CRRT (Continuous Renal Replacement Therapy)

CRRT (Continuous Renal Replacement Therapy)

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Provides renal support to patients. Usually veno-venous, but arterio-venous exists too (not covered here).

Mechanisms

Solute removal mechanisms:

• Diffusion
  • Accomplished by passive movement of solute across a membrane (due to a solute concentration gradient, movement from high to low)
  • Efficient clearance of low-molecular-weight solutes (< 500-1,500 Daltons)
  • Clearance decreases with increasing molecular weight of solute
• Convection
  • Accomplished by active movement of solute (due to a pressure gradient) across a membrane
  • Limited primarily by size of pores in membrane
  • Clearance of different size of solute similar until the size of the solute reaches with size of the pores
  • Overall CVVH with higher clearance rate than CVVHD for solutes 1,000 to 20,000 Daltons

Membrane characteristics:

• Geometric: length, mean inner radius, wall thickness, number of pores, number of fibers (determines membrane surface area)
• Membrane ultrafiltration coefficient: water permeability of filter
• Filter ultrafiltration coefficient: membrane ultrafiltration coefficient multiplied by membrane surface area
• Mass transfer area coefficient: overall capacity of membrane to provide diffusive removal of solute
• Membrane sieving coefficient: ratio of specific solute concentration in ultrafiltrate (only removed by convection) divided by mean plasma concentration in filter
• Cutoff: molecular weight of the smallest solute retained by the membrane

Types of CRRT (see figure 1 for diagrams)

Figure 1: Types of CRRT (Recreated based on Tandukar 2019 & Asahi Kasei Medical Co.)

SCUF (Slow continuous ultrafiltration)

• Convection
• Mainly fluid removal

CVVH (Continuous venovenous hemofiltration)

• Convection

CVVHD (Continuous venovenous hemodialysis)

• Diffusion

CVVHDF (Continuous venovenous hemodiafiltration)

• Combination of convection and diffusion

Anesthetic implications

Indications

Renal replacement for patients with cardiovascular/hemodynamic instability, acute brain injury, cerebral edema, or raised intracranial pressure (for slow electrolyte correction).

References

Tandukar S, Palevsky PM. Continuous Renal Replacement Therapy: Who, When, Why, and How. Chest. 2019 Mar;155(3):626-638. doi: 10.1016/j.chest.2018.09.004. Epub 2018 Sep 25. PMID: 30266628; PMCID: PMC6435902.

Neri M, Villa G, Garzotto F, Bagshaw S, Bellomo R, Cerda J, Ferrari F, Guggia S, Joannidis M, Kellum J, Kim JC, Mehta RL, Ricci Z, Trevisani A, Marafon S, Clark WR, Vincent JL, Ronco C; Nomenclature Standardization Initiative (NSI) alliance. Nomenclature for renal replacement therapy in acute kidney injury: basic principles. Crit Care. 2016 Oct 10;20(1):318. doi: 10.1186/s13054-016-1489-9. PMID: 27719682; PMCID: PMC5056503.

Continuous Renal Replacement Therapy (CRRT) | Asahi Kasei Medical Co., Ltd. (n.d.). https://www.asahi-kasei.co.jp/medical/en/apheresis/product/crrt/about/cure.html

Cronin B, O'Brien EO. Intraoperative Renal Replacement Therapy: Practical Information for Anesthesiologists. J Cardiothorac Vasc Anesth. 2022 Aug;36(8 Pt A):2656-2668. doi: 10.1053/j.jvca.2021.10.002. Epub 2021 Oct 8. PMID: 34750060.