› Assess AV Fistulas

The Basic Assessment of an AV Fistula

Usually AV fistulas are used for patients on dialysis, and so are covered under the Renal System, but I decided since we are dealing with arteries and veins to place it under Cardiovascular. 

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An AV (ArterioVenous) Fistula is also called an AV Shunt. 

While technically there is a difference, most nurses use the term interchangeably unless they know specifically they type the patient has. 

The fistula or shunt is used for patients receiving hemodialysis.

It allows a good access to the patient’s blood, which needs to be removed at a rapid pace, about a pint per minute.

This blood circulates through the dialysis machine, toxins removed and the processed blood returned to the patient. 

There are three principal forms of chronic vascular access for hemodialysis that you may see. They are:

  • Native arteriovenous fistulas (native AVFs), 
  • Arteriovenous shunts using graft material (AV graft),
  • Tunneled double-lumen catheters 

The native AV fistula is when the surgeon takes a large vein, usually in the forearm, though at times it can be in the upper arm and attaches it directly to a nearby artery. 

This short circuits some but not all of the blood flow that normally would continue to flow down the arm, hand and fingers into smaller and smaller arteries until they flowed through capillaries and returned to the heart via the veins. 

The native AVF is preferred for long-term hemodialysis vascular access since it has the best long-term primary patency rate, requires the fewest interventions of any type of access, and most importantly, AVFs are associated with the lowest incidence of morbidity and mortality.

The use of an artificial tube to attach artery to vein is specifically called the shunt as opposed to fistula. While it does have a higher incidence of complications it is still pretty common. 

Once a fistula is surgically created, it must “mature” before being used regularly for dialysis.

Since it may be necessary for a patient to start dialysis before the fistula or shunt is mature and so a tunneled double – lumen central line catheter will be used until them. 

The central line catheter is inserted in the chest wall, tunneled under the skin for a distance until it is inserted in a major vein. I has two lumens or tubes with caps hanging out of the chest wall for removal of blood and return of blood during dialysis.

Maturation of the AV Fistula

Maturation towards use of the AV fistula should occur at 4 to 6 weeks. A fully matured AV fistula is one that can sustain 3 consecutive 2-needle cannulations with no infiltrations at the prescribed needle gauge and blood flow rate. 

Maturation is the process that allows time for the vein to enlarge and thicken enough to withstand the multiple needle sticks every week. (Dialysis is usually 3 times per week.)

These shunts can look large and bulge significantly.

The mature fistula must be superficial enough, large enough (in diameter), thick-walled, straight enough, and have sufficient blood flow to permit routine and safe 2-needle cannulation.

Do not attempt to draw blood from a fistula. Use the other arm completely for lab draws as well as Blood Pressures. Most patients are well aware of the restrictions and will tell you if you start to put a BP cuff on the arm with the shunt. 

Evaluation of the AV Shunt

Check that the temperature of the hand is equal to that of the opposite hand, check for a radial pulse as well as check for capillary refill, which should be < 3 seconds. (The symbol “<” means less than. The symbol “>” means greater than).

The Thrill

The next step to evaluation of the AV Fistula is checking for the thrill. Hold your hand and fingers flat and gently place your fingers on the shunt. You should feel a constant vibration or buzzy feeling. There should NOT be any sense of a pulse like throb, just a steady sensation.

Bruit (pronounced brōō′ē)

Now taking your stethoscope, place it gently on the AV Fistula and listen. You should hear a constant swishing sound. Again no sense of pulsating should be noticed. If there is, that is when you call the dialysis unit to let them know. Yes, you could call the primary doc but s/he will just refer you to the dialysis unit.

If perchance your patient has a dressing over the shunt that was placed by dialysis, there is no need for you to remove it to listen to the thrill and bruit. 

As a matter of fact most dialysis units prefer that their dressings remain intact and undisturbed by other nurses. If there is an obvious problem such as bleeding which is most abnormal, reinforce the dressing with additional gauze over the original and call dialysis immediately.  

Just as an FYI, the AV fistula shown here is relatively small. They can be significanlty larger.

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