GUIDELINES [ back to index ]
9. Post operative control of A/V fistula and graft function (2)
*1 Monitor Vascular Access
In native A/V fistulas, monitoring of the vascular access is important to assess maturation status (dilatation of the fistula sufficient to allow safe needling for dialysis by nurses or other appropriately trained healthcare workers) and to determine the ideal time for first cannulation. Native fistulas need time to mature. However, the decision when to cannulate first should be made individually on clinical assessment of vein diameter and topography.
In general, A/V fistulas mature within 4-6 weeks. Tautenhahn et al. described a shorter average maturation time of three weeks 1. Immature fistulas have fragile veins, are difficult to cannulate and provide insufficient blood flow 2. Failure of maturation is higher in arteries with < 40 ml/min blood flow before fistula creation3, but it is not influenced by the direction of blood flow (antegrade, retrograde) in the distal artery after creation of the fistula 4.
A stenosis was found in all fistulas with delayed maturation in the study of Turmel-Rodrigues et al.5. Dilatation of the stenosis was successfully performed in 97 % of the cases, leading to a primary patency rate of nearly 40 % and a secondary patency rate of 75 % after one year. Further confirmation of these exceptional findings is required.
Faiyaz et al. successfully salvaged 15 out of 17 A/V fistulas with delayed maturation by percutaneous ligation of accessory veins 6. Whilst this procedure maybe of benefit in some cases, such high success with branch ligation is unusual. If a stenosis is the reason for non-maturation, it must be treated. In many cases of inadequate maturation, the problem is a critically reduced arterial inflow, particularly in patients with atherosclerotic disease. On the other hand, venous side branches can form a venous resource over time, a source for surgical repair as patch material, homologous interposition graft and other possibilities. To destroy a venous branch requires considerable reflection and should only be performed after consultation with an experienced surgeon.
Konner published his single-centre experience of 748 consecutive primary direct arteriovenous accesses, of which only 52 % were located in the forearm. Without a single side branch ligated, he observed 99.4 % (96.8 %) functioning fistulae at four weeks in diabetic (non-diabetic) patients, and during a 1 7 year follow-up of his patients he found thrombosis rates as low as 0.03/patient year (0.07/patient year) 7.
In grafts, postoperative swelling should have disappeared before the first cannulation. PTFE grafts should not be used routinely until 14 days after placement 8. Normally, there will be no doubt about the quality of blood flow rates.
In both native A/V fistulas and grafts, the decision when to cannulate first time has to be made very carefully. Otherwise, the vascular access as well as the patients confidence can be damaged substantially. In many cases, the problem does not arise from cannulation, but from subcutaneous bleeding and formation of haematoma after removal of the cannulas.
*2 Start HD when indicated
The start of HD is indicated if GFR falls below 15 ml/min (UK < 10 ml/min) and there is one or more of the following signs: uncontrollable hypertension, fluid overload, malnutrition or uraemic symptoms. In any case, HD should be instituted before the GFR has fallen to 6 ml/min/1.73 m2 9. High risk patients, e.g. diabetics, may benefit from an earlier start 9.
*3 Duplex ultrasound
Due to maturation of an A/V fistula, the blood flow in the arm vessels augments 10 to 20 fold from baseline levels. In well-functioning radiocephalic A/V fistulas flows from 500 to 900 ml/min or higher may be found. In grafts and in upper arm A/V fistulas higher flows are obtained, ranging from 800-1400 ml/min. The maturation and increase in blood flow in native A/V fistulas is gradual and due to remodelling or adaptation of the vessel. After 4 - 6 weeks, radiocephalic A/V fistulas should have matured and be usable for haemodialysis.
Failure to mature is usually due to insufficient blood flow, caused by a stenosis occurring anywhere from the subclavian artery to the outflow vein of the fistula. Blood flow of the radial artery measured by duplex ultrasound one and seven days after the operation may predict maturation. Low blood flow rates and velocities in the native fistula within the first 2 weeks usually result in fistula failure10 11. A radial artery cross-sectional area of > 8.5 mm2 and venous outflow of > 425 ml/min have a positive predictive value of 0.95 and 0.97, respectively, for the outcome of radiocephalic A/V fistulas 12.
In a prospective study, Wong et al. measured blood flow velocity at 24 hours after fistula creation and found a significant difference between successful (average blood flow velocity of 0.53 m/s) and poor (average velocity 0.18 m/s) fistulas 11. Also, duplex scan at 3 weeks revealed clear differences in blood flow and vein diameter between successful and poor fistulas. A combination of the two investigations should give a good clinical likelihood of successful maturation of the fistula and help expedite further investigation to improve fistula flow, surgical fistula revision or angioplasty, or create a more proximal fistula.
*4 Reassess fistula after 4 weeks
A native A/V fistula generally matures within 4 - 6 weeks. Maturation of ulnar-basilic fistulas has been reported to take longer (6 weeks), but maturation time depends on the initial diameter of vein and artery and haemodynamic factors. In case of delayed maturation, an underlying stenosis must be looked for either by colour-coded duplex-ultrasound or by angiography.
Cannulation and catheterisation of an immature fistula can generate stenosis 13.
*5 Consult surgeon / radiologist
In cases of stenosis and, depending on their location, either surgery or angioplasty may be indicated. For planning and performing the appropriate treatment close co-operation between surgeon, nephrologist and radiologist as a team is needed 14.
For arterial inflow stenoses and venous stenoses in the prospective needling segment and more centrally, angioplasty is indicated. In one study by Turmel-Rodrigues et al. an underlying stenosis was identified as the cause of dysmaturation in all cases. Treatment by interventional radiology led to a secondary patency rate of 79 % at one year 5. However, haemodynamics of the A/V fistula as well as quality of inflow artery or outflow vein dictate maturation.
Preference should be given to surgical intervention for isolated stenoses close to the anastomosis of a wrist fistula, where proximal re-anastomosis is likely to give better results than PTA. If the A/V fistula fails because of tiny or heavily calcified arteries, a decision may be made to create a more proximal A/V fistula connected to a larger artery.