The term fistula comes from Latin and in medicine in means a connection between two or more organs that has been formed as a result of pathological processes, iatrogenic artifacts or on purpose in a surgical procedure.
An arteriovenous fistula is formed when an artery and a vein are directly connected. Such consolidation can be congenital or it may result from an injury. However, most often we encounter connections created surgically using the patient’s blood vessels, usually between the radial artery and the cephalic vain in the distal region of the forearm. If it is impossible to form a fistula with the patient’s vessels, the connection is created using plastics. Medicine uses bypasses made of a synthetic material (polytetrafluoroethylene).
There are three basic ways to provide constant access to the patient’s blood vessels: arteriovenous fistula, arteriovenous vascular prosthesis (graft) and central venous catheter. Fistula is the most common type of vascular access, which makes it possible to perform for instance haemodialysis, helpful in the treatment of renal failure. An arteriovenous fistula permits taking blood from the patient’s body, filtering it through a dialyser, where it is purified, and then pumping it back into the patient’s body.
Before creating an arteriovenous fistula certain preparatory procedures must be followed. First of all, the patient’s consent to the surgical procedure needs to be obtained. Additionally, to protect the patient’s health, vaccination against hepatitis B is recommended. It is also necessary to determine the blood type and the coagulation parameters, that is prothrombin time, activated partial thromboplastin time, fibrinogen level and platelet count. Furthermore, complete blood count parameters must be determined. The extremity in which the fistula is to be created should not be pierced for the purpose of blood tests or administration of drugs. The attending physician may also order Doppler ultrasonography.
Of course, not everyone can undergo a surgery involving creation of an arteriovenous fistula. Contraindications include mostly severe circulatory insufficiency, considerable obesity of upper extremities and narrowing of the venous outflow from extremities. The surgery is performed with local conduction anaesthesia or general anaesthesia with the assistance of an anaesthesiologist. It involves dissecting and then naturally connecting blood vessels using a synthetic graft or a vein taken from the patient’s thigh. The time required between the creation of a fistula and the commencement of dialysis is 3 to 6 weeks. In this period the connection matures – the veins taking the arterialised blood dilate and their walls grow together.
Status of arteriovenous fistula (positioning of arteriovenous fistula) ought to be deep enough to prevent the skin from being continuously damaged but shallow enough to ensure easy access for needle insertion. In the case of such a vascular connection, blood flow velocity is of great importance. It ought to permit effective dialysis without overburdening the heart. The maximum blood flow velocity in brachial artery with a fistula is about 1.3 m/s. At the site where a vascular prosthesis is connected with the artery, blood flow velocity slightly increases and the maximum value is about 0.3 to 1 m/s. Estimated, appropriate blood flow in brachial artery on the side of fistula is 350 ml/min.
A dialysis connection requires special care and protection. A person with an arteriovenous connection must adapt their lifestyle to the new requirements of such a solution. It is fundamental that the patient tested their arteriovenous fistula on their own on a daily basis. Such a test should involve placing their fingers on the fistula and sensing the blood pulsating along the whole length of the arteriovenous connection. A fistula works properly if there is a clear, palpable pulsating murmur, usually in one specific place. Checking a vascular connection is quite a simple activity, taking little time but it can prevent unpleasant complications. Arteriovenous fistula ought to be tested at least a few times a day – during each time of the day. If during the examination a patient notices that the fistula is not pulsating or that their skin is reddened or swollen, they must immediately consult a specialist physician. The most frequent reason for improper functioning of an arteriovenous connection is thrombosis, caused by inadequate inflow of arterial blood or insufficient outflow of venous blood. Before falling asleep, a patient ought to make sure that the arm with the fistula is positioned in a way guaranteeing that the arteriovenous connection will not cease to function because of, for instance, being pressed. Additionally, using the arm with a fistula to carry objects weighing over 3 kg is not recommended.
A specialised test of arteriovenous fistula is performed using 5-10 MHz linear probes. Blood vessels are evaluated at cross sections and longitudinal sections, and flow velocity is measured for the artery supplying blood to the fistula. Measurements are carried out wherever the structure of vessels suggests possible narrowing.
