Hemodialysis (HD) is a medical treatment that artificially removes waste products, excess fluids, and toxins from the blood when the kidneys are no longer able to perform these functions effectively. In this process, blood is filtered through a semipermeable membrane, allowing for the selective removal of waste while preserving necessary components like blood cells and proteins. Hemodialysis is typically performed in patients with end-stage renal disease (ESRD) or severe kidney failure.
Hemodialysis uses a semipermeable membrane made from synthetic materials, such as polysulfone or polyamide, which is used to filter the blood inside a device called a dialyzer (or artificial kidney). The blood is brought into contact with a dialyzer that contains this membrane. The primary objectives of HD are to remove toxic nitrogenous substances (such as urea) from the blood and eliminate excess fluid, which leads to restoring balance in electrolyte levels and maintaining proper fluid volume in the body.
During hemodialysis, blood is diverted from the patient's circulation to the machine using a blood pump. The blood, carrying waste products and toxins, enters the dialyzer, where the harmful substances diffuse across the membrane and are removed by a cleansing solution called dialysate. Then, the cleansed blood is returned to the patient's circulation.
Effective hemodialysis requires rapid blood flow, which necessitates access to a large blood vessel. The common types of vascular access include arteriovenous fistulas (AVFs) and arteriovenous grafts (AVGs).
An arteriovenous fistula (AVF) is considered the preferred and most durable form of permanent vascular access for patients undergoing hemodialysis. An AVF is surgically created, typically in the forearm or upper arm, by connecting an artery (such as the radial or brachial artery) directly to a vein (often the cephalic or basilic vein). This anastomosis, either side-to-side or end-to-side, allows arterial blood to flow directly into the vein, leading to increased blood flow in the vein.
The enhanced blood flow causes the vein to enlarge and strengthen over time, a process known as "maturation," which typically takes at least 6 to 12 weeks. Once matured, the vein can accommodate two large-bore needles (14- to 16-gauge), which are used during dialysis. One needle is used to withdraw blood for filtering, while the other is used to return the cleansed blood to the body.
Patients are encouraged to perform hand exercises, such as squeezing a rubber ball, to promote vein dilation and speed up the maturation process. Once established, an AVF has the longest useful life and lowest risk of infection compared to other vascular access methods, making it the best option for ongoing hemodialysis treatment.
A properly functioning AVF produces a thrill (a buzzing sensation) that can be felt by touch and a bruit (a rushing sound) that can be heard using a stethoscope. These indicators reflect the high-velocity blood flow through the fistula. However, AVFs can be more difficult to create in patients with compromised vascular systems, such as those with severe peripheral vascular disease (e.g., diabetes), people with a history of prolonged intravenous drug use, and obese women. In such cases, a synthetic graft may be necessary.
For patients whose veins are not suitable for AVF creation, arteriovenous grafts (AVGs) provide an alternative. AVGs are made from synthetic materials like polytetrafluoroethylene (PTFE), commonly known as Teflon. They are surgically placed under the skin to connect an artery (usually the brachial artery) to a vein (commonly the antecubital vein). This artificial "bridge" provides a conduit for blood flow between the artery and vein.
AVGs typically require a healing period of 2 to 4 weeks before they can be used for hemodialysis, but in some cases, they can be used sooner. Due to their artificial nature, AVGs are more prone to complications such as infections and thrombosis (clotting) than AVFs. If an AVG becomes infected, surgical removal of the graft may be necessary, as it is often difficult to treat infections involving synthetic materials.
A common and serious complication of AVGs is central venous stenosis (CVS), which is the narrowing or occlusion of the central veins, impeding blood flow to the heart. As CVS progresses, it can lead to the loss of vascular access for hemodialysis, making it a significant concern.
Hemodialysis filters waste, excess fluids, and toxins from the blood using a semipermeable membrane in a dialyzer while preserving essential components like blood cells.
It requires vascular access to allow rapid blood flow, usually achieved through an arteriovenous fistula or AVF.
AVF is surgically created by connecting an artery, such as the radial or brachial artery, to a vein, typically the cephalic or basilic vein.
This connection makes the vein enlarge and strengthen, a process called maturation, which usually takes 6 to 12 weeks.
Once the AVF has matured, two large-bore needles, usually 14 to 16 gauge, are inserted into the vein during dialysis to allow blood to flow in and out of the body.
If an AVF cannot be created due to vascular issues, such as peripheral vascular disease, an arteriovenous graft, or AVG, may be used.
AVGs are synthetic polytetrafluoroethylene tubes surgically placed beneath the skin to attach an artery to a vein.
They can be used within 2 to 4 weeks but carry a higher risk of complications, like infection and thrombosis.
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