• Ventricular fibrillation: very rapid beats to the point ventricles don’t pump blood, results in low blood pressure, loss of blood supply to organs, extremely serious and can be fatal without immediate intervention, may be caused by heart attack.
• Long QT syndrome (LQTS): extremely serious and may cause fainting and lead to death, rapid heartbeats, caused by gene mutation, congenital heart defects, and certain medications.
• Sick Sinus: caused by scarring of sinus node or disruption of sinus node impulses, causes slower heart beat.
In addition to tachycardia and bradycardia, some people experience premature heartbeats. A premature heartbeat is simply an extra little heartbeat that occurs in-between normal heartbeats. Common causes for premature heartbeats include caffeine and some medications.
Heart Arrhythmias, The Mayo Clinic, 14 Feb 2009, http://www.mayoclinic.com/health/heart-arrhythmias/DS00290