Principles of hemodialysis

1. PRINCIPLES OF HEMODIALYSIS

2. • DEFINITIONS
• CONCEPT OF CLEARANCE
• UREA REDUCTION RATIO
• Kt/V RATIO

3. Dialysis is a process whereby the solute composition of a solution,
A, is altered by exposing solution A to a second solution, B, through
a semipermeable membrane until they reach equilibrium
Water molecules and low-molecular-weight solutes in the two solutions can
pass through the membrane pores and intermingle, but larger solutes
(such as proteins) cannot pass through the semipermeable barrier,
and the quantities of high-molecular-weight solutes on either
side of the membrane will remain unchanged.

4. blood
membrane
dialysate

5. Diffusion
`
Blood
side
Dialysate
side
Semipermeable
Membrane
Larger solute
difuse
slower
Small solutes
diffuse
easier

6. Factors affecting diffusion
`
Diffusion process is
slower in :
1- Bigger molecules .
2-Thicker
Dialysis
membrane
3-low Temperature
4-Low
concentration
gradient

8. • DIFFUSION REFERS TO SOLUTES MOVEMENT

9. • DIFFUSION DEPENDS ON VELOCITY
IT REFERS TO SOLUTE
OSMOSIS REFERS TO MOVEMENT OF WATER
ULTRAFILTRATION(CONVECTION)-REFERS TO SOLVENT DRAG.IT REFERS
TO BOTH SOLUTE AND WATER
FACTORS- OSMOTIC PRESSURE
-MEMBRANE PRESSURE DIFFERENCE(TMP)

11. Hydrostatic ultrafiltration
a. Transmembrane pressure.
The rate of ultrafiltration will depend on the total pressure difference
across the membrane (calculated as the pressure in the blood
compartment minus the pressure in the dialysate compartment).
b. Ultrafiltration coefficient (KUF). It is the permeability of dialyzer
membranes to water and is a function of membrane thickness and
pore size.
KUF is defined as the number of milliliters of fluid per hour that will be
transferred across the membrane per mm Hg pressure gradient
across the membrane.

12. The number of ml/hour of water the dialyzer can remove for every 1
mm Hg rise in TMP
E.g. a HF with KUF of 1 can remove 100ml/hour with
TMP of 100
For volumetric machines HF with KUF above 4 should be used to give
accurate results

16. Always a
concentration
Gradient

17. Why is ultrafiltration needed for water in presence
of diffusion for solutes inspite of osmosis?
Hemofiltration and hemodiafiltration. Whereas diffusive removal of a solute
depends on its size, all ultrafiltered solutes below the membrane pore size are
removed at approximately the same rate. This principle has led to use of a
technique called hemofiltration, whereby a large amount of ultrafiltration (more
than is required to remove excessive fluid) is coupled with infusion of a
replacement fluid in order to remove solutes.
Although hemodialysis and hemofiltration often show comparable
removal of small solutes such as urea (MW 60), hemofiltration can
effect much higher removal of larger, poorly diffusible solutes, such as
inulin (MW 5,200). Sometimes hemodialysis and hemofiltration are
combined. The procedure is then called hemodiafiltration.

18. NEED OF COUNTERCURRENT MECHANISM
If the blood and dialysate were left in static contact with each
other via the membrane, the concentration of permeable
waste products in the dialysate would become equal to that in
the blood, and no further net removal of waste products would
occur.
In Dialysis, concentration equilibrium is prevented, and the concentration
gradient between blood and dialysate is maximized, by continuously refilling the
dialysate compartment with fresh dialysis solution and by replacing dialyzed
blood with undialyzed blood.
The purpose of “countercurrent” flow is to maximize the concentration
difference of waste products between blood and dialysate in all parts of the
dialyzer.

19. solute blood dialysate direction
UREA high zero To Dx
OTHER TOXINS high zero To Dx
Sodium 135-140 135-140 NO
Potassium Above 5 1.4-3.0 To Dx
Magnesium Above 1 0.5-1.0 To Dx
glucose +/-140 (8) 180 (10) +/-
chloride 100-119 100-119 NO
Ionized Calcium 4.5-5 mg/dl
2-2.5mEq/L
5-6 mg/dl
2.5-3 mEq/L
+/-

20. UREA EXTRACTION RATIO

23. In this case, the flow rate of the cleared
stream is simply 60% of the inlet flow rate. If the inlet flow rate
is 400 mL/min, the flow rate of the cleared stream would be
0.60 × 400 = 240 mL/min, and the flow rate of the unchanged
stream would be 160 mL/min. Thus, a dialyzer extraction ratio
of 60% translates into a dialyzer clearance of 0.6 × blood inflow
rate , or 240 mL/min.

25. Effect of dialyzer blood flow rate on
clearance
we see that when blood flow is very low,
50 mL/min, the blood in the dialyzer is cleaned very well,
due to a long residence time in the dialyzer, and the outlet
SUN is only 1 mg/dL, with an extraction ratio of 99%. However,
the amount of blood cleared is limited by the flow rate
of 50 mL/min; although 99% of the blood is cleared, 99% of
50 mL/min is a low number

26. When the blood flow rate is
increased, the blood is only partially cleared of urea due to
less time spent in the dialyzer, but even though the extraction
ratio falls as blood flow rate is increased, the volume
of blood cleared of urea nitrogen keeps increasing as the
blood flow rate is increased. Ultimately, when blood flow
rate is very high, 20 L/min, the clearance in this particular
example is 600 mL/min, even though only 3% of the inlet
SUN is removed.

