Hemodynamic responses:

The body has 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). Fever, hyperventilation and severe infections can also cause tachycardia, primarily due to increase in metabolic demands.

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. This is the basis for the so-called "Fight or Flight" response, but such stimulation can also be induced by stimulants such as ephedrine, marijuana, amphetamines or cocaine. Certain endocrine disorders such as pheochromocytoma can also cause epinephrine release and can result in tachycardia independent nervous system stimulation. Hyperthyroidism can also cause tachycardia.

Cardiac Arrhythmias:

The Cardiac Arrhythmias lead ECG can help distinguish between the various types of tachycardias, generally distinguished by their site of pacemaker origin:

Sinus tachycardia, which originates from the Sino-atrial (SA) node, near the base of the superior vena cava Ventricular tachycardia, any tachycardia which originates in the ventricles. Supraventricular tachycardia (SVT), which is a tachycardia paced from the Atria or the AV node.

SVT rhythms include:

Atrial fibrillation AVNRT or AV nodal reentrant tachycardia AVRT or AV reentrant tachycardia Junctional tachycardia Classify tachycardias as either narrow complex tachycardias (often referred to as supraventricular tachycardias) or wide complex tachycardias. "Narrow" and "wide" refer to the width of the QRS complex on the ECG. Narrow complex tachycardias tend to originate in the atria, while wide complex tachycardias tend to originate in the ventricles. Tachycardias can be further classified as either regular or irregular.

Sinus tachycardia:

The most common type of tachycardia is sinus tachycardia, which is the body's normal reaction to stress, including fever, dehydration, or blood loss (shock). It is a technical narrow complex tachycardia. In the absence of heart disease, it tends to have a narrow QRS complex on the ECG. Treatment is generally directed at identifying the underlying cause.

Ventricular tachycardia:

Ventricular tachycardiaVentricular tachycardia (VT or V-tach) is a potentially life-threatening cardiac arrhythmia that originates in the ventricles. It is usually a regular, wide complex tachycardia with a rate between 120 and 250 beats per minute. Ventricular tachycardia has the potential of degrading to the more serious ventricular fibrillation. Ventricular tachycardia is a common, and often lethal, complication of a myocardial infarction (heart attack).

Exercise-induced ventricular tachycardia is a phenomenon related to sudden deaths, especially in patients with severe heart disease (ischemia, acquired valvular heart and congenital heart disease) accompanied with left ventricular dysfunction. A 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.