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4. Placement of forearm A/V fistula
Despite the increasing number of patients with diabetes, peripheral vascular disease and of older age, creation of a native A/V fistula is possible in the vast majority of cases 1 2. Konner et al.. created 748 primary A/V fistulas but only 52 % were located in the forearm in patients, of whom 24 % had diabetes as their cause of end stage renal disease, without the use of graft material 3. One year primary access survival ranged from 70 % (in male patients with diabetes mellitus and under the age of 65) to 85 % (male, non-DM, < 65 years), secondary access survival was 84 % (male, DM, < 65 years) to 98 % (male, non-DM, < 65 years).
The early occlusion or failure of the fistula vein to dilate rate, as reported in the literature from 1983 to 1986, were higher, ranging from 10 to 24 % 4 5 6 7 8 9 and the success rates of early revisions were often less than 50 %. This high failure rate may be primarily due to the selection of inappropriate vessels or due to stenosis at the anastomosis, impairing adequate blood flow. Lower early failure rates of 5.6 % and 8.3 % were reported by Mihmanli et al.. in 2001 and by Silva et al.. in 1998, respectively 10 11 but is likely to reflect a very selective policy. In order to select the most appropriate vessels for the creation of the native A/V fistulas, many authors recommend the use of duplex-ultrasound investigation before each fistula creation
12 10 11 13 14 15 16.
It is difficult to compare failure rates, as there are two different schools of thinking. Some teams attempt fistula creation only when the arteries and veins are of optimal quality. If these conditions are not found at the wrist, the A/V fistula is created at the elbow or the upper arm. One significant concern is that if the radiocephalic fistula occludes then there is a risk that the radial artery may also occlude. Steal syndrome may be more likely to occur when an elbow fistula is created later in the same arm, especially in a diabetic patient. In contrast other surgeons, especially those using microsurgery and working in close cooperation with interventional radiologists, will attempt fistula creation even when venography or duplex shows evidence of small but non-stenosed vessels. The latter bears the risk of higher initial failure rates and delay in achieving a functional vascular access for the patient but less risk of mid-term and long-term morbidity and mortality since steal syndromes and excessive high flows are much less frequent in forearm fistulas. Two recently published articles stress the value of interventional radiology in helping failing fistulas to mature 17 18.
*1 Assessment of arteries: Duplex ultrasound
Duplex ultrasound (see appendix) is an accurate method for the investigation of arteries in the upper extremity with a sensitivity of 90% and a specificity of 99% for the detection of obstructive disease and is strongly recommended, pre- and post operatively, for the assessment of vessels and fistula outcome 19. The use of duplex ultrasound influences the choice of access placement 13.
Vessel diameter measured by duplex ultrasound can be potentially underestimated, as the vessel might be in spasm or insufficiently distended during the investigation. The procedure should be performed in a warm room to reduce the risk of vasospasm. In cases of severe calcification, it can be impossible to measure the diameter of the artery.
A standardised examination is performed starting from the infraclavicular subclavian artery down to the brachial, radial and ulnar arteries at the wrist. Arterial diameters, Doppler waveform analysis and sites of stenosis or occlusion are recorded. Adequate arterial inflow is of prime importance for successful functioning of A/V fistulas.
Duplex ultrasonography was used in one study to determine the change in resistance index (RI) at reactive hyperaemia as a test of the functional status of the feeding artery (see appendix 1.2.6 for details). A/V fistulas were successfully created in 95.3 % cases when using feeding arteries with a RI at reactive hyperaemia of < 0.7 and only 38.7 % with an RI of >0.7 20.
The internal radial artery diameter has been used in several studies to predict the outcome of radiocephalic (RC) A/V fistulas (failure or dysmaturation) or to plan strategies for vascular access creation. Wong et al.. observed either thrombosis or failure to maturation in all RC A/V fistulas created in patients with either a radial artery or a cephalic vein with a diameter < 1.6 mm 21. In another study, successful A/V fistulas had a preoperatively measured radial artery diameter of 2.7 mm versus 1.9 mm in failed A/V fistulas 14. Malovrh discriminated between RC A/V fistula created with radial arteries, whose diameter were > 1.5 mm versus < 1.5 mm. Immediate patency rate in the >1.5 mm group was 92 % versus 45 % in the < 1.5 mm group, while the patency rates after 12 weeks were 83 % versus 36 %, respectively 12. However it has to be stressed that lower success rates with smaller vessels do not necessarily mean that A/V fistula creation must not be attempted.
