Continuous arteriovenous hemofiltration in the intensive care unit

REVIEW

Continuous Arteriovenous Hemofiltration in the Intensive Care U n i t William C. Maguire and Robert J. A n d e r s o n

O N T I N U O U S arteriovenous hemofiltration (CAVH) is a powerful technique for blood purification and control of fluid and electrolyte imbalance in critically ill patients. The widespread availability of modern hemofilters and the wealth of published clinical experience with CAVH has elevated this modality from an experimental procedure to a routine part of our therapeutic armamentarium. This review describes briefly the theoretical basis of CAVH, the benelits that can be anticipated, and the complications to be avoided.

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HEMODIALYSIS A N D HEMOPERFUSION

Two extracorporeal methods of fluid removal and blood puritication, hemodialysis and CAVH, are currently used in the intensive care setting (Table 1). With hemodialysis, blood is pumped from the patient into a semipermeable dialysis membrane compartment, which is surrounded by dialysate. Solutes diffuse down a concentration gradient front the blood through the membrane into the dialysis solution. Blood is then returned to the patient. Fluid is removed during hemodialysis by either increasing the hydrostatic pressure within the dialyzer compartment or by reducing the hydrostatic pressure in the compartment surrounding the dialysate. Hemodialysis is an expensive therapeutic maneuver requiring the presence of a specially trained technician to oversee a complex machine. The technique is relatively inefficient for the removal of high molecular weight solutes. Probably because of the more rapid removal of dialyzable osmotically active solutes from extracellular than from intracellular fluid compartments, dialysis disequilibrium (headache, nausea, lethargy, From the Department o f Medicine. University o f Colorado Health Sciences Center, and the k'A Hospital Denver. Address reprint requests to Robert J+ Anderson, MD, University of Colorado Health Sciences Center. 4200 E. Ninth Ave, Box C280, Denver CO 80262. © 1986 b), Grune & Stratton, hw. 0883-9441/86/0101-0007505.00/0

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or seizures), resulting from cerebral edema, is occasionally seen with hemodialysis. Because blood is ultrafiltered at a rate independent of the patient's blood pressure and because of acetate intolerance (acetate is the base commonly used in dialysate), hypotension is frequently a problem in patients who already may be hemodynamicaily unstable. A stable blood pressure of at least 90 mm Hg systolic (to give at least 150 m L / m i n blood flow rate) is required for hemodialysis, and little fluid can be removed at low systemic arterial pressure. Whether acetate or bicarbonate baths are used, dialysis-induced hypoxemia due to pulmonary vascular white blood cell and platelet sequestration, as well as alveolar hypoventilation, can be expected to occur. Finally, the requirement for doses of heparin sufficient to produce systemic anticoagulation can lead to a significant risk of hemorrhage in patients with underlying coagulopathies or ongoing problems with bleeding. Continuous arteriovenous hemofiltration is an extracorporeal process in which fluid, electrolytes, and small- and medium-sized molecules are removed from the patient by ultrafiltration over an extended period of time (Fig !). A small filter containing a membrane highly permeable to water is used for filtration. The arterialto-venous blood pressure gradient of the patient usually generates sufficient flow (30 to 90 m L / min is adequate) and hydrostatic pressure to generate a significant ultrafiltrate. Solute removal in CAVH depends on convection of molecules carried in the bulk flow of fluids in a hydraulically driven ultrafiltrate of blood. In CAVH, it is the hydrostatic difference across the membrane that accounts for transmembrane flux. The hemofilter acts as a molecular seive, allowing soluble plasma elements of sufficiently small size to traverse the membrane along with plasma water while formed elements of the blood are retained. Solutes small enough to pass easily through the filter's pores thus are found in essentially the same concentration in the ultrafiltrate as in the plasma water. Journal of Critical Care, Vol 1, No 1 (March), 1986: pp 54-56

CONTINUOUS ARTERIOVENOUS HEMOFILTRATION Table 1. Comparison off Two E~traoorporeal Methods Currently Used for Emergent Bloot! Purification/Fluid Removal in the Intensive Care Unit

