Arteriovenous Malformation and Arteriovenous Fistula Effect on the Cardiovascular System

Introduction

Cardiovascular diseases, especially ones affecting blood vessels, occupy one of the first places in the structure of diseases characterized by the highest mortality and disability of patients. Among the pathological changes of cerebral vessels to undergo surgical treatment, one of the main positions occupied by arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs). Despite the fact that AVM and AVF have long been not considered as belonging to the category of casuistry, they are currently the object of study of neurologists, neurosurgeons, radiologists, medical ultrasound diagnostics and cardio diseases specialists. AVM and AVF being the diseases of one type can affect the human body causing the great damage not only to the organ affected, but to the cardiovascular system as a whole.

Brief Overview

Arteriovenous malformation is an abnormal vascular plexus in which the blood from the arteries falls directly into a vein, bypassing the capillaries, characterized by the absence of capillary vessels and high blood pressure. AVM may be formed in any part of the body; symptoms have consistent localization. In the brain, arteriovenous malformation may be localized on the surface of the brain (cortical), inside the brain tissue (hypothalamus, basal ganglia, or trunk), and within the dura – tight protective layer of the brain. AVM being located inside the special shell is called arteriovenous fistula (AVF).

Arteriovenous fistula is a pathological connection (bypassing the capillary network) between the artery and vein. Congenital arteriovenous fistulas can be located in any part of the body and are more often multiply acquired – they may be associated with damage to the blood vessels in trauma, surgery, organ biopsy. Sometimes, arteriovenous fistula are created intentionally, e.g. in patients subjected to long-term treatment of hemodialysis. Large arteriovenous fistulas can lead to a deterioration of blood supply to the distal fistula, an increase in cardiac output and heart overload.

Normally, the blood moves through the veins and is transported to the venous sinuses are located in the various body organs, mainly lungs, liver, kidneys and brain, then moves to the heart muscle. At AVM artery is connected directly to the veins or sinuses. In addition to the lining of the organ arteriovenous fistula, carotid-cavernous fistulas (shunts between the carotid artery and venous sinuses) are likely to occur. Spinal AVM is formed either on the surface of the spinal cord (extramedullary) or inside it (intramedullary).

There are four general types of AVM:

• Arteriovenous malformation is an abnormal vascular plexus in which the blood from the arteries falls directly into a vein, bypassing the capillaries in which characterized by the absence of capillary vessels and high blood pressure;
• Cavernous hemangioma is an abnormal accumulation of dilated capillaries; usually veins and arteries feeding absent; characterized by a low pressure;
• Venous malformation is an abnormal accumulation of enlarged veins in the form of wheel spokes; characterized by the absence of feeder arteries and low pressure; usually it does not require treatment as rarely bleeds;
• Capillary telangiectasia is abnormal capillaries with increased surface area (like a cavernous hemangioma); pressure is very low.

The effect of the volume of blood flow in the shunt arteriovenous malformation on central hemodynamics and heart is practically not studied. If to analyze hemodynamic disturbances arising from the operation of the malformation, then they are equivalent to hemodynamic disorders in birth defects or ventricular atrial septum, i.e. a long-term discharge of blood “from left to right”.

The main feature of the AVM is a significant increase in linear and volumetric blood flow in it, while these figures for the rest of the organ blood flow remained normal. Time circulation in the brain is normally about 6, and in the afferent vessels and stroma AVM is reduced to 3-4 seconds. Blood flow in the AVM of the brain may increase by 2-3 times.

By fistula or any other malformation at AVM, venous blood flows from the pulmonary artery in the pulmonary vein without being oxygenation, and then enters the left side of the heart, which leads to arterial oxygen undersaturated. Shunt fistula occurs in the zone at low pressure and low vascular resistance, so in the pulmonary artery pressure remained in the normal range. Cardiomegaly are observed, heart failure is rare.

Clinical presentation depends on the amount and size of the shunt. For large fistulas appear shortness of breath, cyanosis, clubbing syndrome and developing polycythemia. Hemoptysis is rare, but it can be significant. Approximately 50% of patients or their relatives revealed signs of the syndrome Randy – Weber – Rendu, including recurrent nasal and gastrointestinal bleeding. Transient dizziness, diplopia, aphasia, a disorder of motor function or seizures may be the result of cerebral thrombosis, abscesses, or paradoxical embolism. Over the area of the fistula can hear the soft systolic or continuous noise.

