What is the purpose of each parameter in hemodialysis?

Purpose of Each Parameter in Hemodialysis

The key parameters in hemodialysis each serve specific purposes aimed at optimizing toxin removal, maintaining fluid balance, and ensuring patient safety during treatment. 1

Blood Flow Rate (BFR)

• Determines the volume of blood passing through the dialyzer per minute, directly affecting the efficiency of solute removal 1
• Typically ranges from 200-250 mL/min in Japan and can be higher in other countries, depending on vascular access function and patient tolerance 2
• Inadequate BFR significantly reduces delivered dialysis dose and can contribute to treatment failure 1
• Should be balanced with dialysate flow in a ratio of approximately 1:2 for optimal efficiency 2

Dialysate Flow Rate (QD)

• Controls the volume of dialysate passing through the dialyzer per minute, affecting concentration gradients for diffusion 1
• Typically set at around 500 mL/min but can be reduced to 400 mL/min while maintaining efficiency if the blood flow rate is adjusted accordingly 2
• Reduction in dialysate flow can be environmentally beneficial, as hemodialysis uses approximately 500L of water per session 3
• Inadequate dialysate flow can reduce effective clearance and contribute to underdialysis 1

Treatment Time

• Represents the exact duration during which diffusion occurs at the prescribed blood and dialysate flow rates 1
• Directly impacts total solute removal and fluid removal capacity 4
• Must be sufficient to achieve adequate fluid removal without excessive ultrafiltration rates 1
• Interruptions during treatment reduce actual dialysis time and effectiveness 1

Ultrafiltration Rate and Volume

• Determines the rate and total volume of fluid removed during dialysis 5
• Should ideally be ≤10 mL/kg/hour to minimize cardiovascular risk and organ stunning 5
• Excessive rates can lead to intradialytic hypotension and increased mortality 1
• UF volume is determined by interdialytic weight gain and target dry weight 1

Dialyzer Characteristics

• Dialyzer membrane type (cellulosic vs. synthetic) affects biocompatibility and clearance profiles 4
• Ultrafiltration coefficient (Kuf) classifies membranes as low-flux or high-flux 4
• Surface area affects total clearance capacity 4
• Molecular weight cut-off (MWCO) and molecular weight retention onset (MWRO) determine which molecules can be removed 6
• Total cell volume (TCV) is used to assess dialyzer function, especially with reuse 4

Anticoagulation

• Prevents clotting of the dialyzer during treatment, which would otherwise reduce effective dialyzer surface area 1
• Inadequate anticoagulation can lead to dialyzer clotting and reduced clearance 1
• Typically achieved with heparin administration 1

Blood Urea Nitrogen (BUN) Sampling

• Critical for accurate calculation of dialysis adequacy measures like Kt/V and URR 4
• Predialysis samples must be drawn before dialysis begins and without dilution 4
• Postdialysis samples should be drawn using the slow flow/stop pump technique to prevent dilution with recirculated blood 4
• Improper sampling techniques can lead to inaccurate adequacy measurements 1

Kt/V and URR (Urea Reduction Ratio)

• Quantitative measures of dialysis adequacy 1
• Kt/V represents fractional urea clearance (K = clearance, t = time, V = volume of distribution) 1
• Target single pool Kt/V (spKt/V) is 1.4 per session for thrice weekly treatment, with minimum 1.2 1
• Monthly measurement of delivered dose is recommended to ensure adequacy 1

Common Pitfalls and How to Avoid Them

• Targeting minimum values as goals can result in many sessions falling below the threshold; always target higher (1.4 for Kt/V) 1
• Ignoring treatment time reductions due to late starts or early terminations affects dialysis efficacy 4
• Failing to account for access recirculation reduces effective clearance 1
• Inadequate blood sampling techniques lead to inaccurate Kt/V calculations 4
• Focusing solely on small-solute clearance can overlook other critical aspects like fluid balance, blood pressure control, and nutritional status 1

Monitoring Parameters

• Delivered dose of hemodialysis should be measured at least once monthly in all patients 1
• Increase measurement frequency when patients are non-compliant, when frequent problems in delivery occur, when there's wide variability in results, or when the prescription is modified 1
• Regular assessment of dialyzer performance through TCV, ionic clearance, or urea clearance measurements is essential 4