An arrhythmia is an abnormal heartbeat usually caused by an electrical "short circuit" in the heart. Many are minor and not a health threat, but some can indicate a more serious problem.

Learn how this amazing muscle pumps blood throughout the body.

Fainting is a temporary loss of consciousness. Someone who has fainted will usually recover quickly.

Fainting is a loss of consciousness that can be caused by many things. Here's what to do if your child faints or is about to faint.

Long QT syndrome is a condition that affects the electrical system of the heart. Some kids have no symptoms, while others may feel changes in their heartbeat or feel lightheaded.

Postural orthostatic tachycardia syndrome (POTS) happens when the autonomic nervous system — which controls things like heart rate, blood pressure, and breathing — doesn't work as it should.

Supraventricular tachycardia is a type of abnormal heart rhythm in which the heart beats very quickly.

A guide to medical terms about the heart and circulatory system. In an easy A-Z format, find definitions on heart defects, heart conditions, treatments, and more.