A normal heart rate in an adult is between 60 and 100 beats per minute. An elevated heart rate is called tachycardia. Elevated heart rates occur in healthy individuals under certain circumstances. They can also indicate an underlying medical illness that might range from mild to life threatening.
As noted in Arthur C. Guyton's "Textbook of Medical Physiology," tachycardia occurs in generally healthy people with healthy hearts when the body needs an increased supply of blood and oxygen. As the heart beats more rapidly, the blood flow increases and more oxygen is delivered to the tissues. Causes include exercise and dehydration. Too much sun exposure, too little fluid intake and vomiting will all cause tachycardia. Other common causes are fever and anxiety.
The heart rate can be increased but normal in pattern. For example, after a massive heart attack, the weakened heart muscle beats more rapidly. Less blood is delivered each beat, but there are more beats per minute as the heart tries to deliver enough blood to the body.
The heart beat can have an abnormal rhythm in addition to being rapid. According to Malcolm S. Thaler, author of "The Only EKG Book You'll Ever Need," paroxysmal supraventricular tachycardia, or PSVT, is the most common type of this sort of problem. PSVT consists of sudden episodes of rapid heart beat. It is usually benign and sometimes can resolve on its own. Ventricular tachycardia, on the other hand, which starts in the main chamber of the heart, occurs in people with heart disease and can be fatal.
Circumstances of low oxygen in the environment, such as in cases of high altitude, or of low oxygen in the blood, due to conditions like lung disease or severe anemia, can cause the heart to beat more rapidly in an attempt to drive more oxygen to the body. Another situation might be pulmonary embolism, a life threatening condition where a blood clot from the leg travels to the lung and blocks a large portion of it. Pulmonary embolism can occur after a long flight when a traveler is sitting stationary, not moving his legs, for hours on end. A primary symptom of this condition is tachycardia.
Elevated thyroid hormone, or hyperthyroidism, occurs in several types of thyroid disease. One of the symptoms, according to "Harrison's Principles of Internal Medicine," is tachycardia. Hyperthyroidism is diagnosed initially by a simple blood test and later by more complicated scans performed by an endocrinologist.
Diagnosis and Treatment
Diagnosis begins with a thorough history and physical exam. An electrocardiogram, or EKG, will give a picture of the heart rate and rhythm at the time of the exam. A Holter is an electrocardiogram that the patient wears for 24 hours during all his normal activities. It can reveal episodes of intermittent tachycardia that might be missed on the EKG. Other tests include a complete blood count, measurement of oxygen in the blood and a thyroid hormone level. Treatment usually addresses the underlying problem, such as dehydration or a blood clot. Occasionally the heart rate itself needs to be corrected.