HEMODIALYSIS, PERITONEAL DIALYSIS, AND CONTINUOUS THERAPIES
When kidney dysfunction is severe enough to cause homeostatic abnormalities that cannot be corrected with diet or medications, dialysis is performed to artificially replace the kidney’s major functions. The major goals of dialysis are to support the elimination of nitrogenous waste products, restore fluid and electrolyte homeostasis, and restore normal plasma pH. The major indications are listed in the plate.
PRINCIPLES
OF DIALYSIS
Dialysis
employs a semipermeable membrane to alter the composition of blood. Blood is
located on one side of the membrane, whereas a wash solution, known as the
dialysate, is on the opposite side. The objective is for the desirable
electrolytes to move from the dialysate to the blood and for the undesirable
electrolytes to move in the opposite direction. The movement of fluid and
solutes across the membrane depends on two physical forces: diffusion and
convection.
In
diffusion, solute transport is directly dependent on the concentration gradient
of the solute, diffusivity of the solute, permeability of the membrane, and surface
area across the membrane. The smaller the molecule, the more rapidly it will
diffuse. Molecules continue to move across the membrane until equilibrium is
achieved.
In
convection, solutes are dragged across a membrane in the solvent that contains
them. An analogy would be an ocean wave (the solvent) pushing sea shells (the
solute) onto the shore. The solvent carrying these solutes crosses the membrane
in a process known as ultrafiltration, which depends on the pressure gradients
across the membrane.
Diffusion is
more efficient at clearing small molecular weight substances (less than 500 Da),
such as electrolytes. In contrast, convection is more efficient at clearing
medium molecular weight substances (500 to 5000 Da), such as vitamin B12 or
drugs (e.g., vancomycin).
Hemodialysis. In hemodialysis, blood leaves the patient and flows through tubing into a dialyzer. The dialyzer contains numerous hollow fibers composed of semipermeable membranes. As blood flows through these fibers, dialysate flows around them in the opposite direction. Molecules are exchanged across the fiber walls. The blood then returns to the patient.
Because
blood and dialysate flow in opposite directions, concentration gradients are
maintained across the entire length of the dialyzer. As a result, potassium, nitrogenous waste
products, phosphorus, and other substances that have accumulated in the blood diffuse
into the dialysate. Meanwhile, substances that are concentrated in the
dialysate, such as bicarbonate and other electrolytes at specific concentrations, diffuse into the blood to
restore desired levels. As diffusion occurs, the hydrostatic pressure in the
dialyzer leads to ultrafiltration of fluid and convection of larger solutes.
The
patient’s vasculature can be accessed using either a central venous catheter
(CVC) or a connection between an artery and vein (fistula or graft). A CVC used
for dialysis contains two lumens and is inserted into a large central vein. The
rapid and substantial amount of flow (up to 400 to 500 mL/min) drawn from these veins allows blood
to efficiently exit the vein through one lumen, enter the dialysis circuit, and
return to the vein through another lumen. The high blood flow also prevents
stasis, which could lead to clotting, and optimizes the exchange of solutes
across the membrane. Heparin is often used at intervals to prevent clotting
within the dialysis circuit.
The main
disadvantage of CVCs is their infection risk. To decrease this risk, catheters
are often tunneled, meaning they are passed through a subcutaneous tract before being inserted into the central vein. This
process lengthens the distance that skin flora must travel before being able to
cause a systemic infection.
In addition
to the risk of infection, catheters can also clot and kink, and they can incite
an inflammatory reaction that leads to venous stenosis. Thus even tunneled CVCs
should be considered temporary access routes to be used only while awaiting creation
of a more permanent solution, such as an arteriovenous fistula or graft. An
arteriovenous fistula permits the high blood flow of the artery to be shunted
into a neighboring vein. After a fistula is surgically created, the vein will
dilate and thicken over the course of 6 to 8 weeks, after which dialysis
needles can safely be inserted and removed as needed. Fistulas most often join
the cephalic vein and radial artery in a side-to-side or end-to-side
anastomosis, although many other configurations are possible.
If a patient
has diseased peripheral vasculature that would not permit the creation of a
fistula, usually because of complications from diabetes mellitus, an artificial
graft can be implanted to join the artery and vein. These grafts, often made of
polytetrafluoroethylene, can be used for hemodialysis within 1 to 2 weeks of
implantation. Their disadvantages, however, are that they do not remain patent
for as long as fistulas, and that they are more likely to become stenosed or
thrombosed.
Once
vascular access has been established, several different hemodialysis schedules
can be used. A standard schedule consists of 3- to 4-hour sessions occurring
three times per week. Nocturnal hemodialysis, also performed three times per
week, consists of 8- to 10-hour sessions (at reduced blood and dialysate flow
rates) performed while the patient sleeps. Short daily hemodialysis consists of
2- or 3-hour sessions occurring five to six times per week. At-home dialysis is
becoming increasingly common and allows for more flexible schedules compared to
in-center hemodialysis.