The appearance of arteriovenous fistulas or malformations leads to poor blood supply to important parts and organs of the human body. And the blood pressure falls, while the pressure in the veins increases. It increases the load on the heart, which is at the same time begins to experience lack of blood flow due to violation of the loop circulation. Another type of complications caused by arteriovenous fistulas are various cosmetic defects: spots on the skin, and swelling of tissues.

AVM and AVF Effect onto Cardiovascular System

As was said earlier, arteriovenous malformation is a congenital defect, when veins and arteries are not connected by the capillaries. Lack of capillaries leads to the fact that the blood goes through the abnormal vessels in the rapid pace and high pressure so that the nutrients together with the blood enters the tissues and organs. Arteriovenous malformation may develop in various organs of the body, if the disease affects the brain and spinal cord a negative impact on the entire human body.

Changes of cerebral circulation in cerebral AVM is closely associated with the presence of arteriovenous shunts. The lack of capillary barrier leads to a marked decrease of cerebrovascular resistance, resulting in a significant portion of the blood flowing to the brain “resetting” through arteriovenous shunts bypassing the normal capillary network of the organ. Consequently, in the brain are formed to some extent two separate circulation systems: the vessels AVM through normal brain and blood vessels. Both the circulatory system and are connected in a dynamic equilibrium.

A variety of local clinical manifestations of AVM is well known, but the central effects are primarily due to the impact of “shunted” blood on cardiac hemodynamics is not adequately covered in the medical literature. Connection of arterial and venous blood vessels, regardless of location, is always accompanied by a massive discharge of blood from the arterial system to the venous, reaching 20-50% of the flow. A significant increase in venous return, increased venous pressure, right atrial pressure causing tachycardia, increased stroke volume and cardiac output. With the increase in cardiac output increases the systolic blood pressure, a rapid return of blood is accompanied by a decrease in diastolic blood pressure, which is the cause of hyperkinetic hemodynamics, increased pulse pressure, subsequently leading to heart failure.

Consequently, large AVMs have an impact not only on cerebral hemodynamics, but also lead to changes in the cardiovascular system, which can manifest hyperkinetic hemodynamics, tachycardia, and hypertension. The size of the AVM is undoubtedly the most important factor in determining the structure and functional reconstruction of the entire circulatory system of the brain. The size of the AVM marks the increase in the linear and volumetric blood flow in vessels and outlet leading AVM. However, a direct correlation between the volume of the AVM and the size of the blood flow is maintained only up to a certain limit. Thus, when AVM size is about to increase to the4 bigger size (over 50 cm) it corresponds to the fact that the linear and volumetric blood flow to the AVM value is broken and a further increase in blood flow occurs due to expansion, tortuosity and elongation of the afferent and efferent vessels, turbulent blood flow slows down. It is crucial to reducing the tone of resistance vessels, which manifests itself in the first place suffering from myogenic regulation circuit – it causes the “paralysis” of myogenic.

In small AVM arteriovenous “bypass” is expressed negligibly, with no effect on blood flow. As the size of the ABM aspirates its effect on the blood flow increases. This is reflected in a sharp increase in blood flow to the right path and convergence towards the AVM from adjacent vascular beds. Therefore, large AVMs shifts from the dynamic equilibrium of the blood flow in the AVM side, and therefore the circle connecting artery anastomoses loses it general role and becomes an additional way of blood supply to the AVM.

One of the most common consequences of AVM is an isolated systolic hypertension. This is an increased rigidity of blood vessels while reducing the elasticity of the vascular wall. These pathological states with high cardiac output associated with a reduction in systemic vascular resistance, increase in stroke volume and cardiac output, an increase in pulse pressure with increased systolic blood pressure and diastolic blood pressure and peripheral arterial pulse quickening. At these complications appearance, there is a high apical impulse, gain in the I heart tone and pulmonary component of the II heart sound and rise the top III tone. Early systolic ejection murmur is usually audible on the background of the heart or the left edge of the sternum, which is associated with an increase in blood flow through the aortic valve and the pulmonary artery.

Posts vessels in the lungs through the fistula can be enlarged and of limited stretch or multiple, disseminated and small. They may be manifestations of the inherited hemorrhagic telangiectasia syndrome with localization of angiomas in the mucous membranes of the nose and cheeks, the gastrointestinal tract or liver. Rare variants include direct connection of the pulmonary artery to the left atrium. Lung undersaturation, in its turn, can lead to various, including severe, cardiovascular disease, aneurysm – increased load on the vein blood flow can lead to their tensile and tear – and thrombosis, that can develop in areas below the venous fistula appearance.

