Tachycardia overview

# Overview
Tachycardia is a form of cardiac arrhythmia which refers to a rapid beating of the heart. By convention the term refers to heart rates greater than 100 beats per minute in the adult patient. Tachycardia may be a perfectly normal physiological response to stress. However, depending on the mechanism of the tachycardia and the health status of the patient, tachycardia may be harmful, and require medical treatment. In extreme cases, tachycardia can be life threatening.
Tachycardia can be harmful in three ways. Firstly, if the heart is pumping too fast for an extended period of time it will change the balance of oxygen and carbon dioxide in the hemoglobin in the blood; this is normal during exercise but when resting this is quite dangerous. Second, when the heart beats too rapidly, it may pump blood less efficiently. Thirdly, the faster the heart beats, the more oxygen and nutrients the heart requires.  This may leave patients feeling out of breath or cause angina. This can be especially problematic for patients suffering from ischemic heart disease.

# Classification
Tachycardias can be classified in a variety of ways. One form of classification is whether it is a wide-complex tachycardia, or whether it is a narrow-complex tachycardia. Another way of classifying tachycardia is whether the rhythm is regular or whether it is irregular. Tachycardia can also be distinguished by type; whether is is a sinus tachycardia, a ventricular tachycardia, a supraventricular tachycardia, or atrial fibrillation.

# Pathophysiology
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 pathophysiology of abnormal tachycardias vary with the type of tachycardia.

# Causes
Tachycardia can be caused by a wide variety of factors. The most common cause is orthostatic hypotension. Other common causes of tachycardia include endocrine disorders, disorders involving the heart, and situations which cause an excess of catecholamines in the body such as fever and exercise. Various medications and illicit drugs can also cause tachycardia.

# Diagnosis

## Electrocardiogram
Tachycardia is defined as heart rate more than 100 beats per minute and it can be sinus tachycardia, ventricular tachycardia or supraventricular tachycardia. Ventricular tachycardia can be distinguished by broad QRS complexes on an ECG.

# Treatment

## Medical Therapy
Treatment of tachycardia is usually directed at chemical conversion (with antiarrythmics), electrical conversion (giving external shocks to convert the heart to a normal rhythm) or use of drugs to simply control heart rate (for example as in atrial fibrillation). The treatment modality used depends on the type of tachycardia and the hemodynamic stability of the patient.

## Surgery
Surgical methods of treatment for tachycardias include device-based treatment such as the implantation of a permanent pacemaker. There are guidelines on the indications for surgical therapy, depending on the type and severity of the tachycardia.