[OS 206] E03-T10-Integration Physiologic Basis of Dialysis-compressed

• Dialysis is a treatment to replaces the excretory function of the kidney
• Main target of dialysis is solute and fluid removal
• Hemodialysis and Peritoneal Dialysis (PD) are two kinds of dialysis

Functions of the Kidneys & Disturbances

• Excretion of waste products disturbance leads to increased Blood-Urea-Nitrogen/BUN, creatine, and uric acid
• Fluid Balance disturbance is Oligo-anuria & Fluid overload polyuria
• Electrolyte Balance disturbance is Electrolyte imbalance with Hyperkalemia
• Regulation of blood pressure through Renin-Angiotensin-Aldosterone System (RAAS) disturbance leads to hypertension
• Acid-base regulation disturbance is metabolic acidosis
• Production of erythropoietin disturbance is anemia
• Bone metabolism, calcium, and phosphorus regulation disturbance is Secondary hyperthyroidism, hypocalcemia, hyperphosphatemia, bone, and mineral disease/renal osteodystrophy

Mnemonic AEIOU lists Indications for Acute Dialysis

• A: Intractable Acidosis
• E: Electrolyte derangements
• I: Intoxication (such as ethylene glycol, methanol, lithium, aspirin)
• O: Volume Overload
• U: Uremic symptoms

Traditional Indications for Acute Dialysis

• Severe hyperkalemia that is unresponsive to conservative therapy can cause arrhythmia
• Uncontrolled acidosis cannot be safely corrected due to the risk of sodium or volume overload
• Severe volume overload is a result of uncontrolled hypertension, pulmonary edema, or cardiac failure where diuretics are ineffective
• Progressive uremia with deterioration if general condition
• Hypercatabolic states with increase in blood area by >10 mmol/L per day and increase waste products
• Toxic ingestion of certain compounds caused by a non-accidental ingestion of dialyzable medications
• Volume/Fluid overload is an indication for dialysis

Types of Dialysis

Principles of Dialysis & Diffusion

• Solute movement across a semipermeable membrane is from high to low concentration
• The larger the molecular weight the slower the diffusion
• Net diffusion can occur with a one-directional movement for one solute type, or bidirectional for multiple solute types
  • Blood from the body goes through one dialyzer chamber during hemodialysis.
  • The other side has a countercurrent of dialysis fluid maintaining the concentration gradient

Principles of Dialysis & Osmosis

• Water molecules movement is from a high concentration to a lower concentration
• This process occurs through a semipermeable membrane
• Ultrafiltration(UF) is passage of water through a membrane under a pressure gradient during dialysis

Principles of Dialysis & Convection

• Water pressure gradient moves of solutes through a membrane
• Fluid in the blood compartment moves through a hydrostatic pressure gradient during hemodialysis
• Solutes dissolved in the fluid filter out

Components of Hemodialysis

• Solute Clearance restores the extracellular fluid environment to the body composition of healthy individuals.
• Ultrafiltration removes excess water volume
• Extracorporeal blood purification occurs out of the body

Hemodialysis Access

• Blood exits vessels and passes through bloodlines on the hemodialysis machine
• Arteriovenous Fistula (AVF) adheres the vein and artery at the radius, creating an anastomosis
• Advantange: larger vessel easily creates access
• More commonly used Arteriovenous Graft (AVG) for patients with small veins

Dialyzers

• Cylinder that has small straw-like fibers where the blood flows through
• Dialysate fluid surrounds the fibers
• Pore size varies among fibers, small pore size removes small molecules
• Standard Hemodialysis removes urea, creatinine, potassium
• Middle & bigger molecules require larger pores, needs special dialyzer capable of removing them

Hemodialysis set-up

• Blood flows from the Arterial Port, to the Blood Pump, to the Dialyzer, and then to the Venous Port
• Dialysis fluid is countercurrent
• Note that turbulence can create bubbles and air embolisms

Dialysate & Anticoagulant

• Typical blood of a hemodialysis patient with renal failure is a high BUN, Potassium, & Creatinine and low HCO3
• Blood passes from left to right because of countercurrent flow of dialysis fluid
• Countercurrent flow of dialysate against the blood creates a diffusion gradient
• Anticoagulants are injected prior to the dialyzer to prevent clotting
• Blood clots can cause the dialyzer to burst due to pressure

