Dialysis: Indications, Kidney Functions

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• Dialysis is a renal replacement therapy (RRT) used to remove waste and excess fluid.

Functions of the Kidney

• Excretion of metabolic waste products like blood urea nitrogen (BUN), creatinine, and uric acid.
• Regulation of fluid and electrolyte balance.
• Blood pressure regulation via the renin-angiotensin-aldosterone system (RAAS).
• Acid-base balance.
• Production of erythropoietin.
• Bone metabolism, calcium, and phosphorus regulation via calcitriol.

When Dialysis is Considered

• Severe hyperkalemia unresponsive to conservative measures.
• Uncontrolled acidosis.
• Severe volume overload with hypertension, pulmonary edema, or cardiac failure when diuretics are ineffective.
• Progressive uremia with general condition deterioration.
• Hypercatabolic states with increased waste product buildup.
• Toxic ingestion of certain compounds that can be dialyzed.

Mnemonic “ΑΕΙΟΥ” for Dialysis Indications

• A: Intractable Acidosis
• E: Electrolyte derangements, especially potassium, sodium, and calcium imbalances
• I: Intoxication with substances like ethylene glycol, methanol, lithium, and aspirin
• O: Volume Overload
• U: Uremic symptoms

Renal Replacement Therapy (RRT)

• A treatment that replaces the excretory function of the kidney, such as kidney transplant and dialysis.
• During kidney transplants, existing kidneys are typically not removed unless there is a tumor.
• The new kidney is placed in the retroperitoneal pelvic area.

Dialysis

• Achieves blood purification using semi-permeable membranes.
• Main goals include solute and fluid removal.
• Two primary types: hemodialysis and peritoneal dialysis (PD).

Hemodialysis Compared to Peritoneal Dialysis

• Hemodialysis requires vascular access and is typically continuous daily.
• Peritoneal dialysis uses a peritoneal access point and can be intermittent where treatment done at the center for 4 hours 2-3 times a week.
• Hemodialysis involves extracorporeal purification, while peritoneal dialysis is intracorporeal.
• Hemodialysis uses both diffusion and convection for solute clearance
• Peritoneal Dialysis uses diffusion.
• Hemodialysis is often deployed in acute kidney injury.
• Peritoneal dialysis tends to apply to chronic conditions.
• Hemodialysis is driven by hydrostatic pressure.
• Peritoneal Dialysis driven by osmotic pressure.

Principles of Dialysis

• Diffusion involves movement of solutes across a semipermeable membrane from high to low concentration.
• Larger molecular weight solutes diffuse more slowly.
• Hemodialysis involves blood flowing through one chamber of a dialyzer with dialysate fluid in another separated by a membrane, and solutes diffuse from the blood to the dialysate fluid.
• Osmosis drives water molecule movement from high to low concentration across a semipermeable membrane. Ultrafiltration (UF) is the removal of fluid during dialysis under a pressure gradient, either hydrostatic (hemodialysis) or osmotic (peritoneal dialysis).
• Convection moves solutes through a membrane by the force of water/pressure gradient
• Also described as solvent drag, where solutes dissolved in water move with it.
• During hemodialysis, fluid in the blood moves through a hydrostatic pressure gradient, filtering out dissolved solutes.
• Adsorption involves solute removal from the blood by clinging to the membrane, like a magnet.
• Hemofiltration achieves solute clearance by restoring the body’s intracellular and extracellular fluid environment, removing excess water, and purifying blood outside the body.

Hemodialysis Components

• Key components: hemodialysis machine, dialyzer, bloodlines, access, and dialysate fluid.
• Hemodialysis access involves a blood exit point passing through bloodlines to the hemodialysis machine.
• Arteriovenous Fistula (AVF) connects a vein and artery, creating a larger vessel for easier access.
• Arteriovenous Graft (AVG) provides the access which is more suitable for patients with small veins.
• Central Venous Catheter (CVC) is for very small veins, often used in small children.
• Dialyzers are cylinders containing small straw-like fibers, surrounded by dialysate fluid. The fiber pore size determines the size of molecule removed.
• Standard hemodialysis removes urea, creatinine, and potassium.
• Another dialyzer type can remove larger molecules.
• Blood clots can cause bursting due to pressure in the membrane fibers, wear and tear due to the membrane.
• Anticoagulants help prevent clotting in the membrane.
• High BUN, K+, and creatinine and low HCO3 are expected in the blood of renal failure patients.
• Na is physiologic.
• Zero K+ because you want K+ to be removed.
• High HCO3 is used because you want normal levels to return.
• Glucose draws water and exerts osmotic pressure.
• Postfilter returns all components to normal and concentrates hematocrit (Hct), signaling fluid loss.

