PRINCIPLES OF CONTINUOUS
RENAL REPLACEMENT THERAPY (CRRT)
- Access Sites
- Principles of CRRT
- Artificial Kidney
- Pre/Post Dilution Hemofiltration
- Role of Filters in Clearance
- Role of Diffusion in Clearance
- Role of Hemofiltration in Clearance
- Therapies Used
dialysis treatments are provided for brief intervals,
usually every day or every 2-3 days as required (e.g. intermittent
hemodialysis or peritoneal dialysis). Continuous Renal Replacement
Therapies (CRRT) are dialysis treatments that are provided as a
continuous 24 hour per day therapy. This on-line
program will focus on continuous hemodialysis circuits only (versus
continuous peritoneal dialysis).
intermittent hemodialysis and continuous hemodialysis circuits utilize
the same principles. Blood is removed from the patient, pumped through
a dialysis filter and returned to the patient following removal
of surplus water and wastes. The filter performs many of the functions
of the kidney's nephron unit, hence, it is referred to as an "artificial
The major difference between intermittent and continuous therapies
is the speed at which water and wastes are removed. Intermittent
hemodialysis removes large amounts of water and wastes in a short
period of time (usually over 2-4 hours), whereas, continuous renal
replacement therapies remove water and wastes at a slow and steady
rate. While intermittent dialysis allows chronic renal failure patients
to limit the amount of time that they are connected to a machine,
the rapid removal of water and wastes during intermittent treatments
may be poorly tolerated by hemodynamically unstable patients.
early circuits removed blood from arterial access sites and returned
the purified blood via a venous catheter. This promoted blood flow
through the filter by utilizing the patients own arterial to venous
blood pressure gradient.
arterial-to-venous access sites are still used in patients with
end-stage renal failure, arterial-to-venous pressure gradients are
no longer needed. Modern continuous and intermittent hemodialysis
circuits utilize blood pumps to remove blood from the access site,
allowing venous-to-venous catheters to be used.
An example of an arterial-to-venous access site is a fistula. Fistulas
are created surgically using graft material to connect an artery
of a limb directly to a vein. Fistulas take several months to "mature"
or dilate sufficiently before they can be accessed for dialysis.
they are under the skin, fistulas reduce the risk for bleeding or
infection when long term therapy is required. Filter patency can
be confirmed by listening with a stethoscope for a "bruit"
(created by the rapid flow of blood though this large circuit) or
by feeling a tingling sensation known as a "thrill". Never
place a BP cuff or tourniquet above a fistula as this can reduce
the flow through the circuit and lead to clotting.
Lumen Venous Catheter
critical care, temporary double-lumen venous dialysis catheters
are the most common form of access. They can be inserted quickly
at the bedside and used immediately. "Perm" catheters
are double lumen venous catheters that are designed for longer indwelling
use. They are used more frequently in patients with chronic renal
failure and may be used as a bridge until a surgically created fistula
is ready for use.
catheters are easy to differentiate from regular intravenous lines
by their red and blue hubs. The red lumen denotes the side of the
venous catheter that is used to pull blood from the patient, and
is referred to as the access lumen. The blue lumen is the return
site and is used to reinfuse the patient's blood after it passes
through the dialysis filter. If an adequate flow rate cannot be
achieved by removing blood from the access side of a catheter, the
catheter limbs can be reversed. Reversal of the limbs does produce
a small reduction in clearance due to recirculation.
A double-lumen venous dialysis catheter can be used as
a central venous infusion site during an emergency, however, to
ensure the line remains patent for subsequent dialysis treatments,
and to reduce the risk for infection, it is preferrable that these
catheters used for dialysis only. If it is the only vascular access
available in a life-threatening emergency, it can be used as a central
line, HOWEVER, always assume that the catheter contains heparin.
