AV Fistula Volume Flow Measurement Techniques
After surgical creation, the vein destined to become a successful arteriovenous fistula (AV), which undergoes a remodelling process that is referred to as maturation. Although somewhat variable, these changes occur relatively quickly, resulting in a fistula that can be used repeatedly to provide suitable dialysis treatments.
Once an AV fistula is created, the blood vessels involved (both artery and vein) undergo marked changes in the hemodynamic forces that trigger vascular remodelling. Blood flow must increase to a level that provides a sufficient delivery to the dialysis machine. In addition, the vessels must increase in diameter to accommodate cannulation, and the vessel wall must thicken to permit repeated cannulation.
The goal of creating an AV fistula is to achieve vascular access that can be used frequently for effective and efficient hemodialysis.
AV Fistula Measurements techniques – past and present
In the early years of hemodialysis treatment, a graft or fistula was assessed via physical examination and the occasional recording of venous and arterial pressures from the dialysis machine. This process was not well-organized.
Over the last 15 years, a growing number of testing methods have been developed to evaluate the function of the access and the adequacy of the dialysis treatment, and it has been understood that function and adequacy can be related.
The organized testing of static and dynamic venous resistance, kinetic modelling, and ultrasonographic and flow measurements has been used alone and in various combinations to evaluate grafts and fistulas. These are non-invasive tests, a prime directive for surveillance techniques.
The most widely used direct test of hemodialysis access surveillance is the Transonics Doppler ultrasound system which uses an ultrasound dilution method to determine flow. Other techniques use thermal dilution, optical dilution, or direct ultrasound evaluation of the access. Most of these techniques require a skilled technician, and results are operator-dependent.
Doppler ultrasound (DUS)
A Doppler ultrasound test uses reflected sound waves to examine the flow of blood through a blood vessel. It helps doctors assess the blood flow through major arteries and veins in, for example, the arms, legs, and neck. It can show the blocked or reduced flow of blood through narrow areas.
During a Doppler ultrasound test, a handheld device called a transducer is passed lightly over the skin above a blood vessel. The transducer sends and receives sound waves that are amplified through a microphone. The sound waves bounce off solid objects including blood cells. The movement of blood cells causes a change in the pitch of the reflected sound waves, known as the Doppler Effect. If there is no blood flow, the pitch does not change.
The possible applications of DUS during the creation and postoperative follow-up of AVFs place particular emphasis on the following aspects:
- Pre-operative vascular mapping.
- Maturation of the AVF.
- Monitoring/surveillance of AVF (follow-up and early detection of complications).
A thorough evaluation of a new AV fistula should be considered mandatory in order to assess fistula maturation and to detect problems as early as possible. Failure to detect access dysfunction has consequences on morbidity and mortality.