Basic Concepts of Hemodialysis

Basic Concepts of Hemodialysis

Hemodialysis, commonly known as "artificial kidney," is the primary form of renal replacement therapy for patients with acute and chronic renal failure. From 90-year-old seniors to newborn infants, people have gradually come to accept it, a shift from initial fear to widespread acceptance, and it has saved countless lives. The world's longest-surviving maintenance hemodialysis patient has lived for 43 years—an unimaginable feat in the past.

Hemodialysis is a basic extracorporeal circulation therapy that requires the use of anticoagulants to prevent blood clotting. Each treatment session lasts approximately four hours, and is performed 2-3 times per week. The equipment includes a hemodialysis machine, water treatment system, and dialyzer, which together form a hemodialysis system.

The hemodialysis machine is the most widely used treatment instrument in blood purification therapy. It can control the flow rate and temperature of the dialysate, the amount of water removed, and the blood flow rate, and has various monitoring systems for extracorporeal circulation. During treatment, a blood pump draws blood from the body, which then flows through tubing and the dialyzer before finally returning to the body.

Water Treatment System: During a single dialysis session, a patient's blood comes into contact with a large amount of dialysis fluid (120L) through the dialysis membrane, the majority of which is dialysis water. Municipal tap water contains various trace elements, especially heavy metals, as well as disinfectants, endotoxins, and bacteria. Contact with blood will cause these substances to enter the body. Therefore, tap water must undergo sequential treatments including filtration, iron removal, softening, activated carbon adsorption, and reverse osmosis before it can be used to dilute the concentrated dialysis fluid. The water treatment system determines the quality of dialysis and affects the patient's ultimate survival rate.

A dialyzer consists of hollow fibers made of chemical materials, each with numerous tiny pores. The dialyzer is the site of substance exchange, with hollow fiber dialyzers being the most commonly used. Hollow fibers are artificially synthesized semi-permeable membranes. During dialysis, blood flows through the inside of the hollow fibers while the dialysate flows in the opposite direction through the outside. Small solute molecules and water in the dialysate are exchanged through the pores in the hollow fibers. The final result of this exchange is that toxins, electrolytes, and excess water in the blood are removed into the dialysate, while bicarbonate and electrolytes in the dialysate enter the bloodstream for further exchange through diffusion or convection. This process achieves the goals of removing toxins and water, maintaining acid-base balance, and stabilizing the internal environment. Therefore, dialysis can treat significant hyperkalemia and severe acidosis with immediate results. The total surface area of the hollow fibers, i.e., the exchange area, determines the permeability of small molecules, while the pore size determines the permeability of medium and large molecules.

Dialysate: It is obtained by diluting dialysis concentrate containing electrolytes and bases with reverse osmosis water in proportion. It eventually forms a solution with a concentration close to that of blood electrolytes to maintain normal electrolyte levels. At the same time, it provides bases to the body through a higher base concentration to correct the patient's acidosis. It is usually divided into A and B solutions. The commonly used dialysate base is mainly bicarbonate and also contains a small amount of acetic acid. It is mixed with reverse osmosis water to form a 35-fold concentrated dialysate, which is diluted by the machine and then flows into the dialyzer.