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Q: Hemodialysis Machine ( Answered 5 out of 5 stars,   1 Comment )
Subject: Hemodialysis Machine
Category: Health > Conditions and Diseases
Asked by: jram444-ga
List Price: $50.00
Posted: 17 May 2006 15:00 PDT
Expires: 16 Jun 2006 15:00 PDT
Question ID: 729860
How does a hemodialysis in-center machine work?  I need to learn the details of
how it works, what is the dialyzer, how does the dialyzer interact
with the blood? and what are the causes for blood to coagulate on the
machine. What purification method is used for the water, does the
water contin any chemicals? how does the wet / dry weight interact
with the timie on the machine and the level of moisture removed? As an
example if dry weight has been set at 49.0 kg and the wet weight at
startup is 51.0 kg how does this intereact with the calculations of
time and removal rate?
Subject: Re: Hemodialysis Machine
Answered By: boquinha-ga on 18 May 2006 18:11 PDT
Rated:5 out of 5 stars
Hello jram444-ga!

I decided to divide this question into different sections based upon
your various requests. I was able to find a number of useful websites
and I think you?ll find the information that you need.

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As you are probably aware, a dialysis machine attempts to perform the
filtration functions of the human kidney. It is reserved for use in
patients with kidney failure. The primary substance the machine
removes from the blood is the waste product urea, in addition to other
salts. has a simplified description of the apparatus.

?In a dialysis machine, blood from the patient runs through tubes made
of a semi-porous membrane. Outside the tubes is a sterile solution
made up of water, sugars and other components. Red and white blood
cells and other important blood components are too large to fit
through the pores in the membranes, but urea and salt flow through
membranes into the sterile solution and are removed.?

The following is from an article by a biomedical technician, written
for dialysis patients, produced to explain in more detail some of the
machine and how it works.

?The dialysis machine mixes and monitors the dialysate. Dialysate is
the fluid that helps remove the unwanted waste products from your
blood. It also helps get your electrolytes and minerals to their
proper levels in your body. The machine also monitors the flow of your
blood while it is outside of your body. You may hear an alarm go off
from time to time. This is how the machine lets us know that something
needs to be checked.?

For another basic description of how a dialysis machine functions, see
this patient information sheet.

?During dialysis, your blood is cleaned using a fluid called
dialysate, or ?bath.? Wastes and fluid from your blood go into the
bath and are drained away. The dialysis machine controls the flow of
the blood and the bath. The dialysis machine has two systems?the
extracorporeal (outside the body) circuit and the dialysate delivery
system. The extracorporeal circuit is the tubing, blood pump, heparin
(blood thinner) pump, kidney, and monitors for blood flow, blood
pressure, and air bubbles. The dialysate delivery system of the
machine mixes the bath with purified water and checks to be sure it is

Wikipedia has a fairly detailed entry about dialysis and the machine
and chemicals used during the process. There is a nice schematic
showing the dialysis circuit, in addition to a step-by-step
description of the process. Important components of the complete
dialysis apparatus are the dialysis machine, the water delivery
system, and the dialyzer. I will discuss the dialyzer and the water
delivery systems in later sections. The primary purpose of the
dialysis machine is the ?pumping [of] the patient's blood and the
dialysate through the dialyzer.?

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The National Institute of Diabetes and Digestive and Kidney Diseases
(NIDDK), division of the National Institutes of Health (NIH), have
authored a fairly detailed information sheet for dialysis patients. In
it they describe the dialyzer.

?The dialyzer is a large canister containing thousands of small fibers
through which your blood is passed. Dialysis solution, the cleansing
fluid, is pumped around these fibers. The fibers allow wastes and
extra fluids to pass from your blood into the solution, which carries
them away. The dialyzer is sometimes called an artificial kidney.?

So from the above, we understand that the blood directly flows through
the fibrous filter in the dialyzer. The dialysis solution bathes the
filter, but does not directly interact with the blood. Concentration
gradients between the blood and the dialysate help to drive various
minerals and substances either into or out of the blood for
replacement or elimination.

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Rudolf Virchow was a 19th-century German pathologist who was credited
with describing reasons that blood clotting occurs. These three
principles are called ?Virchow?s triad.? These 3 factors are:

* Changes in the vessel wall.
* Changes in the pattern of blood flow/turbulence.
* Changes in the constituents of blood/.

A 1981 article in the New England Journal of Medicine mentions briefly
one rationale for the anticoagulation of dialysis patients during

?Anticoagulation during hemodialysis is necessary to prevent clotting
of the blood on contact with the dialysis membrane. Heparin is the
usual anticoagulant used.?

