Principles and Devices
Hemodialysis (HD) is commonly performed in forms such as conventional hemodialysis, hemofiltration, and hemodiafiltration. Its primary function is to replace the kidney's ability to clear solutes (mainly small-molecule solutes) and fluids. By utilizing the principle of semipermeable membranes, hemodialysis facilitates solute exchanges to remove metabolic waste from the blood, maintain electrolyte and acid-base balance, and eliminate excess fluids. Solute clearance relies mainly on diffusion and convection. Diffusion involves solute movement from the side with a higher concentration to the side with a lower concentration across the semipermeable membrane, based on the concentration gradient. Convection depends on a pressure gradient across the membrane, whereby water and solutes smaller than the membrane's molecular weight cutoff move from the high-pressure side to the low-pressure side. In conventional hemodialysis, diffusion plays a predominant role, while convection is the primary mechanism during hemofiltration.
During hemodialysis, blood flows from the vascular access into the extracorporeal circulation. A roller pump propels the blood into a dialyzer where solute exchange occurs between the blood and dialysate solution, followed by the return of the blood to the body through the vascular access. Hollow fiber dialyzers are commonly used in clinical practice, with dialysis membranes primarily made from modified cellulose or synthetic materials. The dialysis membrane area for adult patients typically ranges from 1.5 to 2.0 m2 to ensure sufficient exchange capacity.
The dialysate commonly uses bicarbonate-buffered solutions and contains substances such as sodium, potassium, calcium, magnesium, chloride, and glucose. Sodium ions are maintained at physiological concentrations, while diabetic patients may receive dialysate with physiological glucose concentrations. Dialysis water is produced by specialized water treatment systems, and its quality affects the efficacy of dialysis and long-term outcomes for the patient.
Vascular Access
The arteriovenous fistula is the most ideal permanent vascular access and includes autogenous arteriovenous fistula (AVF) and graft arteriovenous fistula (GAF). The autogenous AVF typically involves the anastomosis of the radial or brachial artery to the cephalic or basilic vein, causing "arterialization" of the superficial forearm veins. This results in a blood flow rate up to 500 ml/min and a vessel diameter of ≥5 mm, which facilitates needle insertion. It is recommended to perform AVF surgery at least 3 to 6 months before the anticipated initiation of hemodialysis. This allows adequate time for fistula maturation, functional assessment, or repair, ensuring a functional AVF is available for dialysis. For patients unable to establish an autogenous AVF, a graft AVF can be created, although it carries a higher risk of thrombosis and infection.
Another option for vascular access involves the placement of a percutaneous dual-lumen central venous catheter. Depending on their design and use, these catheters can be classified as non-cuffed catheters (NCC) or tunneled cuff catheters (TCC). NCCs are used for short-term, urgent requirements, whereas TCCs are used for long-term hemodialysis in those who cannot undergo AVF surgery or whose AVFs have failed. Catheter placement typically involves the internal jugular, femoral, or subclavian veins. Common complications include infection, thrombosis, and venous stenosis.
Indications and Treatment
Indications
Hemodialysis should be initiated appropriately for patients with acute kidney injury or chronic renal failure. It can also be used for acute poisoning by drugs or toxins, particularly when the molecular weight of the substance is lower than the dialyzer membrane's cutoff, is highly water-soluble, has a low volume of distribution, a low protein-binding rate, and a high free plasma concentration (e.g., ethanol, salicylates). Additionally, hemodialysis may be applied in emergencies such as refractory congestive heart failure, acute pulmonary edema, and severe disturbances in fluid, electrolyte, or acid-base balance.
Anticoagulation Therapy
Anticoagulation therapy is required during hemodialysis to prevent clotting within the dialyzer and bloodlines. The most commonly used anticoagulants are heparin or low-molecular-weight heparin. Heparin is usually administered with an initial dose of 0.3–0.5 mg/kg, followed by a maintenance dose of 5–10 mg/hour, adjusted individually based on the patient's coagulation status. For patients with active bleeding or a marked bleeding tendency, low-dose heparin, regional citrate anticoagulation, or dialysis without anticoagulants may be used.
Dialysis Dosage and Adequacy
Based on the patient's residual renal function, hemodialysis is generally performed three times per week, with each session lasting 4–6 hours. The dialysis dose should be adjusted to ensure adequate dialysis. Inadequate dialysis is a common cause of complications and mortality in long-term dialysis patients. The adequacy of dialysis is assessed primarily through parameters related to protein metabolism. The urea clearance index (Kt/V) is the most commonly used quantitative measure.
In the formula, K represents the dialyzer urea clearance rate, t refers to the duration of a single dialysis session, and the product of K and t represents the urea clearance volume. V is the distribution volume of urea (approximately equivalent to 57% of the patient's dry weight, which refers to their weight after the removal of excess fluids). Kt/V thus represents the ratio of the volume of urea cleared during dialysis to the total distribution volume of urea in the body. A Kt/V between 1.2 and 1.4 is considered optimal for dialysis adequacy.
Complications
Most complications are associated with the intermittent nature of hemodialysis, as the rapid clearance of solutes such as blood urea nitrogen during each session disrupts the osmotic balance between intracellular and extracellular fluids. Hypotension is primarily caused by excessive and rapid ultrafiltration within a short period, resulting in inadequate effective blood volume. Other contributing factors include autonomic neuropathy, the use of antihypertensive medications, eating during dialysis, arrhythmias, pericardial effusion, sepsis, myocardial ischemia, and reactions to the dialysis membrane.
Long-term complications associated with hemodialysis include amyloidosis and protein-energy malnutrition, which are often related to inadequate dialysis or low clearance rates of middle- and large-molecule toxins.
For first-time dialysis patients, low-efficiency dialysis may reduce the occurrence of complications, with adjustments such as lowering blood flow rates, shortening dialysis duration, and using dialyzers with smaller surface areas. The prevention of long-term complications primarily focuses on improving the clearance rates of middle- and large-molecule toxins.
Complications of hemodialysis include:
- Dialysis Disequilibrium Syndrome
- Hypotension
- Thrombosis
- Air Embolism
- Painful Muscle Cramps
- First-Use Syndrome of Dialyzers
- Fever
- Arrhythmias
- Hypoglycemia
- Bleeding and Acute Hemolysis
- Dialysis-Related Amyloidosis
- Protein-Energy Malnutrition
- Thrombocytopenia
Continuous Renal Replacement Therapy
Continuous renal replacement therapy (CRRT) refers to a group of blood purification therapies that provide continuous and slow removal of solutes and fluids. Traditionally, CRRT is carried out over a 24-hour period but can be adjusted according to the patient’s condition.
CRRT differs from conventional hemodialysis in several key aspects:
- It has minimal impact on hemodynamics, with less fluctuation in blood osmotic pressure.
- It allows continuous removal of solutes and fluids, maintaining internal environmental stability and facilitating enteral or parenteral nutrition.
- Convection is the primary means of clearance, enabling the simultaneous removal of both small and middle-sized molecules.
- It supports bedside treatment and emergency care, making it an important organ support measure in critical care settings beyond kidney function replacement.
Indications for CRRT include severe acute kidney injury, chronic renal failure (especially when accompanied by conditions such as acute pulmonary edema, cerebral edema, hemodynamic instability, or hypercatabolism), multiple organ failure, sepsis, cardiopulmonary bypass, acute respiratory distress syndrome (ARDS), congestive heart failure, severe acute pancreatitis, drug or toxin poisoning, and crush syndrome.