When a double lumen catheter is not in use for dialysis, some form
of anticoagulant is always instilled into each lumen to maintain
patency. While citrate is the most common agent, some catheters are blocked with heparin or tPA. If heparin is used, the concentration may be as high as
5,000 - 10,000 units per mL. Because each lumen contains a volume
of ~1 to 2 ml, the two lumens could contain a maximum of up to 40,000 units of
heparin! ALWAYS assume that each lumen contains full strength heparin
(even if it is labeled as containing saline). Always
withdraw at least 5 ml of blood from EACH lumen prior to using the
catheter as an intravenous line.
CCTC, 4% Citrate is used to block all dialysis catheters. Citrate
binds to calcium to prevent clotting and does not affect the aPTT,
therefore, it is useful when anticoagulation is contraindicated.
Citrate is the standard for blocking all CRRT catheters in CCTC,
even when heparin has been used to maintain filter patency.
citrate 4% is the usual catheter blocking agent, heparin may still
be used at the end of intermittent treatments or when lines are
blocked by nephrology residents after insertion. It is always safest
to assume that heparin could be present in all dialysis catheters
employs the principles of diffusion, hemofiltration and convection,
using an external filter to create an artificial nephron unit.
the normal nephron unit:
flows from the Renal Artery (A) to the
Afferent Arteriole (B). The Afferent Arteriole
then enters the Bowman's Capsule (H) and
becomes the Glomerulus (C). Blood leaves
the Glomerulus via the Efferent Arteriole (D),
which continues to become the Peritubular Capillary (E).
and solutes that are filtered through the Glomerular Membrane
collect in the Bowman's Capsule (H) and
drain into the Proximal Tubule (I). Filtrate
continues through the Loop of Henle (J),
Distal Tubule (K) and Collecting Tubule
(L). Water and solutes are reabsorbed from
the filtrate into the peritubular capillaries, while solutes
can also be secreted from the peritubular blood into the
tubule system for elimination in the final urine.
diagram above depicts one nephron unit. Each kidney has approximately
one million of these microscopic units. They collectively maintain
water, electrolyte, waste and acid-base balance.
blood flows from the renal artery, branching into smaller divisions
known as arterioles. Branches of the arterioles eventually carry
blood into small "containers" called Bowman's Capsules,
located in the cortex of the kidney. The arteriole that Arrives
at the Bowman's Capsule is called the Afferent
arteriole. The blood then flows into a specialized capillary (located
inside the Bowman's Capsule), called the GLOMERULUS. Any blood remaining
at the end of the glomerulus Exits the Bowman's
Capsule via the Efferent arteriole.
afferent arteriole is larger in diameter than the narrow efferent
arteriole. This arrangement provides a high rate of blood flow into
the glomerulus, but a high level of resistance to blood flowing
out of the glomerulus. This structural difference produces a hydrostatic
pressure within the glomerulus that is twice that of other capilliariesin
the body. This increased hydrostatic pressure forces more water
to move from the glomerulus, across the semi-permeable glomerular
membrane and into the Bowman's Capsule.
that are small enough to pass through the glomerular membrane will
diffuse from an area of high concentration (from the glomerulus)
to low concentration (to the Bowman's Capsule). When large volumes
of water are forced across the membrane, additioanl particles (or
solutes) are "dragged along with the water". Thus, the
large movement of water across the glomerulus removes even more
solutes than diffusion alone would remove. The "washing"
of additional solutes across the membrane by a large flux of water
is known as convection.
are larger molecules and are too big to fit across a normal glomerular
membrane. Consequently, blood that leaves the glomerulus via the
efferent arteriole has most of the water and electrolytes removed,
but all of the plasma proteins remaining. Thus, blood in the efferent
arteriole has a higher oncotic pressure.
order to adequately eliminate all of the waste products produced
each day, we have to filter very large volumes of water across the
glomerulus. About 1200 ml per minute of filtrate is produced. By
the time enough water has been moved across the membrane to wash
out all of the surplus waste products, over-removal of water, glucose,
electrolytes and other substances has occurred. Consequently, large
amounts of the filtered water and solutes will need to be reabsorbed
from the tubule fluid into the blood. Solutes and water are reabsorbed
into capillaries that are wrapped around the tubules, called peritubular
capillaries. These peritubular capillaries are the continuation
of the efferent arterioles. They are also responsible for perfusing
the kidney. In addition to reabsorbing water and solutes from the
tubule filtrate, surplus solutes can be secreted from the peritubular
capillaries into the tubule filtrate for elimination in the urine.
