What does ultrafiltration mean in continuous renal replacement therapy (CRRT)?

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What is Ultrafiltration in CRRT?

Ultrafiltration in CRRT is the process of removing fluid (plasma water) from the blood across a semipermeable membrane, serving two critical functions: achieving net fluid removal to manage volume overload and enhancing solute clearance through convection. 1

Core Mechanisms and Functions

Ultrafiltration operates through pressure-driven filtration across the hemofilter membrane, where:

  • Fluid removal: The ultrafiltrate volume produced can exceed desired weight loss targets, with the excess replaced by replacement solution 1
  • Solute clearance: Convective transport removes solutes dissolved in the ultrafiltrate, particularly effective for middle and larger molecular weight substances 1
  • Net ultrafiltration (NUF): The difference between total ultrafiltrate produced and replacement fluid administered, representing actual fluid removed from the patient 2, 3

Clinical Application Across CRRT Modalities

The role of ultrafiltration varies by CRRT mode 1:

Pure hemofiltration (CVVH/CAVH): Ultrafiltration is the primary mechanism for both fluid removal and solute clearance through convection. Typical ultrafiltration rates are 1-2 L/hour, with replacement solution administered to maintain desired fluid balance 1

Hemodialysis modes (CVVHD/CAVHD): Ultrafiltration primarily serves fluid removal, while diffusion handles most solute clearance. Dialysate flows at 1-2 L/hour with minimal replacement fluid 1

Hemodiafiltration (CVVHDF/CAVHDF): Ultrafiltration volumes are optimized to exceed desired weight loss, enhancing convective solute clearance while replacement solution maintains fluid balance. This combines both diffusive and convective clearance mechanisms 1

Slow continuous ultrafiltration (SCUF): A specialized form where ultrafiltration occurs without fluid replacement, with the primary aim of achieving fluid removal in volume-overloaded states 1

Prescription and Dosing Considerations

For pure hemofiltration: The ultrafiltration rate directly determines treatment dose. Evidence supports an intensity of 35 mL/kg/hour for improved survival compared to 20 mL/kg/hour in critically ill patients with acute renal failure 1. For high-volume hemofiltration, ultrafiltration volumes exceed 35 mL/hour/kg 1

For fluid management: The net ultrafiltration rate (NUFR) must be prescribed based on:

  • Desired daily fluid balance target
  • Patient body weight
  • Baseline fluid overload status
  • Ongoing fluid inputs (IV medications, nutrition, blood products) 2, 3

Recent evidence suggests optimal NUFR between 1.01-1.75 mL/kg/hour during the first 72 hours of CRRT is associated with lower mortality, with both lower and higher intensities showing increased risk 4

Critical Operational Parameters

Pre-dilution vs post-dilution: Administering replacement fluid before the hemofilter (pre-dilution) enhances achievable ultrafiltration rates—particularly important in high-volume CVVH—and may reduce filter clotting. However, it dilutes blood entering the filter, reducing clearance efficiency 1

Monitoring ultrafiltration: The ultrafiltration rate and sieving coefficient for marker solutes measure clearance in pure hemofiltration. For other modalities, both dialysate and ultrafiltrate flow and concentrations are required 1

Safety and Volume Management

Volume overload should be avoided, especially in patients with acute lung injury, as there is level II evidence that maintaining negative fluid balance decreases ICU length of stay 1. However, active fluid removal should only occur in patients who don't require ongoing resuscitation 5

Common pitfall: Using cumulative fluid balance alone to assess fluid overload is imprecise and can lead to inappropriate ultrafiltration targets 5. The capillary refill rate—how quickly plasma volume is replenished from interstitial fluid during ultrafiltration—varies widely and may be impaired in conditions with vasoplegia, endothelial dysfunction, or low oncotic pressure 5

Precision approach: A tri-parameter framework is emerging where NUFR serves as the weight-adjusted prescription, net fluid balance as the safety outcome, and fluid overload clearance percentage (expressing net fluid balance as proportion of initial overload) as the individualized target 6

The ultrafiltration rate requires daily adjustment based on total fluid intake, achieved fluid removal, and clinical status 7, 2. Devices specifically designed for CRRT should be used rather than adapted IV infusion pumps, which risk significant fluid balance errors 1.

References

7
Research

A new method to control ultrafiltration in conventional continuous renal replacement therapy.

ASAIO journal (American Society for Artificial Internal Organs : 1992), 2001

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.