Arteriovenous fistulas are also associated with large vascular tumors such as hemangiomas and hemangiosarcoma. If they are very large, they may lead to hemodynamic disturbances, such as increased heart rate at rest due to the increased effort required to maintain normal cardiac output. Intracranial arteriovenous fistulas cause the most severe hemodynamic disturbances as striking large vessels and cause a large discharge of blood from left to right. In infancy arteriovenous malformations can cause severe heart failure; it is one of the few cardiovascular disease, which causes fetal hydrops and severe heart failure in the first days of life. Clinical manifestations usually include a constant noise on one side of the head, prancing on the pulse of the carotid arteries and jugular veins. Chest X-ray visible dramatically expanded upper hollow Vienna and dilatation of both ventricles. The peripheral pulse – very tall and fast, jumpy, but very severe heart failure, he becomes weak in all the arteries, except sleepy. With a small reset hemodynamic manifestations are not pronounced and dominant neurological symptoms. Small arteriovenous malformations occur partial seizures, major – headache, seizures and focal neurological symptoms. Sudden severe headache – a sign of intracranial hemorrhage. Approximately 10% of first intracranial hemorrhage leading to death.

The size of the right heart chambers, the diameter of the pulmonary artery and the design pressure in the pulmonary circulation in the patients with AVM are significantly different from mean indicators. The average diameter of arteriovenous fistula of the anastomosis is 0,18 ± 0,03 mm. Mean values of blood flow in arteriovenous fistula made: 6,0 ± 2,1% of the cardiac output, 464,6 ± 161,8 ml / min, which is significantly different from the common one in healthy individual.

If to analyze the existing data on the matter, a high degree of correlation between the amount of blood flow through an arteriovenous fistula (% of cardiac output and in ml / min) and anastomotic diameter (r = 0,69, p <0.01), the main echocardiographic dimensions of right heart chambers (r = 0,56, p <0.01), and the diameter (r = 0,53, p <0.01) and the design pressure (r = 0,57; p <0.01) in the pulmonary artery can be revealed. In patients with a high discharge of arteriovenous fistula (849,3 ± 358,7 ml/min), there is a significant increase in the size of the right atrium (p <0.001) and right ventricle (p <0.01), which is a manifestation of a long-term volume overload . This serves as a strong evidence of the AVM influence on the size, rate and amount of blood incoming and outcoming from the heart, creating greater pressure in the blood vessels.

Aneurysms and aortic sinus fistula are the most severe consequences of the AVM since they appear in the closest position to the heart itself. Congenital aortic sinus of Valsalva aneurysm, in particular the right coronary sinus is rare at the AVM aneurism. This defect is to divide the aorta layers into two, or there is no connection between the core layer of the aorta and the aortic annulus. The progressive aneurysmal extension of this zone can not manifest until a patients’ 30-40 years of age, can occur when her breakthrough into the cavity of the heart. As a rule, formed cardiac fistula opens into the right ventricle and the least – in the right atrium.

Coronary arteriovenous fistula, as the consequence of AVM is a rare malformation, which usually is a connection between the right coronary artery and the right atrium or ventricle. Shunt is usually small, and myocardial blood flow is not affected. Potential complications include bacterial endocarditis, thrombosis or occlusion with distal embolization, rupture of the aneurysmal fistula expanded, and in rare cases – pulmonary hypertension and congestive heart failure. Upon detection of the surface loud constant noise in the lower or middle sternal border area requires a thorough examination of the patient. Doppler echocardiography allows to establish the localization of the shunt. If the place of origin of the fistula proximally located, it can be detected by echocardiography biplane. Retrograde thoracic aortography or coronary arteriography allows to specify the size and anatomical features of the fistulous, which then can be closed.

Conclusion

Summing up, one can say that AVM and AVF as its complication cause a significant damage to the cardiovascular system. AVM consequences can manifest in narrowing and tightening of blood vessels, heart failure, aneurism, increase in the heart size and high blood pressure. In addition, AVM lead to undersaturation of the heart by oxygen and, as the result, undersaturation of all body organs. Taking into account its severe health consequences, the problem of AVM appearance and causes should be researched in details.