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Continuous Arteriovenous Hemofiltration (CAVH)
Contents:
Introducion
ontinuous arteriovenous hemofiltration is an extracorporeal treatment in
which fluid , electrolyte ,and low and middle molecular weight solute are
removed from the body by convective transport. The cellular elements and protein
contents are conserved. The blood enters the exteacorporeal circuit by an
arterial access, flows through a hemofilter, and returns to the patient via
venous access. The technique utilizes the patient's own cardiac output and
arterial pressure to move the blood in the circuit and a large volume of
ultrafiltrate with the same characteristics of plasma is generated. Typically
10-15 liters fluid and solute are removed per day. Therefore concomitant
administration of balanced replacement solution is required. The substitution
of the amount of fluid lost by ultrafiltration with sterile replacement solution
permit correction of electrolyte, acid base abnormalities and lower the
patient’s BUN concentration. The credit of this technique goes to Peter Kramer
when he observed that the arterial blood pressure is sufficient to produce
ultrfiltration [1,2].
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Mechanism of hemofiltration
The mechanism underlying hemofiltration involves the use of a transmembrane
pressure gradient. This pressure gradient is achieved by the net difference
between hydrostatic and osmotic pressures. The hydrostatic pressure consists of
a-The arterial blood pressure(systolic blood pressure more than 80 mmHg), which
drives the fluid across the membrane into the ultrafiltrate compartment. b-The
pressure exerted by the fluid within the ultrafiltrate system, which drives
fluid from the fibers into the ultrafiltrate. The pressure opposing the
hydrostatic pressure is the colloidal osmotic pressure exerted by plasma
proteins.
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Difference between
hemodialysis and hemofiltration
The basic difference between hemodialysis and hemofiltration is in the principle
of solute transport. During hemodialysis the solutes are removed by diffusion
and the fluid by ultrafiltration. During hemofiltration the solute removal is
accomplished by convection that is solvent drag. The fluid removal here
likewise takes place by ultrafiltration but in large amount, therefore fluid
replacement is essential.
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Clinical indication of CAVH:
[3,4]
1-Acute renal failure with cardiovascular instability
2-Acute renal failure in critically ill overhydrated patients who are resistant
to diuretics.
e.g. with
a. Chronic heart disease
b. Non cardiogenic pulmonary overhydration
c. Oligoanuric status
d. Needs for total parenteral nutrition
e. Cerebral edema.
f. Burns.
3-Acute renal failure patients who are critically ill with multiorgan failure.
4-Acute renal failure patients who are critically ill with electrolyte
abnormalities
5-Seriously ill chronic renal failure patients with any of the previous
indications
6-Drug overdosage.
7-Management diuretic-resistant cardiac failure in non-renal failure patients.
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Technical aspect
CAVH does not require complicated equipment [fig 1]. Principally IV infusion
pump to regulate the fluid replacement and heparin infusion pump are required. A
third pump used to control the ultrafiltration rate is required in the two pump
method. The CAVH set contents includes catheters, hemofilter, blood lines, guide
wires, introducing needles, ultrafiltrate line and graduated drainage bag.
a-Vascular access
Vascular access should guarantee adequate arteriovenous pressure gradient, low
resistance, flexible , biocompatible and clinically well tolerated.
1-Percutanuous cannulation of femoral artery and vein are usually used.
2-Scribner A-V shunt
3-Arteriovenous fistulae or grafts are used as arterial line while peripheral or
central vein are used for blood return.
b-Blood lines
The length and the diameter of blood lines are critical in conducting the blood
at a given arteriovenous pressure gradient. Reduction of the arterial line(50
cm) and longer venous line(75 cm) permit high hydrostatic pressure inside the
hemofilter.
c-Hemofilter
Several small hollow fiber highly permeable membranes are used for CAVH. The
membranes are made of polyacrylonitrile or polysulphone with a surface area of
0.2 - 0.5 m2.
d-The ultrafiltration line and drainage bag
The ultrafiltrate flow from the ultrafiltrate
port into the drainage bag through the ultrfiltration line. The difference in
altitude between the hemofilter and the drainage bag must be 40 to 60 cm to
allow the generation of negative pressure inside the hemofilter's ultrafiltrate
compartment thus enhancing the ultrafiltration. In some cases suction may be
applied at the ultrafiltrate port to increase the negative pressure[5]
e-Fluid replacement
Commercially available replacement solution is
typically have the following composition: sodium 140 mmol/L, potassium 0-2
mmol/L, calcium 1.75 mmol/L, magnesium 0.75 mmol/L, chloride 106 mmol/L and
lactate 45 mmol/L or acetate 41 mmol/L. If ready made replacement solution is
not available several formulae can be used depending on patients requirements.
