Continuous Renal Replacement Therapies - Dialysis

CONTINUOUS RENAL REPLACEMENT THERAPIES

Several types of continuous renal replacement therapy (CRRT) are available and are widely used in critical care units. CRRT may be indicated for patients who have acute or chronic renal failure and who are too clinically unstable for traditional hemodialysis, for patients with fluid overload secondary to oliguric (low urine output) renal failure, and for patients whose kidneys cannot han-dle their acutely high metabolic or nutritional needs. CRRT does not produce rapid fluid shifts, does not require dialysis machines or dialysis personnel to carry out the procedures, and can be ini-tiated quickly in hospitals without dialysis facilities.

CRRT methods are similar to hemodialysis methods in that they require access to the circulation and blood to pass through an artificial filter. A hemofilter (an extremely porous blood filter containing a semipermeable membrane) is used in all CRRT methods, which are described below (Astle, 2001).

Continuous Arteriovenous Hemofiltration

Continuous arteriovenous hemofiltration (CAVH) was firstused in 1977 to treat fluid overload. Blood is circulated through a small-volume, low-resistance filter, using the patient’s arterial pressure rather than that of the blood pump as is used in he-modialysis. Blood flows from an artery (usually by an arterial catheter in the femoral artery) to a hemofilter. A pressure gradi-ent is necessary for optimal filtration; cannulation of the femoral artery and vein provides the necessary gradient (difference) in ar-terial and venous pressures. The filtered blood then returns to the patient’s circulation through a venous catheter. Intravenous flu-ids may be administered to replace fluid removed by the proce-dure. With CAVH, there is no concentration gradient, so only fluid is filtered. Electrolytes are eliminated only as they are pulled along and removed with the fluid. Ultrafiltrate is collected in a drainage bag, measured, and discarded. CAVH is usually set up and initiated by trained dialysis staff and then maintained and monitored by critical care personnel.

Continuous Arteriovenous Hemodialysis

Continuous arteriovenous hemodialysis (CAVHD) has manyof the characteristics of CAVH but offers the advantage of a concentration gradient for faster clearance of urea. This is ac-complished by the circulation of dialysate on one side of a semi-permeable membrane. The blood flow through the system depends on the patient’s arterial pressure, as in CAVH; a blood pump is not used as it is in standard hemodialysis. CAVHD is usually set up and initiated by trained dialysis staff and then maintained and monitored by critical care personnel.

Continuous Venovenous Hemofiltration

Continuous venovenous hemofiltration (CVVH) is increas-ingly being used in managing acute renal failure. Blood from a double-lumen venous catheter is pumped (using a small blood pump) through a hemofilter and then returned to the patient through the same catheter (Fig. 44-7). CVVH provides continu-ous slow fluid removal (ultrafiltration); therefore, hemodynamic effects are mild and better tolerated by patients with unstable conditions. CVVH has several other benefits over CAVH in that no arterial access is required and critical care nurses can set up, initiate, maintain, and terminate the system.

Continuous Venovenous Hemodialysis

Continuous venovenous hemodialysis (CVVHD) is similar toCVVH. Blood is pumped from a double-lumen venous catheter through a hemofilter and returned to the patient through the same catheter. In addition to the benefits of ultrafiltration, CVVHD uses a concentration gradient to facilitate the removal of uremic toxins. Therefore, no arterial access is required, hemodynamic effects are usually mild, and critical care nurses can set up, ini-tiate, maintain, and terminate the system.