Tachycardia

Tachycardia is an abnormal condition in which the heart (myocardium) contracts regularly but at a rate greater than 100 beats per minute. The heart rate normally speeds up in response to fever, exercise, or nervous excitement.
 
Pathological tachycardia goes along with lack of oxygen (anoxia), as caused by anemia, congestive heart failure, bleeding, or shock.

A slow heart beat (bradycardia) develops because the heart muscle gets too-little oxygen and cannot maintain the sped-up pace.