Dialysis Fluid Flow

• Countercurrent to blood flow, maintaining the concentration gradient.

Clearance of Solute in Dialysis Depends On

• Solute molecular weight.
• Membrane diffusibility.
• Blood flow.
• Dialyzer flow.

Lower Molecular Weight Components

• Easily dialyzed small molecules: creatinine, phosphate, urea, potassium, phosphorus, and sodium.
• Slowly passed medium molecules include vitamin B12, aluminum, glucose, and uric acid.
• Undialyzed large molecules: albumin, cytokines, and β2-microglobulin.
• Solute removal mechanism: diffusion in small solutes, diffusion and convection in middle molecules, and protein-bound molecules relies mainly on convection.
• A higher membrane cut-off means greater capacity to remove larger molecules.
• High blood flow purifies more blood.

Dialysate Flow

• Countercurrent flow against blood creates the diffusion gradient.

Hemodialysis Machine Dependent Variables

• Diffusion is regulated by blood flow rate, dialysate flow rate, and dialysate composition.
• Convection is primarily driven by ultrafiltration rate.
• Blood flow rate should increase incrementally to avoid dialysis disequilibrium syndrome (DDS).

Prescription Factors

• Target urea clearance: 2-3 ml/min/kg, or 1.5-2 ml/min/kg for the first dialysis.
• Chronic Hemodialysis Duration: 4 hours 2-3 times per week.
• A longer session increases tolerance and control.
• The surface area of the dialyzer should be less than the body surface area.
• Biocompatibility minimizes patient allergic reactions and complement activation.
• Good urea mass transfer coefficient (KoA) is needed remove urea and creatinine where KoA is < 500 ml/min (LOW) and > 600 ml/min (HIGH).
• Good ultrafiltration coefficient (Kuf) is required where Kuf < 10 ml/h/mmHg (LOW FLUX) and Kuf > 15 ml/h/mmHg (HIGH FLUX).
• Dialysate composition includes components such as Sodium, Chloride, Bicarbonate, Calcium, Lactate, Potassium, Magnesium.

Peritoneal Dialysis (PD)

• Form of RRT that uses the peritoneum as a semipermeable membrane to purify blood and to transport and remove waste and fluids.

Peritoneal Dialysis (PD) - Mechamism

• Dialysate fluid is placed in a bag, the fluid introduced into the peritoneum via a catheter, dwells for 4-6 hours.
• The inserted dialysate draws solutes and excess fluid.
• Fluid is drained, replaced about every 4-6 hours.

Peritoneal Dialysis (PD) - Peritoneum

• 2L bag of dialysate fluid is inserted into the peritoneum to allow patients to walk while it cleans.
• Fluid is changed out every 4-6 hours.
• Children may only need a 100 or 200 mL of the 2L bag of fluid and remain connected.
• PD is favored for pediatric patients for home treatment, glucose concentration, and other reasons.
• A pre-sternal catheters avoids infection and a colostomy site

Acute PD Catheters

• Access a stylet catheter (stiff catheter).
• Insertion is done surgically of single cuff straight Tenckhoff catheters. Surgically placed double cuff catheters.

Continuous Ambulatory PD (CAPD)

• A manual type PD.
• Exchange number: 4-5 / day.
• Dwell Time: 4-6 hours for 3 daytime changes.
• 6 -9 hours for night changes.

Automated PD (APD)

• The machine automatically cycles PD dialysate without the manual exchange of fluids.
• Done while they sleep.
• APD patients experienced less discomfort from changing fluid bags.

Nocturnal Intermittent PD (NIPD)

• Dialysis cycles only happen at night.
• For patients with intact kidney excretory function.

Continuous Cycling PD (CCPD)

• One continuous cycle performed during the day.
• Intermittent cycles performed at night.
• A method for patients having trouble excreting with an kidney.

Tidal PD (TPD)

• Intermittent dialysis cycles at night similar to NIPD.
• only 50-75% of the total volume of dialysate is exchanged at each cycle.

Summary

• Provides solute clearance and fuid removal
• Utilizes semipermeable membranes
• Solute clearnece is achieved through diffusion and convection
• Ultrafiltration is fluid removal by a pressure gradient
• Ultrafiltration is accomplished through hydrostatic pressure in hemodialysis and osmotic pressure in peritoneal dialysis

Overview of dialysis as a renal replacement therapy (RRT) for removing waste and excess fluid. Covers kidney functions, dialysis considerations, and the mnemonic 'AEIOU' for dialysis indications.