1. What Dialysis Replaces

Dialysis the way it is used is in a way a misnomer. The term technically refers to clearance (the definition of dialysis is to loosen apart), but it is often used to include both clearance and fluid removal.

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2. How Does Dialysis Work? (Tea vs Coffee)

 

As noted above, one can accomplish clearance in two ways, diffusion and convection

 

 

Diffusion

Occurs when solutes move from high concentration → low concentration. The comparison is to making tea- the tea diffuses out of the teabag into the water

Example:

• Blood urea: 80

• Dialysate urea: 0

Urea diffuses across the membrane to reach

equilibrium.

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Convection

Water is pushed across the membrane

and drags solutes with it. The comparison

is to coffee- you compress the grounds and

force them through the grounds

along with water to make coffee.

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Better for removing:

• Cytokines

• Larger molecules

 

 

 

 

 

 

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Dialysis modalities

Broadly, there are two modalities of dialysis: hemodialysis and peritoneal dialysis. The goals of dialysis (fluid removal and clearance), while occurring in different ways, is the common theme across all modalities.

 

3. Hemodialysis (HD)

Hemodialysis uses an artificial dialyzer membrane.

Blood leaves the patient → enters the dialyzer → passes through tons (thousands!) of thin tubes that have a semipermeable membrane while dialysate flows on outside at a predefined concentration of sodium, potassium, urea,  etc. to create that concentration gradient (i.e. dialysis). (see image above).

     

Ultrafiltration

• Removes excess fluid by exerting a pressure on the blood from one side of the membrane to the other as well as some electrolytes, but less than diffusion. Therefore, one can only do an UF session with some electrolyte changes but not as significant as with dialysis. This is useful if someone only needs/needs more fluid removal. 

 

Typical HD session:

• 3–4 hours

• 3-4 times per week typically.

 

Advantages (vs PD in chronic patients).

                Fewer hours attached to a machine

                Fewer times a week

                No need for equipment at home

Disadvantages

              Have to go to an HD center at a particular time 3-4 times a week for 3-4 hours at a time

              Fluid shifts are significantly more and some patients don’t tolerate it all that well.

              More of an option for older children who are not on a total liquid diet as there is only so much fluid to remove in                      one session

 

Below is a sample HD prescription. This is a prescription for someone who may come 3 or 4 times a week. the dialysate flow rate is typically a 2:1 ratio to the blood flow rate to maximize diffusion. We remove fluid to a target weight (EDW= estimated dry weight) to prevent complications of fluid overload. Access is via AV fistula.

 

Aside: some patients do make significant amounts of urine but the renal function requires them to be on dialysis. some patients need very little fluid removal. some patients who have slightly better function may only need HD twice a week. 

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HD prescription
K+: 2 mEq
Ca++: 3.5 mEq
Bicarb (mEq): 37
Na+: 140 mEq
Dialyzer: F180
Dialysate Temperature (C): 37
Blood Flow  Rate -As tolerated to a maximum of: 350 mL/min
Dialysis Flow Rate: 700 mL/min
Duration of Treatment: 3 Hours
Dry weight (kg): 60kg
Goal Fluid to remove: to Estimated Dry Weight
Tubing: Adult combiSet
Laterality: Right
Access Site: AVF
 

4. Continuous Renal Replacement Therapy (CRRT)

                CRRT is the overarching terminology given to extracorporeal kidney therapy that is run at slower rates than HD. , CRRT is used in ICU patients who may not tolerate HD for typically one of two reasons (or both). There are other reasons as well, but these are two of the most common.

1. Hemodynamic instability- The blood flow of CRRT  that goes from patient to machine is typically run slower than HD circuits

2. Volume requirements: patients in the ICU receive feeds/TPN, antimicrobials/antifungals, pressors etc where the volume that the patient is getting is too large to remove with HD (which only runs 3-4 hours/session and there is only so much fluid you can remove in one session).

The CRRT circuit typically runs continuously over 24 hours

and allows gentle fluid removal. As mentioned earlier, we can

“clean” the blood with diffusion or convection.

With many machines you can do CVVH, CVVHD, or both (CVVHDF).

Much of choice of modality depends on machine capabilities

and center experience.

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The machine setup can be a bit confusing,

but the important points are thus:

                CVVH involves convection, i.e. exerting a pressure

to achieve fluid removal. If this is the only way one is

removing solutes and toxins, then the rates will have to

be fairly high, on say an average ~ 40ml/kg/hr of fluid

and solute removal. You can imagine, after just a few

hours a patient can become volume depleted and have

“wonky” electrolytes fairly rapidly. Therefore, we provide

what is called replacement fluids- typically a standard

out of the box fluid that contains electrolytes that is given

back to the patient to maintain their fluid volume

(you set the rate of fluid removal on the machine that

takes the replacement rate into account, e.g. if

replacement rate is 100ml/hr you program the machine

to remove 150ml/hr to achieve fluid removal). This fluid has two different location to be given back to the patient and is infused in different areas of the machine to achieve specific outcomes, but that is beyond the scope of this primer (see above image).

