How do I know if my access is functioning well?

If you are a hemodialysis patient, your blood access is your connection to life. Preventing access problems can save you a great deal of misery. While the dialysis unit staff should be routinely assessing the condition of your access, you can also use several techniques to check yourself as well.

If an access is not working properly, the cause is usually a stenosis. A stenosis is a narrowing in the width of a blood vessel. Any narrowing leading to, through or from the access can cause a problem, and if severe enough, slow down the access blood flow, allowing it to clot. The most common sites of narrowing are listed below.

Artery leading to the access: Usually seen in people with atherosclerotic disease involving small arteries, such as diabetics and long time smokers.
Arterial connection to veins (anastomosis): This is a connection surgically joining the superficial veins of the arm or the artificial graft with the artery.
In the veins or graft leading from the arterial anastomosis: Stenoses here can come from poor needle sticking or a leakage of blood from the access into the arm, causing a hematoma.
Venous connection with the graft: This is the most common site of blood flow obstruction. The cause of these stenoses are not clear, but may be related to tissue reaction to turbulent blood flow.
Anywhere in the native veins after the fistula anastomosis or after the venous connection with a graft: Usually due to previous catheters placed for IV infusions that have damaged the blood vessels. This is a major complication of subclavian catheters used for dialysis, but is seen less often with catheters placed in the internal jugular veins.

Here are factors influenced by stenoses whereby access performance can be evaluated:

Access blood flow
Venous pressure
Recirculation

Your access is supposed to deliver enough blood to the surface of your arm to be removed for dialysis. If your dialysis blood flow is 400 or 500 ml/min, seen by reading the blood flow gauge, then your artery must deliver more than that amount to the access to prevent recirculation of blood or collapse of the arterial flow. This collapse in a fistula is called "sucking" and is seen when the arterial connection with the vein is too small or the arteries leading to the connection are damaged. If the narrowing occurs in blood vessels leading from the access, then blood flow is slowed as pressure builds up in the access. Eventually, it will clot because blood is not moving fast enough. Recent studies show that when the blood flow in the access drops below 600 ml/min the risk of clotting markedly increases.

Slow flows or drops in pressure within an access indicate that the access will eventually fail. Various machines have been developed to indicate when this happens. These machines are expensive and getting useful information depends on the skills and availability of those running them. Are you at a disadvantage if your unit does not employ one of these machines? Not at all! There are evaluations that the staff and especially you can use to great effect. You probably know best the force or strength of the blood flow in your access under different circumstances: during activity and during rest, before and after dialysis. Any weakening of the flow which you can feel or hear (at night through your pillow) should cause you to bring it to your doctor's attention. If you have a fistula, listen to the anastomosis with a stethoscope. There should be a loud continuous murmur heard for a good ways up the arm, which sounds like machinery or "to and fro" sound. The pitch (to musicologists) should be deep - a rumble. The higher the pitch (like a whistle), the more narrow the opening. If only a single, discontinuous, high pitched whistle is heard over the anastomosis with the heartbeat, then the fistula is too small and will never work properly.

The most common method to detect access problems is the venous pressure. There are special machines to do this, but you can use the venous pressure monitor on your dialysis machine. This is the monitor which connects with the drip chamber in your venous line. The pressure to be concerned about varies with the needle size (gauge) and blood flow. Your nurses and technicians should be familiar with these. You certainly should note any increases of pressure over time when other conditions, like needle size and blood flow, are unchanged. A persistent increase would suggest that the access was about to fail.

Recirculation takes place when not enough blood is entering or leaving your access and occurs more frequently in grafts. It reduces the efficiency of your dialysis treatment. Significant recirculation is usually a late finding in access failure. As it worsens, it can lead to dialysis treatments of lessening efficiency. If you are wondering why you just can't seem to feel better on dialysis, maybe your access is dysfunctional and causing recirculation. Again, there are machines to measure recirculation but it also can be checked out by drawing blood. The dialysis staff should be familiar with this technique. The details are somewhat technical and are contained in the AAKP Advisory: Inadequate Hemodialysis Increases the Risk of Premature Death or at the end of this article.

In my unit, I have been impressed by the results of a "compression" test in detecting access clotting about to happen. This procedure is particularly useful with grafts. With your finger - or the side of a pen or pencil - completely close off the blood flow between your two needles under maximal blood flow while watching the venous pressure monitor. Any change of the venous pressure indicates recirculation in the access and is a sign for intervention to save it. Really impressive changes (increase in pressure followed by a dramatic drop with needle sucking) are seen when the needles are reversed. It was recently reported that up to 25 percent of dialysis treatments take place with the arterial and venous needle switched. Such highly inefficient treatments are immediately detected and prevented with a compression test early in your dialysis treatment. Some argue that this test could be risky for the access, but it takes only a few seconds and I have never heard of a problem with it. The only grafts at risk should be those which are severely malfunctioning.

By feeling and listening to your access and observing venous pressures on your machine and by careful use of the compression test, you should be in an excellent position to know whether or not your access is functioning well.

Measurement of Access Recirculation

The formula for measuring blood recirculation in your access is:

P-A x 100

P-V

The best way to do this is to have the blood flow reduced to 50 ml/min and after a few minutes draw a blood sample for BUN from the arterial line blood port. This maneuver will eliminate access recirculation so that the BUN concentration is more reflective of the total body or peripheral BUN (P). The blood pump is then returned to its normal speed and the arterial (A) and venous (V) samples for BUN are drawn at the same time from the arterial and venous line blood ports. Using this method, there will always be some degree of recirculation due to its occurrence in other areas of the body or lab variation in measuring BUN from sample to sample, but it should not exceed 10 percent.

Question answered by Dr. A. Peter Lundin.

This article originally appeared in Winter 1998 aakpRENALIFE, Vol. 13, No. 4.

The Dear Doctor column provides readers with an opportunity to submit renal related health questions to healthcare professionals who specialize in the area of concern. The answers are not to be construed as a diagnosis and therefore, altercations in current healthcare should not occur until the patient's physician is consulted.