Ventricular tachycardia
Ventricular tachycardia is a medical emergency.  First of all, if the heart is beating too fast, it does not have time to refill with blood between beats.  This means that the heart will not be able to squeeze out enough blood to the rest of the body.  Most people in ventricular tachycardia have a heart rate of at least 170 beats per minute.  This is very fast.  (A normal heart rate is 60-100.)