To understand how atrial fibrillation can lead to stroke, it is important to know exactly what happens in the heart during atrial fibrillation and what this means to the blood circulation system.
The heart and the circulatory system
The heart consists of two atriums and two ventricles. When an electric impulse is sent through the heart as a signal to contract, it happens quickly between the atriums and the ventricles, however with a slight delay in the signal allowing the atrial musculature to contract right before the ventricles. This causes the atriums to empty all their blood into the ventricles, after which the ventricles contract sending the blood out of the heart. This way, the blood is constantly moving in a healthy heart.
The blood coming from the body, which has been supplied with oxygen and nutrients, now runs to the right atrium which empties the blood into the right ventricle sending the blood to the small pulmonary circuit. Here, the blood is saturated with oxygen and it releases large amounts of carbon dioxide which is exhaled. After this, the blood runs to the left atrium, into the left ventricle and into circulation once again, where the oxygenated blood is now able to supply the body’s organs.
The blood coagulates
Most frequently, blood coagulates due to the following two causes: Either because it pools or because it is exposed to certain substances caused by a broken blood vessel wall. These two situations can trigger the coagulation cascade, leading to blood clotting. Stasis of the blood increases the platelets’ contact with the blood vessel wall and at the same time, stasis leads to imbalances between factors which either stimulate or prevent coagulation. Because of these two mechanisms, stasis will lead to blood coagulation. The coagulated blood will form a clot capable of blocking small blood vessels if the clot is sent into circulation in the body. This situation is known as thromboembolism.
Atrial fibrillation leads to blood clots
Atrial fibrillation is a heart disease involving disturbances of the electric impulses in the heart. Usually, the heart’s electrical impulses will arise from a group of specialised muscle cells in the right atrium called the sinoatrial node, and from this point, they are forwarded to the rest of the heart. The electric impulse causes the heart to contract. Thus, the sinoatrial node normally controls the heart rhythm. In the event of atrial fibrillation, these electrical impulses will arise from different parts of the atria and thus not from the sinoatrial node. These irregular impulses will lead to uncoordinated muscle contractions in the atria and a faster heart rhythm. When contractions are this chaotic, the heart cannot contract simultaneously but merely vibrate, which reduces the blood flow from the atria to the ventricles and into the circulatory system.
When you suffer from atrial fibrillation, this vibration in the arteries and the lack of coordinated contractions will result in an increased tendency to blood stasis in the atria, and thereby an increased risk of coagulation and blood clots. In addition, atrial fibrillation is thought to damage the atrial tissue, whereby tissue factors are exposed to the blood. This initiates the coagulation cascade which leads to blood clots. Furthermore, atrial fibrillation is associated with a condition known as hypercoagulation, meaning that there is an elevated level of components in the blood which stimulate coagulation and not enough factors which stop coagulation.
These three elements, stasis, endothelial injury and hypercoagulability are known as Virchow’s triad – a description of the three categories that are thought to contribute to blood clots or thrombosis. Only one element is necessary to the formation of a blood clot, but in atrial fibrillation, all three factors are present, and for this reason, people with atrial fibrillation is at high risk of developing blood clots.
At a random time, these clots travel from the left atrium to the left ventricle and into circulation, where part of the blood travels to the brain. The blood vessels gradually become smaller and smaller, and at one point, the clot will be too big to pass thus forming a blood clot. As a result, the area blocked by the clot will be deprived of oxygen and nutrients, the brain tissue will be damaged, and abilities controlled by that area of the brain will be lost. If a patient experiences a stroke – or apoplexy – he or she will often experience symptoms such as problems seeing or speaking, paralysis of one side of the face or body, numbness etc.
For this reason, patients with atrial fibrillation are often treated using blood thinners which decrease the risk of blood coagulation in the veins and thus prevent blood clots. Therefore, it is important to take your medication even if you feel no apparent effect, as it prevents potentially fatal conditions.
Sources:
1) https://hjerteforeningen.dk/alt-om-dit-hjerte/hjerte-og-kredsloeb/hjertet/
2) http://www.pathwaymedicine.org/thrombosis
3) https://hjerteforeningen.dk/alt-om-dit-hjerte/hjerte-kar-sygdomme/atrieflimren-og-atrieflagren/
5) http://www.mayoclinic.org/diseases-conditions/atrial-fibrillation/symptoms-causes/dxc-20164936