27. Diffusion and Backdiffusion
Movement of
molecules from the
blood side is called
Clearance or
diffusion
Movement of
molecules from the
Dialysate side is
called
BackDiffusion
Removal of Toxins
Removal of excess
K+
Backdiffusion of
Bicarbonate
Glucose and
Calcuim

28. Transmembrane
pressure (TMP) =
Positive pressure in Blood side + Negative
pressure in Dialysate side in mmHg
Maximum UF
Higher
pressure
Lower UF
Lower
pressure

29. The
magnitude
of net
filtration in
Haemodialys
is is
determined
by
1- The
hydraulic
permeability
of the
dialyzer
2-The
surface
area of the
membrane,
and by the
geometry
of the
dialyzer .
3-the
hydrostatic
and
oncotic
forces
acting on
the blood
and
dialysate
sides of

30. The K0A, mass transfer area coefficient
If the extraction ratio remained constant at 60%, a doubling of the
blood flow rate would double the clearance.
However, removal efficiency falls at higher blood flow rates, and so the
clearance does not increase with QB in a 1:1 ratio.
Ultimately, at very high blood flow rate, the clearance will plateau. The
theoretical maximum clearance of a dialyzer ( for a given solute) at
infinite blood and dialysate flow rates is called the K0A and has units of
mL/min.

31. Urea diffuse from Plasma and Blood , creatinine less and Phosphate
Much less
-
8%
-
13%

32. Effect of erythrocytes
urea diffuses into and out of erythrocytes quickly.
For example, if the outlet plasma urea nitrogen level is 40 mg/dL, the
urea concentration in erythrocytes will have been reduced to
about that level also.
When calculating the removal rate of creatinine or phosphorus in mg/min
or mmol/min, one needs to use the plasma flow rate instead of the blood
flow rate.

33. Effect of dialysis solution flow rate
Dialyzer clearance of urea (and other solutes) depends on the dialysis solution flow rate as
well.
A faster dialysis solution flow rate increases the efficiency of diffusion of urea from blood
to dialysate although the effect is usually modest.
The usual dialysis solution flow rate is 500 mL/min.
A flow rate of 800 mL/min will increase urea clearance by about 5%–8% when a
highefficiency dialyzer is used and when the blood flow rate is greater than 350 mL/min.
The optimum dialysis solution flow rate is 1.5–2.0 times the blood flow rate.

34. Dialysate delivered at a rate of 500ml/min
◦ 120 liters of dialysate / 4-hoursession
Concentrated solutions mixed with water
Usually 1:34 or 1:40
Conductivity is a measurement of electric conductivity of Na to check
if dilution is correct
With proper dilution conductivity = 13-15
Serious hyponatremia or hypernatremia occurs if dilution is incorrect

35. H+ neutralized by Na HCO3 in the body
Acetate
◦ Transformed in LIVER to HCO3 (10-15 min)
◦ BUT is a potent vasodilator
Hypotension especially with liver disease
Acetate intolerance in high flux dialyzers
Bicarbonate
◦ Immediately neutralizes H+
◦ BUT precipitates Calcium salts (CaCO3)
Should be delivered separately as NaHCO3
Short life span of machine
Needs a strong post dialysis acid rinse (citric acid)

36. Attempts are made to increase the surface area of contact
between dialysate and dialyzer
• The Hollow fiber
◦ The parallel plate dialyzer

37. ◦ Surface area.
◦ Low flux vs high flux.
◦ Biocompatibility.
◦ Technique of manufacture including hemo- adsorption.

38. Cellulose membrane (Cuprophan)
◦ Is the first membrane to be used
◦ Contains free hydroxyl radicals
◦ They are bio-incompatible (BIC)
They are able to activate complement a inflammatory reaction a
chronic inflammation a protein catabolism + anorexia + malnutrition
a Cardiovascular accidents
Cause dialysis relatedAmyloidosis
Increased incidence of infection
Rapid loss of residual kidney function
◦ Cuprophan is BIC BUT this effect can be abolished after 2nd
use

39. Substituted Cellulose
◦ Chemically bonding the free hydroxyl group
Cellulose di acetate
Cellulose Triacetate
◦ Addition of a synthetic material to cellulose
Hemophane (semi synthetic)
Synthetic modified cellulose (SMC)
Synthetic material
◦ Contains no cellulose
Polysolphone
PMMA
PAN

40. Ability of the dialyzer to clear urea from
blood
The more clearance the better the dialyzer
Clearance can be calculated in vivo=
Qb x [BUN ART – BUNVEN]
BUNART
Clearance is closely related to the surface area

42. HF with a high urea clearance
◦ They contain pores bigger in number and size
◦ Must be with bicarbonate dialysis
◦ They perform more adequate dialysis
◦ Clearance of bigger molecules toxins e.g. (B2 microglobulin)
◦ expensive

44. The more the patient’s weight the larger surface area (and clearance)
you need
Patients with increased weight gain (volume overload) need adialyzer
with high KUF