Hemodialysis

# Overview
The goal of hemodialysis is removing toxins and aim to maintain euvolemia. Ninety three percent of ESRD patients in the United States and 89% worldwide, are under hemodialysis. Solute diffusion across a membrane is the basic principle for hemodialysis. Metabolic waste products move across a semipermeable membrane depending on their concentration gradient between plasma and dialysate. Concentration gradient, membrane surface area, the membrane thickness, and size of solute molecule are important factors determining rate of diffusion. Small molecules clear more efficiently than larger molecules. Fluid removal is another advantage of hemodialysis that could be achieved by ultrafiltration. In-center hemodialysis and home hemodialysis are available for ESRD patients requiring renal replacement therapy; the choice of modality is based on patient condition, patient preference, and the availability of equipments.

# Components

## The Dialyzer
Dialyzer is usually made of bundles of hollow fibers permitting a high flow rate of blood and dialysate simultaneously. Parallel plates are another type of dialyzer that are barely used in recent times. Most of the dialyzers are synthetic with a variety of materials including polyamide, polyarylethersulfone, polyvinylpyrrolidone, polyacrylonitrile, and polysulfone. Biocompatible membranes have the advantage of not activating complement system.

## The Dialysate
The dialysate is a paramount composition in hemodialysis. Solutes diffuse across the dialyzer between blood and dialysate. The dialysate composition should be individualized to restore plasma normal values. The main solutes in dialysate include sodium, potassium, calcium, magnesium, chloride, bicarbonate , and glucose. These electrolytes are treated with water during the process. About 100 liters of water is needed for each dialysis session. The water used for hemodialysis should be processed in order to have a balanced concentration of solutes. Also, contaminants including bacteria, viruses, and heavy metals, such as aluminium should be removed from water. This removal could be done either by using reverse osmosis or deionization. Filters may be used to improve water quality by removing particles. The dialyzate temperature could also be adjusted to cause vasoconstriction and improve patient's hemodynamics.
The choice of the dialysate is important to maintain or correct the electrolytes.
- Sodium: Usually, sodium concentration of 140-145 mEq/L is suitable for most of the patients but, it could be adjusted based on patient's sodium level.
- Potassium: The potassium concentration in dialysate solution depends on patient potassium level. For example: For patients with hyperkalemia, a potassium concentration of 2-3 mEq/L is appropriate and for patients with hypokalemia, a potassium concentration of 4 mEq/L is appropriate.
- Calcium: Dialysate concentration for calcium are available with 2.5 ,3, or 3.5 mEq/L. In most of the cases a 2.5 mEq/L solution which is equivalent to 5 mg/dl ionized calcium is used.

## Blood Delivery System
The blood pump delivers blood from the arterial line to the dialyzer and then return it to the venous line. It's speed could be adjusted based on patient condition typically between 200 to 600 mL/min. This pump allows creating a transmembrane pressure in dialyzer by sucking dialysate. The dialysate flow rate is between 500 to 800 mL/min.

# Vascular Access
- The establishment and maintenance of reliable vascular access is crucial for long-term hemodialysis.
- Creating arteriovenous (AV) access by establishing arteriovenous fistula (AVF) is the most reliable vascular access. On the other hand, arteriovenous graft (AVG) may provide access in certain circumstances by placing a prosthetic or biograft. However, majority of patients need temporary access by tunneled catheters for initiating hemodialysis.
- Insertion of central catheter may cause complications, such as arterial and ventricular dysrhythmias, arterial puncture, hemothorax, pneumothorax, air embolism, perforation of central vein or cardiac chamber, and pericardial tamponade.
- Infection is another concern regarding catheter care. Migration of bacteria from patient's skin is the mechanism of catheter infection. Skin preparation before procedure by using chlorhexidine at the catheter exit site could prevent catheter infection. Staphylococcus epidermidis, is the most common isolate. Prompt catheter removal is recommended if evidences of site infection is present even in the absence of systemic signs.
- Catheter thrombosis may cause block blood flow is another aspect of catheter care. Instilling alteplase in the affected catheter lumen for 30 to 120 minutes is the preferred treatment.
- Central vein stenosis: Subclavian vein catheters have a higher risk of stenosis. Using a dilator and ultrasound guided inserting may decrease the risk of vessel trauma and consequently minimize the likelihood of stenosis.

# Anticoagulation
Dialysis patients tend to have greater risk of thrombosis due to increased factor VII activity, increased fibrinolytic activity, and elevated fibrinogen levels. However, uremic state may poses patients to bleeding diathesis. Also, dialysis procedure may increase turbulent blood flows with high shear stress which may activate platelets. All of these concepts emerge the use of anticoagulants during hemodialysis. Heparin (UFH) is the most frequently used form of anticoagulants. The usual heparin dose is 50 to 100 U/kg bolus at the initiation which followed by 100 U/hour. Activated clotting time (ACT) is measured in certain circumstances to assess the level of anticoagulation by maintaining it above 200 to 250 sec. Some patients have a higher risk of bleeding which requires a regional anticoagulation. In this method, extracorporeal dialyzer is heparinized in the arterial line and protamine is administered in the venous line. Another strategy is not using any forms of anticoagulation which hemodialysis is initiated with a high blood flow rate to decrease the risk of thrombosis and using saline flush technique which, the dialyzer is flushed with 50 mL of saline every 15 to 60 minutes.

# Monitoring and Adequacy
The dialysis adequacy is measured by using two factors, Kt/V and the urea reduction rate:
- Kt/V: measures the ratio of cleared plasma (Kt) to the volume of urea distribution (V). The goal is to keep the ratio above 1.2 however, Kt/V higher than 1.4 may show more efficient dialysis.
- Urea reduction ratio (URR): It reflects the removal of urea. URR is calculated by using the pre and post dialysis BUN measures. URR = (BUN pre-BUN post) / BUN pre  It is recommended to keep URR 65% to 70%.
A typical hemodialysis is usually between 3 to 4 hours and three times a week. Blood flow is between 400 to 500 mL/min. However, for central venous catheter it might be lower about 350 to 400 mL/min. Dialysate flow rate is between 500 to 800 mL/min. Large size dialyzer may improve the adequacy.