**1. Introduction**

Supraventricular tachycardia (SVT) is a type of arrhythmia that originates from areas above the bundles of His, i.e., the sinus node, atria, and atrioventricular node (AVN), manifesting with heart rate > 100 beats/min. In terms of duration, it can be sustained, non-sustained (<30 seconds), paroxysmal, and persistent. Tachycardias are usually classified according to anatomical structure like atrial, AV nodal, and due to accessory pathway. Nonetheless, since an electrocardiogram (ECG) is used for diagnosis, it is better to approach the diagnosis and classify SVTs according to the characteristics of the ECG. Generally, SVTs fall into two categories; narrow QRS complexes and wide QRS complexes. This chapter will focus on ECG approach to narrow QRS complex SVTs, **Figure 1**.

**Figure 1.** *Algorithm to define the supraventricular tachycardia.*
