Physicochemical fundamentals of dialysis
The basis of most renal replacement therapy methods is the removal of substances through a semipermeable membrane into a specialized dialysis solution (most commonly bicarbonate-based). The driving forces of this process are hydrostatic and osmotic gradients.
During the procedure, the following are removed from the body:
- Nitrogenous waste products (uric acid, creatinine).
- Excess electrolytes (allowing the management of hyperkalemia).
- Toxic "middle molecules" (substances with a molecular weight of 300–5200 Da).
The difference between the basic methods lies in the type of filtering barrier. Hemodialysis utilizes an extracorporeal (located outside the patient's body) synthetic membrane inside the dialyzer. In the case of peritoneal dialysis, an intracorporeal biological membrane is used—the patient's own peritoneum.
Hemodialysis: machine and vascular access
Regular chronic hemodialysis requires a stably high volumetric blood flow. Arteriovenous connections are surgically created, most commonly using the radial artery (a. radialis) and the cephalic vein (v. cephalica). There are two main types of access:
- External arteriovenous shunt. The vessels are connected using an external system of polymer tubing. At rest, blood flows from the artery to the vein through a closed U-shaped loop, bypassing the capillaries (this prevents thrombosis). During the session, the loop is disconnected: the arterial limb delivers blood to the machine, and the venous limb receives the purified blood.
- Internal arteriovenous fistula. This is a surgical anastomosis directly between the artery and vein, in which the distal end of the vein is ligated. Under the pressure of arterial blood, the vein "arterializes"—its walls thicken and its lumen widens. This option is more convenient for regular punctures since all elements are hidden under the skin. Two needles are placed during the procedure: one for blood extraction (closer to the anastomosis) and one for return (further up the course of the vein).
The artificial kidney machine itself includes a blood circuit (with a heparin buffer to prevent clotting, a pressure monitor, and an air bubble trap) and a dialyzer. The efficiency of blood purification depends on blood flow velocity, the surface area of the membrane, and the duration of the session. In acute kidney injury (AKI), permanent access is not created; instead, temporary catheterization of the femoral or subclavian vessels is used.
Peritoneal dialysis and transplantation
Peritoneal dialysis utilizes the anatomy of the abdominal cavity. Through a catheter in the anterior abdominal wall, 2–2.5 liters of dialysis solution are infused. The peritoneum acts as a natural filter between the blood in its capillaries and this fluid. After a specific dwell time, the solution is exchanged. Although the filtration rate is lower here than in machine hemodialysis, the method has significant advantages:
- It clears "middle molecules" significantly faster.
- It less frequently causes the endocrine and neurological complications characteristic of chronic hemodialysis.
- It improves quality of life since the patient performs the procedure independently and is not tethered to a stationary machine.
A radical solution is kidney transplantation. This method has experienced significant progress in recent decades thanks to the introduction of cyclosporine. By suppressing the rejection reaction, cyclosporine has radically improved the lives of patients with end-stage renal disease.
Other extracorporeal blood purification methods
In addition to standard dialysis, specific blood purification techniques are used:
- Hemofiltration. Operates on the basis of hydrostatic pressure and high-permeability membranes. The removal of fluid and dissolved solutes occurs via convection, and losses are compensated by a specialized solution (e.g., Ringer's solution). The method efficiently clears "middle molecules" and controls fluid balance, but is less effective at reducing urea and creatinine, so it is often combined with hemodialysis.
- Hemoperfusion. Blood is passed through activated charcoal and ion-exchange resins. It effectively eliminates creatinine, uric acid, and exogenous toxins, but does not remove urea. In uremia, it is used in combination with hemodialysis.
- Isolated ultrafiltration. The main goal is the rapid removal of excess fluid. Indicated for anasarca, uncontrolled arterial hypertension, and refractory edema (in nephrotic syndrome or heart failure).
- Plasmapheresis. Removal of the patient's plasma and replacement with donor plasma. Used to eliminate substances that do not pass through membranes: proteins, circulating immune complexes (CICs), inflammatory mediators, and coagulation factors.
Briefly: answers to topical questions
What complications can develop from the arteriovenous fistula in patients on chronic dialysis?
The following structural and hemodynamic complications can develop in the arteriovenous fistula in hemodialysis patients:
- Aneurysm — a complicated false or true aneurysm with a risk of rupture, hemorrhage, and infection.
- Thrombosis — acute thrombosis of the arteriovenous fistula, requiring emergency thrombectomy.
- Reduction of the cannulation area — a technical issue that complicates procedures.
- Heart failure decompensation — a high risk of developing severe heart failure after the creation of the anastomosis.
What is the exact pharmacological mechanism of action of cyclosporine in preventing transplant rejection?
In the cytoplasm of a T-lymphocyte, cyclosporine binds to a specific intracellular immunophilin protein, cyclophilin. The resulting complex blocks the activity of the enzyme calcineurin, a key link in T-cell activation. As a result of calcineurin blockade, the production of interleukin-2 (IL-2) by T-lymphocytes is reduced.
The factors of hemodialysis efficacy are easy to remember as the "rule of threes": Speed (blood flow through the dialyzer), Schedule (duration of the procedure from 3.5 to 6 hours), and Size (membrane surface area).
Class and exam questions
Why is hemoperfusion rarely used as a standalone method in uremia?
Hemoperfusion effectively clears the blood of creatinine, uric acid, and "middle molecules", but it is completely incapable of removing urea. Therefore, for complete detoxification in uremia, it must be combined with hemodialysis.
What clinical advantages does peritoneal dialysis have over hemodialysis?
Peritoneal dialysis clears the body of "middle molecules" better, less frequently leads to neurological and endocrine complications, and gives the patient more freedom since the procedure can be performed independently without being tied to a clinic.
What else to study on this topic
- Mechanisms of formation and typical complications of the arteriovenous fistula
- Comparison of the clearance of various substances during hemodialysis and hemoperfusion
- Indications and contraindications for plasmapheresis in nephrology
- Operating principles of the air bubble trap and pressure monitor in the artificial kidney circuit
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