What is the difference between ventricular and supraventricular arrhythmia?

Degree of danger: Although supraventricular arrhythmias (rhythm disturbances that originate in the atria) may lead to serious health problems over time, they are rarely an immediate life threat. In contrast, ventricular arrhythmias are truly life threatening and if not rapidly treated can lead to cardiac arrest and death. Automatic external defibrillators (aed's) were developed to treat ventricular arrythmias.

Bottom and top: Ventricle is the bottom part of the heart, or the main pumping chamber. Rhythms that start here are not normal, and in some situations can be fatal. Rhythms that originate anywhere else are called supraventricular. While less associated with sudden death, are still not good when sustained.