Continuous Veno-Venous Hemofiltration, or CVVH, is a continuous blood purification therapy used to support critically ill patients in the intensive care unit. It supports kidney function during acute kidney injury (AKI). Unlike traditional, short-burst treatments, CVVH operates around the clock, offering a gentler, more sustained approach to managing fluid and waste levels. This continuous nature is particularly beneficial for medically fragile patients who cannot tolerate the rapid shifts in fluid and solute concentrations caused by conventional methods. The goal of CVVH is to mimic the natural function of the kidneys by slowly and steadily removing excess fluid, waste products, and toxins from the bloodstream.
Defining Continuous Veno-Venous Hemofiltration
CVVH is a form of Continuous Renal Replacement Therapy (CRRT) designed to provide ongoing kidney support. It is employed when a patient’s kidneys cannot filter the blood effectively, leading to a dangerous buildup of waste products and fluid overload. It is primarily indicated for acute kidney injury, especially in patients who are hemodynamically unstable (low or fluctuating blood pressure).
The continuous nature of the therapy prevents large, rapid fluid and electrolyte shifts that can strain the cardiovascular system. This gentler pace helps maintain stable blood pressure and a more consistent internal balance. CVVH is also used to correct severe electrolyte imbalances, such as high potassium levels, or to manage significant fluid retention that does not respond to diuretic medications.
The treatment is temporary, used until native kidney function recovers or the patient stabilizes enough for other dialysis forms. The slow, steady removal of solutes and fluid supports the patient’s body while allowing the injured kidneys time to heal. Clinicians prescribe the therapy based on the patient’s body weight, with an effluent flow rate often targeted between 20 to 25 milliliters per kilogram per hour.
The Principle of Convective Clearance
CVVH cleans the blood using convection, often described as “solvent drag.” This differs from standard dialysis, which primarily uses diffusion. Convection relies on moving a large volume of plasma water across a semipermeable membrane; as the water moves, it physically pulls dissolved toxins and waste products along with it.
This movement is driven by hydrostatic pressure applied to the blood as it flows through the hemofilter. The pressure gradient forces the plasma water through the filter’s pores, creating a fluid waste product known as ultrafiltrate. This mechanism is effective at removing larger molecules, often referred to as middle molecules, which are not cleared as efficiently by diffusion-based methods.
Since a large volume of fluid is removed, a sterile replacement fluid must be added back to the circuit. This solution maintains fluid balance and replaces beneficial electrolytes removed during filtration. The replacement fluid can be infused either before or after the filter, a choice that affects the efficiency of solute clearance and the lifespan of the filter.
Treatment Logistics and Setup
CVVH requires a vascular access device, typically a dual-lumen central venous catheter. This catheter is usually inserted into a large vein, such as the internal jugular or femoral vein, to ensure a high and consistent blood flow rate. The dual-lumen design uses one channel to draw blood out and another to return the cleaned blood.
The blood flows through a continuous external circuit including a blood pump, the hemofilter, and a waste fluid collection system. Anticoagulation medication, such as citrate, is commonly infused into the circuit to prevent the blood from clotting within the filter and tubing.
Continuous patient monitoring of blood pressure, fluid balance, and electrolyte levels is required to ensure the therapy is effective and safe. Treatment continues day and night, sometimes for several days to weeks, until the patient’s own kidneys are functioning well enough to take over blood purification.