The dialysis filter is referred to as an artificial kidney. Blood
is pulled from the patient and carried into the filter. Once inside,
the blood travels through many tiny tubules called hollow fibers.
Water and solutes can pass across the semi-permeable membrane between
the blood and the fluid that surrounds the hollow fibers. Any fluid
or solutes that enters the filter canister will be drained out as
of dialysis filter (artificial kidney)
how the dialysis filter has structural similarities to the nephron
unit. Blood arrives at the filter via the access tubing (afferent
arteriole). Blood enters the small hollow fibers within the filter
(glomerulus). Water and solutes diffuse across the semi-permeable
membrane of the hollow fibers and collect in the canister (Bowman's
Capsule). Collected fluid (filtrate or effluent) is then removed
via the drainage tubing (collecting tubule). Blood that remains
in the hollow fibers is returned to the patient via the return side
of the filter (efferent arterial).
similarities exist between the nephron unit and the artificial kidney,
the artificial kidney has limited capabilities. In the nephron unit,
filtered water and waste enters the proximal tubule. Because the
nephron unit removes significantly more water and solutes than needed,
most of the water and electrolytes that enter the tubule system
the nephron unit, the artificial kidney cannot reabsorb water or
solutes that enter the filter canister Any filtrate that enters
the filter canister will be removed via the drainage tubule. Consequently,
one of the differences in the artificial kidney is the absence of
the proximal tubule, loop of henle and distal tubule where water
and solute reabsorption and secretion occurs. Thus, the drainage
tubule that exits the filter is similar to the collecting tubule
of the nephron unit, not the proximal tubule. To compensate for
the inability to reabsorb water and solutes following removal from
the blood, the artificial kidney is manipulated to restrict the
actual removal to only surplus water and wastes. This is done by
adjusting dialysis solutions and ultrafiltration rates. If more
water or solutes are removed than desired, they may need to be given
back via intravenous infusions.
artificial kidney does not replace other important kidney functions,
including stimulation of red blood cell production (erythropoietin),
blood pressure and sodium regulation (renin) and calcium uptake
by the GI tract (vitamin D synthesis). The nephron normally traps
and recycles bicarbonate to maintain acid base balance. Bicarb is
given to patients during hemodialysis to compensate for bicarb deficits.
principles used during hemodialysis are reviewed below:
is the movement of particles (solutes) across a semi-permeable membrane.
Diffusion is the movement from the side with the highest concentration
of particles, to the side with the lowest concentration.
Dialysate is the fluid that is pumped into the filter canister,
surrounding the hollow fibers. The concentration of solutes in the
dialysis fluid determines diffusion gradients. The removal of surplus
solutes from the blood is achieved by infusing dialysate fluid that
contains a lower solute concentration than the serum concentration
(e.g. dialysate does not contain urea or creatinine).
maintain normal serum electrolyte levels, dialysate fluid contains
sodium, chloride and magnesium levels that are equal to serum concentrations
(thus, removal of these electrolytes should only occur if the blood
level exceeds normal serum concentrations). In renal failure, potassium
is often high at the start of a treatment, therefore, we may begin
dialysis with a low concentration of potassium in the dialysate.
