Tachycardia in Wikipedia
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(Source - Retrieved 2006-09-07 14:20:03 from http://en.wikipedia.org/wiki/Tachycardia)
Tachycardia is an abnormally rapid beating of the heart, defined as a resting heart rate of 100 or more beats per minute in an average adult.
It can have harmful effects, in two ways. First, when the heart beats too rapidly, it performs inefficiently (since there is not enough time for the ventricles to fill completely), causing cardiac output to diminish. Second, it increases the work of the heart, causing it to require more oxygen while also reducing the blood flow to the cardiac muscle tissue, increasing the risk of ischemia and resultant infarction.
Tachycardia is a general symptomatic term that does not describe the cause of the rapid rate. Common causes are autonomic nervous system or endocrine system activity, hemodynamic responses, and various forms of cardiac arrhythmia.
Autonomic and endocrine causes
An increase in sympathetic nervous system stimulation causes the heart rate to increase, both by the direct action of sympathetic nerve fibers on the heart, and by causing the endocrine system to release hormones such as epinephrine (adrenaline) which have a similar effect. Increased sympathetic stimulation is usually due to physical or psychological stress (the so-called "fight or flight" response), but can also be induced by stimulants such as amphetamines.
Endocrine disorders such as pheochromocytoma can cause epinephrine release and tachycardia independent of the nervous system. Hyperthyroidism is also known to cause tachycardia.
The body contains several feedback mechanisms to maintain adequate blood flow and blood pressure. If blood pressure decreases, the heart beats faster in an attempt to raise it. This is called reflex tachycardia.
This can happen in response to a decrease in blood volume (through dehydration or bleeding), or an unexpected change in blood flow. The most common cause of the latter is orthostatic hypotension (also called postural hypotension), a sudden drop of blood pressure that occurs with a change in body position (e.g., going from lying down to standing up). When tachycardia occurs for this reason, it is called postural orthostatic tachycardia syndrome (POTS).
Fever and infection leading to sepsis are also common causes of tachycardia, primarily due to increase in metabolic demands and compensatory increase in heart rate.
An electrocardiogram tracing can distinguish several different forms of rapid abnormal heartbeat:
If the heart's electrical system is functioning normally, except that the rate is in excess of 100 beats per minute, it is called sinus tachycardia. This is caused by any of the factors mentioned above, rather than a malfunction of the heart itself.
Supraventricular tachycardia (SVT) occurs when an abnormal electrical impulse originates above the ventricles, but instead of causing a single beat and a pause, it travels in circles and causes many rapid beats.
To distinguish SVT from Sinus Tachycardia one must simply look at the rate: If the rate of contraction is more than 150 bpm, then it is considered SVT. Otherwise it is Sinus Tachycardia.
Ventricular tachycardia (VT or V-tach) is a similar phenomenon occurring within the tissue of the ventricles, causing an extremely rapid rate with poor pumping action.
Abnormal accelerated ventricular rhythm with a usual rate of 150-200 beats/minute. Because ventricular tachycardia originates in the ventricle, it appears as a wide complex rhythm on EKG. A potentially unstable rhythm that may result in fainting, low blood pressure, shock, or sudden death. Ventricular tachycardia has the potential of degrading to the more serious ventricular fibrillation. Ventricular tachycardia is a common, and often lethal, complication of acute myocardial infarction (heart attack).
Exercise-induced ventricular tachycardia
Exercise-induced ventricular tachycardia is a phenomenon related to sudden deaths, especially in patients with severe heart disease (ischaemia, acquired valvular heart and congenital heart disease) accompanied with left ventricular dysfunction. A famous case of a death from exercise-induced VT was the death on a basketball court of Hank Gathers, the Loyola Marymount basketball star, in March 1990.
Both of these rhythms normally last for only a few seconds to minutes(paroxysmal tachycardia), but if VT persists it is extremely dangerous, often leading to ventricular fibrillation.
The vagus reflex may help as a first-aid measure.
Arrhythmias can be treated using drugs, ablation, intervention, or implantable devices.
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