Atrial fibrillation. Artwork of a section through a human heart during atrial fibrillation, a rapid, irregular heartbeat. In the background are ECG (electrocardiogram) traces, showing irregularity of the heart rhythm (arrhythmia). The electrical impulses that coordinate heart contractions start at the sinoatrial node (red, upper left) and run down to the atrioventricular node (red, upper centre). When the atria (upper heart chambers) contract irregularly and too rapidly, the ventricles (lower heart chambers) have trouble keeping pace. The main symptom of atrial fibrillation is palpitation. ECGs are used to diagnose heart conditions such as arrhythmia.