**2. NCTs**

The NCTs are common problems encountered in clinical situations [1–5, 14–21]. The key to approaching the diagnosis of these arrhythmias is identifying atrial activity (P waves) on the ECG and classifying these tachycardias according to the presence of AV dissociation (**Figure 2**) and then re-classifying according to long RP or short RP (**Table 2**) [1–5, 14–21]. On the basis of these algorithm, a differential diagnosis can

#### **Figure 2.**

*Differential diagnostic algorithm of NCTs with regular rhythm.*


#### **Table 2.**

*Differential diagnosis of NCTs according to RP interval.*

**Figure 3.** *Schematic demonstration of short RP and long RP.*

be generated, logical therapy can be delivered for termination of the tachycardia, and a plan can be developed to prevent recurrence.

Short RP tachycardias are defined as regular tachycardias in which interval from QRS complex to P wave (upper arrows, **Figure 3**) much less than interval from P wave to subsequent QRS complex, whereas long RP tachycardias are defined as regular tachycardias in which interval from QRS complex to P wave much more than interval from P wave to subsequent QRS complex (lower arrows, **Figure 3**) [1–5, 14–21].

### **2.1 NCTs with regular rhythm**

#### *2.1.1 Sinus tachycardia*

Sinus tachycardia is defined as an increase in sinus rate to more than 100 bpm with regular rhythm. The rate increases gradually and may show beat to beat variation. Although generally identifiable by a P wave of normal morphologic features that precedes each QRS complex, sinus tachycardia can be difficult to recognize when the P wave begins to fuse with the T wave of the preceding QRS complex. Sinus tachycardia is usually a physiological response such as fever, anxiety, pain, hyperthyroidism but may be precipitated by sympathomimetic drugs or endocrine disturbances [5, 14–21].

### *2.1.2 Sinus nodal reentrant tachycardia*

The morphologic appearance of sinus nodal reentrant tachycardia is identical to that of sinus tachycardia. In contrast to sinus tachycardia, the rate is very regular and initiation and termination are abrupt without an underlying physiological stimulus. Vagal maneuver may be successful in stopping the arrhythmia [5, 14–21].

### *2.1.3 Atrial tachycardia*

Atrial tachycardia (AT) is usually a NCTs accounting for 5–15% of SVT. Other than sinus tachycardia, AT is the most common long RP tachycardia. In AT, an atrial source

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

outside the sinoatrial node due to focal automatic activity or re-entry circuit activates the atria. Accordingly, P-wave morphologic characteristics vary depending on the site of this source. Digitalis toxicity should be suspected in patients with paroxysmal AT with AV block [5, 14–22].

#### *2.1.4 Atrial flutter*

Atrial flutter is a reentrant rhythm of the right atrium typically with an atrial rate of 250 to 350 beats/min. The flutter may circulate in a counterclockwise direction around the tricuspid annulus in the frontal plane (typical, counterclockwise flutter) or in a clockwise direction (atypical, clockwise flutter). P waves have a characteristic "sawtooth" appearance, and 2:1 AV block is common. Because one flutter wave occurs in the ST-T segment and another flutter wave occurs before each QRS complex in atrial flutter with 2:1 AV conduction, atrial flutter is neither a short RP nor a long RP tachycardia [5, 14–22].

#### *2.1.5 Junctional tachycardia*

Non-paroxysmal junctional tachycardia (NPJT) is a tachycardia that arises in the AV junction. Although often described as a short RP tachycardia, because NPJT causes ventricular activation almost concurrently with atrial activation, a substantial portion (25%), which is described as a long RP tachycardia, actually show P waves that slightly precede the QRS complex. and in some cases, AV dissociation may be present. Unlike AVNRT and AVRT, initiation and termination are gradual. NPJT is often associated with digitalis intoxication, inferior myocardial infarction, myocarditis, and mitral valve surgical procedures [5, 14–23].