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| VASCULAR ACCESS FOR HEMODIALYSIS |
Peritoneal
Dialysis. In peritoneal dialysis, dialysate is instilled into the intraperitoneal
space. Blood in the peritoneal capillaries exchanges material with the
dialysate using the peritoneal membrane as a natural semipermeable membrane.
The dialysate dwells in the peritoneum for 2 to 12 hours and is then removed.
Each sequence of instillation, dwelling, and draining is known as a cycle (or
exchange).
The
dialysate is a sterile solution that contains variable concentrations of
glucose. While the dialysate is in the peritoneum, substances such as urea and
potassium diffuse from the capillaries into the dialysate, whereas glucose and
lactate diffuse in the opposite direction. Fluid ultrafiltration occurs because
of the osmotic gradient
established by the glucose, resulting in the convective clearance of larger
molecules.
The
dialysate is instilled into the abdominal cavity through a surgically tunneled
catheter (Tenckhoff), which remains in place between sessions. Given the risk
of peritonitis with an indwelling catheter, patients must be instructed to
perform each exchange using sterile technique.
Several
different schedules may be used. In continuous ambulatory peritoneal dialysis
(CAPD), approximately four
exchanges are performed per day. During each session, 1.5 to 3 L of dialysate
dwell in the peritoneal cavity for 6 hours. The patient must manually instill
and then drain or remove the dialysate. In automated peritoneal dialysis (APD),
4 to 5 exchanges occur overnight. During each session, 1.5 to 3 L of dialysate
dwell in the peritoneal cavity for 2 hours. In this case, a machine performs
the exchanges. Continuous cyclic PD (CCPD) is a regimen in which 3 to 4
exchanges are performed automatically overnight, while 1 to 2 long dwell exchanges are performed manually during the
day.
Peritoneal dialysis is
less efficient than hemodialysis; however, because it is performed daily,
patients still attain adequate clearance and generally feel better and have
fewer dietary restrictions than with in-center hemodialysis. In addition, the
patient can perform peritoneal dialysis at home and with less equipment than
required by hemodialysis.
CONTINUOUS THERAPIES
Continuous renal
replacement therapy (CRRT) is similar to hemodialysis in some respects;
however, sessions are continuous, rather than discrete, and the fl ow rate is
lower (100 to 300 mL/min). CRRT is performed when patients require dialysis but
are hemodynamically unstable or have homeostatic abnormalities that cannot be
addressed with an individual hemodialysis session. For example, if a patient in
renal failure is expected to receive a large volume load (in the form of
transfusions or antibiotics), it may be advantageous to receive continuous
renal replacement therapy.
As in hemodialysis, a
connection is established between the patient’s vasculature and an
extracorporeal apparatus. Access is established using a central venous
catheter. AV fistulae or grafts cannot be used because the constant presence of
needles in these vessels (in contrast to the episodic presence associated with
hemodialysis sessions) could lead to damage and infection, which would prevent
future use.
CRRT uses the principles
of hemodialysis (diffusive clearance) and hemofiltration (convective clearance)
either alone or in combination. A variety of different configurations may be
used. The most common include:
Slow continuous
ultrafiltration (SCUF) In
this modality, fluid is removed from the blood by hemofiltration alone. No
dialysate is used. This modality is generally used for fluid-overloaded patients
(e.g., congestive heart failure) who are not responsive to diuretics but who
have preserved electrolyte balance.
Continuous venovenous
hemofiltration (CVVH) In
this modality, convective clearance is achieved by using hydrostatic pressure
to ultrafilter plasma across a membrane, as with SCUF. In this case, however, a
replacement fluid is added either before or after the blood enters the filter
cartridge; it is similar in content to dialysate and, when mixed with blood,
brings its electrolyte composition into a desirable range.
Continuous venovenous
hemodialysis (CVVHD) CVVHD
is similar to hemodialysis as described earlier but is continuous rather than
episodic. This modality consists primarily of diffusive clearance of small
molecules. Some convective clearance occurs, but to a lesser extent than
diffusive clearance.
Continuous venovenous
hemodiafiltration (CVVHDF) CVVHDF
combines both CVVHD and CVVH.
In this modality, a replacement solution infuses into the blood either
prefiltration or postfiltration. At the same time, a dialysate solution runs
countercurrent to the blood in the filter cartridge. This modality removes both
small and medium-sized molecules from the blood.
Studies are currently
being performed to compare the relative advantages of and indications for CVVH,
CVVHD, and CVVHDF.
