Hemodialysis Principles and Diffusion

Hemodialysis Principles

• Clearance in hemodialysis measures the plasma volume from which a solute is removed per unit time (ml/min). It indicates how efficiently the dialysis system removes a specific solute.
• Clearance depends on blood flow rate (Qb), dialysate flow rate (Qd), and KoA (dialysis membrane's mass transfer capacity).
• KoA is calculated using the diffusion coefficient (D), membrane area (A), membrane thickness (X), and concentration gradient across the membrane (ΔC).

Diffusion

• Diffusion is the movement of solutes from high to low concentrations across a membrane. It's crucial for removing small solutes (urea, creatinine) during dialysis.
• Influencing factors include the concentration gradient (larger differences lead to faster diffusion), molecular size (smaller molecules diffuse faster), membrane surface area (larger areas increase the rate), membrane thickness (thinner membranes facilitate faster diffusion), temperature (higher temps increase molecular kinetic energy), and pressure (generally constant during dialysis).

Convection

• Convection is solute movement along with water flow across the membrane due to solvent drag during ultrafiltration.
• Influencing factors include transmembrane pressure (TMP), driving the water movement influencing solute transport, solvent drag (water carrying solutes), molecular size (convection is effective for larger molecules), and membrane pore size (larger pores enable more solute transport).

Kinetics

• First-order kinetics describes solute removal where the rate is proportional to the solute's blood concentration. This is common in hemodialysis because higher concentrations lead to faster removal initially. This rate slows as concentration declines.
• Zero-order kinetics describes a constant removal rate, irrespective of solute concentration—less common in dialysis.

Ultrafiltration Coefficient (Kuf)

• Kuf measures the dialyzer's ability to remove fluid. It’s the fluid removal rate per hour per 1 mmHg of transmembrane pressure.
• Determinants of Kuf include membrane composition (synthetic membranes generally have higher Kuf), pore size (larger pores increase fluid transfer), membrane thickness (thinner membranes yield higher Kuf), and oncotic pressure (higher protein concentrations reduce water movement).

Dialyzer Types

• High-efficiency dialyzers are optimized for removing small solutes and can have a higher KoA and can be more effective in 24 hours.

• High-flux dialyzers have larger pores to enable the passage of more molecules (including middle-sized ones) and have higher Kuf values.

Hemodialysis Process

• Blood is extracted, processed through a dialyzer (artificial kidney), and returned to the patient.
• IHD treatment involves 4-5 hours, 3 times per week; CRRT is continuous for critically ill patients.

Dialysis Types (Effectiveness and Methods )

• Intermittent hemodialysis (IHD) is highly effective for acute and chronic kidney failure, with scheduling flexibility, and effectively removes small solutes (urea, creatinine).

• Continuous renal replacement therapy (CRRT) runs continuously for critical patients who can't tolerate rapid fluid shifts, but it's less effective at removing solutes than IHD.

• Peritoneal dialysis (PD) uses dialysate in the peritoneal cavity; it is less effective than hemodialysis.

Solute Removal Mechanisms

• Diffusion, convection, and ultrafiltration are the major solvent dragging mechanisms for solute removal in hemodialysis.

• Adsorption, while present, plays a minor role in dialysis, where some solutes adhere to the membrane surface.

Dialyzer Structure and Function

• Dialyzers use synthetic membranes (polysulfone, polyamide, polyacrylonitrile).
• Blood flows through hollow fibers, and dialysate surrounds them.
• The apparatus circulates blood from the vascular access (arterial and venous lines), and dialysate, with monitoring, water purification, and purification system components ensuring the proper composition for the dialysate.
• Water purification techniques include particulate filters, activated carbon, and reverse osmosis for removing contaminants.

Key Dialyzer Terms

• KoA (Mass Transfer Area Coefficient) describes a dialyzer's maximal clearance capacity. It's influenced by membrane surface area and thickness.
• Ultrafiltration Coefficient (Kuf) determines the dialyzer's fluid removal ability per hour for each mmHg of transmembrane pressure. It's influenced by membrane composition, pore size, and thickness.

Water Treatment for Dialysis

• Dialysis requires purified water with various treatments, including particulate filters, carbon sorbents, water softeners, deionization, and reverse osmosis.

Principles of Solute Removal

• Diffusion is driven by concentration gradients, influenced by molecular size, temperature, and pressure.
• Convection is directly influenced by solvent drag during ultrafiltration.
• Clearance, the volume of plasma cleared per unit time, is influenced by concentration gradients and dialyzer properties and parameters such as membrane thickness and pore size.

Membrane Resistance

• Reduced membrane resistance contributes to dialyzer efficiency, with high-efficiency dialyzers having a high KoA suggesting high removal ability for small solutes