Principles of Hemodialysis

What are the two concentrates used to make dialysate so solids do not form?

- Acid concentrate has CALCIUM AND MAGNESIUM - Bicarbonate concentrate has BICARBONATE AND CARBONATE

What happens during a dialysis treatment?

- Urea (smal molecule) is quickly removed from the blood. - The blood urea level drops. This forms a gradient between the blood and fluid in the interstitial space. - The gradient forces urea to move from the interstitial space into the blood. - Now, a gradient forms between the interstitial space adn fluid inside the cells. - The gradient forces urea to move from inside the cells into the interstitial space.

What factors affect mass transfer rate?

-Constant temperature - effective membrane surface area - membrane permeability - BFR DFR - flow geometry

What factors affect the diffusion or removal of toxins in dialysis?

-Dialysate temperature: The higher the temperature, the greater the solute removal. - Dialysate Blood Flow Rate: The greater the dialysate flow rate, the greater the removal of solutes. - Blood flow Rate: The greater the blood flow rate, the greater the removal of solutes. - Molecular weight of solutes: The smaller the molecular weight, the greater the removal of solutes. - Concentration Gradient: the greater the concentration gradient, the greater the amount of diffusion. - Membrane permeability: The more permeable the membrane, the greater the removal of solutes.

Hemodialysis can do what?

-Remove wastes and excess fluid - Facilitate control of blood pressure - Restore electrolyte and acid base balance

What is an example of a isotonic solution?

.9 Normal Saline

Most of the water in our bodies is inside our cells, and so are most wastes. To be removed by dialysis, wastes must do what?

1. Diffuse through cell walls into the interstitial space. 2. Diffuse through blood vessels walls and into the blood stream. 3. Flow into the blood compartment of the dialyzer. 4. Pass throught the dialyzer membrane and into the dialysate.

When blood passes through the blood pump, it is pushed against the resistance of what?

1. The tubing. 2. Tiny hollow fibers (the membrane) in the dialyzer 3. The venous needle or CVC

What is reverse filtration?

A blood/dialysis fluid profile within the dialyzer that is positive (Blood to fluid) near the blood inlet but that may, under some circumstances, become negative (fluid to blood) toward the outlet. The movement of dialysis fluid into blood is known as reverse filtration.

What is a concentration gradient?

A concentration gradient is necessary to accomplish solute removal in dialysis. It simply means a difference in concentration.

What is biocompatibilty?

A measure of the reaction between the blood and the dialyzer membrane.

What is a solution?

A mixture of a solvent and a solute.

The lowest positive pressure in the blood path (tubing) is found where?

After blood leaves the dialyzer

What is mass transfer rate, or solute flux?

Artificial kidneys, dialyzers, are designed to remove metabolic wastes from the body, restore water and electrolye balance, and correct acid-base disturbances. The dialysis process involves transport of unwated or excess solute and excess water from the blood across a semipermeable membrane. The engineering term for such transport is mass transfer, and the rate of movement is mass transfer rate, or solute flux.

Another use for concentration gradients include what?

Balance electrolye levels in patient's blood.

Solubility can be affected by what?

Concentration - only so much solute can dissolve before a solvent is saturated (full) pH - acid or base level of the solvent and solute Temperature of solvent - Solutes dissolve best in warmer termperatures Other solutes- Solids (called precipitate) can form when some solutes mix. We made dialysate out of two solutions, then mix them, to avoid this.

What is the TMP?

Determines how much water crosses the membrane. The TMP reflects both positive and negative pressures in the dialyzer. Positive pressure is applied to the blood side of the dialyzer, which pushes the plasma fluid out. Negative pressure is applied to the dialysate side of the dialyzer to pull the plasma fluid out of the blood compartment necver exerts a pressure more positive than the blood compartment. (Reverse Filtration)

What is diffusion?

Diffusion also known as conductive transport is the movement of solutes from an area of greater concentration of solute to an area of lesser concentration of solutes.

What is the basis of dialysis?

Diffusion of particles across a semipermeable membrane

How is acid base balance achieved during hemodialysis?

During the dialysis treatment, there is a transfer of bicarbonate from the dialysate tot he blood. The diffusion of bicarbonate helps the patient to achieve acid-base balance by buffering the hydrogen ions.

What is meant by clearance?

Empirical measure indicating a calculated volume of blood completely cleared of a substance in a given time. Clearance is expressed in mL/min.