1.Ringer's lactate + calcium and magnesium as
required
2.For hypotensive, acidotic patients or with
liver disease infuse bicarbonate -buffered solution by alternating the following
two solutions:
a.0.9 saline + calcium and magnesium as required
b.5% dextrose/0.45% saline + 66 mmol sodium
bicarbonate ( with 23% sodium chloride if needed to attain the desired sodium
level 130-140 mEq/L)
3.For patients with lactic acidosis acetate
containing Ringer's solution can be used.
Replacement solution can be infused into the
arterial line [predilution] or into the venous line [postdilution]
-
In postdilution blood in the hemofilter can
become very concentrated which lead to poor blood flow and clotting specially
at high fluid removal rate.
-
In predilution the blood is diluted with the
replacement solution before it reaches the hemofilter. Predilution is
recommended when fluid removal of more than 10 liters/day are required.
The ultrafiltrate contains a lower concentration
of waste product in postdilution than in prediluation fluid replacement.
The fluid removal and replacement rates are
controlled by a- Gravity method which is suitable for ultrfiltration rate
of up to 10 liters/day. b- Two pump method in which the ultrafiltrate and
fluid replacement are dialed by IV infusion pumps. It is suitable for
ultrafiltration rates up to 10-20 liters /day
f-Anticoagulation
Prolonged anticoagulation is required in CAVH to prevent clotting in
extracorporeal circuit and to extend hemofilter span life and performance with
minimal side effects to the patient. A loading dose of 500 - 2000 heparin is
usually injected into the arterial line followed by maintenance dose of 10
units/kg/hour [6]. PTT to be measured every 6-12 hours keeping the systemic PTT
about 150% of the baseline.
Heparin-free, regional heparinization, and regional citrate anticoagulation are
also applicable to patients with liver disease, active or recent bleeding,
heparin induced thrombocytopenia or in postoperative patients.
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Prescription of CAVH
[7]
The rate of urea production by the body is directly dependent on the
rate at which the protein (ingested and endogenous ) and amino acids are broken
down. Urea nitrogen generation rate ranges from 5 to 10 gm / day. The higher the
urea nitrogen generation rate the great the need to increase urea clearance per
day.
Plasma urea clearance in postdilution CAVH urea concentration of the
ultrafiltrate is approximately equal to the plasma. Urea clearance is simply
equal to ultrafiltrate volume. In predilution however urea clearance can be
calculated by the following formula:
Urea clearance = Ultrfiltrate volume X Ultrafiltrate urea concentration
X 0.85
Plasma urea concentration
Table 1 shows CAVH schedule according to the patient catabolic state
|
Patient category |
Desired plasma urea clearance /day* |
|
Not hypercatabolic and has
residual renal function |
5-10 liters |
|
Not hypercatabolic and has
no residual renal function |
8-15 liters |
|
Hypercatabolic with minimal
or no residual renal function |
15-25 liters |
* multiplied by 1 on post
dilution and by 1.2 on prediluation
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Nursing care in CAVH
·
Patient and or family education
about the purpose and function of CAVH
·
A baseline assessment , including
clinical history, the patient's weight and physical examination,.
·
Hamodynamic profile is essential.
It includes vital signs, central venous pressure, pulmonary artery pressure.
pulmonary capillary wedge pressures and arterial pressures
·
Baseline laboratory data;
hemogram, coagulation profile, and chemistry .
·
Establish the vascular access
aseptically.
·
The hemofilter and lines are
primed, heparinised and attached to the patient (according to manufacture's
instruction)
·
The hemofilter must be secured
carefully to the patient to avoid accidental disconnection.