This contact with the dialyzer filter (membrane) changes the pattern
of blood flow, which would fall under the second of Virchow?s three

In a lecture for the National Kidney Foundation, Zbylut J. Twardowski,
M.D., Ph.D. of the University of Missouri spoke about a number of
issues regarding kidney dialysis. In this he mentions that many of the
large-bore catheters in the arm can fit too tightly in the vein, cause
blood vessel damage (another of Virchow?s triad), and predispose the
blood to clot formation

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Wikipedia actually has an excellent article about hemodialysis.
According to their article,

?An extensive water purification system is absolutely critical for
hemodialysis. Since dialysis patients are exposed to vast quantities
of water . . . even trace mineral contaminants or bacterial endotoxins
can filter into the patient's blood. Because the damaged kidneys are
not able to perform their intended function of removing impurities,
ions that are introduced into the blood stream via water can build up
to hazardous levels, causing numerous symptoms including death. For
this reason, water used in hemodialysis is typically purified using
reverse osmosis.?

Gambro is one of the major manufacturers of water treatment systems
used in dialysis centers. The specifications sheet discusses its
reverse osmosis process of filtration, along with its efficacy.

Minerals and electrolytes replaced during dialysis include potassium,
calcium, magnesium, sodium, chloride, lactate, bicarbonate, and
glucose. Here is an article discussing optimal replacement of these

Gambro also produces a number of dialysate solutions for patient use.
Each has a varying amount of compounds and minerals necessary in
dialysis, and each can be selected depending on specific patient
needs. Here are specification sheets for two of their products.

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Dry weight is an important measure that is monitored with each
dialysis treatment. This weight is critical to know because the
patient?s going below the ideal dry weight can cause low blood

?The amount of fluid to be removed is set by the dialysis nurse
according to the patient's ?estimated dry weight.? This is a weight
that the care staff believes represents what the patient should weight
without fluid built up because of kidney failure.?

Apparently, actually determining the ?dose? of each dialysis session
is an extremely complex mathematical mindbender. It is dependent upon
a parameter known as the fractional clearance of urea as a function of
its distribution volume (Kt/V). It is defined as: Kt/V = -Ln(R - 0.008
 t)+(4 - 3.5 R) UF/W

Ln = natural logarithm
R = BUN (blood urea nitrogen) predialysis / BUN postdialysis
t = dialysis session length in hours
UF = ultrafiltration volume in liters
W is the patient?s postdialysis weight in kilograms

The Kt/V value can also be extrapolated by using what is known as
formal urea kinetic modeling (UKM).

?Because of the complexity of the formulae that provide the
information for calculation of Kt/V by UKM, computational software is
necessary to compute Kt/V using formal UKM. Formal UKM can be used to
calculate the exact treatment time required to deliver a particular
hemodialysis dose at specified blood and dialysate flows with a
particular dialyzer. Formal UKM requires accurate measures of:

* Predialysis and postdialysis BUN for the first dialysis treatment of
the week and the predialysis BUN for the second dialysis session of
the week in a thrice-weekly hemodialysis schedule.
* Predialysis and postdialysis weights at the time of the first
hemodialysis treatment of the week.
* The actual treatment time, i.e., the exact number of minutes during
which the hemodialysis treatment was delivered on the first dialysis
treatment of the week (not the prescribed length of treatment time or
the time elapsed between putting the patient on the machine and taking
him or her off).
* The effective clearance of the dialyzer as measured in the
hemodialysis unit (not the in vitro clearance value reported by the
manufacturer alone).?

Making statements regarding adjusting time and removal rates during
dialysis based upon a patient?s wet and dry weights do not seem to be
simply done. For a complete discussion of these topics see:

This article from the American Journal of Nephrology discusses
adjusting dialysis time and dry weight as a way to better control
blood pressure.

Here is another article discussing weight and dialysis time, and their
roles in blood pressure control. It appeared in Nephrology Dialysis

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Here is more information about kidney disease and dialysis from The
Nephron Information Center.

This website is from the National Kidney Foundation and is an
excellent patient information resource.

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This should be the information that you need. If you have need of any
further clarification, please let me know how I can help.


Search terms:

dialysis machine
dialysis description
why coagulation dialysis patient
dry weight dialysis time
dialysis duration formula
formal UKM
Virchow?s triad
jram444-ga rated this answer:5 out of 5 stars
Thank you for answering my question in such detail and also thanks for
the many links to further investigate.

Subject: Re: Hemodialysis Machine
From: boquinha-ga on 21 May 2006 12:11 PDT
Thank you very much for the 5 stars! I'm so glad that you're pleased!


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