The presence of an abnormal channel between the arteries is called an arteriovenous fistula and is of two types: congenital and acquired. It can occur anywhere in the body, but is more common in the extremities. Congenital arteriovenous fistulas often involve numerous small arterial and venous branches, so the fistulas are multiple. Acquired arteriovenous fistulas can occur in large, medium, and small arteries and veins. The fistulae are usually solitary. Due to the abnormal channels created between the arteries and veins, blood shunts in the arterial ducts can produce varying degrees of systemic and local hemodynamic changes, with the following clinical manifestations. 1. Local lesions: local continuous vascular murmur: blood flow through the fistula can produce a murmur, usually the loudest at or near the fistula site, such as a “rolling machine murmur” or a systolic murmur. A murmur may be accompanied by a palpable tremor in the vicinity of the fistula, a feature that is often diagnostic. Arteriovenous fistula disorders often have pressure variations, and the venous pressure proximal to the fistula may be low due to low outflow resistance and good adaptation of the venous wall to its blood flow. Distal to the fistula, the venous lumen enlarges and the vein wall thickens due to the impact of arterial pressure, and the venous pressure increases considerably. The arteries and veins surrounding the fistula, due to blood flow and pressure, make the vein diameters dilate and distort, the vessel walls become thinner, and degenerative changes and aneurysmal enlargement of the vessel walls may occur in advanced stages. The closer the fistula is to the heart, the more pronounced the changes in the vessel wall. Both arterial and venous lumen, can be enlarged more than 2-3 times. As a result of blood flow and blood pressure, the venous pressure near the fistula rises significantly and venous pulsation may occur. The enlargement of the venous lumen results in loss of valve function and backflow of venous blood, causing swelling of the limb, venous tortuosity, venous stasis, skin pigmentation, and even ulceration. The arterial blood flow in the distal part of the fistula is reduced, and ischemia can occur, such as pale skin, cold, pain, numbness, and a weakened or absent pulse, and claudication can also occur. 2. Effects of systemic circulation The abnormal passage between arteries and veins causes a decrease in peripheral resistance. As the decrease of peripheral resistance inevitably causes the central arterial pressure to decrease, the central venous pressure to increase, and the blood flow to perfuse the surrounding tissues to decrease. The effect of blood flow inevitably leads to changes in normal physiological function and body metabolism. The impact of arteriovenous fistulas on systemic blood flow depends on the site, size, and duration of presence of the fistula and the degree of fibrosis around the fistula. Fistulas between the aorta and the vena cava can develop heart failure earlier; arteriovenous fistulas in the limbs often do not necessarily develop significant cardiac complications even after several years. Congenital multiple arteriovenous fistulas, which are small and diffuse in character and associated with some resistance, are less likely to develop heart failure than acquired solitary fistulas. The large arteriovenous fistula shunts will persistently contribute to the ineffective pulsation of the heart, constantly increasing the burden on the heart. In addition, the decrease in arterial blood flow in the distal segment of the fistula and the decrease in peripheral vascular resistance often contribute to an increase in the heart rate and an increase in mean arterial pressure. The enlargement of the vascular bed around the fistula and the increase in blood volume may, over time, cause hypertrophy and enlargement of the heart to the point of heart failure. As the higher pressure arterial blood flows through the fistula to the lower venous system, the impact of this blood flow inevitably causes friction and damage to the endothelium, which may be subject to bacterial attack and reproduction, resulting in localized endarteritis and even endocarditis, and a few patients may experience recurrent fever and sepsis. 3. Congenital arteriovenous fistula: Congenital arteriovenous fistula is due to the abnormal channels remaining between the arteries and veins during the developmental evolution of the embryonic mesoderm. (1) Etiology: In the embryonic mesoderm of the early embryonic yolk sac wall and body tip, some cells form clusters of cells of different sizes called blood islands. The blood islands gradually extend and interconnect to form a primitive vascular plexus. As the embryo develops and grows, this reticulated capillary plexus evolves into small capillary-like vessels and larger vessels, which eventually become arteries and veins. Initially, arteries and veins traffic directly with each other and replace each other functionally. The embryonic development of the vasculature is broadly divided into 3 stages: the plexiform phase, the reticular phase, and the tubular stem formation phase. The adult vascular system is formed from the extension, anastomosis, atrophy and neogenesis of vessels in these 3 stages. Sometimes, the vasculature fails to follow this pattern of growth and decline during development, resulting in variability or malformation and the formation of abnormal channels between arteries and veins. The size of the fistula between the arteries and veins varies, but if the fistula is small and cannot be seen by the naked eye, it is called a microscopic arteriovenous fistula. If the fistula is slightly larger and can be detected by the naked eye, it is called a slightly larger arteriovenous fistula. Congenital arteriovenous fistulas are formed during embryonic life, but can also continue to develop after birth and can manifest clinically as angiomas, trabecular aneurysms, venous dilatation, or abnormal traffic between arteries. These malformations often occur in combination and can sometimes be combined with malformations of the lymphatic system. Congenital arteriovenous fistulas can occur anywhere in the body, but are commonly found in the extremities, with the lower extremities being the most prevalent, especially the ankles. In the