Assessing Clinical Utility of the Criteria for Accessory Vein Obliteration for Failing Arteriovenous Fistula.

Hemodialysis (HD) vascular access dysfunction is a huge clinical problem which results in significant morbidity amongst patients with End Stage Renal Disease (ESRD), causing a severe economic burden on any health care system. Native arteriovenous fistula (AVF) is the preferred form of permanent dialysis access since it is associated with the best long term outcome as compared to other forms of dialysis access. Despite the clinical benefits, many patients on HD do not have AVF as there dialysis access. One of the reasons for low AVF rates is early fistula failure (EFF). EFF is defined as an AVF that never develops adequately for dialysis (failure to mature) or which fails within 3 months of starting dialysis. An adequate AVF for dialysis according to Dialysis Outcome Quality Initiative (DOQI) guidelines is the one which a) Has a flow of greater than 600ml/min, b) Has a diameter of 0.6cm or greater and c) Is approximately not deeper than 0.6cm from the skin surface. Between 23%-46% of newly constructed AVF have problems with early failure resulting in a dismal one year patency of 60-65%.

In order to devise a strategy to prevent EFF, one needs to understand the physiology of fistula maturation. Creation of an AVF leads to an immediate increase in flow through the vein due to the pressure gradient created. This increase in flow leads to increase wall shear stress which is defined mathematically by the formula 4ηQ/πr3, where η is blood viscosity, Q is blood flow and r is vessel radius. Shear stress thus is directly proportional to blood flow while inversely proportional to vessel diameter. After the creation of the AVF, the flow mediated increase in shear stress is mitigated by vessel dilatation through biological mediators. Consequently the shear stress is brought back to pre-anastomosis levels leading to vessel dilation. It seems that this positive remodeling of the vein leading to AVF maturation is dependent on increase in blood flow rather than the increase in pressure. Any pathology affecting the blood flow through the newly constructed AVF can thus lead to failure to mature.

The two most important causes for EFF amenable to intervention are stenosis anywhere in the circuit and/or presence of accessory veins. While stenosis development is pathological and accessory vein (av) presence is natural, both lead to decreased blood flow through the main AVF circuit, which may be responsible for failure to mature. Addressing these two entities in a timely fashion can lead to salvage of many AVF, which otherwise would have been abandoned. Although management of stenosis is well established with relatively clear guidelines, the management of av lacks clear scientific approach. It has been anecdotally recommended that any av with a diameter greater than 25% of the AVF diameter should be considered for ligation. It is also suggested that the significance of an av can be ascertained by assessing the flow of contrast through the av during an angiogram or by assessing the augmentation of AVF after manually occluding the av during the procedure. These current recommendations are very subjective and none have been adequately studied to date leading to considerable debate on the issue of when to obliterate an av. A scientific approach to obliteration of av is essential to avoid an unnecessary procedure, which may even be harmful. Also, obliterating a non-significant av and waiting for AVF to mature may cause a delay in making a definite decision for the patient. These patients may have other factors and not av as the cause of EFF. Conversely, not obliterating a significant av may also delay the use of AVF for dialysis by causing EFF. In a recent study researchers recommended a hemodialysis arteriovenous flow quantification-diameter (HAQ) criteria for accessory vein obliteration written below:

1. 60% or greater diameter of the main AVF
2. 50% diameter of AVF with at least one more av>40% in diameter.
3. 50% in diameter and divides into branches of same size.
4. av likely to interfere with cannulation on physical examination.
5. >30% in diameter and associated with stenosis at site of origin.

The HAQ criteria for av obliteration is based on computational flow dynamic (CFD) models of AVF created using 2 dimensional CFD software. This criteria has not been tested in clinical trials and its clinical utility has not been established to date. The purpose of this controlled, blinded, prospective trial is to assess the clinical utility of the HAQ criteria as compared to current recommendations for av obliteration.