The heart contracts when stimulated by electrical impulses that it produces on its own. Any part of the heart muscle can produce electrical impulses that will cause heart contractions. Normally, the sinus node acts as the heart rhythm guide. The sinus node is found in the right heart's pre-chamber. The sinus node self-creates electrical impulses (auto-excitation); the pulse rate, however, is controlled by the neuron-vegetative system (simpaticus and parasimpaticus). Electrical impulses from the sinus node are then conducted to another node (atrial-ventricular) and from there are transmitted through Hiss’s fascicle into the heart chamber’s muscles. This means that one electrical impulse produced in the sinus node causes one contraction of the heart muscle. In atrial fibrillation, many electrical impulses are produced from various places in the heart’s atria. Not all those impulses will reach the chambers and cause contractions because some will be stopped in the atrio-ventricular node. But if there are many impulses produced in the atria(over 200 per minute), many of them will pass through the atrio-ventricular node and will reach the chambers causing the heart to contract too quickly (over 160 per minute). This may occur suddenly. it is called "supra-ventricular paroxysmal tachycardia". Such a heart rate is not effective and manifests with chest pain and breath shortness. While the chambers contract quickly, the atria are only trembling. This atrial trembling creates a condition for the development of blood clots. Blood clots created in the heart’s atria can cause thrombo-embolism in distant organs (lungs, brain, extremities, intestines etc.). Your doctor prescribed you Coumadin in order to prevent blood clotting in the atria. Atrial fibrillation is treated with digoxin, verapamil or another anti-arrhythmic drug.