Presented at Renal Trainee Seminar October 2012 - Belfast City Hospital

1. Principles of Haemodialysis
Richard McCrory
10/10/2012

2. Summary
• A description of the HD technique
– Physics of solute and fluid removal
• Diffusion / Convection
– Haemodialysis vs. Haemofiltration vs.
Haemodiafiltration
• The importance of water purity
• Dialysate
– Sodium Profiling

3. The Aim of Haemodialysis
‘To deliver blood in a fail safe manner from the
patient to the dialyzer, to enable an efficient
removal of uraemic toxins and fluid, and to
deliver the cleared blood back to the patient’

4. Components of Haemodialysis
Blood Circuit
Patient | Vascular Access
Blood Tubing and HD Machine
Dialysate Circuit
Dialysate / Dialysis Tubing
Water Treatment System
The Dialyzer

5. The Dialyzer
Composition
1)Blood Compartment
2)Dialysate Compartment
3)Semipermeable membrane
4)Membrane support structure

6. Dialyzer Design
Hollow Fibre – ‘Cylinders within cylinders’
An ‘ideal’ dialyzer
– Maximises surface area
– Smallest amount of volume at given time

7. And now for some mathematics…
The Hagen-Pouseille Equation
‘Blood flow more strongly dictated by radius
rather than length’

8. Diffusive Clearance
• Solute transfer across semipermeable
membranes along concentration gradients
– Mass transport without bulk motion

9. Factors influencing Diffusion Rate
• Concentration gradient
• Molecular weight/size
• Membrane surface area
• Membrane Permeability
• Blood flow rate (Qb)
• Dialysate flow rate (Qd)

10. Relationship between blood flow
and solute clearance is non-linear

11. Convective Clearance
Solutes move independent of solute activity
(size, concentration) but dependent on the
rate of solvent flow
– Provided size of pore permits movement
Solutes travel at the same rate
irrespective of size

12. The Flux of a Dialyzer
Determined by Ultrafiltration Coefficient
Quantity of pressure that must be exerted
across dialysis membrane (transmembrane
pressure) to generate a given volume of
ultrafiltrate per unit time
Low Flux – Less leaky to water
High Flux – More leaky to water

13. The Sieving Co-efficient
S = Cfiltrate / Cplasma
Any value between 0 (no transport) and 1 (unrestricted
transport)
For HIPS 20
SBeta-2Microglobulin = 0.8
SAlbumin = 0.005

14. Summary: Overall Clearance of Solute
Small Molecules
– Dictated mostly by diffusion
– Efficacy limited by fluid flow
Middle Weight Molecules
– Dictated mostly by convection
– Efficacy limited by properties of the membrane

15. Techniques of Haemodialysis

16. Haemodialysis

17. Isolated Ultrafiltration

18. Haemofiltration

19. Haemodiafiltration

20. Haemodiafiltration
• HDF combines diffusive, convective and
adsorptive clearances in the same unit
• More clearance of middle weight molecules
without failure of small molecule removal

21. Question
Haemodialfiltration as a modality of dialysis has
been associated with:
a) Improved EPO response
b) Improved Phosphate Clearance
c) Reduction in rates of intradialytic
hypotension
d) Improved Mortality Risk
e) All of the above

22. Question
Haemodialfiltration as a modality of dialysis has
been associated with:
a) Improved EPO response
b) Improved Phosphate Clearance
c) Reduction in rates of intradialytic
hypotension
d) Improved Mortality Risk
e) All of the above

23. Question
Which component of a HD water system is most
important in ensuring that HD patients are not
exposed to microbiological or chemical
contamination?
A)Good quality water from water provider
B)Carbon Filters
C)Ion Exchange Filters
D)Reverse Osmosis
E)Effective Disinfection of the HD machine

24. Question
Which component of a HD water system is most
important in ensuring that HD patients are not
exposed to microbiological or chemical
contamination?
A)Good quality water from water provider
B)Carbon Filters
C)Ion Exchange Filters
D)Reverse Osmosis
E)Effective Disinfection of the HD machine

25. ‘As human beings,
you and I need
fresh, pure water
to replenish our
precious bodily
fluids.’

26. Water Exposure in Haemodialysis
• Haemodialysis patients may be exposed to
350 to 500 L of water per week through a non-
selective membrane, depending upon their
treatment time and dialysate flow rate.
– In contrast to an average 12 litres per week
through a highly selective membrane (intestinal
tract) in healthy individuals.

27. Regulation of Water Purity
‘Guideline on water treatment facilities,
dialysis water and dialysis fluid quality for
haemodialysis and related therapies’ – Jan
2012
Association of Renal Technologists
http://www.artery.org.uk

28. International Regulation of Water
Purity for RRT
• BS ISO 13959; 2009: Water for haemodialysis and
related therapies,
• BS ISO 11663; 2009: Quality of dialysis fluid for
haemodialysis and related therapies,
• BS ISO 26722; 2009: Water treatment equipment for
haemodialysis and related therapies.
• BS ISO 23500; 2011: Guidance for the preparation
and quality management of fluids for haemodialysis
and related therapies

29. Standards (From ART Guidelines 2012)
‘…dialysis water shall contain a total viable microbial count of less than
100 CFU/ml and an endotoxin concentration of less than 0.25 EU/ml‘
HOWEVER
‘…The concentrations of microbial contaminants and endotoxin in
ultrapure dialysis fluid shall be < 0.1 CFU/mL and < 0.03EU/mL
respectively when used for high flux haemodialysis…’

30. Water Treatment System

31. Aluminium Toxicity
Aluminum, in the form of alum, is added to water to
remove suspended colloidal matter; this process is
called flocculation.
Chronic exposure to aluminum is also associated with
severe bone disease and erythropoietin-resistant
anemia
– Speech abnormalities
– Myoclonic muscle spasms
– Personality changes
– Seizures
– Dementia

33. Dialysate
‘Dialysate characteristics influence the final
concentration of blood solute, intermediary
protein, carbohydrate, and lipid metabolism
and affect systemic vasomotor tone, cardiac
contractility and rhythm, pulmonary gas
exchange, and bone turnover.’
AJKD, Vol 46, No 5 (November), 2005: pp 976-981

34. Composition
• Sodium (standard or ‘profiled’)
• Potassium (variable)
• Calcium (variable)
• Buffer
– Bicarbonate
– Kept separate from the remainder of dialysate to
prevent precipitation of calcium carbonate
– Mixed in machine to achieve pH=7.40

35. Sodium Profiling
Considered a means of reducing intradialytic
hypotension
2 hypotheses:
1) High dialysate sodium at the beginning of dialysis
combats movement of extracellular water into the
intracellular space.
2) Diffusion of sodium from dialysate into plasma
water would exert an osmotic effect tending to
increase plasma volume filling.

36. Fluid shifts during Ultrafiltration

37. Fluid Movement during ‘Standard’ HD

38. Sodium Profiling
• Requires techniques to accurately follow the
changes in plasma sodium levels during the
session.
• Only advantageous for the patient in the long-
term if it results in a neutral sodium balance.
– Otherwise excess sodium will lead to higher IDWG,
thirst, hypertension

39. Summary
• Haemodialysis achieves its aims by a
combination of diffusive and convective
clearance.
• High standards of water purity required for
safety of the HD patient
• HDF holds promise for improved dialysis
outcomes but hurdles for implementation
need overcome