Preparation for Dialysis: AV Access

Preparation for Dialysis: AV Access

As the patient reaches Stage 4 CKD, and the GFR falls below 30 cc/min, it often becomes necessary to face the decision of whether to choose hemodialysis, peritoneal dialysis or a kidney transplant.

For patients who choose hemodialysis, an access becomes their lifeline. The key to a successful outcome with any of the therapeutic modalities depends upon the level of awareness and preparation. One of the challenges of hemodialysis is the late or improper placement of an arteriovenous (AV) access. This article will outline the strategies leading to the best outcomes with respect of AV access placement.

Hemodialysis always requires some sort of “access” to be placed to enable the blood stream access to the filtration system (dialysis machine) required to remove uremic toxins and excess electrolytes. The three major choices are a catheter, an AV fistula and an AV graft.

Catheter

A catheter (plastic tube) is placed into a large vein to allow dialysis. It is generally placed when there is not enough time to allow a fistula to mature prior to beginning dialysis. The current preferred catheter has cuffs made of Dacron. This allows it to be secured in place, and also acts as a barrier, blocking the transfer of bacteria into the circulation. It has the advantages of immediate convenience and the avoidance of the pain of a needle stick. However, the tradeoff is that it becomes infected easily, can lead to stenosis (narrowing) or scarring inside the major blood vessels in which it is placed. It is also associated with greater complications and shortened patient survival. The quality of dialysis filtration is often below standard level when using a catheter.

The AV fistula is created when an artery and vein are connected together (anastamosed) in such a way as to create a direct connection between them. This increases the pressure against the connected veins, and they compensate by expanding and hardening. It also increases the flow and diameter in the artery. After a few weeks, the large, thickened vein can be easily cannulated or have a needle placed in it. The AV fistula is the most successful type of access in use today. It was originally developed in 1966, and was popular in the early days of dialysis. With proper care, the fistula can last for many years – some have been used for 20 to 30 years. The figure on the next page is from a lady whose fistula was placed in 1975 and is still in use today. (Ref: Taking matters into your own hands – cannulating your own fistula – Judy Weintraub, http://www.nephron.org/nephsites/nic/self_cannulation.htm , 2001)

Graft

Another type of access is called the arteriovenous graft. This uses a popular synthetic material, GORE-TEX. This Teflonlike plastic became very popular for access in the United States in the 1970s, but not in Europe. It has the advantage of being placed with less skill than an AV fistula, and will “mature” or be ready to use in less time (around two weeks). However, being synthetic, it is considered a foreign body and easily develops stenosis and clotting. It is more prone to infections. Direct pressure from the cardiac system enlarges and hardens veins.

Two buffering systems exist in nature – the capillary tree and the elasticity of the blood vessel walls which absorbs the energy (like the mat in gym class). In both the graft and the fistula, the capillary tree is removed as a buffer, leaving only the vessel wall of the fistula, and nothing at all in the graft. This puts stress on the veins, creating scarring and narrowing. Frequently, an intervention is necessary to dilate (expand) the graft so it can continue functioning. In general, grafts lasts less than two years, and thus must be replaced more frequently.

The preferred access is the AV fistula. AV fistulae are direct connections between an artery and a vein. There are several types of AV fistulae. The Brescia-Cimino fistula is preferred, and connects blood vessels at the wrist. Theupper arm fistula, and a variant, the Gracz fistula are also popular. 2 Sometimes, veins must be relocated from their position deep in the arm to beneath the skin to make them more accessible.

Saving veins

In order to achieve a successful fistula prior to dialysis, adequate preparation is necessary. A successful access is dependent upon the condition of blood vessels - arteries and veins. Veins, in particular are subject to damage from the excessive needle sticks that accompany clinical procedures. Veins are used for sampling blood and for the administration of fluids and medications. Some fluids and medications are corroding and may lead to scarring. In order to minimize this, patients and their healthcare providers must draw blood and use IVs only when necessary. Ideally, attempts should be first to draw blood from the back of the hand with a small-gauge needle. Those who require frequent blood draws or IV access should have their sites rotated to prevent infection or excess damage to vessels. The cephalic vein (superficial vein of the upper limb) and the veins in the antecubital fossa (crease of the elbow) should be avoided. The IV line should be removed as soon as possible, and medications that are well tolerated and absorbed orally should not be given by IV.

