When should blood flow be adjusted during hemodialysis (HD)?

When to Adjust Blood Flow During Hemodialysis

Blood flow rate should be decreased to 50-100 mL/min for 15 seconds at the end of dialysis specifically for accurate post-dialysis BUN sampling, then either maintained at this slow flow or stopped completely depending on your sampling technique. 1

Primary Indication: Post-Dialysis Blood Sampling

The most clearly defined situation requiring blood flow adjustment occurs at treatment completion for laboratory sampling 1:

Slow Flow/Stop Pump Sampling Technique

At dialysis completion:

• Turn off dialysate flow and reduce ultrafiltration to 50 mL/h or minimum setting 1
• Decrease blood flow to 50-100 mL/min for 15 seconds to clear the arterial line dead space of any recirculated blood 1
• You may need to manually adjust venous pressure limits downward to prevent pump shut-off 1

Purpose: This fills the arterial needle tubing with non-recirculated blood, eliminating access recirculation artifact that would falsely lower the BUN and artificially inflate Kt/V calculations 1

Then proceed with either:

• Slow flow technique: Maintain blood pump at 50-100 mL/min and draw sample from arterial port 1
• Stop pump technique: Immediately stop the pump, clamp lines, and draw sample 1

Critical timing: Sampling at 0.25-0.50 minutes post-dialysis (corresponding to this slow flow/stop pump technique) provides the most accurate measurement for single-pool urea kinetic modeling, before significant urea rebound occurs 1

Situations Requiring Blood Flow Optimization During Treatment

Inadequate Vascular Access Flow

When access flow is compromised (<600 mL/min), you should still increase blood flow rate to maximize Kt/V 2:

• Even with low access flow rates, increasing dialyzer blood flow from 200 to 300 to 400 mL/min generally increases delivered Kt/V 2
• Access recirculation remains minor (0.9% ± 0.6%) even at higher blood flow rates in patients with low access flow 2
• The primary solution for inadequate blood flow is extending treatment time, not reducing blood flow 3

Key threshold: Blood flow <300 mL/min indicates catheter dysfunction and requires intervention 4:

• Prepump arterial pressure more negative than -250 mmHg signals inadequate flow 4
• For high-efficiency dialysis, blood flow >300 mL/min is required to achieve target spKt/V of 1.2 4
• 15% of catheter treatments have blood flow <300 mL/min, leading to underdialysis 4

Balancing Blood Flow with Dialysate Flow

Maintain a blood flow to dialysate flow ratio of 1:2 for optimal efficiency 5:

• If dialysate flow must be reduced (e.g., from 500 to 400 mL/min), calculate the appropriate blood flow using the K₀A equation to maintain equivalent dialysis efficiency 5
• Example: When reducing dialysate flow from 500 to 400 mL/min with blood flow of 200 mL/min, increase blood flow to 210 mL/min to maintain clearance 5

Catheter-Specific Considerations

Central venous catheters may require blood flow adjustment based on position and design 4:

• Femoral catheters have significantly greater recirculation than internal jugular catheters (13.1% vs 0.4%) 4
• Femoral catheters <20 cm have recirculation of 26.3% vs 8.3% for those >20 cm 4
• Newer catheter designs can achieve 400 mL/min or greater when properly placed 4
• Exception: Pediatric or smaller adult catheters are not designed for flows >300 mL/min 4

Common Pitfalls to Avoid

Do not sample immediately at time zero (blood pump still running at full speed): This causes major BUN reduction from recirculated blood still in the access, falsely elevating Kt/V 1

Do not assume that reducing blood flow will improve Kt/V in patients with low access flow: Studies demonstrate that even with compromised access, higher blood flow rates generally increase delivered dose 2

Do not reduce blood flow as the primary solution for inadequate dialysis: The prescribed dose should be Kt/V 1.3 (URR ~70%) to ensure minimum delivered dose of 1.2 is achieved 1. When blood flow is limited, extend treatment time instead 1, 3

Monitor for factors that compromise effective blood flow 1:

• Access recirculation reducing concentration gradient
• Blood pump/dialysate flow calibration errors
• Dialyzer clotting during treatment
• High pre-pump negative pressure causing tubing collapse