The threshold of a normal heart rate (pulse) is generally based on the person's age. Tachycardia can be dangerous; depending on how hard the heart has to work. In general, the adult resting heart beats between 60 and 100 times per minute (some doctors place the healthy limit at 90, so some of them may diagnose tachycardia at slightly lower than 100 beats per minute). When an individual has tachycardia the upper or lower chambers of the heart beat significantly faster - sometimes this happens to both chambers.

When the heart beats too rapidly, it pumps less efficiently and blood flow to the rest of the body, including the heart itself is reduced. The higher-than-normal heartbeat means there is an increase in demand for oxygen by the myocardium (heart muscle) - if this persists it can lead to myocardial infarction (heart attack), caused by the dying off of oxygen-starved myocardial cells. Some patients with tachycardia may have no symptoms or complications. Tachycardia significantly increases the risk of stroke, sudden cardiac arrest or death. Our heart rates are controlled by electrical signals which are sent across heart tissues. When the heart produces rapid electrical signals tachycardia occurs.
Causes of tachycardia

Tachycardia is generally caused by a disruption in the normal electrical impulses that control our heart's pumping action rhythm - the rate at which our heart pumps. The following situations, conditions and illnesses are possible causes:
-- A reaction to certain medications
-- Congenital (present at birth) electrical pathway abnormalities in the heart
-- Congenital abnormalities of the heart
-- Consuming too much alcohol
-- Consumption of cocaine and some other recreational drugs
-- Electrolyte imbalance
-- Heart disease which has resulted in poor blood supply and damage to heart tissues, including coronary artery disease (Atherosclerosis), heart valve disease, heart failure, heart muscle disease (cardiomyopathy), tumors, or infections.
-- Hypertension
-- Hyperthyroidism (overactive thyroid gland)
-- Smoking
-- Certain lung diseases