                CVVHD involves diffusion only, i.e. standard out of the box dialysis fluid is run countercurrent flow to blood flow moving through a semipermeable membrane (similar to HD) at a specific rate with pressure placed on the blood flow in the filter to achieve fluid removal.

                CVVHDF is the combination of both, which we do at my center. In which case, we divide our total desired clearance between the two modalities running concurrently (diffusion with dialysate fluid and convection achieved through transmembrane pressure in the filter with replacement fluid given back to the patient) to achieve CRRT.

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Below is a simplified sample prescription of a patient receiving CRRT-CVVHDF

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The filter size is dependent on body surface area.. The dialysate fluid comes in various concentration of Ca++ and K+ (in addition to  other electrolytes), in this case we are using a bag with 4mEq/l of potassium and 2.5meq/L of Calcium (physiologically active calcium) for dialysis/dialysate (does not come in contact  with the patient, see image) and Prismasol as replacement fluid (since we are using convective therapy, see image). Clearance rates are divided between dialysis and convection therapies. 

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5. Peritoneal Dialysis (PD)

Although PD is a different modality, the concepts are similar.

We need to:

1) remove fluid and 2) provide clearance. This can be used in the acute or chronic setting.

 

Peritoneal dialysis uses the peritoneal membrane as the dialysis membrane. Dialysate fluid is placed inside the abdomen.

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Solutes move between:

• Blood in peritoneal capillaries

• Dialysate in the peritoneal cavity

To achieve fluid removal and solute clearance, we need to have diffusion and convection.

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Diffusion

• Removes electrolytes and toxins. see sample prescription below. The dianeal bags contain sodium, magnesium, calcium, lactate (as a base)

Osmosis (i.e. UF)

• Removes water via glucose osmotic gradient. If you recall the osmotic equation (2NA + BUN/2.8 + Glucose/18) glucose is osmotically active (remember DKA and HHS!). the dianeal contains 1.5% glucose (15g/L, Osmolality 346mOsm/L) even 4.25% glucose (42.5g/L= 485mOsm/L) which exerts an osmotic draw and removes fluid.

 

5a. PD Cycle

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The cycle then repeats.

Dialysis can occur via:

• Manual exchanges (CAPD)

• Automated cycler (CCPD/APD)

 

Advantages of APD

                Can be done at home, typically overnight for 7-12 hours

                A bit more “gentle” than HD, but one can overdo it on the prescription

                Good for those on liquid diets and who are anuric as the therapy is nightly vs HD

Disadvantages

                One needs space to hold a lot of equipment – the cycler, bags, supplies etc

               A trained caregiver needs to learn how to use the cycler, attach the patient, and understand warning signs as                          well as troubleshooting.

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In our institution we typically do CCPD using a cycler (see image with fluid above).  This mean a patient will have a machine at home. Each fill-dwell-drain is one cycle. Patients typically require multiple cycles over 7-12 hours. Manipulating the cycles and time is a bit beyond the scope of this primer, but a starting PD prescription for a child is around 5 cycles over 7 hours and adjustments are made from there. 

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Below is a sample PD prescription. There are other components that have been removed, but this is the basic idea.

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Dialysate #1: 1.5% Low Ca
Total dialysate  volume (ml): 5000
therapy time (hours): 7
therapy time (minutes): 420
 fill volume (ml): 1,000
Baxter last fill volume (ml): none

# of cycles: 4

Baxter patient weight: 19.5

 

 

 

 

6. Hemodialysis Access

   Temporary HD Catheters

Common sites:

• Internal jugular

• Femoral

Avoid:

• Subclavian due to the possibility of developing  central vein stenosis and then be unable to place permanent HD catheter

 

    Permanent Access

AV Fistula

Best long-term option. It takes 4-6 weeks to be able to use it after placement, so sometime patients receive a catheter first as they wait for the fistula to mature. 

Advantages:

• lowest infection risk

• longest survival

Tunneled HD Catheters

Used when fistula not available (patient too small etc). Ideally placed in Internal Jugular vein

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HD catheters differ from regular central lines:

Feature                                                                                 Reason

Larger diameter                                               High blood flow

Dual lumen                                                Simultaneous inflow/outflow

Special (thicker) material                                Prevent collapse

 

 

 

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7. PD Access

Tenckhoff Catheter

Features:

• silicone

• cuffs to prevent infection

• tunneled exit site

This catheter made long-term PD possible in children.

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