Continuous Renal Replacement Therapy (CRRT) is a specialized method of blood purification for severely ill patients. It functions as a gentle, continuous form of dialysis, filtering waste products and excess fluid from the blood. This approach differs from standard, intermittent dialysis, which performs similar functions over much shorter sessions. CRRT’s continuous nature is particularly beneficial for maintaining stability in patients who cannot tolerate rapid changes in fluid balance or blood pressure.
Conditions Requiring CRRT
CRRT becomes necessary for critically ill patients, particularly those experiencing acute kidney injury (AKI) where the kidneys suddenly lose their ability to filter blood effectively. These patients often have unstable blood pressure, a condition known as hemodynamic instability, which makes them unable to endure the abrupt fluid shifts associated with traditional, intermittent hemodialysis.
CRRT also manages severe fluid overload, common in patients with conditions like congestive heart failure or sepsis. It addresses electrolyte imbalances, such as high potassium (hyperkalemia) or abnormal sodium (hyponatremia or hypernatremia), by slowly adjusting these concentrations. Additionally, CRRT removes certain toxins or medications in cases of overdose or poisoning. This includes substances like lithium, salicylates, methanol, and ethylene glycol.
How CRRT Operates
CRRT operates as a continuous extracorporeal process, meaning blood flows outside the body through a circuit. A catheter is placed into a large vein, typically in the neck or groin, to draw blood from the patient. This blood then travels through the CRRT machine, where it passes through a filter known as a hemofilter.
Within the hemofilter, the blood is cleaned by removing excess fluid, waste products, and electrolytes. This purification occurs through two primary mechanisms: diffusion and convection. Diffusion involves the movement of solutes from an area of higher concentration in the blood to an area of lower concentration across a semi-permeable membrane, into a specialized fluid called dialysate. Convection involves the “dragging” of solutes along with water as it is pulled across the membrane under hydrostatic pressure, removing both fluid and dissolved substances. The filtered blood is returned to the patient through the same catheter.
Different CRRT Modalities
Various CRRT modalities are employed to optimize the removal of specific substances from the blood. Continuous venovenous hemofiltration (CVVH) primarily uses convection to remove both fluid and solutes, particularly larger molecules. This method involves a high volume of fluid removal, which is then replaced with a balanced solution to maintain the patient’s fluid status.
Continuous venovenous hemodialysis (CVVHD) focuses on solute removal through diffusion, utilizing a dialysate fluid that flows countercurrent to the blood across the filter. This technique is effective for clearing small to medium-sized waste products and correcting electrolyte imbalances. Continuous venovenous hemodiafiltration (CVVHDF) combines both convection and diffusion, offering a comprehensive approach to remove a wide range of waste products and manage fluid balance simultaneously. This combined modality utilizes both replacement fluid and dialysate. Slow continuous ultrafiltration (SCUF) is a simpler modality for removing excess fluid from the body, without significant removal of waste products. It employs only ultrafiltration to gently pull off fluid at a controlled rate, making it suitable for patients with severe fluid overload who do not require substantial solute clearance.
The Patient Experience During CRRT
Patients undergoing CRRT are typically cared for in an intensive care unit (ICU) due to the continuous nature of the therapy and close medical supervision. A catheter is inserted into a large blood vessel, which connects to the CRRT machine positioned at the bedside. This setup allows for the continuous circulation of blood through the machine for purification.
Throughout the treatment, which can last for several days or even weeks, patient vital signs, fluid balance, and blood work are continuously monitored by nurses and medical staff. Patients may be sedated to ensure comfort and minimize movement, which helps the CRRT machine operate smoothly, though some patients may remain awake. Nurses play a central role in managing the CRRT circuit, troubleshooting alarms, changing fluid bags, and adjusting machine settings to tailor the therapy to the patient’s changing needs. This constant oversight helps ensure the gentle and effective removal of waste and fluid, supporting the patient’s recovery until their kidneys regain function.