Why does predilution in Hemodialysis (HD) not exceed 30ml/hr in the device?

Predilution Flow Rate Limitations in Hemodialysis Devices

Direct Answer

The question appears to reference a specific device limitation regarding predilution rates in hemodialysis, but the provided evidence does not contain information about a 30 ml/hr predilution limit in any particular device. However, I can explain the general principles and practical limitations of predilution in hemodialysis based on the available evidence.

Understanding Predilution in Hemodialysis

What Predilution Does

• Predilution hemodiafiltration dilutes the blood before it enters the dialyzer, which reduces the concentration gradient and decreases small molecule clearance compared to postdilution or standard hemodialysis 1, 2
• The dilution effect means that for urea and creatinine, predilution clearance is approximately 15% lower than postdilution at equivalent flow rates 2

Device-Specific Flow Rate Limitations

Current hemodialysis machines have specific technical constraints on dialysate and substitution fluid flow rates:

• The Prismaflex and Fresenius devices with pediatric circuits limit dialysate flow (Qd) to 1 liter/hour (approximately 17 ml/min), which restricts the efficiency of continuous renal replacement therapy 3
• The Asahi Sigma Plasauto allows dialysate flow up to 6 liters/hour 3
• These limitations directly impact the achievable clearance rates and may necessitate switching between treatment modalities 3

Practical Considerations for Predilution

Blood Flow Requirements

To prevent circuit clotting and maintain adequate clearance with predilution, specific blood flow rates must be maintained:

• Blood flow less than 250 ml/min is associated with increased risk of extracorporeal circuit thrombosis during heparin-free predilution hemodiafiltration 4
• At low blood flow rates (60 ml/min typical in pediatrics), predilution clearance is significantly reduced compared to other modalities 2

Dialysate Flow Considerations

• When substitution fluid is separated from dialysate in predilution mode, the effective dialysate flow is reduced, which decreases clearance of small molecules like urea, creatinine, and phosphate 1
• Predilution with dialysate flow of 500 ml/min is significantly less effective for small molecules than standard hemodialysis 1

Clinical Implications

When Predilution May Be Preferred

Despite lower small molecule clearance, predilution offers specific advantages:

• Allows higher substitution volumes at lower blood flow rates 5
• Reduces shear stress and improves hemodynamic stability during treatment 5, 6
• May enhance removal of larger low-molecular-weight proteins 5, 6
• Reduces risk of filter clotting in heparin-free settings when blood flow exceeds 250 ml/min 4

Adequacy Requirements Remain Unchanged

Regardless of the dilution mode used, minimum adequacy standards must be maintained:

• The same minimum levels of spKt/V apply to hemodialysis, hemofiltration, and hemodiafiltration 3
• Prescribed Kt/V should be 1.3 to ensure delivered dose does not fall below the minimum of 1.2 3

Common Pitfalls

• Do not assume predilution automatically provides adequate small molecule clearance - it requires higher substitution volumes to compensate for the dilution effect 1, 2
• Avoid using blood flow rates below 250 ml/min with predilution as this significantly increases thrombosis risk 4
• Do not reduce dialysate flow excessively when using predilution, as this further compromises small molecule removal 1

Note: If you are asking about a specific 30 ml/hr limitation in a particular dialysis device, this information is not contained in the provided clinical guidelines and research evidence. You may need to consult the device's technical manual or contact the manufacturer for device-specific operational parameters.