What is the relationship between the hydrostatic pressure and the ultrafiltration rate?

For a particular dialyzer, at any given TMP, a certain amount of fluid will be removed per unit time at specific blood and fluid flow rates. During the investigational phase of a new dialyzer, an average ultrafiltation rate per mm Hg TMP is calculated. This is the ultrafiltation coefficient (K UF) and is unique to each dialyzer. The K UF is expressed as mL/hr of fluid removed for each mm Hg. Ther the K UF, the greater the amount of fluid that can be removed with less pressure being applied to the semipermeable membrane.

Viscosity is largely a matter of __________.

Hematocrit. The viscosity of blood of 30% hematocrit is approximately 2.3 to 2.5 centripose, about 2 to 2.5 times that of water.

What does hemodialysis mean?

Hemo means blood. Dialysis connotes a separation or filtration process. Metabolic wastes or toxins are filtered from the blood by a semipermeable membrane and carried away by the dialysis fluid. The goals of hemodialysis are to manage the uremia, fluid overload, and electrolyte imbalances that occur as a result of chronic kidney failure.

What does clearance mean?

How fast any solute moves across a membrane

What is solubility?

How well a solute will fully dissolve in a solvent.

How does ultrafiltration occur?

Hydrostatic pressure is the pressure that forces plasma fluid out of blood compartment adn into the dialysate compartment of the dialyzer. The rate of fluid removal is influenced by the difference in hydrostatic pressure of the blood and dialysate compartments. The difference in the hydrostatic pressure of the blood and fluid represents the transmembrane pressure (TMP). T

What is an example of a hypertonic solution?

Hypertonic saline is a >.9 solution

What does interstitial space refer to?

In between cells - about 20% of body water

What is the Ultrafiltation coefficient?

KUF

What are the goals of hemodialysis?

Manage some of the various conditions that are a result of renal failure: - uremia - fluid overload - electrolyte imbalance

What is osmosis?

Movement of solvent (fluid) from an area of lesser concentration of solutes to are of greater concentration of solutes, through semipermeable membrane. OCEAN (WATER --- OOOOHH LOOWWW)

Hemodialysis cannot do what?

Replace hormones that the kidney can no longer produce such as renin, erythropoeitin, and calcitriol.

Are membranes permeable to middle and large molecules?

Several synthetic materials are used for high flux dialysis. These incluse polyacrylonitrile (PAN), polucarbonate, polysulfone, polyamide, polymethyl-methacrylate (PMMA), and other membrane materials.

What is sodium modeling?

Sodium modeling or sodium variation is a tool that may be used to minimize some of the complications associated with the hemodialysis treatment. It helps maximize the refilling of the vascular space during ultrafiltration.

What is solute?

Solute is a substance that can be dissolved

What is meant by diffusive transport?

Solute particles diffuse through the dialysis membrane from the side of higher concentration to lower. This movement is knowns as diffusive transport or conductive transport.

In dialysis.. What is the solution? What is the solvent? And what are the solutes?

Solution in dialysis: Dialysate Solvent in dialysis: Treated water Solutes in dialysis: Electrolytes and glucose

What is solvent?

Solvent is a fluid.

What is an example of a hypotonic solution?

Sterile water

What are the components of the hemodialysis system?

THE DELIVERY SYSTEM - the machine and dialysate THE EXTRACORPOREAL CIRCUIT - Fistula needles - Arterial bloodline - Venous bloodline - Dialyzer VASCULAR ACCESS

What affects the resistance in the blood circuit?

The 2 major components are 1. viscosity of the blood 2. deometry of the blood pathway

What is sieving coefficient?

The amount of solute convected across a memnbrane in proportion to the quantity of fluid ultrafiltrated depends on particle size related to poe size. If the pore to particle ratio is high, there is no restriction of solute transfer and the sieving coefficient is said to be 1. If none of the particles can be squeezed through, the sieving coefficient is 0.

How is bicarbonate used in dialysis fluid production?

The concentrate to be used is packaged in two parts. The "acid concentrate" contains chemicals other than sodium bicarbonate, plus a small amount of acid. The "bicarbonate concentrate" has the sodium bicarbonate and some sodium chloride (necessary to increase conductivity for monitoring purposes). Three streams of fluid are blended by the proportioning equipment: 1. water (34 parts) 2. acid concentrate (1 part) and 3. bicarbonate concentrate (1.8 parts)

Why are all the solutes and water in blood not removed by the dialyzer?

