Principles of Dialysis Combined

8/9/2019 Principles of Dialysis Combined

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Principles of Dialysis



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Hemodialysis

Dialysis Can

Remove wastes and excess fluid

Facilitate control of blood pressure Restore electrolyte and acid base balance

Cannot Replace hormones that the kidney no longer

produces Renin Erythropoeitin Calcitriol



Goals of Hemodialysis

!anagement of some of the variousconditions that are a result of renal failure "remia Fluid overload Electrolyte imbalance



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Components of the Hemodialysis System

$he delivery system $he machine % dialysate

$he extracorporeal circuit Fistula needles &rterial bloodline 'enous bloodline Dialy(er )artificial kidney*

'ascular access



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Hemodialysis

Removes metabolic wasteproducts through diffusion

Removes excess fluidthrough ultrafiltration

,oth take place using asemipermeable membrane



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Principles of Hemodialysis

Definitions .emipermeable / allows passage of some

)but not all* substances and fluid



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.olutes / !olecules and electrolytes)particles* suspended in fluid the solids in asolution

Clearance / Rate of removal of specificsolutes Dialysate / a solution in which water is the

solvent and many dissolved substances such

as electrolytes and glucose are the solutes Electrolytes )34 Ca34 !g334 5a3* are present in

dialysate in concentrations similar to the levels inblood





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Definitions Diffusion / movement of solutes )particles*

through a semipermeable membrane from an

area of greater concentration to an area oflesser concentration



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Diffusion

$he movement ofparticles )solutes*through a

semipermeablemembrane from anarea of greater soluteconcentration to anarea of lesser soluteconcentration

semiermeable

membrane

dialysate

urea

bloodstream

creatinine



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Diffusion

Diffusion also depends

on: blood flow rate dialysate flow rate treatment time



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Diffusion

Salt particles pass

through cellophane

Solutions reach equal

concentration



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Principles of Dialysis

,oth osmosis anddiffusion can occur atthe same time across asemipermeable

membrane $he end result is that

movement continuesuntil the concentrationof moleculesequilibrates )becomese;ual* on both sides ofthe membrane



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Convection (Solute Drag)

<hen fluid crosses asemipermeable membrane ).=!*

some solutes are pulled along with it !ost effective way to remove large

solutes



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Adapted with permission of Amgen Inc.



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Diffusion occurs continuously in the patient>sbody as well as in the hemodialysis system 7nce dialysis has changed the chemical

concentrations of the blood4 that blood isreturned to the patient>s body where it slowlydilutes the rest of the blood

<aste products and electrolytes pass out ofthe patient>s cells and into the vascular system ? which in turn can be dialy(ed



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Factors Affecting Rate of Diffusion

$he nature of thesolution concentration

gradients molecular weight of the

solutes

temperature

$he nature of themembrane membrane

permeability surface area flow geometry



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Concentration Gradient

Difference in concentration of a given solutein a patient>s blood compared to fresh

dialysate Differences in concentration of solutes

between intercellular4 extracellular andvascular fluids



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Concentration Gradient

.olute movement can bein either direction

.olutes move toward thearea with lowerconcentration

$he larger the difference inconcentration across themembrane4 the higher therate of diffusion



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olecular !eight

.mallmoleculesmove fasterthan large

molecules



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"emperature

.olute particles

move faster athigher temperatures



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em#rane Permea#ility

@ow numerous are theporesA

@ow large are theporesA

@ow thinBthick is themembraneA



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Surface Area

s the amount ofmembrane in direct

contact with the bloodand dialysate

arger surface areasallow more diffusion

Range 9+ 22 .;uaremeters of surface area



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Flo$ Geometry

Countercurrent ,loodflows one way whiledialysate flows the

opposite way oal is to maintain a

concentration gradientand movement of

solute out of the blood



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Fluid Removal

"ltrafiltration <ater removal from the blood due to a

pressure gradient across a membrane "sed to remove excess water that has

accumulated in the tissue spaces betweendialysis treatments



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Filtration

$he movement of fluidthrough asemipermeablemembrane based on

the hydrostatic pressureexerted by the fluidagainst the membrane



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%ltrafiltration (%F)

$he removal of additionalfluid through asemipermeablemembrane as a result of

increasing the hydrostaticpressure differenceacross that membrane



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Fluid Removal

Adapted with permission of AmgenInc.



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%ltrafiltration

Pressure

Cellophane



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Positive Hydrostatic Pressure

=ositive pressure can be illustrated withan ordinary sprinkler hose =ositivepressure is exerted against the sides of

the hose as the water moves through it<ater is forced through the holes



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Fluid Removal

=ressure is exerted on the walls of themembrane on both sides of themembrane

,lood side pressure is positive Dialysate side pressure can be negative

or positive and is regulated by the dialysismachine

$he difference in pressure4 )blood sideminus dialysate side* is called the$ransmembrane =ressure or $!=



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Dialysis Concepts




$he primary purpose of dialysis is to removewaste products4 remove excess fluid andmaintain a patient>s electrolyte balance

"nderstanding the principles of hemodialysiswill help us to do Gust that safely andeffectively



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Dialysis vs& Functioning 'idney

idney Functions <aste removal <ater removal Electrolyte balance

&cid base balance ,lood pressure control

Endocrine functions Erythropoietin Renin Calcitriol

@emodialysis H H H

H H )partially controlled with

ultrafiltration medicationsoften re;uired*

5o !ust be controlledusing medications

Documents

Principles of Dialysis Combined