Tachycardia, also called tachyarrhythmia, is a heart rate that exceeds the normal resting rate. In general, a resting heart rate over 100 beats per minute is accepted as tachycardia in adults. Heart rates above the resting rate may be normal (such as with exercise) or abnormal (such as with electrical problems within the heart). Complications
Tachycardia can lead to fainting.When the rate of blood flow becomes too rapid, or fast  blood flow passes on damaged endothelium, it  increases the friction within vessels resulting in turbulence and other disturbances. According to the Virchows triad, this is one of the three conditions that can lead to thrombosis (i.e., blood clots within vessels). Causes
Some causes of tachycardia include:

Diagnosis
The upper threshold of a normal human resting heart rate is based on age. Cutoff values for tachycardia in different age groups are fairly well standardized; typical cutoffs are listed below:
1–2 days: Tachycardia >159 beats per minute (bpm)
3–6 days: Tachycardia >166 bpm
1–3 weeks: Tachycardia >182 bpm
1–2 months: Tachycardia >179 bpm
3–5 months: Tachycardia >186 bpm
6–11 months: Tachycardia >169 bpm
1–2 years: Tachycardia >151 bpm
3–4 years: Tachycardia >137 bpm
5–7 years: Tachycardia >133 bpm
8–11 years: Tachycardia >130 bpm
12–15 years: Tachycardia >119 bpm
>15 years – adult: Tachycardia >100 bpmHeart rate is considered in the context of the prevailing clinical picture. When the heart beats excessively or rapidly, the heart pumps less efficiently and provides less blood flow to the rest of the body, including the heart itself.
The increased heart rate also leads to increased work and oxygen demand by the heart, which can lead to rate related ischemia. Differential diagnosis
An electrocardiogram (ECG) is used to classify the type of tachycardia. They may be classified into narrow and wide complex based on the QRS complex. Equal or less than 0.1s for narrow complex. Presented order of most to least common, they are:
Narrow complex
Sinus tachycardia, which originates from the sino-atrial (SA) node, near the base of the superior vena cava
Atrial fibrillation
Atrial flutter
AV nodal reentrant tachycardia
Accessory pathway mediated tachycardia
Atrial tachycardia
Multifocal atrial tachycardia
Cardiac Tamponade
Junctional tachycardia (rare in adults)
Wide complex
Ventricular tachycardia, any tachycardia that originates in the ventricles
Any narrow complex tachycardia combined with a problem with the conduction system of the heart, often termed "supraventricular tachycardia with aberrancy"
A narrow complex tachycardia with an accessory conduction pathway, often termed "supraventricular tachycardia with pre-excitation" (e.g. Wolff–Parkinson–White syndrome)
Pacemaker-tracked or pacemaker-mediated tachycardiaTachycardias may be classified as either narrow complex tachycardias (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
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.