Because potassium is easily removed during dialysis, and continued
dialysis will be required to ensure removal of other wastes such
as urea and creatinine, potassium concentrations in the dialysate
often require upward adjustment as the potassium level in the blood
falls. Although in theory, potassium levels should not fall below
4 mmol/L in the serum if the dialysate contains 4 mmol/L, a number
of factors influence serum potassium levels in critical care. Insulin
therapy and the use of sympathomimetic drugs promotes the movement
of potassium from the blood into the cells. This can lower serum
levels. Additionally, potassium loss through the GI tract can increase
the potential for hypokalemia. Low magnesium levels will also suppress
the serum potassium levels, therefore, magnesium deficits should
be replaced as needed. Additionally, high hemofiltration rates can
lead to additional potassium clearance. Potassium levels must be
monitored closely and adjusted to maintain normal serum concentrations.
renal failure, serum bicarbonate levels are generally low, therefore,
a source of bicarbonate is added to the dialysate to facilitate
diffusion of bicarbonate into the blood. Lactate based formulas
provide one source (e.g., Gambro's LG formulas). Higher concentrations
of lactate in the dialysate promote diffusion into the blood. If
hepatic function is normal, lactate is quickly converted to bicarbonate
in the liver. Prisma(TM) and
Prismaflex(TM) both use premixed
bags of sterile dialysate. Lactate based preparations have a long
stability, making them less expensive to prepare. Because bicarbonate
is only stable for a short period in solution, it must be added
to the dialysis bags before using.
If the patient is unable to convert lactate to bicarb at a rate
that is fast enough, serum lactate levels will rise. This occurs
in both hepatic insufficiency and shock states were the patient
already has excess lactate production due to anaerobic metabolism.
In these instances, a bicarb containing product is used to deliver
bicarbonate (e.g. B.O or Normocarb).
1 L of dialysate is administered per hour, one L of dialysate fluid
will collect in the drainage collection bag per hour. This will
be in addition to any fluid removed; dialysate doesn't normally
cross into the bloodstream.
gradients play a major role in diffusion. These will be explored
further in the discussion on clearance. The other factor that influences
diffusion is the type of filter used. Diffusion of solutes cannot
occur across a concentration gradient if the pore size is too small
to permit passage.
Ultrafiltration is the movement of water across a semi-permeable
membrane because of a pressure gradient (hydrostatic, osmotic or
oncotic). The increased blood pressure in the glomerulus creates
a favourable driving pressure to force water across the glomerular
pressure within the hollow fibers is positive, while the pressure
outside the hollow fibers is lower. Increased negativity can be
generated outside the hollow fibers by the effluent pump by either
increasing the fluid removal rate, or by increasing the replacement
flow rate. The difference between the blood pressure in the hollow
fibers and the surrounding pressure is the TransMembrane Pressure
(TMP). The TMP determines the ultrafiltrate production.
filter membrane properties can produce different ultrafiltration
rates at a constant TMP. A filter that is more permeable to water
will allow more water to travel across the membrane at a given TMP.
A filter with a high permeability to water is called a high flux
hemodialysis circuits, pulling large volumes of water across the
semi-permeable membrane creates a convective current that "drags"
additional solutes. While diffusion is effective at removing most
small molecules, convection enhances the removal of small and mid-sized
molecules. Thus, convection can be added to hemodialysis therapy
to enhance solute removal. To prevent hypovolemia, any water removed
during hemofiltration must be returned to the blood before it reaches
the patient. This is called "replacement" fluid. Hemofiltration
rates of 1 L/hr mean that one liter of fluid is removed from the
patient's blood and eliminated in the drainage fluid AND 1 L of
replacement fluid is returned to the circuit before it reaches the
patient. We set hemofiltration rates by adjusting replacement rates.
Any fluid removed during hemofiltration is given back to maintain
a net neutral fluid balance. Replacement fluid must be sterile intravenous
fluids with concentrations of electrolytes similar to plasma.
example, if the CRRT therapy includes a hemofiltration rate of 1
L per hour, and the fluid removal is set at 200 ml per hour, 1200
ml will be pulled from the patient and introduced into the drainage
collection bag each hour. Because the 1 L of hemofiltration is replaced,
the net fluid removed is 200 ml. Whether hemofiltration is used
or not, the net fluid removed is equal to the fluid removal setting.
