Continuous Renal Replacement Therapy (CRRT) is a blood purification method used for individuals with acute kidney injury (AKI), particularly those who are hemodynamically unstable. Unlike intermittent dialysis, CRRT works slowly over a 24-hour period, which helps maintain stability in critically ill patients. Among its specific methods are Arteriovenous Veno-Hemofiltration (AVVH) and Continuous Veno-Venous Hemofiltration (CVVH). Each uses a distinct technique to filter waste and fluid, offering different benefits and challenges in the intensive care setting.
The Mechanics of Arteriovenous Veno-Hemofiltration (AVVH)
Arteriovenous Veno-Hemofiltration operates by using the body’s own power to run the system. This technique requires access to both an artery and a vein. Blood is diverted from the patient through a catheter placed in a large artery, and the natural force of the patient’s blood pressure propels the blood through the external circuit and into a hemofilter.
Inside the hemofilter, a process called convection occurs where waste products and excess water are pushed across a semipermeable membrane and removed. This process does not require an external pump. After passing through the filter, the cleansed blood is returned to the patient through a separate catheter placed in a large vein. The system’s efficiency is directly linked to the patient’s cardiovascular stability, as a sufficient blood pressure is necessary to maintain adequate flow.
The Mechanics of Continuous Veno-Venous Hemofiltration (CVVH)
Continuous Veno-Venous Hemofiltration is a more technologically advanced approach that only necessitates access to the venous system. This is achieved with a single, specialized catheter that has two separate internal channels, called lumens, inserted into a large central vein. One lumen is used to draw blood from the patient, while the second lumen returns the filtered blood.
The defining feature of CVVH is its reliance on a mechanical blood pump. This pump actively pulls blood from the patient and pushes it through the hemofilter at a consistent and controllable rate. This external pump ensures that the therapy can proceed effectively, independent of the patient’s own blood pressure, allowing clinicians to tailor the treatment.
Comparing Vascular Access and Blood Flow Drivers
The differences in vascular access between the two modalities are significant. AVVH requires the cannulation of both an artery and a vein, a procedure that involves accessing two separate major blood vessels. In contrast, CVVH consolidates the process into a single site using a dual-lumen venous catheter, which simplifies the access procedure and provides two conduits within one device.
The driving force behind blood movement also starkly differs. AVVH is a passive system, entirely dependent on the patient’s mean arterial pressure (MAP) to push blood through the filter. Consequently, the rate of filtration and fluid removal can fluctuate if the patient’s blood pressure changes, making the treatment less predictable. CVVH operates as an active system, driven by an external blood pump. This mechanical pump provides precise control over the rate of blood flow, allowing clinicians to set and adjust this rate to achieve a consistent and predictable level of filtration.
Associated Risks and Clinical Considerations
The requirement of arterial access in AVVH introduces a specific set of risks. Placing a catheter in an artery carries a higher potential for complications compared to venous access. These include:
- Limb ischemia, a condition where blood flow to the limb is dangerously reduced
- Hemorrhage if the catheter dislodges
- Thrombosis (blood clots) at the insertion site
- Infection
AVVH’s dependence on the patient’s blood pressure is another clinical limitation. In hypotensive patients, blood flow through the filter can be insufficient, rendering the therapy ineffective. The passive nature of the system means that fluid and waste removal cannot be precisely controlled.
CVVH, while avoiding the hazards of arterial cannulation, has its own set of potential complications. These are related to its technology and venous access, such as air embolism if air accidentally enters the system or circuit clotting. The use of a large central venous catheter also carries its own risks, such as bloodstream infections and the formation of clots within the vein.
The Evolution Toward Veno-Venous Therapies
The progression from arteriovenous to veno-venous methods reflects a significant evolution in critical care medicine. The clinical community has largely moved away from AVVH due to its higher risk profile and lack of precise control. The complications associated with placing and maintaining a catheter in a major artery, such as bleeding and reduced blood flow to a limb, were substantial drivers for seeking safer alternatives. The unreliability of the therapy in patients with low blood pressure further limited its utility.
The development of reliable blood pumps and sophisticated dual-lumen venous catheters made CVVH a much safer and more effective option. By eliminating the need for arterial access, CVVH immediately reduced the most severe risks associated with the older technique. The ability of the pump-driven system to provide consistent and highly controllable blood flow allows for a more predictable and tailored treatment, solidifying veno-venous therapies as the standard of care.