Some one also mentioned atrial fibrillation.
Atrial fibrillation and atrial flutter are very fast electrical discharge patterns that make the atria contract extremely rapidly, thus causing the ventricles to contract faster and less efficiently than normal. These abnormal rhythms may occur sporadically or may persist. Durning fibrillation or flutter, the contractions of the atria are so fast that the atrial walls simply quiver, so blood isn't pumped effectively to the ventricles. In fibrillation, the atrial rhythm is irregular, so the ventricular rhythm is also irregular; in flutter, the atrial and ventricular rhythms usually are regular. In both cases, the ventricles beat more slowly than the atria because the atrioventricular node and the bundle of His can't conduct electrical impulses at such a fast rate, and only every second to forth impulse gets through. Still the ventricles beat too fast to fill completely. Therefore inadequate amounts of blood are pumped out of the heart, blood pressure falls, and heart failure may occur.
The heart may go into atrial fibrillation or flutter with no other sign of heart disease, but more often the cause is an underlying issue/problem, such as high BP, alcohol abuse, thyroid hormones.(hyperthyroidism).
Symptoms of atrial fibrillation or flutter depend largely on how fast the ventricles beat. A modest ventricular rate-- less than about 120 beats per minute-- may produce no symptoms. Higher rates cause unpleasant palpitations or chest discomfort. With atrial fibrillation, the person may be aware of the rhythm irregularities.
The diminished pumping ability of the heart may make a person feel weak, faint, short of breath, chest pains, and shock. The diagnose of atrial fibrillation or flutter is suspected from the symptoms and confirmed by an electrocardiogram (ECG). With atrial fibrillation, the pulse is irregular. With atrial flutter, the pulse is more likely to be regular but rapid.