Principles of Haemodialysis

Richard McCrory10/10/2012

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

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’

Components of Haemodialysis

Blood CircuitPatient | Vascular AccessBlood Tubing and HD Machine

Dialysate CircuitDialysate / Dialysis Tubing

Water Treatment System

The Dialyzer

The Dialyzer

Composition

1)Blood Compartment2)Dialysate Compartment3)Semipermeable membrane4)Membrane support structure

Dialyzer Design

Hollow Fibre – ‘Cylinders within cylinders’An ‘ideal’ dialyzer

– Maximises surface area– Smallest amount of volume at given time

And now for some mathematics…

The Hagen-Pouseille Equation

‘Blood flow more strongly dictated by radius rather than length’

Diffusive Clearance

• Solute transfer across semipermeable membranes along concentration gradients– Mass transport without bulk motion

Factors influencing Diffusion Rate

• Concentration gradient• Molecular weight/size• Membrane surface area• Membrane Permeability• Blood flow rate (Qb)

• Dialysate flow rate (Qd)

Relationship between blood flow and solute clearance is non-linear

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 rateirrespective of size

The Flux of a Dialyzer

Determined by Ultrafiltration CoefficientQuantity 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 waterHigh Flux – More leaky to water

The Sieving Co-efficient

S = Cfiltrate / Cplasma

Any value between 0 (no transport) and 1 (unrestricted transport)

For HIPS 20

SBeta-2 Microglobulin = 0.8SAlbumin = 0.005

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

Techniques of Haemodialysis

Haemodialysis

Isolated Ultrafiltration

Haemofiltration

Haemodiafiltration

Haemodiafiltration

• HDF combines diffusive, convective and adsorptive clearances in the same unit

• More clearance of middle weight molecules without failure of small molecule removal

Question

Haemodialfiltration as a modality of dialysis has been associated with:

a) Improved EPO responseb) Improved Phosphate Clearancec) Reduction in rates of intradialytic

hypotensiond) Improved Mortality Riske) All of the above

Question

Haemodialfiltration as a modality of dialysis has been associated with:

a) Improved EPO responseb) Improved Phosphate Clearancec) Reduction in rates of intradialytic

hypotensiond) Improved Mortality Riske) All of the above

QuestionWhich 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 providerB)Carbon FiltersC)Ion Exchange FiltersD)Reverse OsmosisE)Effective Disinfection of the HD machine

QuestionWhich 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 providerB)Carbon FiltersC)Ion Exchange FiltersD)Reverse OsmosisE)Effective Disinfection of the HD machine

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

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.

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 Technologistshttp://www.artery.org.uk

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

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…’

Water Treatment System

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

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

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

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.

Fluid shifts during Ultrafiltration

Fluid Movement during ‘Standard’ HD

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

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