Tachycardia vs Bradycardia vs Heart Attack Tachycardia. When our heart pumps very fast without any physical activity and causes shortness of breath and trouble breathing. Bradycardia. When our heartbeat goes lower than normal and causes tiring and slow breathing. Heart Attack. When the heart ceases its activity. Causes may include the following: Disease, Cholesterol, Blockage, Lack Of Blood or Oxygen, Reaction and Shock.