TACHYCARDIA

in physiology and medicine, a heart rate of more than 100 beats per minute. Tachycardia occurs normally during and after exercise or during stress and represents no danger to healthy individuals. In some cases, however, tachycardia occurs without apparent cause and is an arrhythmia, i.e., a pathological deviation from the normal heartbeat rhythm. Most arrhythmias are caused by irregularities in the electrical stimuli that cause the heart to beat. Normally these pacemaking stimuli originate in the sinoatrial node. When the point of origin of pacemaking shifts to some other site in the heart, arrhythmias result, and arrhythmias in which the heart beats at a rapid rate are termed tachycardias. Tachycardias can be atrial, ventricular, or nodal, depending on the faulty site of origin of the pacemaking stimulus. The chief symptoms of all tachycardias are fatigue, faintness, dizziness, shortness of breath, and a sensation of thumping or palpitation in the chest. The heart can beat as many as 240 times per minute in tachycardia. Some tachycardias, such as paroxysmal tachycardia, are harmless and transitory phenomena that last a few minutes or hours, subside spontaneously, and cause no lasting ill effects. Other types, such as multifocal atrial tachycardia or ventricular tachycardia, usually occur in persons with serious heart, lung, or circulatory disease and demand immediate medical attention. The more serious tachycardias may be precursors of atrial fibrillation or even a heart attack. Tachycardias can be terminated by lying down and other basic physical measures, by administering an electrical shock to the heart to restore regular heart rhythm, or by the administration of such antiarrhythmic drugs as quinidine, atropine, lidocaine, or procainamide.