What is the relationship between dialysate flow and clearance in hemodialysis for adult patients?

Relationship Between Dialysate Flow and Clearance in Hemodialysis

Direct Answer

Increasing dialysate flow rate directly increases urea clearance and Kt/V in hemodialysis, with the most clinically significant gains occurring when dialysate flow is increased from 300 to 500 mL/min, though further increases to 800 mL/min provide additional meaningful improvements in dialysis adequacy. 1, 2

Mechanism of the Relationship

Fundamental Principles

• Clearance can be increased by raising blood flow rates, dialysate flow rates, or increasing dialyzer surface area 1
• The rate of diffusion across the dialyzer membrane is driven by blood concentration and follows first-order kinetics, with clearance remaining relatively constant during treatment as both blood concentrations and solute removal rates decrease proportionally 1
• Dialysate flow affects the concentration gradient across the membrane—higher dialysate flow maintains a steeper gradient by more rapidly removing solutes from the dialysate side 3

Mass Transfer Coefficient Changes

• Increasing dialysate flow rate from 500 to 800 mL/min significantly increases the urea mass transfer-area coefficient (K₀A) by approximately 5.7% during clinical dialysis 4
• This finding challenges the traditional assumption that K₀A remains constant for a given dialyzer, demonstrating that dialysate flow rate directly influences membrane transport efficiency 4
• The increase in K₀A with higher dialysate flow is less pronounced in vivo (5.7%) compared to in vitro studies (14.7%), likely due to blood cells and proteins affecting blood-side mass transfer resistance 4

Clinical Evidence for Dialysate Flow Effects

Quantitative Impact on Kt/V

In a prospective study of 23 maintenance hemodialysis patients, increasing dialysate flow produced the following results: 2

• At 300 mL/min: mean Kt/V = 1.19
• At 500 mL/min: mean Kt/V = 1.32 (11.7% increase)
• At 800 mL/min: mean Kt/V = 1.45 (9.9% additional increase from 500 mL/min)

The proportion of patients failing to achieve adequacy (Kt/V ≥1.2) decreased from 56% at 300 mL/min to 30% at 500 mL/min, and further to 13% at 800 mL/min 2

Practical Optimization

• A model-based approach using an "AutoFlow" factor (ratio of dialysate flow to blood flow) can provide adequate Kt/V while consuming lower amounts of dialysate, water, and energy 5
• In a study of 33 stable hemodialysis patients, Kt/V was 1.52 at 700 mL/min, 1.50 at 500 mL/min, and 1.49 with AutoFlow, demonstrating that increasing dialysate flow beyond model predictions has diminishing returns 5

Clinical Algorithm for Dialysate Flow Selection

Step 1: Initial Assessment

• Measure baseline delivered Kt/V with current dialysate flow (typically 500 mL/min standard) 1
• If Kt/V <1.2, proceed to Step 2 2

Step 2: Optimization Hierarchy

Before increasing dialysate flow, first optimize: 1

1. Blood flow rate (if not already maximized)
2. Treatment time (avoid ultrashort <3 hour sessions)
3. Dialyzer surface area/K₀A

Step 3: Dialysate Flow Adjustment

If Kt/V remains <1.2 after optimizing blood flow and time: 2

• Increase dialysate flow from 500 to 800 mL/min
• Expect approximately 10% increase in Kt/V
• This is particularly valuable for patients with limited vascular access preventing higher blood flows

Step 4: Cost-Benefit Consideration

• Use model-based AutoFlow systems when available to balance adequacy with resource consumption 5
• Standard 800 mL/min may be excessive for patients already achieving Kt/V >1.4 at lower flows 5

Important Caveats and Limitations

Solute-Specific Effects

• Increasing dialysate flow does NOT improve removal of highly sequestered solutes like phosphate 1
• Removal ratios for creatinine and β₂-microglobulin are not significantly affected by increasing dialysate flow from 500 to 800 mL/min 2
• The benefit is primarily for small, freely diffusible solutes like urea 6

Diminishing Returns

• The observed gain in clearance from increasing dialysate flow is greater than predicted by mathematical modeling, but the incremental benefit decreases at higher flow rates 2, 4
• The most cost-effective increase occurs in the 300-500 mL/min range rather than 500-800 mL/min 5, 2

Monitoring Requirements

• Monthly measurement of delivered Kt/V is mandatory to ensure adequacy regardless of dialysate flow rate 1, 7
• Conductivity (ionic) clearance can be measured with each treatment to provide real-time assessment without blood sampling 1
• Multiple ionic clearance measurements throughout treatment are needed to avoid errors from clearance changes during dialysis 1

Practical Pitfalls to Avoid

• Do not rely on increasing dialysate flow alone to achieve adequacy—it provides only modest improvements (10%) and should be part of a comprehensive prescription optimization 2
• Do not assume K₀A is constant when calculating clearances at different dialysate flows—it increases with higher flows 4
• Do not use dialysate flow increases to compensate for inadequate treatment time—longer treatment times correlate with improved mortality independent of Kt/V 1
• Avoid assuming that higher dialysate flow will improve removal of all uremic toxins—the benefit is limited to small, diffusible solutes 1, 2