Vascular access in hemodialysis
Fashioning the fistula – timing
Early presentation of potential hemodialysis patients to a surgeon or nephrologist is strongly recommended, so that there is enough time to protect the vein, fashion the access, perform any secondary procedures and allow the access to mature. [1, 2, 3, 4] Therefore, an AV fistula (AVF) should be fashioned three months before the scheduled start of hemodialysis. Preparations for fashioning the AVF should begin as soon as the glomerular filtration rate (GFR) falls below 30 ml/min.
Preoperative ultrasonography of the veins and arteries
A clinical examination and ultrasound study of the arteries and veins of the arm should be performed prior to fashioning the vascular access. Diagnostic imaging of the central veins is useful in patients with a history of central venous implants (catheters, ports, cardiac pacemakers).
Preoperative ultrasonography of blood vessels increases the likelihood that the AVF can be fashioned successfully and increases the intermediate-term patency rate. In a randomized study, the primary occlusion rate of 25% in cases where the AVF was fashioned solely based on clinical examination was significantly reduced to 6% when the vessels underwent preoperative ultrasound assessment. [5] Other studies demonstrated that additional ultrasonography increased the AVF patency rates from 17–35% to 58–85% [6, 7, 8, 9] and reduced the early occlusion rate from 36% to 8%. [10]
Veins with a diameter of less than 1.6 mm suffer from frequent early-onset occlusion. [11] In radiocephalic AVF, good patency rates have been reported if the preoperative diameter of the cephalic vein at the wrist was greater than 2.0-2.6 mm or greater than 3 mm in brachial veins. [12]
The diameter of the radial artery is also a predictor of successfully fashioned AVFs. Fashioning radiocephalic AVFs in patients with a radial artery diameter of less than 1.6 mm almost always results in primary thrombosis or inadequate maturation of the fistula. [13, 14, 15]
A minimum diameter of 1.6 mm (better 2.0 mm) of the radial artery and the cephalic vein is recommended for AVFs fashioned at the wrist; no data that could be interpreted as a selection criterion is available for AVFs in the antecubital fossa.
Strategies for fashioning AVFs
The primary patency rates of AVFs and synthetic grafts differ significantly. After 1 and 2 years, these are 90% and 85% respectively for AVFs and 60% and 40% for synthetic grafts. [16] Approximately 0.2 interventions per patient per year are required to preserve native AVF function, and 1.0 per patient per year in synthetic grafts. Native AVF is therefore the vascular access of choice.
First-choice vascular access – fashioning native AVFs
The first choice is a radiocephalic AVF at the wrist, which will work for years with a minimum of complications, revisions, and interventions. The major shortcoming of this site is the high rate of early-onset occlusion (5–30%). [17, 18] Long-term function rates range from 65 to 90% at 1 year, and from 60 to 80% at 2 years. The incidences of thrombosis (0.2 events per patient per year) and infections (2%) are low.
If the AVF cannot be fashioned at the wrist due to inadequate vascular conditions, a more proximal anastomosis between the radial artery and the cephalic vein on the middle forearm up to the crook of the elbow or even at the level of the antecubital fossa is an option. AVFs at the level of the antecubital fossa allow highly efficient dialysis with a low incidence of thrombotic and infectious complications and good long-term outcomes. [19–27] One shortcoming of this high-flow AVF is the risk of inferior peripheral perfusion with symptomatic ischemia of the hand, hypercirculatory heart failure especially in patients with CHD, or cardiac insufficiency. [28]
Second-choice vascular access – grafts
If autologous AVF is not possible, grafts (alloplastic, xenogeneic, autologous) may be an option. Synthetic grafts are made of polyurethane, polyester (Dacron®) and polytetrafluoroethylene. [29] The primary patency rates in synthetic grafts range from 60% to 80% after 1 year, but drop to 30–40% after 2 years. [30, 31, 32, 33] While the primary patency rate is thus generally good, stenosis at the venous anastomosis often results in fistula thrombosis if left untreated. These stenoses are marked by intimal hyperplasia due to the immigration and proliferation of smooth muscle cells and increased matrix deposits. The etiology of the intima hyperplasia is multifactorial. [34, 35, 36]
The outcomes of autologous grafts utilizing the great saphenous vein have not been convincing. [37] Xenogeneic materials on the market with acceptable patency and low infection rates are bovine mesenteric veins and bovine ureters. [38]
Third-choice vascular access - central venous catheter (CVC)
There are only few indications for tunneled CVC as permanent vascular access: severe access-related ischemia of the upper extremities, severe heart failure, advanced malignancy.
Prognosis post AVF
Patient-related factors impacting the AVF patency rate
Patient-related characteristics can have a significant impact on the choice and prognosis of vascular access. Age may affect blood flow in an AVF, resulting in a slightly increased occlusion rate compared with young patients (18.9% vs. 13.6%). [29] The combination of age and diabetes results in a significantly higher AVF failure rate (28.6%). Epidemiologic studies have revealed a higher share of synthetic grafts in elderly patients. In Europe, for example, the use of synthetic grafts rose from 5% in patients younger than 45 years to almost 9% in patients older than 75 years. [39, 40] Since synthetic grafts are second-line vascular access devices, the frequency of revisions was correspondingly higher. [41, 42, 43]
In women with their more delicate vessels, AVFs nevertheless do not manifest poorer maturation and functional rates than in men. [44] However, studies have demonstrated that female sex was associated with increased use of synthetic grafts and a correspondingly greater number of vascular access revisions. [40, 42, 43, 45, 46, 47]
Impact of concomitant diseases on AVFs and patency rates
The most important causes of end-stage renal failure and dialysis are diabetes and arteriosclerosis. Affected patients usually have poor, thickened, and calcified arteries with proximal and/or distal obstructions [48], complicating the fashioning of vascular access and increasing the risk of access-related symptomatic limb ischemia. Patients with diabetes also suffer from a higher incidence of AVF thrombosis. [49]
Native AVFs with good outcomes have also been described in diabetic patients, but unlike in patients without diabetes more proximal forearm and antecubital fistulae were fashioned. [27] In patients without diabetes primary patency rates were comparable, but secondary rates were better at 2 years. Ischemia was significantly more common in diabetic patients (7 vs. 0.6 per 100 patient-years).