Factors Affecting Solute Dialysis

• Clearance of solute in dialysis depends on Solute molecular weight, Membrane diffusibility, Blood Flow, Dialyzer Flow, and Dialysate Flow
• Smaller solute sizes are easier to dialyze
• Small molecules are creatinine, phosphate, urea, potassium, phosphorus, and sodium
• Medium molecules are vitamin B12, aluminum, glucose, uric acid
• Large molecules are albumin, cytokines, and β2-microglobulin which don't pass through the membrane

Blood Flow & Dialysate

• High blood flow purifies more blood
• Countercurrent flow of dialysate creates the diffusion gradient.
• Fast blood flow and dialysate flow maintains the concentration gradient

Solute Removal

• Diffusion is easily removed by dialysis strategy with Urea and Creatinine which are considered non-toxic
• Large array Molecules include Large array of biological impacts of Leptin removed only through large-pored membranes
• Bound Molecules include Phenols, and Indoles removed via Convection, adsorption, and less diffusion

Hemodialysis Membrane Diffusibility

• The higher the blood flow, the more that blood is purified
• Dialyzer Surface Area should be less than Body Surface Area otherwise that patient will develop hypovolemia
• The membrane chosen depends on solute size to be removed

Blood Flow Rate

• Prescribed at 5-7 ml/kg/min or 150-200 ml/m2/min
• Adolescents = 150-200 ml/min
• Children = 75-150 ml/min
• Higher BFR equates to greater solute clearance
• Reduce at intervals to prevent dialysis disequilibrium syndrome (DDS)

Hemodialysis Duration

• Aim for urea clearance of 2-3 ml/min/kg, and first dialysis should be1.5-2 ml/min/kg
• Chronic HD duration is about 4 hours, done 2-3x/week, can be shorter or longer depending on patient
• Longer sessions better controls BP and removes time-dependent solutes like Phosphate, and β2-microglobulin

Hemodialysis - Dialyzer

• Surface Area should be less than the Body Surface Area
• Patient will develop hypovolemia otherwise
• Important for anemic patients and pediatric patients
• Improves biocompatibility of the following: Cellulose, Substitute cellulose, Cellulosynthetic, Synthetic
• Has good Urea Mass Transfer Coefficient (KoA) and removes urea and creatinine.
• Has good Ultrafiltration Coefficient (Kuf), removing middle and large molecules which can cause back diffusion

Dialysate Composition

• New dialyslate is bicarbinate base, has sodium, chlorite, calcium, magnesium
• Old dialysates are lactate-based.

Dialysate Flow Rate

• Increased DFR decreases in dialysate increases diffusion and clearance
• Effective DFR doubles BFR, then solute removal is minimal
• DFR is 500 mL/min

Ultrafiltration

• UF should NOT exceed 1.5±0.5% body weight per hour
• Do NOT exceed 5% of child or adult's body weight per session to avoid severe hypotension
• Driven by hydrostatic pressure due to the pump speed
• Primed in a circuit wth blood, removed at normal hemaglobin or kept if anemic

Peritoneal Dialysis

• Form of renal treatment using peritoneum for transport and removal of fluid and solutes
• Intracorporeal blood purification
• Dialysate fluid inserted into the peritoneum collects fluid and waste, then drained and repeated in sessions

Peritoneal accesses

• Acute stylet catheter punctures below the umbilicus
• Chronic uses a double-cuff Tenckhoff catheter
• There are also pre-sternal catheters and manual and twin-bag set

Peritoneum

• Serous membrane from the mesenchyme, with parietal and visceral layers
• Total surface area of peritoneum in adults = 1-2 m2
• Parietal space contains 50-100 mL of fluid
• Large in both surface area & vascularity
• Semi-permeable/selective membrane Barrier is the mesothelium, insterstitium, and capillary wall or perioneal microcirculation
• UltraSmall(Small Pore / 40-60 Å /95%) / Large Pores

Principles / Transport

• Main driving force is osmotic pressure gradient in dialysis solution. Increased gradient > Increased fluid removal
• Optimal = 4 hours. A general UF decreases past 4 hours in all glucose concs
• Optimal Fill Volume is a balancing act of solute clearance and ultrafiltration
• Peritoneal dialysis is achieved by CAPD, APD, NIPD, CCPD, COPD, TPD OS 206 Integration: Physiological Basis of DIalysis