Continuous hemofiltration and hemodialysis procedures filter and dialyze blood without interruption. (See Overview of Renal Replacement Therapy Overview of Renal Replacement Therapy Renal replacement therapy (RRT) replaces nonendocrine kidney function in patients with renal failure and is occasionally used for some forms of poisoning. Techniques include continuous hemofiltration... read more 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 Hemodialysis In hemodialysis, a patient’s blood is pumped into a dialyzer containing 2 fluid compartments configured as bundles of hollow fiber capillary tubes or as parallel, sandwiched sheets of semipermeable... read more and its intermittent removal of large volumes of fluid. These procedures are therefore indicated for managing patients with acute kidney injury Acute Kidney Injury (AKI) Acute kidney injury is a rapid decrease in renal function over days to weeks, causing an accumulation of nitrogenous products in the blood (azotemia) with or without reduction in amount of urine... read more 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 Arteriovenous Fistula An arteriovenous fistula is an abnormal communication between an artery and a vein. An arteriovenous fistula may be congenital (usually affecting smaller vessels) or acquired as a result of... read more .)
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; particularly those with acute liver failure Acute Liver Failure Acute liver failure is caused most often by drugs and hepatitis viruses. Cardinal manifestations are jaundice, coagulopathy, and encephalopathy. Diagnosis is clinical. Treatment is mainly supportive... read more , due to decreased hepatic citrate metabolism which causes increased risk of citrate toxicity (1 General reference Continuous hemofiltration and hemodialysis procedures filter and dialyze blood without interruption. (See Overview of Renal Replacement Therapy for other renal replacement therapies.) The principal... read more ).
1. Acute Kidney Injury Work Group: KDIGO (Kidney Disease: Improving Global Outcomes) clinical practice guideline for acute kidney injury. Kidney Int Suppl 2(1):89-115, 2012.