If your heart rate is over 100 beats per minute this is called tachycardia. There are normal and abnormal fast heart rhythms. Exercise, mental or emotional stress, and some illnesses can cause your heart rate to rise normally, above 100 beats per minute.

When your heart rate is too fast, however, or occurs without cause, or is too fast and irregular, it is called a tachyarrhythmia.

The different types of tachyarrhythmia include Ventricular Tachycardia (VT) and Ventricular Fibrillation (VF), and are described below.

VT is an abnormal rapid heart rate that originates in the ventricles. The heart pumps less blood with each beat because there isn’t enough time for the chambers to refill with blood between beats. This causes symptoms such as dizziness, lightheadedness, near fainting symptoms, fainting, and loss of consciousness. For most patients, VT is dangerous if not treated.

In VF, the heart beats very fast and irregularly. This is due to chaotic electrical activity of the ventricles.

Little or no blood is pumped. Your brain, heart and the rest of your body are quickly starved of oxygen. Patients usually pass out within a few seconds. VF almost never stops on its own, and is therefore fatal unless the normal rhythm is restored with an electric shock to ‘reset’ the heart (defibrillation). Defibrillation can be given by doctors or emergency medical personnel using paddles held to the chest (external defibrillation), or automatically by an implantable defibrillator (ICD or CRT-D).

VT and VF have a number of causes, the most common of which is scarring of the heart due to a previous heart attack. Cardiac enlargement as a result of heart failure is also an important cause.

The electrical signal is not following the normal pathway. Instead, multiple electrical outbreaks are firing at the same time. The heart cannot contract properly, making the pump inefficient until a normal rhythm is restored.