**1. Introduction**

Differential diagnosis and treatment of tachycardias is a common dilemma encountered by physicians or cardiologists. Although such tachycardias often occur in patients with a normal heart, they may cause bothersome symptoms and rarely represent life-threatening conditions. Among these tachycardias with a heart rate greater than 100 beats per minute (bpm), the narrow QRS complex tachycardias (NCTs) are defined by the presence in a 12-lead electrocardiogram (ECG) of a QRS complex duration less than 120 ms and the wide QRS complex tachycardias (WCTs) are defined by the presence in a 12-lead ECG of a QRS complex duration more than 120 ms (**Figure 1**) [1–10]. The NCTs are typically of supraventricular origin above or within the His bundle, although rarely narrow complex ventricular tachycardias (VT) have been reported in the literature in which early activation of the His bundle can also occur in high septal VT, resulting in relatively narrow QRS complexes of 110–140 ms (**Table 1**, [1–5]). The WCTs can be VT or supraventricular tachycardia (SVT) with right or left bundle branch block (BBB) or right or left accessory pathway (**Table 1**, [6–10]). Because administration of medications based on misdiagnosis of these tachycardias can be harmful and sometimes fatal, diagnosis

#### **Figure 1.**

*Differential diagnostic algorithm of NCTs and WCTs.*


*SVT; supra-ventricular tachycardia, AVNRT; atrio-ventricular nodal re-entrant tachycardia, AVRT; atrio-ventricular reciprocating tachycardia, AV; atrio-ventricular, VT; ventricular tachycardia, BBB; bundle branch block, AF; atrial fibrillation, VA; ventriculo-atrial.*

#### **Table 1.**

*Differential diagnosis of NCTs and WCTs.*

*Electrocardiographic Differential Diagnosis of Narrow QRS and Wide QRS Complex… DOI: http://dx.doi.org/10.5772/intechopen.102568*

of these tachycardias is critical [11–13]. The accurate, rapid diagnosis in patients with these tachycardias still remains a significant clinical dilemma, because the published numerous ECG algorithms and criteria are complicated and difficult to recall in urgent clinical situations [11–13]. We have reviewed ECG findings of the NCTs and WCTs in order to reduce the possible diagnostic errors on the ECGs.
