Acquired Arteriovenous Fistula

Acquired arteriovenous fistula is an abnormal communication between an artery and a vein that follows injury, disease or a procedure. It diverts blood from the arterial to the venous system, and its effects depend on the size of the shunt, its site and its duration. Intentional haemodialysis fistulae are not covered.

Causes by frequency:

• Iatrogenic: femoral puncture and catheterisation, renal and hepatic biopsy, spinal surgery (lumbar disc surgery causing iliac artery to vein fistula), and surgical dissection.
• Trauma: penetrating injury (stab, gunshot), and blunt injury with fracture.
• Spontaneous rupture of an arterial aneurysm into an adjacent vein (aorto caval fistula from abdominal aortic aneurysm in about 3 to 4% of ruptures, and iliac and popliteal fistulae), and erosion from infection or tumour.

Fistula after femoral catheterisation occurs in about 0.1 to 1% of procedures, and many small ones close within weeks. Large fistulae from trauma and aneurysm rupture enlarge with time.

Pathophysiology.

Blood takes the path of least resistance from the high pressure artery to the low pressure vein, which lowers peripheral resistance, raises venous return and increases cardiac output. A large, long standing fistula causes high output heart failure.

Venous pressure rises distal to the fistula, causing limb swelling, varicose veins, skin changes and venous ulceration. The feeding artery enlarges and may become aneurysmal, while the artery distal to the fistula can be hypoperfused (steal), causing ischaemia.

Increased flow causes vein wall arterialisation and dilatation, and the aorto caval fistula causes sudden volume load with right heart failure, renal hypoperfusion and pulmonary hypertension.

A continuous machinery murmur with a thrill over the fistula, a pulsatile mass, dilated and pulsatile superficial veins, limb swelling, warmth and venous stasis changes are typical. Reflex slowing of the heart rate on compression of the fistula (the Branham sign) occurs in large fistulae.

Systemic signs are tachycardia, wide pulse pressure, dyspnoea, oedema and raised jugular venous pressure from high output heart failure.

Aorto caval fistula adds sudden abdominal or back pain, an abdominal pulsatile mass with a continuous bruit, bilateral leg oedema, haematuria, oliguria, and heart failure. Distal limb ischaemia (cool, pale limb with weak pulses) indicates steal.

• Duplex ultrasound: turbulent high velocity flow at the communication, an arterialised (pulsatile) venous waveform, low resistance flow in the feeding artery, a dilated draining vein and the size of the fistula.
• CT angiography (arterial and delayed phases) shows early venous opacification, the fistula site and size, and the aneurysm or injury behind it. MR angiography is an alternative.
• Catheter angiography defines the fistula before endovascular treatment.
• Echocardiography for chamber enlargement, high output, pulmonary pressure and right ventricular function. NT-proBNP.
• Blood tests: full blood count, creatinine, electrolytes, lactate and coagulation screen. Group and cross-match for aorto caval fistula.

Differential diagnosis.

• Pseudoaneurysm and arterial aneurysm.
• Congenital arteriovenous malformation and haemodialysis access.
• Other causes of high output failure (thyrotoxicosis, anaemia, Paget disease, beriberi).
• Deep vein thrombosis and lymphoedema.

Decide by size, site and effect.

• Small, asymptomatic iatrogenic fistula of a peripheral artery with no haemodynamic effect: observe with duplex at 2 weeks and 3 months, because about a third close spontaneously. Ultrasound directed compression may close early post-catheter fistulae.
• Indications for repair: heart failure or high output state, limb ischaemia (steal), venous hypertension with swelling or ulceration, enlarging fistula or aneurysmal change, associated pseudoaneurysm, infection, bleeding, and aorto caval communication.

Aorto caval fistula (emergency).

• Resuscitation with controlled fluid (avoid overload, which worsens right heart failure), blood products, and a hybrid or operating room. Intubation with a stable induction (ketamine 1 to 2 mg/kg or etomidate 0.2 to 0.3 mg/kg) and low airway pressures.
• Immediate repair: open repair with direct closure of the fistula from within the aneurysm sac, or endovascular repair with an aortic stent graft that excludes the fistula (preferred in suitable anatomy). Control venous bleeding with pressure (sponge sticks) and give heparin only as the surgeon directs. Expect pulmonary embolism of thrombus or air from the aneurysm.
• Intensive care with monitoring of cardiac output, urine output and lactate.

Repair options for other fistulae.

• Covered stent placement across the fistula in a large artery (iliac, femoral, subclavian, popliteal), with preservation of the main artery. After stenting: aspirin 75 to 100 mg and clopidogrel 75 mg daily for 3 months, then aspirin alone.
• Coil, plug or liquid embolisation for fistulae of small or expendable branches, including renal and hepatic lesions after biopsy (selective embolisation to preserve parenchyma).
• Surgical repair: ligation and division of the fistula with direct arterial and venous repair, patch angioplasty, or interposition graft of the artery. Cefazolin 2 g intravenously within 60 minutes before incision (3 g when weight is over 120 kg); heparin 70 to 100 units/kg intravenously before clamping, reversed with protamine 1 mg for each 100 units given in the previous 2 to 3 hours (maximum 50 mg, given slowly).
• Infected fistula: vancomycin 25 mg/kg intravenously as a loading dose then 15 to 20 mg/kg every 8 to 12 hours (trough 15 to 20 mg/L) with piperacillin and tazobactam 4.5 g intravenously every 6 hours, adjusted to cultures, with excision and reconstruction, for at least 6 weeks.

Heart failure and venous hypertension (before and after repair).

• Furosemide 20 to 80 mg orally once or twice daily, titrated to weight, with electrolytes and creatinine within 1 week of changes.
• Spironolactone 25 to 50 mg daily (potassium under 5.0 mmol/L, eGFR over 30 mL/min/1.73 m2; check at 1 and 4 weeks).
• Compression stockings (class 2, 23 to 32 mmHg) for chronic venous hypertension once ischaemia is excluded.
• High output failure usually resolves within days to weeks after closure, so long term heart failure drugs are rarely needed unless ventricular damage remains.

Anticoagulation. For deep vein thrombosis complicating the fistula: apixaban 10 mg twice daily for 7 days, then 5 mg twice daily for at least 3 months; or enoxaparin 1 mg/kg every 12 hours (once daily at creatinine clearance under 30 mL/min). Hold for the procedure as the surgeon directs.

Monitoring. Duplex at 1 week, 1 month, 3 months and 12 months after repair. Echocardiography and NT-proBNP at 1 month and 3 months in patients with high output failure. Limb perfusion and venous symptoms at each visit.

Referral and escalation. Refer every confirmed fistula to a vascular surgeon. Immediate referral for suspected aorto caval fistula, heart failure, limb ischaemia or bleeding.

Special groups.

• Pregnancy: high output states add to the physiological rise in cardiac output, so repair promptly under specialist care; avoid warfarin and direct oral anticoagulants (use enoxaparin 1 mg/kg every 12 hours with anti-Xa monitoring).
• Children: repair is delayed only when the lesion is small and stable; weight based dosing for any drugs.
• Kidney impairment: limit contrast volume with hydration, and adjust enoxaparin and apixaban by creatinine clearance.
• Region specific: penetrating trauma is a leading cause, and late presentation with heart failure occurs when imaging is unavailable.