·
Continuous care; Hemodynamic
profile, patients fluid status, pulses distal to the access, blood flow, blood
lines, hemofilter, ultrafiltration rate, laboratory values.
·
Hourly record of ultrafiltrate.
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Positives and Negatives of CAVH
Positives of CAVH:
[8,9,10]
·
The main advantage of CAVH over
intermittent conventional hemodialysis is the patient's cardiovascular stability
during the procedure.
·
CAVH provides an alternative
option to peritoneal dialysis for patients with intra-abdominal sepsis or recent
abdominal surgery
·
Continuous and smooth blood
purification with clearance rate of about 9.5 ml/minute are achieved .
·
Precise control of body fluid,
electrolyte and acid base abnormalities in oligoanuric patients are easier. CAVH
has been shown to be beneficial when parenteral nutrition is indicated.
·
CAVH is a simple and safe
procedure to treat critically ill patient - ease of initiation and technically
less demanding.
Negatives of CAVH
[7,9,10]
·
Access related:
Thrombosis of femoral artery Emolization of
atherosclerotic plaque
Retroperitoneal or local hematoma False aneurysm
Femoral arteriovenous fistula
·
Require strict fluid and
electrolyte monitoring to avoid fluid and electrolyte imbalance.
·
Bleeding ; local -
overhepranization. In patient with systemic bleeding the advantages and the risk
must be weighted.
·
Hemofilter malfunction ;
Clotting should be
suspected when blood lines becomes dark and or reduction of
ultrfiltrate volume.
Hemofilter leak and rupture also may occur.
Decreased blood flow may indicate vascular access problem and or low blood
pressure.
·
Inadequate solute clearance in
severely hypercatabolic and hyperkalaemic patients.
·
Access site infection
Conclusion
Acute renal failure and the associated water overload are common problems in
critically ill patients and contribute significantly to mortality. Such patients
are often hemodynamically unstable and can not tolerate conventional
hemodialysis. Continuous arteriovenous hemofiltration provides a simple and safe
renal replacement therapy for such critically ill patients in Intensive Care
Units. It is possible that patients who develop acute renal failure could be
managed using Continuous Arteriovenous Hemofiltration in hospitals which do not
have an associated dialysis unit.
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References
1-Kramer P, Wigger W, Rieger J, et al :Arteriovenous hemofiltration : A new
simple method for treatment of overhydrated patients resista nt to diuretics.
Klin Wochenscher 1977;55:1121-1122.
2-Kramer P, Schrader J, Bohnsack W, et al Continuous arteriovenous
hemofiltration.A new kidney replacement therapy. Proc Eur Dial Transplant Assoc.
1981;18:743-749.
3-Morgan SH, Mansell MA, Thompson FD. Fluid removal by hemofiltrationin diuretic
resistant cardiac failure. Br Heart J. 1985;54:218-219
4-Ronco C, Brendolan A, Bragantini L, Chiaramonte S, et al Continuous
arteriovenous hemofiltration with AN69S membrane; procedures and experience. Kid
international 1988,33 s24:150-153.
5-Bjork S, Delfin K, Mathillas O et al , Continuous arteriovenous
hemofiltration: A simple method to improve the efficiency. A technical note.
Scan J Uro Nephrol 1985;19:29:295-296
6-Kramer P, Bohler J, Kehr A, et al Intensive care potential of continuous arteriovenous hemofiltration. Tran Am Soc Artif Intern Organs .1982;28:28-32.
7-Miles H, Sigler, Brenan P, et al Slow Continuous therapies. Handbook f
dialysis 1994;169-197.
8-Kramer P, Kaufhold G, Grone HJ, et al.. Management of anuric intensive-care
patients with arteriovenous hemofiltration. Int J Artif Organs 1980;3:225-230.
9-Lauer A, Saccaggi A, Ronco C et al. Continuous arteriovenous hemofiltration in
critically ill patient. Clinical use operational characteristics. Ann Int Med
1983;99:455-490.
10-Dodd NJ, Turney JH, Parsons V, et al. Continuos hemofiltration maintains
fluid balance and reduces hemodialysis requirements in acute renal failure. Proc
Dial Transplant Assoc. 1982; 19:329-333.
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