Normal cardiac rhythm results from electrical impulses that control the heart muscle. Any change fro the nomral sequence of impulses is termed arrhythmia. Some arrhythmias are so brief (for example, a temporary pause or premature beat) that the overall heart rate isn't affected greatly. If arrhytmias last for some time, though, they may cause the heart rate to be too slow or too fast. A rate of less than 60 beats per minute is referred to as bradycardia. Tachycardia usually describes a rate of more than 100 beats per minute.
Arrhythmia;s may develop because of abnormalities in how impulses are conducted. Delays in the spreading of impulses can occur anywhere in the conduction system. When the transmission of impulses is blocked intermittently or completely, bradycardia may result.
In another type of abnormal conduction, impulses get caught in a merry-go-round-like sequence. This process, called reentry, is a common cause of tachycardias.
Arrhythmias can produce a broad range of symptoms, from barely perceptible to cardiovascular collapse and death. When they are very brief, arrhythmias are most likely to be almost without symptoms. For instance, a single premature beat may be perceived as a "palpitation" or "skipped beat." Premature beats that are frequent or occur in rapid succession during tachycardia may cause a greater awareness of heart palpitations or a fluttering sensation in the chest or neck.
When arrhythmias last long enough to affect how well the heart works, more serious symptoms may develop. At slower rates, the heart may not be able to pump enough blood to the body. This can cause fatigue, light-headedness, loss of consciousness, or even death. Death occur if the heart rate is zero or so slow that the heart and brain stop working.
Tachycardias can reduce the heart's ability to pump by interfering with the ventricular chambers' ability to fill with blood properly. They do this by reducing the time for such filling or by interfering with the booster effect normally provided by timely contraction of the atria (or both).
Loss of this atrial "kick" during tachycardia may be caused by a change from the usual sequence of atrial and ventricular activity as well as rapid chaotic electrical activity in the upper chambers. The reduced pumping efficiency that can develop during tachycardia may be made worse by underlying heart muscle abnormalities or atherosclerotic blocks in the coronary arteries.
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