In the study performed by Silva et al.. strategy for vascular access creation was based on preoperative duplex scanning. Patients with a radial artery diameter of > 2 mm and a cephalic vein of > 2.5 mm received RC A/V fistulas, while grafts were used in patients with insufficient radial arteries and cephalic veins and an outflow vein in the elbow with a diameter of > 4 mm. The percentage of RC A/V fistula creation increased from 14 % to 63 %, while the early failure rate decreased from 36 % to 8 % 11. The percentage of A/V fistulas in this study could have been higher, if antecubital veins of 4 mm or more had been used to create an autogenous fistula at the elbow.
Table 1 summarises the findings of several authors on minimal arterial diameters necessary for successful creation of radiocephalic A/V fistulas. These are still debatable and controversial as cut-off points for A/V fistula creation. The lack of consensus also indicates that vessel diameter alone is insufficient to guarantee fistula success as pointed out by Wong et al..
Table 1:
Minimum radial artery diameter for successful creation of radiocephalic A/V fistulas in adults:
|
Author |
Radial artery diameter |
|
Wong et al.. 21 |
1.6 mm |
|
Silva et al.. 11 |
2.0 mm |
|
Malovrh et al.. 12 |
1.5 mm |
*2 Soft tissues X-rays
A simple film of the forearms can show calcifications of the forearm arteries, which does not contraindicate fistula creation but may warn the surgeon of potential difficulties. Palpation of the artery is usually sufficient, however.
*3 Digital angiography or MRA
Contrast arteriography remains the gold standard for the evaluation of a suspected arterial stenosis or occlusion. When results of clinical examination and colour-coded duplex-ultrasound are suggestive of arterial run-in or run-off problems, it is advisable to obtain an arteriogram that shows the entire arterial system from the origin of the subclavian artery to the distal forearm branches. Magnetic resonance angiography can also be used for the same purpose 22.
*4 Veins: Duplex ultrasound
The veins in the upper extremity are investigated in the supine position with a tourniquet placed on the forearm, and subsequently moved to the upper arm. The cephalic and basilic veins at the wrist are assessed for compressibility and diameter. Furthermore, the forearm veins are followed proximally for continuity and size. At the antecubital fossa, vein continuity and diameters are verified. After removal of the tourniquet, the continuity of the deep system is determined through the axillary and subclavian veins. The predictive value of cephalic vein diameters for successful RC A/V fistulas remains uncertain. Vein diameters of < 1.6 mm have been associated with A/V fistula failure 21, while good patency rates were obtained in patients with A/V fistulas that were created on the basis of the selection of adequate veins (diameter of the cephalic vein at the wrist > 2-2.5 mm or upper arm veins > 3 mm) 23.
However, despite the lower success rates with smaller vessels, some groups would still attempt fistula creation. In children microsurgery allows for the successful use of even smaller veins and arteries 24 25. Children's vessels are different form those of adults, however, and have a much better capacity to develop. It has been speculated that a skilful surgical technique, possibly including microsurgery, may also enhance patency rates in adults with marginal vessel quality, although there has been no randomised trial to corroborate this conviction.
The size of the increase in vein diameter after proximal vein compression before fistula creation could also be an important predictor of success. In a recently published study, in the group of successfully created A/V fistula, the vein diameter increased by 48%. In contrast, vein diameter only increased by 11.8 % in the group with early failure of the A/V fistula 20.
Table 2 summarises the recommendation on minimum vein diameter:
Table 2:
Minimum cephalic vein diameter for successful creation of radiocephalic A/V fistulas in adults:
Author |
Cephalic vein |
|
Wong et al.. 21 |
1.6 mm |
|
Malovrh et al.. 20 |
1.6 mm |
|
Silva et al.. 11 |
2.5 mm |
|
Ascher et al.. 23 |
2.0 mm |
*5 Venography (see also appendix)
Venography of the arm veins with iodine-contrast may cause further deterioration of renal function in the pre-dialysis patient. Therefore, the patient will need adequate hydration with urinary diuresis and as little contrast as possible. Alternatively, CO2-venography can be used, although it raises other risks and should be performed by experienced radiologists only.
