The upper EKG shows atrial fibrilation. The P-wave is shown by the purple arrow in a normal patient below. Courtesy Wikimedia commons.
Atrial fibrillation is the most common type of arrhythmia. Although it clearly shows up on an EKG with the lack of a P-wave (see sinus rhythm), and can be detected by a diligent physician when taking a pulse, it may be otherwise asymptomatic. However, it is often associated with palpitations, fainting, chest pain, and congestive heart failure. In most cases, it has no obvious cause, or is caused by otherwise benign conditions.
People with atrial fibrillation are about seven times more likely to suffer a stroke as anyone else in the general population.
Treatment concentrates on regularizing the rhythm rather than attempting to correct any underlying conditions. Drugs that slow heart rate can often be effective, although there are surgical options. Sufferers also often take a blood thinner to prevent clotting.