The dialysis fluid is an electrolye solution similar in compostition to normal plasma water. Water molecules cross the membrane in both directions, as do electrolytes and other small particles. Only if the concentration of a particular kind of particle is greater on one side than on the other will there be a net flow from the side of high concentration to the side of lower concentration. Solutes and waste products of a small molecular size diffuse from the blood side concentration (high) to the dialysate concentraion. (low) This the concentration gradient.

What is meant by flow geometry?

The direction the flow of the blood and dialysate. Countercurrent flow is when the blood and dialysate flow in opposite directions, creating optimal concentraion gradient. Concurrent flow is when the blood and dialysate are both flowing in the same direction, creating a much smaller concentration gradient.

What does extracellular refer to?

The extracellular space is OUTSIDE the cells which includes, interstitial space and intravascuar space.

What does intracellular refer to?

The intracellular space is INSIDE THE CELLS - about 70% of body water

What is filtration slide?

The movement of fluid through a semipermeable membrane based on the hydrostatic pressure exerted by the fluid against the membrane.

How does the semipermeable membrane function in hemodialysis?

The patient's blood is passed through a compartment formed by the membrane. Dialyzing fluid surrounds this compartment. Red cells. white cells, platelets, adn most plasma proteins are too large to pass through the pores of the membrane. Water and small particles such as electrolytes cross by diffusion, creatinine, and glucose.

What determines the rate of diffusive transport?

The rate of transfer depends on the following: -Concentration gradient across the membrane for each solute. -The surface area of the membrane; the greater the area, the more solute moved per unit time. - The mass transfer coefficient for the solute of interest for the particular membrane. The mass transfer coefficient increases for thinner or more porous membranes. It is also affected by the flow rate of both blood and dialysis fluid.

What does isotonic refer to?

The same level of solutes as blood. For example, normal saline has the same concentration of sodium chloride as blood.

What is a semipermeable membrane?

The semipermeable membrane is a selective membrane and acts as a sieve. The semipermeable membrane used in dialysis allows passage of some substances and fluid, but not all.

What is concection and when does it occur in dialysis?

The transfer of heat and solutes by physical circulation or movement of the parts of a liquid or gas. In dialysis, convective transport leads to solvent drag. As a solvent crosses a semipermeable membrane, it drags smaller solutes along with it.

What is ultrafiltration profiling?

UF profiling is a technology available on some dialysis machines to vary the volume of fluid removes during the course of the dialysis treatment. Normally the dialysis nurse or technician will endter the volume of fluid to be removed or the patient's goal. The mac hine will automatically divide the total volume based on what profile has been chose. All of the profiles will remove the required total volume during the patient's treatment, but at different intervals and rates.

What shifts occur between intracellular and extracellular fluid compartments during hemodialysis?

Ultrafiltation can rapidly remove fluid from the vascular compartment. If the rate of removal exceeds the repletion rate from interstitial space, hypovolemia and hypotension occur. Infusion of hypertonic saline will increase saline will increase osmolarity in both the vascular and extravascular spaces. This in turn attracts fluid from the much longer intracellualar pool and avoids the hypovolemia that causes the low blood pressure.

How do we reduce the loss of electrolytes out of the blood during dialysis?

We match electrolyte levels in the dialysate closet to the patient's normal blood levels.

When does filtration occur?

When Fluid is pushed through a filter by hydraulic pressure. Fluid always moves from higher pressure to a lower one. The filter traps any matter that is too larger to pass through it.

What is convective transport?

When water moves across a membrane because of a pressure gradient (ultrafiltration), there is a friction effect on solute molecules, called solvent drag. "Carried together"

The highest positive pressue is in the arterial header of the dialyzer, this is where?

Where blood enters the hollow fibers (Post pump arterial pressure)

What does hypotonic refer to?

a lower level of solutes than blood. For example, pure water has less sodium chloride than blood

In dialysis, what do we create between dialysate and the patient's blood to force wastes to move out of the blood?

concentration gradient

What can occur because the vascular refilling is delayed?

hypotension cramping right-sided heart failure hypoalbuminemia

What complications can be prevented with the use of sodium modeling?

hypotension and cramping

What does intravascular space refer to?

inside the blood vessels - about 10% of body water

What is an estimated dry weight EDW?

patient's "real" weight without any excess water

In dialysis, only what can be removed from the bloodstream?

water and solutes