**3.1 Atrial fibrillation**

Atrial fibrillation is one of the most common supraventricular tachycardias it is characterized by chaotic rapid atrial electrical activity with and without variable ventricular rate, **Figure 12**. Its pathophysiology usually varies depending upon the type of atrial fibrillation. Ectopic foci are generally the cause of paroxysmal atrial fibrillation, whereas sustained atrial fibrillation is caused by arrhythmogenic substrates formed by fibrosis of myocytes and arrhythmogenic stimulation by triggers [11].

#### **Figure 11.**

*A reentrant circuit forms through the normal fast atrioventricular nodal conduction pathway (yellow drop sign) and the slow perinodal accessory pathway (blue arrow).*

**Figure 12.** *Atrial tissue generates multiple impulses in a chaotic manner (yellow drop signs).*

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Irregular RR intervals (RR intervals are not equal). There is no distinct presence of P waves, however chaotic fibrillation waves could be seen.

#### **3.2 Multifocal atrial tachycardia**

Multifocal atrial tachycardia (MAT) is an uncommon SVT characterized by an irregular rhythm and discrete P waves with varying morphologies, which are often observed in patients with pulmonary disease during acute exacerbations of the disease. It is the most common arrhythmia to be confused with atrial fibrillation due to varying morphology of the P waves. An increased intracellular calcium store is thought to trigger the arrhythmia from different atrial locations, which may occur as a consequence of hypokalemia, hypoxia, acidemia, and increased catecholamines, **Figure 13** [12].

Approach to ECG findings: Heart rate > 100 → Narrow QRS complex (QRS < 120 ms) → Irregular RR intervals (RR intervals are not equal) → discrete P waves with at least three different morphologies (including the sinus P wave).

#### **3.3 Atrial flutter with or without variable block**

An atrial flutter, also known as macro-reentrant atrial tachycardia, is caused by macro reentrant circuits within the annulus of the tricuspid valve with impulse conduction along the Cavo-tricuspid isthmus, **Figure 14**. It is possible for this macro reentrant circuit to be found in other areas of the atria. Therefore, it can be a typical atrial flutter (through the Cavo-tricuspid isthmus) or an atypical atrial flutter (in another part of the atria). Usually, this circuit develops as a result of fibrosis of atrial myocytes. There is an organized atrial activity, which is commonly observed as sawtooth flutter waves with a rate exceeding 200 beats per minute. Atrial flutter may manifest as an irregular SVT if there is a variable block, however it may also manifest as a regular SVT if there is no variable block.

*ECG Approach to Narrow QRS Complex Supraventricular Tachycardia DOI: http://dx.doi.org/10.5772/intechopen.112151*

**Figure 13.** *The atrial tissue (drop signs) generates impulses in different places.*

**Figure 14.** *A macro-reentrant circuit develops along the Cavo-tricuspid isthmus (yellow arrows).*

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Irregular RR intervals (RR intervals are not equal) → QRS complex is preceded by multiple P waves (two to three sawtooth-like waves) within a single lead with the same morphology. When the circuit is counterclockwise, it produces negative sawtooth flutter waves in leads II, III, and aVF. When the direction of the circuit is reversed, a positive P wave is produced in leads II, III, and aVF [13].