This practice is helpful not only for the prospective dialysis patient, but for anyone. As we age, the likelihood of requiring more medical procedures increases. Those who have not wisely planned soon run out of venous sites and simple procedures, such as a blood draw, become difficult. The placement of an IV access becomes more challenging, as well.

Hand exercises will help make blood draws easier for the technician and consequently minimizing or avoiding the trauma of repeated attempts. It is also ideal when going for blood work to try to maintain some degree of hydration when possible. This makes the veins easier to draw blood.

Arteries are important too

Arterial blood flow is also critical for the success of an AV fistula. In patients with atherosclerosis, the development of calcific plaques inside blood vessel walls blocks the flow of blood. Without adequate flow, the fistula will never mature, let alone be useful for dialysis. Thus, the well-established principles of preventing atherosclerosis apply to successful creation and maintenance of an AV fistula. These include the avoidance of cigarette smoking, getting adequate exercise, watching one’s diet and using lipid-lowering medications when indicated. Much has been written and said about the non-classical factors that lead to arterial disease in dialysis patients. These factors may result in calcification developing in the middle of the arterial wall instead of the inside wall. Since they reduce the vessel wall elasticity, they reduce the “shock absorber” quality of the vessels. This can lead to wear and tear on blood vessels and the heart.

The Office Visit

It is optimal for all patients who begin dialysis to have had adequate time to prepare. This preparation includes a physical examination and the ultrasonic mapping of veins and arteries prior to the procedure. Ideally, the patient can start dialysis on an outpatient basis without having to endure the temporary placement of a catheter, avoiding the potential complications outlined.

During the physical examination, the physician will first inspect the arms, and will assess veins with and without a tourniquet. An ultrasound machine characterizes the diameter of veins and arteries in several locations in the arm with the goal to find the best vessels for access creation. The goal is to find veins that are superficial and are widely open. Transposition or moving of a vessel may be needed if the vessels are too deep. The vessels must be long enough to support the fistula. Duplex sonography allows one to visualize the blood vessel anatomy and blood flow in the same image. The examination starts with the cephalic veins at the wrist and the radial artery at the wrist. It works up to the basilic and cephalic veins in the upper arm, along with the brachial artery. Using proximal vein compression with a blood pressure cuff inflated to 50 mm Hg for at least two minutes, the vein should dilate.

With deep inspiration the venous flow should increase, but if there is venous obstruction in the central veins, there will be no change in the venous blood flow measured in the arms. Venograms may also be necessary if the patient has had a central catheter placed. This is to exclude a central vein occlusion that might impede the flow of blood returning to the heart. An occlusion would increase the pressure in the veins and accelerate stenosis. It would jeopardize the dynamics of blood flow and interfere with the ability to dialyze.

The arteries are also evaluated, and the internal diameter of the radio artery must be at least 1.6 mm to 2.5 mm. The artery must increase in flow for an AV Fistula to be successful, and this is challenged by vascular disease, hypertension and diabetes. The arteries can be evaluated by having the patient clench the fist for two minutes then open it. This should double the flow and thus indicate the artery will respond normally when a fistula is created.

Properly mapped vessels enable surgeons to properly place the AV fistula, and can give the health team a better idea as to the prognosis of fistula survival. The successful creation of an AV fistula, already mature and ready to use prior to beginning dialysis, depends upon selecting the modality of therapy well into Stage 4, and following through with vessel mapping and the surgical procedure.

References

1. U.S. Renal Data System, USRDS 2006 Annual Data Report. 2006

2. Bender MH, Bruyninckx CM and Gerlag PG: The Gracz arteriovenous fistula evaluated. Results of the brachiocephalic elbow fistula in haemodialysis angio-access. Eur J Vasc Endovasc Surg 10:294-297, 1995

3. Malovrh M: Approach to patients with end-stage renal disease who need an arteriovenous fistula. Nephrol Dial Transplant 18 Suppl 5:50-52, 2003

Stephen Fadem, MD, FACP, FASN, serves as vice president of the AAKP Board of Directors, Co-Medical Editor of Kidney Beginnings: The Magazine and is a member of the AAKP Medical Advisory Board. Dr. Fadem is a practicing nephrologist in Houston, TX. This article originally appeared in the April 2007 issue of Kidney Beginnings: The Magazine.

http://www.aakp.org/aakp-library/The-AV-Fistula/