The heart suddenly and unexpectedly stops beating resulting in death within a short time period (generally within 1 h of symptom onset).

Delayed or lack of conduction of atrial depolarizations through the atrioventricular node to the ventricles.

Disturbance in the atrial activation that is caused by transient failure of impulse conduction from the sinoatrial node to the cardiac atria.

Syncope is a syndrome in which loss of consciousness is of relatively sudden onset, temporary (usually less than 1 to 2 minutes), self-terminating, and of usually rapid recovery. Syncope leads to a generalized weakness of muscles with loss of postural tone, inability to stand upright, and loss of consciousness. Once the patient is in a horizontal position, blood flow to the brain is no longer hindered by gravitation and consciousness is regained. Unconsciousness usually lasts for seconds to minutes. Headache and drowsiness (which usually follow seizures) do not follow a syncopal attack. Syncope results from a sudden impairment of brain metabolism usually due to a reduction in cerebral blood flow.

Bradycardia related to a mean resting sinus rate of less than 50 beats per minute.

Episodes of atrial fibrillation that typically last for several hours up to one day and terminate spontaneously.

Inability of the heart to increase its rate commensurate with increased activity or demand.

An anomaly of the complex formed by the Q, R, and S waves, which occur in rapid succession on the electrocardiogram.

Any anomaly of the time interval between the start of the Q wave and the end of the T wave as measured by the electrocardiogram (EKG).

An anomaly of the PR interval, which is the portion of the ECG from the onset of the P wave to the beginning of the QRS complex. A normal PR interval in adults is 0.12-0.2 seconds.