Some people have minor arrhythmias that persist for long periods and are not dangerous—in fact, they are simply heartbeats that are normal for that particular person’s heart. A temporary arrhythmia can be caused by alcohol, caffeine, or simply not getting a good night’s sleep. Often, damage to the heart muscle results in a tendency to develop arrhythmias. This heart muscle damage is frequently the result of a heart attack, but can also develop for other reasons, such as after an infection or as part of a congenital defect.

Arrhythmias may involve either abnormally slow or abnormally fast rhythms. An abnormally slow rhythm sometimes results from slower firing of impulses from the SA node itself, a condition doctors call sinus bradycardia. An abnormally slow heartbeat may also be due to heart block, which arises when some or all of the impulses generated by the SA node fail to be transmitted to the ventricles. Even if impulses from the atria are blocked, the ventricles continue to contract because fibers in the ventricles can generate their own rhythm. However, the rhythm they generate is slow, often only about 40 beats per minute. An abnormally slow heartbeat is dangerous if the heart does not pump enough blood to supply the brain and the rest of the body with oxygen. In this case, episodes of dizziness, lightheadedness, or fainting may occur. Episodes of fainting caused by heart block are known as Stokes-Adams attacks.

Some types of abnormally fast heart rhythms—such as atrial tachycardia, an increased rate of atrial contraction—are usually not dangerous. Atrial fibrillation, in which the atria contract in a rapid, uncoordinated manner, may reduce the pumping efficiency of the heart. In a person with an otherwise healthy heart, this may not be dangerous, but in a person with other heart disease the reduced pumping efficiency may lead to heart failure or stroke.

By far the most dangerous type of rapid arrhythmia is ventricular fibrillation, in which ventricular contractions are rapid and chaotic. Fibrillation prevents the ventricles from pumping blood efficiently, and can lead to death within minutes. Ventricular fibrillation can be reversed with an electrical defibrillator, a device that delivers a shock to the heart. The shock briefly stops the heart from beating, and when the heartbeat starts again the SA node is usually able to resume a normal beat.

Most often, arrhythmias can be diagnosed with the use of an ECG. Some arrhythmias do not require treatment. Others may be controlled with medications such as digitalis, propanolol, or disopyramide. Patients with heart block or several other types of arrhythmias may have an artificial pacemaker implanted in their chest. This small, battery-powered electronic device delivers regular electrical impulses to the heart through wires attached to different parts of the heart muscle. Another type of implantable device, a miniature defibrillator, is used in some patients at risk for serious ventricular arrhythmias. This device works much like the larger defibrillator used by paramedics and in the emergency room, delivering an electric shock to reset the heart when an abnormal rhythm is detected.