Dialysis is a life-sustaining treatment for individuals suffering from kidney failure, acting as an artificial replacement for the natural filtering function of the kidneys. The dialyzer, often called the artificial kidney, is the component where the patient’s blood is cleansed of waste products and excess fluid. High flux dialyzers represent a significant technological advancement in this treatment, offering a more efficient method of blood purification than older systems, improving the overall quality of treatment.
Defining the Dialyzer and the Concept of Flux
The dialyzer is constructed from thousands of microscopic hollow fibers made of a semipermeable membrane, which separates the patient’s blood from a specialized cleaning solution called dialysate. Blood flows through the inside of these fibers, while the dialysate flows in the opposite direction around the outside. This counter-current flow maximizes the efficiency of waste removal across the membrane.
The term “flux” in dialysis refers to the permeability of this membrane, specifically its capacity to allow water and solutes to pass through. Dialyzers are categorized as either low flux or high flux based on this permeability. A high flux dialyzer has a significantly higher ultrafiltration coefficient, meaning it is much more permeable to water and dissolved substances. High flux dialyzers are those with an ultrafiltration coefficient often greater than 14 mL/h/mmHg, and a greater capacity to clear larger molecules.
How High Flux Membranes Change Molecule Removal
The removal of waste from the blood relies on two primary mechanisms: diffusion and convection. Diffusion is the movement of solutes from an area of high concentration (the blood) to an area of low concentration (the dialysate) and is the main mechanism for clearing small molecules, such as urea. Convection, also known as solvent drag, occurs when the movement of fluid across the membrane carries dissolved solutes along with it.
High flux membranes are manufactured with larger pore sizes than low flux membranes, which fundamentally alters the balance between these two removal processes. The larger pores allow a greater volume of fluid to be moved across the membrane, which significantly increases the role of convection in waste removal. While diffusion remains the workhorse for small solutes, the increased contribution of convection is what defines the performance of a high flux dialyzer.
Key Differences and Clinical Use
The most significant clinical distinction between low flux and high flux dialyzers lies in the size of the molecules they can remove. Low flux membranes are primarily limited to clearing small molecules, typically those with a molecular weight less than 500 Daltons. In contrast, the larger pores of a high flux dialyzer enable the effective clearance of larger compounds, which are often referred to as “middle molecules,” generally ranging from 500 to 5,000 Daltons.
The ability to remove these larger toxins is the primary reason for choosing high flux dialysis. A key example of a middle molecule cleared more effectively is Beta-2 microglobulin, a protein with a molecular weight of nearly 12,000 Daltons. Chronic accumulation of this molecule can lead to dialysis-related amyloidosis, a condition where the protein deposits in joints, bones, and tendons. By removing these larger molecules more efficiently, high flux dialysis aims to reduce the risk of such long-term complications and improve patient outcomes.
Operational Requirements for High Flux Dialysis
The increased permeability of high flux membranes, while beneficial for toxin removal, introduces a potential safety concern known as back-filtration. Back-filtration is the unwanted movement of fluid and any contaminants from the dialysate back into the patient’s blood due to pressure differences across the membrane. The large pores that enable higher flux also make the membrane more susceptible to this reverse flow.
To counteract this risk, high flux dialysis necessitates the use of a highly purified dialysate, often referred to as ultrapure dialysis fluid. This fluid must adhere to stringent water quality standards to ensure that any fluid potentially back-filtered into the blood is microbiologically safe. According to international standards, ultrapure dialysate must have a bacterial count below 0.1 colony-forming units per milliliter (CFU/mL) and an endotoxin level below 0.03 endotoxin units per milliliter (EU/mL). Maintaining these strict standards minimizes the risk of inflammation and infection that could occur if contaminants were to enter the patient’s bloodstream.