Acquired arteriovenous fistula
(1) Etiology and pathology
The most common etiology of acquired arteriovenous fistulas is caused by penetrating injuries such as various puncture wounds, especially high-speed bullets, shrapnel, steel and glass fragments struck at speed. At the same time of injury, arteries and veins within the same sheath are injured together. Closed fractures, percutaneous puncture arteriograms and trauma during surgery are common causes. Usually the external opening of penetrating injuries is very small, as the adjacent muscles and soft tissues prevent a large amount of bleeding and a hematoma forms within the local tissues, and the hematoma muscularizes to form a cystic wall of movable and static walls. In addition blunt contusions, crush injuries, squeeze soft tissue on the bone such as shoulder, hip cause extensive small arteries and veins in the area between the penetrating traffic, forming arteriovenous fistula. Sclerotic aneurysms can also cause arteriovenous fistulas by progressive adhesions and erosion that eventually penetrate the accompanying veins.
When adjacent arteries and veins are injured at the same time, the wounded edges quickly close to each other and a hematoma forms around the vessel, which later becomes a sac and tube between the arteries and veins.
The artery proximal to the fistula is dilated and tortuous, with initial thickening of the arterial wall, followed by degenerative changes, atrophy of smooth muscle fibers, reduction of elastic fibers, thinning of the wall, and atheromatous plaque formation. In arteries distal to the fistula, the arterial pressure decreases, the blood flow decreases, the lumen becomes thin or the clot obstructs the blood supplying tissue or organ to which it belongs, causing ischemic and hypoxic changes due to insufficient blood supply.
The lesion of the vein varies according to the size of the fistula. If the fistula is large, a sudden increase in pressure in the vein can be seen after a few weeks of trauma and a pulsating mass can be formed, like a pseudoaneurysm. When the fistula is small, the vein at the fistula gradually expands, the vein lining thickens and the fibrous tissue proliferates, and after a long time, the vein can become arterialized. However, the elastic layer within the vein breaks down and disappears. The distal segment of the fistula is dilated and extended, and the venous valves are damaged or destroyed forming venous valve insufficiency syndrome such as superficial varicose veins, stasis, hyperpigmentation, and ulcers.
According to the form of fistula, it is divided into direct type and indirect type.
(2) Clinical manifestations.
In acquired arteriovenous fistula, clinical symptoms develop gradually, mainly swelling, pain, numbness, and weakness of the affected limbs. Sometimes they are accompanied by chest tightness, palpitations, shortness of breath, and even manifest heart failure. The severity of these symptoms depends on the size of the fistula and its proximity to the heart. Some acquired arteriovenous fistulas, also known as acute arteriovenous fistulas, are characterized by heavy, jet-like bleeding at the time of injury, followed by localized pulsating masses or palpable tremors and murmurs at the affected area. The distal arterial pulsation of the limb is still palpable but weaker than the healthy side. Traumatic arteriovenous fistulas occur mainly in the extremities, more in the lower extremities than in the upper extremities, and more in the superficial femoral artery than in the deep femoral artery.
(3) Signs.
(1) Murmur and tremor: a rough and persistent rumbling “machine rolling murmur” can be heard near the fistula. The murmur increases during cardiac contraction and propagates along the proximal and distal segments of the vessel; the larger the fistula, the stronger the murmur. A tremor can be palpated on the body surface corresponding to the fistula.
(ii) Increased local skin temperature: The surface skin temperature of the affected limb at the site of the arteriovenous fistula is high, 3-5°C higher than that of the healthy side. The skin temperature of the limb is normal or below normal at the site distal to the fistula. It even causes ischemic changes.
(3) Venous valve insufficiency in the affected limb: the high pressure blood flow in the artery flows through the fistula into the vein and impinges on the venous lining, causing damage and thickening of the lumen and widening of the venous lumen, and incomplete closure of the venous valve, causing the distal and proximal veins of the fistula to dilate and migrate. Due to increased venous pressure and obstruction of venous return, edema, depressed dermatitis, hyperpigmentation, and ulcers appear in the distal limbs.
④ Heart enlargement and heart failure: Arterial blood flow through the fistula to the vein, increased venous pressure, increased blood return to the heart, causing the heart to be overburdened and enlarged, which can lead to heart failure with prolonged action. However, the degree of heart enlargement and heart failure is closely related to the size and location of the fistula and the length of time it exists. The closer the fistula is to the heart, the more severe the heart failure and the earlier the symptoms appear. For fistulas in the limbs, heart failure appears later.
(4) Ancillary tests.
(1) Measurement of fistula by finger pressure: The heart rate and blood pressure are measured before and after blocking the shunt by pressing firmly on the fistula to block the blood flow. After blocking the blood shunt, the heart rate slowed significantly. This is because the fistula closes, forcing blood flow through the positive capillary network and thus increasing peripheral resistance. At the same time, after the fistula is suddenly blocked, the shunted blood volume is forced to flow into the peripheral arterial system. The increase in peripheral resistance and the sudden increase in additional blood volume in the arterial system increase the blood pressure, which correspondingly stimulates the aortic decompression nerve and the nerve endings in the carotid sinus, causing the vasodilatory center to act as an inhibitor and slowing down the pulse rate.
(ii) Measurement of venous oxygen: blood is drawn from a vein at the site of the arteriovenous fistula lesion or from a vein proximal to the fistula, and compared with venous blood from the same part of the contralateral limb, the venous blood on the affected side is redder than the venous blood of the normal limb, and the oxygen is significantly higher or even equivalent to that of the arterial blood.
③Ultrasound examination: the arterial blood shunt can be observed, the site of the fistula can be clarified, and whether there is a systolic or diastolic murmur can be identified.
(iv) Arteriography: immediate visualization of veins, as well as abundant collateral vessels around the fistula. The artery proximal to the fistula is thickened and tortuous. The location and size of the fistula can be shown on serial radiographs. The arteriogram not only clarifies the site of the fistula, its size and the extent of the surrounding vascular disease, but also identifies whether there is an associated aneurysm or other disorder.
(5) Treatment.
Because of the large difference between arterial and venous pressure, it is difficult for the fistula to heal on its own and for a thrombus to form in the fistula. The only treatment is to surgically remove or close the abnormal traffic between the arteries and veins.
The only treatment is to surgically remove or close the abnormal traffic between the arteries and veins. ①Surgical indications: In principle, surgery should be performed before serious consequences (heart failure, serious blood flow disorders in the distal extremities of the affected limbs, etc.) occur, and as long as the diagnosis is clear, surgery should be performed as early as possible to obtain satisfactory results.
②Surgical methods.
a, fistula ligation closure: for non-trunk vessels, if used for trunk vessels, can aggravate the distal limb ischemia;
b. Cutting off the fistula and repairing the arteriovenous lateral wall incision respectively: applicable to those who have no surrounding adhesions and clear anatomy;
c.Cut the vein and repair the fistula;
d. Excision of the arteriovenous fistula and suturing of the arterial counterpart.
(6) Surgical complications: bleeding, infection, swelling, recurrence, etc.
(7) Prevention: avoid all kinds of injuries, once the injury occurs, promptly seek medical examination and treatment, and after clear diagnosis, strive for early surgery to prevent complications.