Continuous Arteriovenous

Primary mode of purification Duration Solute removalefficiency Small Large Volume removalat low perfusionpressure Cardiovascularinstability Hypoxemia Anticoagulantrequirement Dysequilibrium Need for specializedpersonnel Cost

Hemodialysls Hemoperfusion Diffusion Convection Intermittent Continuous High Lower

Lower

None Frequent Frequent High Frequent Yes Higher

Moderate None None Lower None No Lower

Higher

Through the process of ultrafiltration alone, large quantities of excess volume can be removed from a fluid overloaded patient. It is only by the administration of replacement fluid that the process of clearance is actually achieved. With replacement fluid, plasma concentrations of electrolytes, such as bicarbonate or potassium, can be made to increase or decrease as desired. Because of a smaller extracorporeal circuit and the lack of dialysate and blood pumps, CAVH is less expensive and requires less specially trained personnel. Another advantage of the CAVH system is that ultrafiltration rate is proportional to the patient's blood pressure. When blood

R e p l a c e m e ~ p a r i n solution ~, . . . . . . . u= I infusion ~ ([ [ ('~_.~.._ Venous lithe ~_~

~.~

infusion

~ " Arterial line

Filtrate line

To collection bags

~~

I

Urine meter

Fig 1. Schematic diagram of continuous arteriovenous hemofiltration system.

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pressure falls, ultrafiltration rate decreases. Because of the lack of a fixed rate of ultrafiltration, as well as the lack of surrounding dialysate and a smaller extracorporeal circuit, CAVH rarely results in hemodynamic instability. Moreover, blood flow rates as low as 20 to 50 m L / m i n can result in ultrafiltration rates as high as 300 to 500 mL/h. Unlike hemodialysis, there is no disequilibrium and no fall in platelets, leukocytes, complement components, or PO2 with CAVH. I Clinical experience with CAVH has shown that less heparin is needed to keep hemofilters clot free than that required for conventional hemodialysis. Adjusting heparin to keep Lee White clotting times between 30 and 45 seconds, Paganini and Nakamoto reported no change in systemic prothrombin time or PTT measurements and only two clotted filters in 142 hours of CAVH. 2 Filters lasted from 12 to 144 hours for Henderson et al, ~ who used heparin at 500 to 1,000 units/h. Using the same heparin dose, Synharvsky et al 4 reported an absence of clotting or systemic increase in PTT. The lower requirement for heparin in CAVH compared to hemodialysis is probably related to the smaller volume of extracorporeal blood and the smaller surface area of hemofilters compared to dialysis filters. In addition to rapidly removing extracellular fluid volume, CAVH can remove substantial amounts of solute. When used in the patient with renal failure, the level of the BUN in plasma during CAVH depends on the rate of urea nitrogen production and removal (residual renal function) and the amount of replacement fluid given. In patients who produce _< 10 g of urea nitrogen/ d, an exhange of 10 to 12 L in 24 hours is sufficient to maintain a BUN < 90 m g / d L and prevent the need for hemodialysis. In catabolic patients producing > 10 g / d of urea nitrogen, CAVH is often used in conjunction with hyperalimentation. The removal of a large volume of fluid by CAVH often allows use of sufficient alimentation fluid to promote anabolism and reduce the rate of urea nitrogen production. 5 TECHNICAL CONSIDERATIONS AND COMPLICATIONS

At present, CAVH is usually performed using a small filter (Amicon Diafilter, Danvers, Mass). Arterial and venous access can be obtained by

