Atrial flutter is a cardiac arrhythmia characterised by rapid, regular atrial contractions typically around 250–350 beats per minute, caused by a reentrant circuit in the right atrium. It matters clinically because it can lead to inefficient atrial contraction, reduced cardiac output, and predisposes patients to thromboembolic events. Atrial flutter is often associated with atrial fibrillation and increases the risk of stroke if untreated.