Continuous hemofiltration and hemodialysis procedures filter and dialyze blood without interruption. (See Overview of Renal Replacement Therapy for other renal replacement therapies.) The principal advantage is the ability to remove large volumes of fluid while avoiding the hypotensive episodes caused by intermittent hemodialysis and its intermittent removal of large volumes of fluid. These procedures are therefore indicated for managing patients with acute kidney injury who are hemodynamically unstable, who must receive large volumes of fluid (eg, patients with multiple organ system failure or shock who require hyperalimentation or vasopressor drips), or both.
In continuous hemofiltration, water and solutes up to 20,000 daltons in molecular weight filter from the blood by convection through a permeable membrane; the filtrate is discarded, and the patient must receive infusions of physiologically balanced water and electrolytes. A dialysis circuit can be added to the filter to improve solute clearance. Procedures may be
In arteriovenous procedures, the femoral artery is cannulated, and arterial pressure pushes blood through the filter into the femoral vein. Filtration rates are typically low, especially in hypotensive patients. (See Arteriovenous Fistula.)
In continuous venovenous procedures, a pump is required to push blood from one large vein (femoral, subclavian, or internal jugular) through the dialysis circuit and back into the venous circulation. Using a double-lumen catheter, blood is drawn from and returned to the same vein.
The arteriovenous route has the advantage of a simple system without the requirement of a pump but may give unreliable blood flows in hypotensive patients. Advantages of the venovenous route include better control of blood pressure and filtration rate with smoother removal of fluid. Also, the venovenous route requires cannulation of only one vessel. Neither procedure is proven more effective than the other.
Both procedures require anticoagulation, most commonly regional rather than systemic. With regional citrate anticoagulation, blood leaving the patient is infused with citrate, which binds calcium to prevent coagulation; calcium is then reinfused as the blood returns from the machine to the patient. This method avoids the complications of systemic heparinization. However, not all patients can receive citrate (1).