upper extremities the most common are the ulnar artery branch, the palmar artery and the finger arteries. Lesions occur primarily in the superficial skin and soft tissues, but can also occur in organs such as the muscles, bones, digestive tract, brain, lungs, and kidneys. In congenital arteriovenous fistulas, the fistulae are small and frequent, and they develop and spread, often extensively invading adjacent tissues and organs, such as muscles, bones, and nerves, and even spreading to the entire limb or trunk. Depending on the size of the fistula and the location of the fistula, there are three types of fistulas: ① Stem arteriovenous fistulas: most of the fistulas are located between the main arteries and veins of the limb, and there are traffic branches in the transverse axis. Most of them have one fistula, but there are several small fistulas and branches. In larger fistulas, there are more blood shunts between the arteries and veins and higher venous pressure, which often results in clinical murmurs, tremors, varicose veins and sinuous aneurysms. If the fistula is tiny, the clinical symptoms are more transcendent. (ii) Aneurysmal arteriovenous fistula: the fistula site is on the branch between arterial and venous trunk, local tissue with aneurysmal vasodilatation, generally less blood shunt, no local murmur and no tremor. (iii) Mixed type: mixed trunk arteriovenous fistula with multiple traffic and aneurysmal lesions between trunks. Small arteriovenous fistulae have little hemodynamic alteration, while if they are large they may involve the function of the heart. Although congenital arteriovenous fistulas are pathomorphologically benign lesions, a few cases have a malignant tendency to grow rapidly. (2) Clinical manifestations: congenital arteriovenous fistulas are generally latent or hypermobile during infancy and childhood, and usually have no obvious clinical symptoms; by school age or adolescent development, the stimulation of endocrine hormones, labor and trauma contribute to the rapid growth of arteriovenous fistulas, which gradually show clinical symptoms. The arteriovenous fistula exists before the epiphysis of adolescents is closed, and there are extensive arteriovenous anastomoses around the epiphysis, resulting in increased blood flow, rich circulation in the bone marrow, and increased blood oxygenation, which leads to thickening and growth of the affected limb, and the patient feels heavy, swollen, and painful limbs. Due to the rich blood and venous congestion in the limb, the local temperature increases significantly, generally 3-5℃ higher than the healthy side. Due to the unequal length of the limb, pelvic tilt and spinal curvature can occur. (ii) Venous valve insufficiency: high pressure blood flow in the artery, through the fistula to the vein, resulting in increased intravenous pressure, enlarged venous lumen, venous valve damage, venous blood backflow, forming superficial venous tortuosity, stasis, hyperpigmentation, eczema, infection and even depressed ulcers. ③Inadequate arterial blood supply: the arterial blood of the affected limb is shunted to the vein, the distal arterial blood flow of the fistula is low, and the tissue is inadequately supplied with blood, producing muscle atrophy, coldness at the end of the finger (toe), low skin temperature at the distal end, and ulceration or gangrene at the end of the finger (toe) due to inadequate blood supply. ④Changes in the heart: abnormal traffic between the arteries and veins, a significant decrease in peripheral vascular resistance, thus making a significant increase in heart beat output, which affects for a long time and leads to heart failure. ⑤ Local lesions: Congenital arteriovenous fistula and congenital hemangioma coexist in the same area. The hemangioma is blue-red, either flat or elevated on the skin surface, and varies in size. A verrucous arteriovenous fistula can be locally swollen or accompanied by a cavernous hemangioma. In large fistulas, vascular murmurs and tremors can be heard locally, as well as high local skin temperature. The lesion may be occupying in the brain, and in hepatic and gastrointestinal arteriovenous fistulas, there may be gastrointestinal bleeding. For intrarenal arteriovenous fistulas, blood in the urine may be present, and for pulmonary arteriovenous fistulas, cough, chest tightness, shortness of breath, and cyanosis may be present. (3) Examination: ① Peripheral venous pressure measurement and oxygen analysis: In arteriovenous fistulas, venous pressure is elevated and intravenous oxygen levels are elevated. ②Color Doppler ultrasonography: It can detect arterial blood shunts and the presence of systolic or diastolic murmurs. (iii) Arteriography: This can be done with a rapid serial radiograph and can show the site of the fistula and the extent of the lesion. In the case of arteriovenous fistulas, the proximal artery may appear dilated and distorted. The corresponding vein may be visualized early. There may also be angiomatous dilatation and a mass of arteriovenous branches. (4) Treatment: Limited congenital arteriovenous fistulas, which affect the function of the limb, may be considered for surgical excision with good results. However, most congenital arteriovenous fistulas are complicated and difficult to treat because of the many small branches of communication between the arteries and veins and the extensive lesions, which sometimes involve the entire limb. If resection is not complete, not only can the lesion recur, but it may stimulate further development of the lesion, so careful consideration must be given to whether to use surgery. ①Surgical indications: Rapidly growing arteriovenous fistulas with obvious clinical symptoms should be operated early; lesions involving surrounding tissues, such as nerve pressure pain, bleeding, ulcers or complications of infection, even affecting the heart, causing heart failure; visceral arteriovenous fistulas, liver and gastrointestinal arteriovenous fistulas, causing bleeding, or intrapulmonary arteriovenous fistulas, cyanosis, shortness of breath, etc., should be operated early. ②Surgical methods: Depending on the lesion, different options are available such as embolization therapy, arteriovenous fistula excision, ligation of the main arteriovenous branches of arteriovenous fistula, and amputation.