Septal Defect: A hole in the wall between the atria or ventricles (upper or lower heart chambers.

Septum: A muscle wall between the atria or ventricles.

Sinus Node: The heart’s natural pacemaker, produces electrical impulses to keep the heart beating at a healthy pace by causing the heart to contract and pump blood at regular intervals.

Sinus Rhythm: A normal heart rhythm of 60-100 beats per/min at rest originating from the sinoatrial node.

Sinus Tachycardia: A normal heart rate greater than 100 beats/min. It occurs as a desirable response to exertion, pregnancy, and emotion. It may also occur with, illness, circulatory problems, or drug use.

Stenosis: A narrowing or constriction of a blood vessel or valve in the heart.

Stroke: The sudden disruption of blood flow to the brain. Many strokes are caused by a clot of blood traveling into the brain (thrombosis). A stroke may also be caused by a rupture of a blood vessel (hemorrhage).

Sudden Cardiac Arrest: The sudden, unexpected loss of heart function, breathing and consciousness. This is usually caused when abnormal heart rhythms prevent the heart from pumping blood to the brain and other organs. Also known as SCA. Often confused with a heart attack. (see Heart Attack)

Sudden Cardiac Death: Arrhythmias caused by electrical events originating in the atria, i.e. above the ventricles.

Syncope: Loss of consciousness and postural tone caused by diminished cerebral blood flow usually associated with aging.