Gadolinum may also serve as an alternative to jodine as a contrast agent. Rieger et al.. performed 32 angiographic procedures (for other reasons than A/V fistula creation) in 29 CRI patients with gadopentetate dimeglumine 26. In only one patient, serum creatinine concentration increased by more than 0.5 mg/dl after the procedure, although this may have been due to cholesterol embolism as a complication of renal artery stenting, rather than caused by the contrast medium.
While gadoterate meglumine is regarded as an effective and safe contrast agent for upper extremity venography for the planning of an A/V fistula 27, others argue that Gadolinum is more toxic than iodine at equal attenuating concentrations 28. Gadolinum is also more expensive than conventional contrast media or CO2.
*6 Magnetic resonance angiography (MRA) (see also appendix)
Magnetic resonance angiography (MRA), with either time-of-flight (TOF) or contrast-enhanced (Gadolinum) technique, is a promising new diagnostic tool. The latter technique results in a good visualisation of arm veins. Conventional and MRA venographic diameter measurements were closely correlated overall (r = 0.91) and on a vein-to-vein basis (r = 0.84 - 0.98) 29. However, this study has been criticised, since veins were imaged without placement of a tourniquet , which is essential to produce good venous filling and dilatation. In addition, upper arm and central veins were not studied 30.
*7 Create forearm A/V fistula
Forearm fistulas, especially wrist fistulas, show several advantages over upper arm fistulas: They preserve more proximal vessels for future access placement, have lower complication rates such as vascular steal syndrome, thrombosis and infection, better long-term patency rates 31 and a longer vein length for dialysis needling . Such fistulas rarely cause excessive high flow due to dilatation of the feeding artery, even when they remain patent for many years.
After thrombosis and declotting by interventional radiology, the one-year secondary patency rates in forearm A/V fistulas and upper arm A/V fistulas were 81 % and 50 %, respectively in one study32.
An increasing percentage of patients nearing ESRD do not have a forearm vein and a forearm artery suitable for vascular access creation. In these patients, the A/V fistula should be created using the veins and arteries of the middle of the forearm, the region below the elbow or at the elbow region 1. In summary, in the vast majority of patients with a damaged arterial and venous vascular system, the question is not only to ask for the dominant or non-dominant arm, but to evaluate the best location where an arterio-venous access can be placed successfully. The selection of the left or right arm primarily depends on the quality of the vessels although in principle the non-dominant arm is preferred in order to keep the dominant arm free for the patient to use during dialysis. The selection of the anastomotic site in these problematic cases must not be done based merely on the results of a clinical examination, but only after adequate pre-operative non-invasive or invasive vascular mapping.
Ligation of venous side branches should be avoided during access creation. The pattern of venous dilatation/arterialisation cannot be foreseen at the time of the first operation. In case of a formerly undetected obstruction of the proximal "main vein, collateralisation via branches can occasionally provide successful cannulation sites and act as run off to prevent fistula thrombosis.
When the main access vein and its collaterals fail to dilate over time, imaging by duplex ultrasound or by angiography must be performed and the underlying stenosis must be then treated by percutaneous dilatation or by creation of a new anastomosis if the stenosis is close to the anastomosis. Sometimes, there are two main veins taking fistula flow and neither may develop sufficiently for dialysis needling. If one of such veins is ligated then the other vein is likely to mature suitable for needling. Otherwise, there is little room if any for ligation or embolisation in correctly performed autogenous A/V fistulas.
End artery to end vein should be avoided as only in end vein to side artery or in side to side anastomoses, distal arterial flow can contribute to the flow of the A/V fistula, ranging from 0-50 % 7 20 33 34. As side to side anastomoses are more likely to cause distal venous hypertension due to possible retrograde venous drainage 35-37, end vein to side artery seems to be the most appropriate form of A/V fistula anastomoses.
*8 Consider vein transposition if necessary
Superficial venous transposition increases the potential to create forearm A/V fistulas. Silva et al.. transposed suitable forearm veins which were not in immediate proximity to a suitable artery. The primary cumulative patency rates after one and two years were 84 % and 69 %, respectively 38.
Venous transposition in a primary access operation in the forearm should be practised with caution. During maturation, the vein undergoes a process of dilatation, elongation and increase in thickness. These factors are not predictable in cases when the vein is largely mobilised, as this causes trauma to the vasa vasorum and removal of the adventitia 39. Venous transposition is easier and more successfully achieved in revisions using a dilated, elongated and wall-thickened venous segment, as in a two stage brachiobasilic transposed A/V fistula.