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

An atrial arrhythmia characterized by disorganized atrial activity without discrete P waves on the surface EKG, but instead by an undulating baseline or more sharply circumscribed atrial deflections of varying amplitude an frequency ranging from 350 to 600 per minute.

Hypertrophic cardiomyopathy (HCM) is defined by the presence of increased ventricular wall thickness or mass in the absence of loading conditions (hypertension, valve disease) sufficient to cause the observed abnormality.

A conduction block of the left branch of the bundle of His. This manifests as a generalized disturbance of QRS morphology on EKG.

A sensation that the heart is pounding or racing, which is a non-specific sign but may be a manifestation of arrhythmia.

Syncope refers 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.

A tachycardia originating in the ventricles characterized by rapid heart rate (over 100 beats per minute) and broad QRS complexes (over 120 ms).

Enlargement or increased size of the heart left ventricle.

Hypertrophic cardiomyopathy with an asymmetrical pattern of hypertrophy, with a predilection for the interventricular septum and myocyte disarray.

A diminution of the volumetric fraction of blood pumped out of the ventricle with each cardiac cycle.

Difficult or labored breathing. Dyspnea is a subjective feeling only the patient can rate, e.g., on a Borg scale.

A sensation of breathlessness in the recumbent position, relieved by sitting or standing.