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insertion of pliable i 4 to 16 gauge plastic cannulae in the femoral artery and vein using the Seldinger technique. Femoral catheters provide the greatest rate of blood flow while A-V fistulas provide the least. ~ It has been suggested that Scribner shunt placement be considered when long-term CAVH is anticipated. Femoral catheters are considered preferable for short-term therapy although they may routinely be left in place for weeks. ~ Occluding thrombosis occurred less with large bore (0.3 cm internal diameter) catheters with minimal taper and no side holes than was seen when conventional catheters were used. Because most complications with femoral catheters occurred when being removed, it's suggested that routine changing be avoided and the catheters left in place for as long as possible, s'7 The lilter is usually primed with 1 or 2 L of normal saline containing 5,000 to 10,000 units of heparin. Subsequcntially, heparin at 10 I U / k g . h is infused into the arterial port of the circuit. The duration of CAVH and the type and quantity of the replacement fluid depends on the clinical circumstances. However, CAVH has been performed continuously for 50 days. Standard replacement fluid often consists of modified Ringer's lactate. Replacement fluid may be given in the venous line. However, it can be given into the arterial line, which will decrease hematocrit and thereby reduce filter clotting and enhance solute and volume clearance. Complications usually arise from vascular access (vascular compromise, bleeding, and infection) or from hemorrhagic complications arising from the anticoagulation. The main group at risk for bleeding complications from CAV H are patients with preexisting coagulopathies as with cirrhosis or burns, or those with previous hemorrhagic diathesis a n d / o r bleeds. Some general principles regarding bleeding complications of CAVH have been suggested by Kaplan et alg: (1) patients who bleed usually have a greater than 50% prolongation of PT or PTT, (2) patients who : we greater than 100 K platelets need heparin regardless of PT, PTT, and (3) patients with less than 100 K platelets may not need heparin at all. Significant infectious complications of CAVH have rarely been reported, although fevers are not uncommon. Catheter site infection is usually transient and treated by changing the access site.

MAGUIRE AND ANDERSON

Blood leaks can occur through separation of tubing, but this may be minimized by careful taping at connection sites. Breaks within the hemofilter can occur during saline priming with excessive pressure. INDICATIONS FOR CAVH Continuous arterial venous hemofiltration is a technique that can be kept in place for hours or days and is most useful for removal ofextraceilu-

lar fluid volume in patients refractory to standard methods of fluid removal, such as diuretics or hemodialysis. This technique has predominantly been used in the setting of tluid overload accompanying either acute renal failure or multiple organ failure. Overall, CAVH has been found to Oe a relatively safe and convenient method for providing continuous fluid, electrolyte, and acid-base balance in patients with inadequate renal function. This treatment technique is especially helpful in critically ill patients with hemodynamic instability or severe fluid overload. The techniques will control volume overload and facilitate electrolyte management in patients with need for large volumes of hyperalimentation fluid and either concomitant cardiac a n d / o r renal failure. REFERENCES I. Dodd N J, Vergani D, Turney JH, et al: Complement activation anti CIq binding activity in haemodialysis. Proc

Eur Dial Transplant Assoc 18:300-304. 1981 2. PaganiniEP, NakamotoS: Continuousslowultratiltralion in oliguricacute renal failure.Trans Am Soc Artif Intern Organs 26:201-204, 1980 3. HendersonAE, Donald LL. Levin NW: Clinical use of the amicon diafilter. Dial Transplant Assoc 12:523-525. 1983 4. SyndaivskyA, Kurtz SB. Wochos DN. et al: Acute renal failure treated by slowcontinuousultrafiltration.Mayo Clin Proc 58:729-733, 1983 5. Kramer P. Bbhler J. Kehr A. el a|: Intensive care potential of continuous arteriovenous hemotiltration.Trans Am Soc Artif Intern Organs 28:28-32, 1982 6. Lauer A, Saccaggi A. Ronco C. et al: Continuous arteriovenous hemofiltrationin the criticallyill patient. Ann Intern Med 99:455-460, 1983 7. Kramer P, KauflloldG. Gr0ne HJ. et al: Management of anuric intensivecare patients with arteriovenous hemofiltration, lnt J Artif Organs 3:225-230, 1980 8, SwartzRD: Hemorrhageduring high-riskhemodialysis usingcontrolledheparinization. Nephron 28:65-69, 198 l 9. KaplanAA, LongneckerRE. FolkertVW: Continuous arteriovenous hemofiltration.Ann Intern Med 100:358-366, 1984