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Course Sample:
Introduction.
Prior to the patient needing renal replacement therapy (dialysis) the patient will need to be prepared to have some form of access placed whether it is for peritoneal dialysis (catheter placement in the abdomen) or hemodialysis (angioaccess ? ability to reach the blood). This course will discuss angioaccess. During dialysis blood is removed and returned via angioaccess method. There are several types of vascular accesses. This can be through the permanent placement (via surgery) of an access in the blood vessels or via a catheter. These are all vascular access and should be prepared weeks or months before dialysis is required. Studies indicate that only 43.9% of patients had a permanent access placed or attempted before the onset of end-stage renal disease, while in 48.2%, no permanent access was attempted.
Timely placement of a permanent access is critical. The National Kidney Foundation Dialysis Outcome Quality Initiative (NK-DOQI) guidelines recommend that patients should be referred for surgery to attempt construction of a primary arteriovenous (AV) fistula when their creatinine clearance is < 25 mL/min, their serum creatinine level is > 4 mg/dL, or within 1 year of an anticipated need for dialysis. This will allow adequate time for the fistulae to mature; sufficient time to perform another vascular access procedure if the first attempt fails, thus avoiding the need for temporary access; and better healing of polytetrafluoroethylene (PTFE) grafts.
Arteriovenous fistula.
The arteriovenous (AV) fistula is the best approach for all patients as dialysis therapy can be best delivered through this large "created" blood vessel. While any superficial vein of adequate length may be used for construction, a side-to-side radial artery to cephalic vein anastomosis of 0.5 to 1.0 cm is preferred. If the veins are large enough and there is time to prepare it all potential dialysis patients should have one placed. Otherwise, a graft or a catheter may be needed. The AV fistula requires advance planning because a fistula takes a while after surgery to develop or mature, the term most health care providers use when defining that an access can be used for dialysis.
Prior to the surgery, an Allen test should be performed because the normal ulnar artery is difficult to palpate. Another way to check for patency is to perform the Allen Test. The patient rests the hand in his lap clenching firmly his fist. The radial Artery is compressed which produces a decrease in perfusion turning the hand pale. The patient will then open his hand and the skin will recover its normal pink appearance if the Ulnar artery is patent. Do the same process with the ulnar artery.
Allen Test
Figure 1
A vascular surgeon is able to create an AV fistula by surgically connecting an artery directly to a vein (see Figure 2 ). Most of the time this type of surgery is readily performed under local anesthesia and is often performed as an outpatient procedure. The best anatomical location for placement of the AV fistula is in the forearm of the patient. The hand is able to withstand even complete diversion of radial arterial blood flow and its ease of venipuncture in the forearm makes this the preferred access. It can also be placed in the upper part of the arm and or the leg. By connecting the artery to the vein one allows more blood to flow into the vein, thereby the vein grows larger and stronger. This will allow for the repeated needle insertions that occur when the patient is on dialysis. AV fistulas can generally last for years (not requiring surgery to repair or remove it) and is clearly the first approach for all patients.
Maturation of the veins to allow blood flows of 450 mL/min and thereby maintain adequate dialysis can take from 6 to 12 weeks to occur. Risk factors for failure include:
- Low blood pressure
- Diabetes
- Small caliber veins
The creation of an Arterio-Venous Fistula will cause arterial blood to enter the venous low resistance system creating a laminar flow with a continuous turbulence with a systolic peak at the site of the anastomosis -the area with the highest pressure gradient allowing successful AVF maturation. One should wait at least 4-6 weeks before using the new access. Most stenosis will occur in the efferent vein near the arterial puncture. Stenosis may occurs as early as 2 weeks after access placement and they are responsible of early access failure. If after 8 weeks the AVF fails to mature a fistulogram should be done. The ultrasound, being very sensitive and specific for detecting graft dysfunctions, is less reliable for the evaluation of AVF anatomy.
A properly formed fistula still can form thrombosis, or clots (the most common problem) and also become infected. A clot is formed when the AV fistula starts to narrow (termed stenosis). The stenosis results in decreased flow (blood) and it is a mechanical problem. However, over time cells can accumulation on the narrowing and this increase in cells growing on the fistula, called neointimal hyperplasia, further narrows the lumen. This narrowing of the lumen is the step prior to the attachment of blood borne cells (platelets) that start to attach to the narrowing further reducing the lumen of the fistula to the point when dialysis becomes ineffective. Quite often the stenosis becomes critical and with cell attachment, there is the possibility of a clot forming. Once the fistula develops a thrombosis it will become completely non-functioning; there is no blood flow through the access and the "buzz-thrill" that one feels when palpating an access is lost, and the functionality of the fistula. At this time the fistula needs to be ?declotted? as soon as possible. The longer the person waits to have the clot removed the greater chance for the AV fistula to never work again. This entire process, while it occurs in those with a fistula, is significantly more common in those with a ?graft? (see below.)
There are methods of detecting when an AV fistula may develop a thrombosis. Simple listening with a stethoscope for a thrill (often called buzz or a bruit) is one method, although the least sensitive. Doppler ultrasound is a significantly more sensitive method for studying and analyzing the flow through the fistula and allows one to ascertain how well the access is conducting blood flow. Another less sensitive but less laborious way to determine if there is decreased flow is to see how well the patient is being dialyzed, that is, the adequacy of dialysis. In other words the patient?s dialysis treatment becomes less efficient as the lumen narrows and the patient?s blood work may show this occurring through less efficient removal of potassium, decrease in how well fluid is removed, and BUN elevation, etc). It is important to try to fix the narrowing prior to it clotting as once it clots off revising the access is more difficult.
Reduced blood flow will suggest that the patient needs to have an angiogram or termed a fistulogram. An angiogram, where dye is inserted into the vein, will definitely show if the fistula is truly narrowing and if it needs to be surgically revised to prevent the access from completely clotting. Thrombectomy (removal of the clot) involves the use of a balloon tipped catheter and requires hospitalization since these patients require heparinization for one to two days following the procedure. Although thrombectomy remains the mainstay of therapy in most institutions the treatment with thrombolytics has been met with some success. This involves the use of urokinase or streptokinase to dissolve the clot. It should be realized that these approaches are not as successful as thrombectomizing the clotted fistula. However, in some patients who have a history of frequent clotting, post procedure they may require oral coumadin therapy for an indefinite period of time.
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