VERSUS POSTDILUTION HEMOFILTRATION
fluids can be returned either pre or post filter. This is referred
to as predilution or post dilution sets. Predilution means that
the replacement solution is returned to the blood before it reaches
the filter, diluting the blood in the hollow fibers. Postdilution
means that the replacement fluid is returned to the blood after
the filter (but before the return side of the access catheter).
Predilution dilutes the blood in the filter, reducing clotting.
Postdilution concentrates the blood in the filter, enhancing clearance.
is a byproduct of muscle protein metabolism that is completely filtered
by the glomerulus and 100% eliminated. None of the filtered creatinine
is reabsorbed from the tubules nor is any additional creatinine
secreted into the tubule lumen post glomerulus. This makes it the
best indicator of renal failure. Because it is completely eliminated
during normal renal function, measurement of creatinine clearance
is the best measure of glomerular filtration.
Urea is another byproduct of protein metabolism, however, it is
a byproduct of all protein metabolism (not just muscle protein metabolism).
It is filtered into the glomerular filtrate. Unlike creatinine,
a percentage of filtered urea is reabsorbed from the tubules. Consequently,
urea levels can become increased in the presence of a normal creatinine
level. For example, urea can increase due to increased urea production
(e.g., anabolic or catabolic states) or increased tubule reabsorption
of urea (e.g., due to dehydration). Creatinine only increases when
renal filtration decreases, or the production of creatinine becomes
so high that it exceeds glomerular filtration capabilities. Excessive
creatinine production can occur when significant muscle death has
occurred, for example in rhabdomyolysis.
is the rate at which solutes are cleared from the body. Clearance
is abbreviated by the letter K. The clearance
(or K) of a solute is the volume of blood from which the substance
is completely removed per unit time (Gambro training manual). It
is calculated as follows:
= excretion rate of solute / blood concentration of solute
translate this to dialysis: if a dialyzer has the ability to clear
170 ml/min of urea at a blood flow rate of 200 ml/min, it means
that for every 200 ml of blood that flows through the filter, 170
ml will be returned urea free. The remaining 30 ml will have the
same concentration of urea as the blood entering the filter. The
200 ml of blood being returned each minute to the systemic circuit
will have significantly less urea than without dialysis, but will
still have to mix in with the systemic volume. Thus, blood must
continually circulate through the filter before the total systemic
level will begin to fall.
following formula can be used to calculate the clearance of a solute
in ml/min at the dialysis membrane. To calculate the rate of clearance
of a solute, the following formula can be used, where Q(blood)in
is the flow of blood into the filter, Q(blood)out is the flow of
blood out of the filter, C(blood)in is the concentration of the
solute in the prefilter serum and C(blood) is the concentration
of the solute in the post filter blood. Q(blood)in and Q(blood)out
are the same and equal to the blood flow rate.
can be simplified to:
At a blood flow rate of 150 ml/min, with a prefilter creatinine
of .980 and post filter concentration of .343, the creatinine
clearance by the filter is 97.5 ml/min. This assumes that
no hemofiltration is being used.
membranes need to be efficient at clearing wastes, but must also
be biocompatible with human blood. Compatibility means that exposure
of blood to the dialysis membrane produces minimal of adverse effects.
Filter permeability is influenced by pore size, the number of pores
and the thickness of the membrane. Generally, high flux membranes
which have more or larger pores allow more solutes and ultrafitrate
to move across the membrane. Thinner membranes offer less resistance
to solute movement by decreasing the distance the solute must travel
across the membrane and also favours increased filtration.
