Blog from the Guest Lecture Series at East Carolina University – Brody School of Medicine
Lecturer: Tushar Vachharajani, MD
Blogger: Tejas Desai, MD
A fistula is a circuit that goes beyond the anastomoses. The circuit starts and ends at the heart. In other words, patients with poorly functioning hearts will have a poorly functioning fistula. As a result, it is important to examine the entire fistula circuit and not simply the points of cannulation.
Monitoring versus surveillance: monitoring occurs through the physical exam. Surveillance occurs through the use of ultrasound, pressure measurements, etc. CMS mandates monitoring, but not surveillance, on a regular basis. Monitoring, if done correctly, can identify problems that can avoid or limit the number of catheter-days. A large number of studies have shown that monitoring provides similar outcomes as any surveillance modality. Unfortunately, monitoring (the physical exam) is often not taught and done.
Optimal frequency of physical examination should be done at least once within 4-6 weeks after creation. Most fistulas will show evidence of failure-to-mature in this time period. If caught early, interventions can be planned and catheters can be avoided. In the dialysis unit, the examination should be performed prior to cannulation at each session.
A mature fistula is (rule of 6’s):
- 6 mm in diameter
- 6 mm or less in depth
- 6 cm of a straight segment so 2 needles can be placed with limited recirculation
- minimum flow of 600 ml/min
The good thing about fistulas is that all you need is a Doppler flow of 400-500 ml/min in order to achieve a blood flow (pump flow) of 350 ml/min. Unlike grafts that require 800-1000 ml/min of Doppler flow to achieve a similar blood flow (pump flow).
Other aspects of the physical exam can tip you off for fistula malfunction. For example, detecting a swollen limb would suggest a central vein stenosis or thrombosis. Prolonged bleeding time after removal of the needles (for greater than 10 minutes) suggests an outflow obstruction. Multiple upper chest scars or keloids would suggest prior catheter use and central vein stenosis. Painful arm upon cannulation would suggest venous stenosis as a result of venous hypertension. Aneurysms or dilated veins suggest a proximal outflow stenosis because blood is trying to flow through alternate channels. Elevated access pressures (for a blood flow of 400 ml/min, arterial pressure of -180 and venous pressure of 250 are generally expected) suggest obstruction and recirculation.
Type of fistula:
- A radiocephalic fistula can be detected with a scar at the ventral surface of the wrist.
- Radiocephalic grafts do not exist.
- A horizontal scar at the elbow suggests a brachiocephalic fistula.
- A long scar extending from the axilla to the elbow on the medial aspect of the arm is a transposed brachiocephalic fistula.
- A long scar along the back of the arm suggests a transposed basilic vein fistula.
Understanding the anatomical nature of the fistula can help you determine the anastomosis location.
Inflow stenosis is defined as stenosis at either the anastomosis or juxta-anastomotic region. The anastomosis is identified as the region of the fistula that has the strongest thrill. This is because the greatest amount of turbulence occurs here. Juxta-anastomosis is defined as a region 3-5 cm proximal from the anastomosis. This is the region that is most mobilized by the surgeon during fistula creation. 65-70% of stenoses occur in the juxta-anastomotic region.
Identifying the anastomosis and assessing the strength of the pulse as you move 3-5 cm proximally along the outflow vein can detect true inflow stenosis. (Note: any stenosis in the proximal portion of the outflow vein is considered an inflow stenosis). A progressively weaker pulse and high-pitched sound (whistle) would indicate inflow stenosis [listen].
True outflow stenosis can be detected through an augmented pulse. Distal to the stenosis the pulse will be very strong (a Water-Hammer pulse). Once you cross the stenosis, the pulse will drastically drop in strength.
Occluding the access in the outflow tract to assess inflow segment patency. If the inflow is patent and healthy, the inflow segment will be hyper-pulsatile. If the pulse does not increase (augmented), there is an inflow stenosis (in the artery itself, the anastomotic or juxta-anastomotic region).
A normal fistula will have a low-pitched bruit throughout the entire cardiac cycle [listen].
Raising the arm will cause a collapse of the fistula. (In the upper arm, the fistula may not collapse because of the underlying muscle but the fistula will become soft). If either of the 2 do not occur, there is a venous outflow obstruction.
All fistulas require a tourniquet before cannulation. W/o a tourniquet, there is a high risk of puncturing through the fistula and causing blood extravasation. Tourniquets are not required for grafts because the graft has an inherent structural integrity.