Tachycardia describes a fast heart rate, generally defined as over 100 beats per minute.
There are several causes of tachycardia, from heart defects to abuse of stimulant drugs (including caffeine). It is normal in a person who is exercising, where a sustained heart rate of 170 beats per minutes is not unusual. It should not occur in a person at rest unless they have an underlying condition. However, even severe anxiety can cause a mild tachycardia.
Tachycardia is usually not a severe symptom unless it is combined with other symptoms. For example, tachycardia combined with an arythmia usually indicates a very serious condition. However, all cases of tachycardia at rest generally indicate a medical emergency, and immediate care should be sought.
If the heart rate continues to increase, the patient can suffer from other symptoms. In most cases, the blood gases become unusual as oxygen saturates the blood and appears in veins as it cannot be used by the body's cells. If the heart rate increases further, fibrilation is a possibility.