are pass through the membrane according to solute size. Imagine
taking a flour sieve and filling it with a mixture of sand, small
rocks and debris. Shaking up the contents would cause the smallest
particles to move towards the bottom, passing through the openings
easily. Particles would be filtered through according to increasing
size until you are left with particles to large to fit through the
sieve. Dialysis membranes act the same way, allowing small and mid
sized molecules to pass across the membrane, without the loss of
larger proteins. High flux membranes that have a larger pore size
increase clearance by allowing larger molecules to pass through
the membrane, and by allowing more ultrafiltrate flow. The standard
AN69 filter used with CRRT is a high flux membrane. Sieving properties
of a membrane describe the membrane's permeability to solutes during
ultrafiltration. Permeability of solutes decrease as the the molecular
size increases. The cut-off point for a membrane is defined by the
molecular weight where only 10% of the solute is filtered.
surface area of the membrane determines the available area for diffusion
and ultrafiltration. The internal volume of the dialysis filter
should be small enough to limit the amount of blood that is outside
of the vascular compartment at any given time. This volume is important
if the filter clots before blood can be returned to the patient.
adsorption is the ability of larger solutes to adhere to the surface
of the dialysis membrane. AN69 filters used in CRRT have strong
adsorptive properties. Adsorption of mid sized molecules including
inflammatory mediators have been demonstrated by a drop in serum
concentrations following initiation of a new filter. The greatest
benefit appears to occur in the first few hours; once the filter
becomes saturated with proteins, further removal from the serum
is limited. While these proteins are too large to pass through the
filter and be removed in the filtrate, the removes the cytokines
from the blood by allowing the to collect (like a sponge) in the
is the pressure exerted on the dialysis membrane during operation
and reflects the difference between blood and fluid compartments.
A TMP above +350 mmHg will produe an advisory alarm. A TMP >
450 will produce a "TMP excessive" alarm. The amount of
increase and the rate of TMP increase contribute to the "Filter
is Clotting" alarm.
Pressure drop is another indicator of clotting. It is an indication
of the pressures in the hollow fibers of the filter. It will slowly
rise with filter use as the hollow fibers become filled with microscopic
clot. The amount and rate of increase determines the activation
of the "filter is clotting alarm".
molecular weight solutes are easily removed by diffusion (dialysis).
The higher the concentration gradient, the higher the diffusion
rate. Solutes will move across a semipermeable membrane until the
two solute concentrations become equal.
solutes move into the dialysate fluid, the dialysate concentration
of the solutes increase, reducing the diffusion gradient. Once the
dialysate concentration of a solute becomes equal to the blood concentration,
diffusion stops. To maintain a high diffusion gradient, the difference
between the blood and dialysate concentrations must be maintained.
Clearance can be increased by higher dialysate or blood flow rates.
Increasing the dialysate rate maintains a low concentration of solutes
on the dialysate side by increasing their removal from the dialysate
fluid. Increasing the blood flow rate brings more solutes to the
filter, promoting continuous diffusion. The smaller the molecule,
the greater the clearance by dialysate/blood flow increases.
higher blood flow rates will increase the rate of clearance, CRRT
circuits have limitations. The smaller filter size (compared to
hemodialysis circuits) limits the blood flow rates. Blood flows
can be increased substantially with hemodialysis, however, blood
flow rate adjustments are limited with CRRT.
While increased dialysate flow rates enhance the clearance of small
molecules, middle sized molecule clearance is more dependent upon
the size of the filter pores. The only way to increase the clearance
of middle sized molecules is to add convection (hemofiltration).
solute clearance is produced when dialysate flow rates are approximately
double that of the blood flow rates. CRRT blood flow rates are typically
150 ml/min. A dialysate flow rate of 1 L per hour, provides a dialysate
flow of 16 ml/min. Increasing the dialysate flow will have a greater
effect than any increase in blood flow rates with CRRT.
flows countercurrent, or in the opposite direction to blood flow.
This promotes continual clearance by ensuring an adequate diffusion
gradient is maintained. Dialysate fluid is introduced at the return
end of the filter, where the serum concentration of solutes has
begun to fall (due to removal from the blood within the filter).
