**2. Regular rhythm**

## **2.1 Sinus tachycardia**

### *2.1.1 Narrow QRS complex, regular rhythm with long RP interval*

In general, sinus tachycardia occurs as an appropriate response of the body to stress (illness, exercise, pulmonary embolism, hypovolemia, pain) or as a pathological abnormality of the SA node (sinus node re-entry tachycardia) [1]. Typically in this type of arrhythmia, there is an increased rate of firing of the SA node. This is demonstrated by an impulse that starts from the SA node and moves to the right atrium then to the left atrium and finally reaching the AV node, **Figure 6**.

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Regular RR intervals (RR intervals are equal) → Long RP interval (RP interval > half of RR interval) → Upright P wave in leads I, II and aVL (showing sinus origin), and a negative P wave in lead aVR.

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

#### **Figure 6.**

*The impulse is generated in the sinus node (yellow drop sign), followed by a normal conduction pathway to the atria (yellow arrow) and ultimately to the AVN.*

#### **Figure 7.**

*An impulse is generated in the atria but not in the sinus node (the orange drop signs), which travels to the AVN.*

### **2.2 Atrial tachycardia (focal atrial tachycardia)**

#### *2.2.1 Narrow QRS complex, regular rhythm with long RP interval*

The term atrial tachycardia, also known as atrial ectopic tachycardia, refers to an arrhythmia that originates from an atrial site other than the SA node and it is

commonly paroxysmal in nature. Focal atrial tachycardia (Focal AT) is relatively uncommon, accounting for between 5% – 15% of arrhythmias in adults [2]. The most common sites of activation are the crista terminalis, tricuspid annulus, and pulmonary vein. Atrial myocytes (any focal site) are activated (either by an automatic, triggered, or micro-reentrant event) and spread centrifugally to reach the AV node, **Figure 7** [3].

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Regular RR intervals (RR intervals are equal) → Long RP interval (RP interval > half of RR interval) → if origin is near SA node, there will be an upright P wave in leads I, II and aVL and a negative P wave in lead aVR.

It is important to compare the P wave morphology with the previous ECG, particularly in leads V1 and II, to be able to differentiate the etiology. Secondly, even though focal ATs are regular, the heart rate may increase in the first few beats of the tachycardia and gradually decelerate in the last few beats (warming up phenomenon). Notably, if there is an abrupt onset or termination of the AT (e.g., over a period of three to four beats), it is more suggestive of focal atrial tachycardia [4].

Moreover, it can be diagnosed with the greatest degree of accuracy if two or more of the following findings are present: (a) RP/PR ratio ≥ 1.65, (b) no P waves in inferior leads, and (c) P wave duration >96 ms [5].

Using the following approach, it is possible to determine the location of the focus of AT as well [6].


#### **2.3 Atrioventricular reentrant tachycardia**

#### *2.3.1 Narrow QRS complex, regular rhythm with long RP interval*

Atrioventricular reentrant tachycardia (AVRT) is an anatomically defined reentrant tachycardia characterized by the presence of the normal AV conduction system and the accessory AV pathway. Atrial premature beats initiating an orthodromic AVRT are blocked in the accessory pathway but conduct antegrade to the ventricles over the AV node/His-Purkinje system. As the impulse is conducted through the ventricles, it then travels back via the AV accessory pathway into the atria in a retrograde fashion. This completes the reentrant loop, **Figure 8** [7]. This reentrant loop leads to tachycardia.

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Regular RR intervals (RR intervals are equal) → Long RP interval (RP interval > half of RR interval).

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

**Figure 9.** *AV node (yellow drop sign) generates impulses at a higher rate than sinus node.*
