The normal resting adult heart beats regularly at an average rate of 60-100 times per minute. How fast the heart beats (heart rate) is governed by the speed of electrical signals originating from the pacemaker of the heart, the SA node. The electrical signals from the SA node travel across the atria and cause these two upper heart chambers to contract, delivering blood into the lower heart chambers, the ventricles. These electrical signals then pass through the AV node to reach the ventricles. Electrical signals reaching the ventricles cause these chambers to contract, pumping blood to the rest of the body, generating the pulse. This regular flow of electricity from SA node to AV node causing a regular contraction of the heart muscle is known as a "sinus" beat. During rest, the speed of electrical signals originating from the SA node is slow, thus the heart beats slowly. During exercise or excitement, the speed of signals from the SA node increases, and heartbeat quickens.
Tachycardia occurring because of rapid firing by the SA node is called sinus tachycardia. Sinus tachycardia is usually a rapid contraction of a normal heart responding to a condition or disease state. Sinus tachycardias can cause palpitations. Causes of sinus tachycardia include pain, fever, excessive thyroid hormone, exertion, excitement, low blood oxygen level (hypoxia), caffeine, and drugs such as cocaine and amphetamines. Under these circumstances, sinus tachycardia represent "appropriate" responses of the heart to stress and stimulation, and do not reflect underlying diseases of the heart muscle, heart valves and electrical conduction system. In some other patients, however, sinus tachycardia may be a symptom of heart failure or significant heart valve disease.
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