The dialysate fluid flows towards the access end of the filter where
the fluid drainage tubing is located. Diffusion of solutes along
the filter makes the concentration of wastes highest in the dialysate
at the access end of the filter. At the access end, the blood concentration
of the solute is highest , counterbalancing the rising dialysate
effectively removes small (e.g. electrolytes) and small to mid size
molecular weight solutes (e.g. glucose, urea, creatinine). The pore
size limits the ability to diffuse middle sized molecules. One way
to increase the clearance of all small and more of the mid sized
molecules is to pull large quantities of water across the semi-permeable
membrane, “dragging” additional solutes by convection.
Higher hemofiltration rates are of interest in critical care. Higher
pre dilution rates may be a successful alternative to anticoagulant
therapy, although, research is needed to examine this option. There
is also interest in the potential clearance of mid sized molecular
weight solutes including inflammatory mediators. In a European trial,
hemofiltration rates of 35 ml/kg/hr were associated with the best
survival rates. Although higher hemofiltration rates have been used
in CCTC, our current practice uses predilution therapies. In this
trial by Ronco, post dilution was used. The significance of high
hemofiltration rates using predilution replacement is not known.
increased ultrafiltration rates during hemofiltration help to remove
molecules too large to travel by diffusion, hemofiltration can also
lead to excessive removal of small molecules. Consequently, electrolyte
removal can be increased beyond that produced by the diffusion gradient
alone (e.g., despite a dialysate concentration of sodium that is
equal to normal serum levels, sodium levels can fall with high hemofiltration
high infusion rates of replacement fluids containing 0.9 NaCl can
lead to hypernatremia. It can also increase chloride levels leading
to hyperchloremic acidosis (chloride and bicarbonate are both negatively
charged, increased chloride levels can cause a decrease in bicarbonate
to maintain anionic balance).
hemofiltration rates are high, careful monitoring is required to
maintain normal electrolyte balance. Replacement fluids may need
to be adjusted to keep serum levels within range. Alternatively,
intermittent boluses of electrolytes may be required.
continuous hemodialysis circuits required arterial to venous access
sites, because they did not utilize a blood pump to pull blood through
the filter. Consequently, they were referred to as CAV (Continuous
arterial-venous) circuits. Today's technology uses a blood flow
pump, therefore, most continuous circuits are CVV (continuous venous-venous).
(Slow Continuous Ultrafiltration):
SCUF is the removal of water from the patient's blood as it travels
through the filter. Water removal is referred to as ultrafiltration.
SCUF is a therapy designed to only remove surplus water. The amount
of water removed is not sufficient to remove wastes.
(Continuous Venous-Venous Hemofiltration)
CVVH is the removal of large amounts of water across the filter
membrane for the purpose of clearing wastes. When large volumes
of water are washed across the membrane, solutes are dragged along
with the water (convection). Hemofiltration is the removal of water
over and above the surplus water removed during ultrafiltration.
To prevent hypovolemia, water removed during hemofiltration must
be given back before the blood is returned to the patient. This
is referred to as replacement. CVVH is the use of replacement fluid
without dialysis fluid, plus or minus fluid removal.
(Continuous Venous-Venous Hemodialysis):
CVVHD is the infusion of dialysis fluid into the filter canister
The dialysis fluid (dialysate) surrounds the blood filled filter
segments. Solutes that are small enough to fit through the membrane
of the dialysis filter will move from an area of high concentration
to low concentration (diffusion). The dialysate determines the solutes
that will be removed. If we want to remove solutes, the concentration
in the dialysate is lower than the blood concentration. If we want
to give something to the patient, the concentration in the dialysate
is higher than the blood. CVVHD is the removal of wastes by diffusion
only, without the use of hemofiltration (replacement fluid). It
can be administered with or without fluid removal from the patient.
(Continuous Venous-Venous HemoDiaFiltration):
CVVHDF is the use of dialysis AND hemofiltration.
Therapy will include the use of both dialysate and replacement fluids
and can be administered with or without fluid removal from the patient.
Gambro Training Manual 1 and 2
Slides from Gambro Training package, reproduced with permission
Update: April 28, 2006
Last Reviewed: July 2, 2013