Continuous Renal Replacement Therapy (CRRT) is a blood purification process for critically ill patients with kidney failure, often within an Intensive Care Unit (ICU). Unlike traditional dialysis, CRRT is a slow and continuous process, frequently running 24 hours a day. This gradual approach is better tolerated by patients who are hemodynamically unstable, meaning they have fluctuating blood pressure and heart rates. The therapy uses an external circuit of tubing and a filter to remove waste products and excess fluid, taking over the function of the kidneys.
Anatomy of the CRRT Circuit
Blood purification begins with vascular access through a specialized central venous catheter, often placed in a large vein in the neck, chest, or groin. This catheter has two separate lumens, or ports. One port, color-coded red, is the “arterial” access line that draws blood from the patient, while the blue port is the “venous” return line for the cleaned blood.
From the catheter, blood enters sterile tubing connected to the CRRT machine. A segment of this tubing fits into a blood pump, which pulls blood from the patient and propels it through the circuit at a controlled rate. This prevents the rapid fluid shifts that can occur with other forms of dialysis.
The hemofilter, sometimes called a dialyzer, is the circuit’s artificial kidney where blood is cleaned. Inside are thousands of microscopic, semi-permeable hollow fibers. As blood flows through these fibers, waste and excess water pass through the walls, while blood cells and important proteins are retained because they are too large to cross the membrane.
Connected to the CRRT machine and circuit are several bags of sterile fluids. One bag contains replacement fluid, used to replenish volume and essential electrolytes that might be lost during filtering. Another may contain dialysate, a special fluid that helps pull toxins from the blood. An effluent bag collects all the waste fluid, toxins, and excess water removed from the patient’s blood, which is then measured and discarded.
The Process of Blood Purification
The purification process begins when the CRRT machine’s pump pulls blood from the patient’s access port. The blood is directed through the tubing and into the hemofilter for cleansing. It is then guided through the return tubing and back to the patient, completing the circuit.
One purification mechanism is ultrafiltration, the removal of water from the blood. A pressure gradient created by the machine pushes fluid across the hemofilter’s membrane, similar to squeezing water from a sponge. The rate can be precisely controlled to slowly remove excess fluid from overloaded patients.
Another mechanism is convection, where dissolved waste products (solutes) are dragged along with the water removed during ultrafiltration. This “solvent drag” is effective at clearing medium-sized molecules. This is the dominant principle in a CRRT mode known as Continuous Venovenous Hemofiltration (CVVH), where large volumes of fluid are filtered from the blood and then replaced with a sterile replacement solution.
Diffusion is a third method, prominent in Continuous Venovenous Hemodialysis (CVVHD). This process works on concentration gradients, where waste products move from an area of high concentration (blood) to low concentration. To achieve this, dialysate is run on the outside of the filter’s hollow fibers, causing waste to diffuse into it, much like tea infusing from a teabag into hot water. Many therapies combine these principles in a mode called Continuous Venovenous Hemodiafiltration (CVVHDF) for broad-spectrum solute removal.
Monitoring Circuit Pressures
The CRRT machine has multiple pressure sensors that constantly monitor the circuit for safety. Alarms trigger if pressures go outside a preset range, alerting staff to potential problems. The primary pressure readings include:
- Access pressure: Measures how easily blood is being pulled from the patient’s catheter. This is a negative pressure reading, and if it becomes too negative, it can indicate a problem such as the catheter resting against a vessel wall, a kink in the line, or a small clot obstructing flow.
- Pre-filter pressure: Measures the force required to push blood into the hemofilter. As the filter clogs with microscopic clots, more pressure is needed to get blood through it. A steadily rising pre-filter pressure is a reliable indicator that the hemofilter is nearing the end of its functional life.
- Return pressure: Measures the pressure as cleansed blood is sent back to the patient. An unusually high return pressure signals a downstream obstruction. This could be caused by a kink in the venous return line or a clot at the catheter’s venous port, making it difficult for blood to re-enter circulation.
- Transmembrane pressure (TMP): Represents the pressure difference between the blood and fluid compartments within the hemofilter. A rapidly increasing TMP is a strong sign that the filter’s pores are becoming clogged, which reduces filtration efficiency and indicates the circuit is failing.
Circuit Lifespan and Common Issues
A CRRT circuit does not last indefinitely and its lifespan is variable. The most common reason for a circuit to fail is clotting. Blood contacting the foreign surfaces of the plastic tubing and filter membrane can trigger the body’s natural clotting cascade, forming small clots that clog the hemofilter.
To combat this, patients on CRRT are given anticoagulants, which are medications that help prevent blood from clotting. These drugs, such as heparin or citrate, are continuously infused into the circuit to help keep the blood flowing smoothly and extend the life of the filter. Even with effective anticoagulation, however, clotting will eventually occur.
A circuit’s lifespan can range from a few hours to the manufacturer’s maximum, often around 72 hours. The main factor is the rate of clotting. Once rising pressure readings indicate the filter is too clotted to function, the entire circuit is replaced.
Beyond clotting, other issues can shorten the circuit’s use. Air can get into the lines, which triggers an air detector alarm and stops the machine to prevent air from reaching the patient. Additionally, problems with the patient’s vascular access catheter, such as positional issues or partial clots, can cause persistent pressure alarms that make it impossible to run the therapy effectively.