Applying Scientific Principles to Dialysis

Tuesday, October 19, 2010

Applying Scientific Principles to Dialysis

Article by a Renal Nurse

Sections of Applying Scientific Principles to Dialysis

• Blood itself is a solution. Water is the solvent, and electrolyte, glucose, and many other substances are
the solutes. The principles of fluid dynamics, diffusion, UF, and osmosis apply to each dialysis treatment.
• To understand how dialysis removes fluid, you must know how fluids work inside the body.

Fluid in
•Food
• Beverages
• Intravenous fluids (if any)

Fluid Out
• Respiration
• Stool
• Perspiration and skin water loss
• Dialysate loss
• Residual urine
•Vomiting

FLUID COMPARTMENTS
• Intracellular compartments is fluid inside the cells
• Interstitial compartments is fluid in between cells
• Intravascular compartment is blood inside the vessels
• The body tries to keep equilibrium - to have the same level of osmotically-active solutes in all three compartments.
• During dialysis, only fluid from the intravascular compartment - the bloodstream – can be removed.

FLUID DYNAMICS IN DIALYSIS

• Fluid dynamics create changes in pressure as blood is pumped out of the patient's body and through tubing and the dialyzer.
• Together, the tubing and the dialyzer are called the extracorporeal (outside of the body) circuit
• When the dialysis machine is switched on and treatment starts, the blood pump speeds the flow of blood from the patient.
• Blood passes through the needle - the first restriction in the circuit.
• Because the blood pump is pulling - rather than pushing - blood through this restriction, the pressure created is usually negative: less than zero.
• The amount of flow and restriction determine negative pressure, just as with positive pressure. As the flow or the restriction increases, the pressure will decrease. (the dialysis machine checks this pressure in the blood tubing before the pump [pre-pump arterial pressure]).
• As the blood pa5ses through the blood pump, it is pushed against the resistance of:
     • The tubing
     • The tiny hollow fibers in the dialyzer
     • The small opening of the (venous) blood return needle (or catheter)
• This resistance creates positive pressure inside the lines and dialyzer fibers. As blood passes through these resistance, the pressure changes.
• The highest positive pressure is measured in the arterial header, where blood enters the dialyzer fibers (post-pump arterial pressure).
• As blood moves through the fibers, resistance drops, so pressure drops too.
• The pressure measured after blood leaves the dialyzer (venous pressure) is the lowest positive pressure in the blood path.
• The average pressure of blood entering and leaving the dialyzer fibers is the true amount of force (positive hydraulic pressure) that aids UF of water out of the blood, through the membrane, and into the dialysate
• Dialysate flows through the dialyzer and around the hollow fibers in one direction.
• Blood flows in the opposite direction for countercurrent flow
• The machine can control the pressure differential between the blood and dialysate compartment as needed to reach the desired fluid removal.
• This pressure difference across the dialyzer membrane is called transmembrane pressure (TMP).

DIFFUSION IN DIALYSIS

• The hollow fibers in the dialyzer are the semipermeable membrane.
• Blood passes through the insides of these tiny fibers (capillaries); dialysate surrounds them on the outside.
• Molecules of a certain size range pass back and forth between the blood and dialysate, always moving from an area of higher concentration to an area of lower concentration.
• Wastes in the patient's blood diffuse across the u membrane and into the dialysate.
• Used dialysate is sent to a drain and replaced with fresh dialysate, to maintain a high concentration gradient.
• This gradient allows as much waste as possible to be removed from the blood during each pass through the dialyzer.
• Electrolyte balance is also maintained with diffusion.
• It is vital to patient's health to keep the right level of electrolytes in the blood.
• To control the balance, electrolytes can be added to the dialysate.
• Electrolytes will move until the concentration is equal on both sides of the membrane.
• Keeping a constant low level of an electrolyte in the dialysate ensures that the excess is removed without allowing the levels in the blood to drop too low,
• As cleansed blood is returned to the patient, it slowly dilutes the rest of the blood.
• The drop in the concentration of solutes in the blood creates a gradient between the blood plasma (liquid
portion of blood) and the fluid in the cells and tissues.
• Because these cells have their own membranes, solutes - such as wastes and certain electrolytes -slowly pass out of the patient's cells and into the bloodstream. From there, they are dialyzed.
• This process allows some of the wastes from other body compartments to be cleared from the body by dialysis.
• This slow process of diffusion is why dialysis treatments require more than one pass of blood through the dialyzer to clear wastes from the blood
• The nephrologist factors in diffusion when prescribing a treatment. • He/she can choose a large or small dialyzer, based on the patient's body size, the length of the treatment needed, and size of molecules to be removed,

ULTRAFILTRATION IN DIALYSIS

• UF requires pressure to force fluid through the membrane.
• The dialysis machine can create a hydraulic pressure difference, with higher pressure in the blood compartment than in the dialysate compartment.
• This TMP pushes excess water out of the blood and into the dialysate.
• A dialysis prescription calls for taking off enough fluid to bring a patient an estimated dry weight (EDW) by the end of the treatment.
• To figure out how much fluid you need to remove, just subtract a patient's EDW from the predialysis weight. Then add the amount of any fluids the patient will receive during the treatment.

CONVECTION IN DIALYSIS
• As water moves from the blood compartment to the dialysate compartment, molecules of dissolved solute are dragged along too (solvent drag).
• This process of solvent movement is called convection.
• The ease with which the solute is dragged along by the solvent is determined by the size of the solute molecule compared to the size of the membrane pores.
• Smaller solutes move easily, so the solution can sieve across the membrane without any change in concentration.
• Larger solutes move more slowly and the rate of convective transport is also slower.
• The convective transport of a solute depends on how porous (both size and number of holes) the membrane is.
• This measurement of porosity is known as the sieving coefficient (SC) of the membrane.

OSMOSIS IN DIALYSIS
• Osmotic forces decide which way water will move from one body compartment to another.
• In hemodialysis, diffusion lowers the solute concentration in the blood.
• Higher solute concentration in the tissues and cells then pulls water out of the blood.
• Rapid drops in blood volume car occur, which causes drops in blood pressure And other symptoms.
• Often, sodium is added to the dialysate, so it diffuses into the blood.
• The higher blood sodium draws water from other body compartments into the blood, so it can be removed by UF.
• The sodium in the dialysate is then lowered towards the end of the dialysis treatment to pull the sodium back out of the bloodstream.