In a clinical context Dialysis is a method for removing waste such as
urea from the blood when the kidneys can no longer do the job. The two
types of dialysis are: hemodialysis and peritoneal dialysis.
In hemodialysis, the patient's blood is passed through a tube into a
machine that filters out waste products. The cleansed blood is then
returned to the body.
In peritoneal dialysis, a special solution is run through a tube into
the peritoneum, a thin tissue that lines the cavity of the abdomen.
The body's waste products are removed through the tube.
There are three types of peritoneal dialysis:
- Continuous ambulatory
peritoneal dialysis (CAPD), the most common type, needs no machine and
can be done at home.
- Continuous cyclic peritoneal dialysis (CCPD) uses
a machine and is usually performed at night when the person is
- Intermittent peritoneal dialysis (IPD) uses the same type of
machine as CCPD, but is usually done in the hospital because treatment
Hemodialysis and peritoneal dialysis may be used to
treat people with diabetes who have kidney failure.
It works by having the blood flow along one side of a semi-permeable
membrane, with the dialysis solution (usually a highly concentrated
saline) flowing along the other side. Due to the difference in
osmolarity between the two liquids, water traverses the membrane in
order to dilute the dialysis liquid, carrying along the unwanted blood
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How Hemodialysis is typically done
- Dialysis is conducted in a dedicated facility, either a special
room in a hospital or clinic that specializes in hemodialysis
- Nurses and technicians working in the facility have special
training specific to dialysis.
- A dialysis patient will be given a prescription by a
(a doctor specializing in kidney issues. All dialysis treatment
issues are ultimately referred back to this doctor or alternate,
though the attending nurse will often make minor care decisions
without referring to the doctor.
- The dialysis prescription will specify various parameters for
setting up dialysis machines. It will also specify times and durations
of dialysis sessions. In the US, 3-4 hour sessions, 3 times a week are
- The dialysis center to be used by the patient is contacted and
schedules the patient for a specific time period.
- Before or around the time the patient arrives for his/her scheduled
session, a dialysis machine will be prepared. There are many models of
dialysis machines, but typically in modern machines there will be a
computer, CRT, a pump, and facility for disposable tubing and filters.
The filters (the actual artificial kidneys) are cylindrical, clear
plastic outside with the filter material visible inside (looks like
thick paper). They are perhaps 15-18" long, and 2-3" thick. They have
connectors at both ends. The technician or nurse will setup plumbing
on the machine in a moderately complex pattern that has been worked
out to move blood through the filter, allow for saline drip (or not),
allow for various other medications/chemicals to be administered. How
the plumbing is setup may vary between models of machine and they
types of filters. For some filters, it is necessary to clear
sterilizing fluid (Renaline, or others) from the filter before
connecting the patient. This is done by altering the plumbing to push
saline through the filter, and carefully checked with a type of litmus
test. The pump does not directly contact the blood or fluid in the
plumbing - it works by applying pressure to the tubing, then moving
that pressure point around. Think of a disk with a protrusion in it.
Put this into a close fitting 270 degree enclosure. Put plastic tubing
between the enclosure and the disk, entering and exiting in the 90
open degrees. Now imagine the disk turning. It will put pressure on
the tubing, and the pressure point will roll around through the 270
degrees, forcing the fluid to move. It is characteristic of dialysis
machines that most of the blood out of the patients body at any given
time is visible. This facilitates troubleshooting, particularly
detection of clotting.
- The patient arrives and is carefully weighed. Standing and sitting
blood pressures are taken. Temperature is taken.
- Access is setup. For patients with a fistula (a surgical
modification to an arm or leg vein to make it more robust, and
therefore usable for high capacity blood movement required by
dialysis) this means inserting to large gauge needles into the
fistula. (Yes, it hurts.) Fistulas are widely considered the desirable
way to get access for hemo-dialysis, but they take time to setup and
mature. For other patients, access may be via a catheter installed to
connect to large veins in the chest. (This means no needles, but there
are other severe downsides to a catheter). There are some other
arrangements that can be made as well.
- When access has been setup, the patient is then connected to the
preconfigured plumbing - creating a complete loop through the pump and
filter. The pump and a timer are started. Hemodialysis is underway.
- Periodically (every half hour, nominally) blood pressure is taken.
As a practical matter, fluid is also removed during dialysis. Most
dialysis patients are on moderate to severe fluid restrictive diets
(in addition to other dietary restrictions). This is because kidney
failure usually includes an inability to properly regulate fluid
levels in the body. A session of hemodialysis may typically remove 2-5
kilograms (5-10 pounds) of fluid from the patient. The removal of
fluid done to achieve a predetermine "dry weight" of the patient. This
is a weight that the care staff believes represents what the patient
should weigh without fluid built up because of kidney failure.
Removing this much fluid can cause or exacerbate low blood pressure.
Monitoring is intended to detect this before it becomes too severe.
Low blood pressure can cause cramping,
lightheadedness, and unconsciousness.
- At the end of the prescribed time, the patient is disconnected
from the plumbing (which is removed and discarded, except perhaps for
the filter, which may be sterilized and reused with the same patient
at a later date). Needle wounds (in case of fistula) are bandaged with
gauze, held for 5-10 minutes with direct pressure to stop bleeding,
then the taped in place. It's just like getting blood draw, only it
takes a lot longer, and more fluid is lost.
- Temperature, standing and sitting blood pressure, and weight are
all measured again. Temperature changes may indicate infection. BP
discussed in point 10 above. Weighing is to confirm the removal of the
desired amount of fluid.
- Care staff verifies that the patient is in condition suitable for
leaving. The patient must be able to stand (if previously able),
maintain a reasonable blood pressure, and be coherent (if normally
coherent). Different rules apply for in-patient treatment - in those
cases the patient isn't leaving the facility.