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Solute and Water Kinetics in Continuous Therapies


Objectives

This chapter will:

  • 1.

    Describe the basic mechanisms of fluid transport and solute removal (ultrafiltration, diffusion, convection, and adsorption) and the factors influencing these processes in continuous renal replacement therapy.

  • 2.

    Describe the manner in which the basic principles of solute and water removal apply in the clinical application of the different continuous renal replacement therapy modalities.

  • 3.

    Apply the above principles in relation to the concept of dose for a dialytic treatment, with specific reference to the various terms used to quantify therapy delivery.

Renal replacement therapy is required in a significant percentage of patients developing acute kidney injury (AKI) in an intensive care unit (ICU) setting. Continuous renal replacement therapy (CRRT) is any extracorporeal blood purification therapy that aims to support kidney function over an extended period of time. One of the foremost objectives of CRRT is the removal of excess fluid and blood solutes that are retained as a consequence of decreased or absent glomerular filtration. In addition to CRRT, hybrid therapies can be performed. They share characteristics of intermittent and continuous techniques with respect to frequency and duration, having the goal of maximizing the desirable and minimizing the undesirable characteristics of each technique.

Because prescription of CRRT requires goals to be set with regard to the rate and extent of both solute and fluid removal, a thorough understanding of the mechanisms by which solute and fluid removal occurs during CRRT and of related therapies is necessary. From the combination of the different transport mechanisms, a number of CRRT modalities are identified. Finally, these principles are applied to provide a brief overview of the concept of CRRT dose.

Transport Mechanism

Fluid Removal

Ultrafiltration

Ultrafiltration describes the transport of plasma water (solvent) through a semipermeable membrane driven by a pressure gradient between blood and dialysate/ultrafiltrate compartments. (The non-blood chamber of a CRRT filter typically is designated as the effluent compartment, and the rate at which fluid exits the filter from this space is equal to the sum of the dialysate flow rate and/or replacement fluid rate along with the patient's net fluid removal rate.) Quantitatively, ultrafiltration is defined by the ultrafiltration rate:


Q UF = DK UF ⋅ TMP

where DK UF is the filter ultrafiltration coefficient and TMP the transmembrane pressure.

Solute Removal

Diffusion

Diffusion is the process of transport in which molecules that are present in a solvent and can pass freely across a semipermeable membrane tend to move from the region of higher concentration into the region of lower concentration. In reality, molecules present a random movement. However, the number of particles crossing the membrane toward the region of lower concentration is statistically higher. Therefore this transport mechanism occurs for solutes that are not restricted completely in diffusion by the porosity of the membrane. In addition to the concentration gradient (dc), the diffusive flux (J d ) is influenced by membrane thickness (dx) and the diffusion coefficient of the solute (D). The diffusive flux (by definition normalized to membrane surface area ) is defined as the solute mass removal rate resulting from diffusion, and it is described mathematically by Fick's law:


J d = D ( d c d x )

Diffusion is an efficient transport mechanism for the removal of relatively small solutes, but as solute molecular weight increases, diffusion becomes limited and the relative importance of convection increases.

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