe kidney failure. The process involves removing waste products, excess fluid, and balancing electrolytes.
The Underlying Need for Renal Replacement
Healthy kidneys perform several biological processes that maintain the body’s internal stability. They constantly filter the blood, removing metabolic wastes like urea and creatinine. Kidneys also regulate the balance of electrolytes, such as sodium, potassium, and calcium, and manage the body’s overall fluid volume. This careful regulation keeps blood pressure stable and allows nerves and muscles to function correctly.
When kidney function drops significantly, these wastes and fluids begin to accumulate, a toxic state known as uremia. This failure can be sudden (Acute Kidney Injury, AKI) or progress slowly over time, leading to End-Stage Renal Disease (ESRD) or stage 5 Chronic Kidney Disease (CKD). ESRD is defined by a glomerular filtration rate (GFR) typically below 15 mL/min/1.73 m². RRT becomes necessary to prevent life-threatening complications that arise from this systemic imbalance, such as dangerously high potassium levels or fluid buildup in the lungs.
Hemodialysis: Mechanism and Delivery
Hemodialysis (HD) is the most common form of RRT, where a machine cleanses the blood outside the body. The process relies on a dialyzer, often referred to as an artificial kidney, which contains thousands of tiny, hollow fibers. Blood flows on one side of a semi-permeable membrane while a specialized cleaning fluid called dialysate flows on the other side. Waste products and excess fluid move from the blood into the dialysate through the principles of diffusion and ultrafiltration.
Diffusion is the movement of solutes, like urea and potassium, from the area of higher concentration (the blood) to the area of lower concentration (the dialysate). Ultrafiltration is the process of removing excess water, achieved by applying pressure across the membrane. The cleansed blood is then returned to the patient’s bloodstream. This procedure requires a vascular access site to allow high volumes of blood to flow continuously to the machine.
The long-term vascular access methods include an arteriovenous (AV) fistula, a surgical connection between an artery and a vein, or an AV graft, which uses a synthetic tube to join them. The fistula is preferred because it has a lower risk of infection and clotting. A temporary venous catheter is sometimes used when immediate dialysis is required before a permanent access is ready. HD treatments are most often performed in a center, typically three times a week for several hours, but can also be done at home.
Peritoneal Dialysis: Process and Types
Peritoneal Dialysis (PD) uses the body’s own peritoneal membrane, the lining of the abdominal cavity, as a natural filter. A soft tube, or catheter, is surgically placed into the abdomen to deliver and drain the dialysate fluid. The dialysate is introduced into the peritoneal cavity, where it “dwells” for a specific period. During this dwell time, waste products and excess water pass from the blood vessels in the peritoneum, across the membrane, and into the dialysate.
The process of filling the abdomen, allowing the fluid to dwell, and then draining the fluid is called an exchange. The used dialysate, which now contains the filtered wastes, is drained and discarded before new dialysate is put in. This method allows for continuous cleansing of the blood, offering greater flexibility than intermittent hemodialysis.
There are two primary types of peritoneal dialysis. Continuous Ambulatory Peritoneal Dialysis (CAPD) is performed manually without a machine, requiring the patient to perform three to five exchanges throughout the day. Each exchange takes approximately 30 to 40 minutes. Automated Peritoneal Dialysis (APD), also known as Continuous Cycling Peritoneal Dialysis (CCPD), uses a machine called a cycler to perform the exchanges automatically, usually at night while the patient sleeps. The cycler delivers and drains the fluid, allowing the patient to be dialysis-free during the day.
Continuous Therapies and Timing for Initiation
Continuous Renal Replacement Therapy (CRRT) is a specialized form of RRT primarily used in the Intensive Care Unit (ICU) setting. Unlike the intermittent schedule of standard hemodialysis, CRRT operates continuously, often over 24 hours a day. This slower, gentler method of filtration is designed for patients with Acute Kidney Injury (AKI) who are hemodynamically unstable. Their blood pressure is too low or variable to tolerate the rapid fluid and solute shifts of conventional dialysis. CRRT slowly removes fluid and waste, which minimizes the risk of complications such as hypotension.
The decision to initiate RRT is guided by specific medical criteria. These indications are life-threatening complications that cannot be managed with medication alone. Absolute indications include severe uremia (causing symptoms like pericarditis or encephalopathy) and refractory fluid overload leading to pulmonary edema. Severe electrolyte imbalances, such as high potassium levels (hyperkalemia) or uncorrectable metabolic acidosis, also prompt immediate RRT initiation. For chronic kidney disease, RRT is typically prepared for when the GFR falls below 15 mL/min/1.73 m², though the presence of these severe symptoms often dictates the final timing.