Encyclopedia of Health and Medicine

enal dialysis 217
long term. Though in theory renal dialysis could
sustain life indefinitely, in practice most people
experience a steady decline of overall health with
long-term dialysis because artificial methods of
cleansing toxins from the blood are not as effective,
efficient, or thorough as the natural processes
the kidneys perform. However, it is not uncommon
for people to use renal dialysis for 10 to 20
years or longer. There are two general types of
renal dialysis: hemodialysis and peritoneal dialysis.
Hemodialysis
Hemodialysis filters toxins directly from the blood.
The person goes to a hemodialysis center for each
dialysis treatment. A catheter inserted into a blood
vessel, usually in the arm, routes the blood circulation
externally through a machine that removes
toxins. The cleansed blood then returns to the
body through a second catheter. When hemodialysis
is long term, the doctor places a permanent
arteriovenous (AV) shunt that connects an ARTERY
and a VEIN. The dialysis machine’s cannulas then
connect to the shunt.
The hemodialysis machine consists of a pump
and a container, called the dialyzer with a semiporous
membrane inside. The membrane looks
somewhat like the filter inside a water purification
canister. On one side of the membrane is a solution
called the dialysate. The dialyzer pumps blood
into the container on the other side of the membrane.
The dialysate attracts certain substances—
minerals, electrolytes, and waste byproducts—to
cross the membrane from the blood. The dialysate
absorbs these substances. Fresh dialysate circulates
through the dialyzer at the same rate as the blood.
The blood and the dialysate never come into direct
contact with one another. Another type of filter
traps any air bubbles that are in the blood before
the blood returns to the body. The dialyzer holds
only a few ounces of blood at a time. It takes three
to five hours for the blood to circulate through the
dialyzer enough times to remove an appropriate
amount of waste and toxins. Most people need
three hemodialysis sessions every week.
In the United States hemodialysis is the standard
renal dialysis method. Many nephrologists
feel it more thoroughly cleanses the blood. However,
hemodialysis entails significant risks. Key
among these risks are INFECTION with HEPATITIS and
other bloodborne conditions, injury to the blood
vessels used to shuttle blood between the person
and the dialysis machine, and microscopic damage
to the blood cells.
Peritoneal Dialysis
Peritoneal dialysis makes use of a natural membrane
in the body, the peritoneum, which encloses
the abdominal cavity. Peritoneal dialysis is a continuous
process. Two catheters surgically inserted
into the abdominal cavity serve as the portals
through which dialysate enters and leaves the
cavity. The doctor prescribes the dialysate, which
comes premixed in single-DOSE bags.
The molecules of the dialysate are to large to
pass through the peritoneum so the solution
remains contained in the abdominal cavity. The
blood’s natural circulation carries blood through
the blood vessels (capillary networks) within the
peritoneum. As with hemodialysis, the dialysate
attracts certain molecules to cross the membrane
into the dialysate. A second catheter carries
dialysate out of the abdominal cavity. There are
two stages to peritoneal dialysis, the exchange
(draining the dialysate into and out of the abdominal
cavity) and the dwell (the time during which
the dialysate remains in the abdominal cavity.
There are two types of peritoneal dialysis:
• Continuous ambulatory peritoneal dialysis
(CAPD) instills dialysate into the abdominal
cavity using gravity to pull the dialysate into
the catheter. The dialysate remains in the
abdominal cavity for about four hours, then the
person drains it out through the second
catheter. Most people who use this method
need four treatments each day. Aside from the
30 minutes it takes to instill the dialysate and
the 30 minutes it takes to drain the dialysate,
the person is unencumbered and goes about his
or her regular activities.
• Continuous cycler-assisted peritoneal dialysis
(CCPD) uses a pump to rapidly infuse and
extract the dialysate at night when the person
is sleeping, with dwell times of about two
hours. The person then infuses the abdominal
cavity with dialysate upon awakening, and
retains the solution all day for a single long
dwell time.