An elevated heart rate can come on suddenly. In some cases, the cause of faster than normal heart rate, known as tachycardia, is never determined. There are a variety of conditions that can cause the heart rate to increase above normal. Rapid heart rate can cause decreased blood flow to the brain and major organs, leading to symptoms of weakness, shortness of breath, and dizziness, or no symptoms at all, depending on the severity and general state of health.
Emotions
Emotions that induce fear, anxiety and anger cause adrenalin to be released in the body. Adrenalin is a hormone that stimulates the sympathetic nervous system that controls heart rate and blood pressure. It should be noted that extreme emotions can also slow the heart rate. When adrenalin is released from fear the heart rate becomes elevated as the body's natural response, known as the "fight or flight" response. Over time, extreme emotions can take a toll on the heart and increase the risk of heart disease and heart attack.
Low Oxygen Levels
Low oxygen levels in the bloodstream can cause an elevated heart rate, such as what might happen during an asthma attack, or in patients with chronic obstructive pulmonary disease (COPD). Other causes of low oxygen levels include severe bleeding, anemia that lowers the oxygen carrying capacity in the blood cells, trauma to the lungs or airways, and slow breathing from brain injury or medications that depress the central nervous system. Pneumonia, viruses or fungal infections that attack the lungs can also impair oxygen exchange and lead to tachycardia.
Fever
Heart rate increases when fever is present. Temperature in the body becomes higher than normal in response to bacteria or viruses, and in the presence of brain injury. Dehydration and low blood pressure elevate the heart rate even further. Treating fever with aspirin, ibuprofen or acetaminophen will bring the heart rate back to normal in the absence of other complications.
Heart Disease
Sometimes the heart rate inappropriately becomes elevated. Pathways in the heart that transmit signals can behave in an aberrant fashion, leading to various types of fast heart rhythms that occur in the top or lower part of the heart. An elevated heart rate can occur when the coronary arteries that supply blood flow to the heart are blocked. Heart enlargement, structural heart abnormalities at birth, and heart valve disorders can also produce a faster than normal heart rate.
Hyperthyroidism
A symptom of overactive thyroid (hyperthyroidism) includes a fast heart rate from disturbance in metabolism. Other symptoms of hyperthyroidism include an irregular heart rate that may feel like a fluttering in the chest, and weight loss.
Stimulants
Stimulants that include caffeine, amphetamines, cocaine, alcohol, cough and cold medicines with pseudoephedrine, and drugs used to treat ADHD such as Ritalin, can lead to an elevated heart rate. Stimulating drugs affect the central nervous system that controls heart rate. Other symptoms include anxiety, dilated pupils, restlessness and dry mouth.
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