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Chapter 51: Continuous Renal Replacement Therapy

Solute clearances in CRRT are dependent on which of the following?

A. Sieving coefficient of the solute

B. Effluent flow rates

C. Ultrafiltration rate

D. The percent saturation of the effluent

E. The dialysate flow rate

The answer is D. Solute clearances in CRRT depend on the permeability of the membrane for the particular solute defined by its sieving coefficient (SC) (ratio of solute in the effluent versus plasma), and the transport mechanisms for the modality (convection for hemofiltration (SCUF, CVVH), diffusion for hemodialysis (CVVHD) and both convection and diffusion for hemodiafiltration (CVVHDF). Effluent volume content is determined by the modality and includes both UF and dialysate. Thus clearance = SC × effluent volume. For small solutes like urea nitrogen SC is usually 1 and consequently the clearance depends on effluent volume. However as the SC changes due to changes in filter permeability clearance will be reduced proportionally. Thus it is important to monitor the SC through the course of therapy as effluent volume alone does not reflect clearance.

Filtration fraction in CRRT represents which of the following?

A. The ratio of effluent volume to blood flow rate

B. The ratio of ultrafiltration volume to blood flow rate

C. The ratio of plasma flow rate to ultrafiltration rate

D. The ratio of ultrafiltration rate to effluent volume

E. The ratio of dialysate flow to blood flow rate

The answer is C. Filtration fraction represents the proportional concentration of blood as it traverses the membrane. The ratio of ultrafiltration rate to plasma flow rate reflects the amount by which hematocrit will rise from the inlet to the outlet of the membrane. Thus FF = UFR/min/plasma flow rate/min. Plasma flow rate: The plasma flow rate = blood flow rate – hematocrit. So for a blood flow rate of 100 mL/min and a hematocrit of 30 the plams flow rate is 100 – 30 = 70 mL/in. If the ultrafiltration rate is 1000 mL/h = 16.7 mL/min the FF = 16.7/70 = 23.8. Since effluent volume includes ultrafiltration and dialysate it would overestimate FF in modalities where dialysate is used. Similarly utilizing blood flow rate in the calculation would underestimate the FF. In general it is recommended to keep the FF below 25–30% to minimize chances of filter clotting.

Daily fluid balance in CRRT is best represented by which of ...

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