2016 AJKD--Continuous Dialysis Therapies Core Curriculum 2016--CRRT principle

2016 AJKD--Continuous Dialysis Therapies Core Curriculum 2016--CRRT principle

1. Studies designed to compare continuous versus intermittent therapies have not shown a beneficial effect on mortality
2. With greater hemodynamic stability and a higher likelihood of kidney recovery compared to standard iHD
3. Lack of consensus on several aspects of RRT (eg, timing of initiation, dose, session lenth, and standards for monitoring.

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1. Solute transport
1. convection, diffusion, and adsorption, and replenishing depleted solutes
2. CVVH, relies solely on convection; CVVHD, CVVHDF, SCUF

3. These mechanisms can by manipulated by the type of membrane and blood and fluid flow rates to selectively influence solute clearances of moledules of different size.

Adsorption of solutes occurs to varying degrees in all CRRT circuits and can be a contributor for large molecule removal, depending on membrane characteristics.

4. Effluent flow rate (Qef) is the final result of the filtration process and is composed of the net ultrafiltration (Qnet) + substitution fluid rate (Qs) in CVVH and CVVHDF + dialysate flow rate in CVVHD and CVVHDF.

=> CVVH and CVVHDF: Qef = Qnet + Qs

=> CVVHD and CVVHDF: Qef = Qnet + Qs + dialysate flow rate (????)

• Filter clearance for most CRRT = Qef * S
• S: sieving coefficient = Cuf / Cp, regulated by reflection coefficient of membranes
• reflection coefficient of memebranes => S = 1 - σ, S = 1, solute freely passes through membrane; S = 0, solute cannot freely pass through membrane
• For middle molecules: clearance depends on [membrane permeability] * UF volume
• For adsorption: overall blood clearance can be grater than filter clearance, even when S is low => blood-side clearances will not match filter clearances

5. Techniques: differ in [various driving force for solute removal] and [membrane use]
1. CVVH: via convection, driven by TMP:
1. Uf  = Kf * TMP,
1. Kf = coefficient of hydraulic permeability, and
2. TMP = (Pb - Puf) - 𝝿, Pb = blood hydrostatic pressure; Puf = UF/dialysate hydrostatic pressure, 𝝿 = plasma proteins oncotic pressure
2. Cx = Quf * S, Cx: convection clearance, S: sieving characteristics of membane = Cuf/Cp
3. Qnet = UF - substitution fluid infusion
2. CVVHD: vis diffusive clearance, by solute concentration gradient acrosss the dialysis membrane
1. Sd = (Cg / Mt ) * D * T * A
2. Sd: solute diffusion, Cg: concentration gradient, Mt = membrane thickness, D = diffusion coefficient of the solute, T = solution temperature, A = membrane surface area
3. gradient across membranes: Qd、Qb; Qd is the rate limiting factor for solute removal => Qd 0.5~3 L/h (8~50 mL/min) vs. Qb 100~200 mL/min
3. CVVHDF: via diffusion + convective technique => dialysate + substitution sol.
1. removal of small + middle molecules
4. Slow continuous ultrafiltration (SCUF): use exclusively the principle of UF w/o substitution. => safely treat fluid overload
1. solute removal is minimal because it is limited by total UF volume
6. CRRT: plasma composition + fluid amout in the body => 分開處理

Substitiuion fluid:

7. Citrate is used as an anticoagulant => provides a buffer base cecause each citrate colecule is metabolized in the liver and muscle to 3 molecules of bicarbonate

=> but  may leads to markedly hypotensiion

8. CRRT’s key feature is the flexibility in maintaining a specific level of any electrolyte and calibrating the rate of correction to accommodate the clinical need
9. The technique to achieve fluid management:
1. Most common: vary Qnet to meet the anticipated fluid balance needs over 8~24 hours:
1. Effluent vol + solute clearance => vary with each adjustment in net ultrafiltration
2. To keep a fixed rate of UF exceeds the hourly intake from all sources and to vary the amount of post-dilution substitution fuid administered:
1. ensures a constant effluent volumen and solute clearance level.
2. Post-dilution fluid can be given outside the CRRT circuit through a peripheral IV line.
3. Third method: fluid valance is tailored to achieve a targeted hemodynamic parameter every hour.
1. such as: CVP, MAP, PAWP, etc.