#### *2.1.6 Paroxysmal SVT (AVNRT/Orthodromic AVRT)*

#### *2.1.6.1 AVNRT*

AV nodal reentrant tachycardia (AVNRT) is characterized by a tachycardia with supraventricular origin, with sudden onset and termination generally at rates between 150 and 250 beats/min and is the most common cause of SVT except atrial fibrillation, atrial flutter, and sinus tachycardia. In majority of patients (noted as the "typical" or "slow-fast" AVNRT), anterograde conduction to the ventricle occurs over the "slow" pathway and retrograde conduction to the atrium occurs over the "fast" pathway and the atria are activated either simultaneously with or just after activations of the ventricles and this common type is classified as a short RP tachycardia. Rarely, in "atypical" or "fast-slow" AVNRT, the reentry occurs in the opposite direction in which anterograde conduction occurs over the "fast" pathway, while retrograde conduction occurs over "slow" pathway, and this rare type is classified as a long RP tachycardia [24–31].

#### *2.1.6.2 AVRT*

AV reentrant tachycardia (AVRT) involves reentry between the atria and ventricles with use of the AV node-His bundle conduction as the anterograde and slow pathway and an accessory conduction as the retrograde and fast pathway. This pattern is also known as orthodromic reciprocating tachycardia (ORT). This type is not apparent by analysis of the ECG during sinus rhythm because the ventricle is not pre-excited and

#### *Clinical Use of Electrocardiogram*

the accessory pathway is said to be "concealed". In tachycardia, retrograde conduction over the accessory pathway is fast and yields a short RP tachycardia [24–31].

In contradistinction to ORT resulting in NCTs, antidromic AVRT has anterograde conduction over the accessory pathway and retrograde conduction over the AV node-His bundle resulting in WCTs [24–31].

The following factors are important differences between AVNRT and AVRT [24–31]:

In contradistinction to AVNRT, an 1:1 relationship is necessary for AVRT because both the atria and the ventricles are part of the reentry circuit. Therefore, if AV block occurs during tachycardia, AVRT is excluded.

If bundle branch block occurs during ORT and the length of the tachycardia cycle increases, AVNRT is excluded because the His-Purkinje system is not part of the tachycardia reentry circuit in AVNRT. The converse is not necessarily true because the absence of cycle length change with the occurrence of bundle branch block does not exclude AVRT.

#### *2.1.7 Permanent junctional reciprocating tachycardia (PJRT)*

As discussed with AVRT, certain types of reentrant circuits exist in which the accessory AV connection has AV nodal properties such as slow conduction. In PJRT, excitation over the postero-septal accessory pathway conducts very slowly, because of a long and tortuous route of pathway. Tachycardia is maintained by anterograde AV nodal conduction and retrograde conduction over slow accessory pathway. Because of slow conduction property of accessory pathway, retrograde atrial activation is delayed, and a long RP tachycardia results. Patients with this type of accessory pathway almost never have preexcitation (a delta wave) on ECGs during sinus rhythm [5, 14–23].

#### **2.2 NCTs with irregular rhythm**

#### *2.2.1 Atrial tachycardia with variable AV conduction*

Atrial tachycardia with atrioventricular block is typically seen with digoxin toxicity. The ventricular rhythm is usually regular but may be irregular if atrioventricular block is variable [5, 14–21].

#### *2.2.2 Multifocal atrial tachycardia (MAT)*

MAT is characterized by P waves with variable morphologies and variable PR intervals. Differential diagnosis between MAT and atrial fibrillation can be possible by the presence of isoelectric baselines between the P waves in MAT. MAT is seen typically in patients with chronic obstructive pulmonary disease or digoxin toxicity [5, 14–21, 32].

#### *2.2.3 Atrial flutter with variable AV conduction*

Atrial flutter is due to a re-entry circuit in the right atrium with secondary activation of the left atrium. This produces atrial contractions at a rate of about 300 beats/ min as flutter (F) waves. F waves show broad and saw-tooth appearances and are best seen in lead V1 and the inferior leads [5, 14–21].

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

### *2.2.4 Atrial fibrillation*

This is the most common sustained arrhythmia with overall prevalence is 1% to 1.5%. Atrial fibrillation is caused by multiple re-entrant circuits or "wavelets" of activation sweeping around the atrial myocardium without effective atrial contraction. Atrial fibrillation is seen on the ECG as irregular baseline undulations of variable amplitude and morphology (called f waves) discharging at a frequency of 350 to 600 beats/min.

With normal conduction, ventricular rate shows frequency between 100 and 150 beats/min. Atrial fibrillation with slow ventricular responses or AV block is seen typically in patients with digoxin toxicity [5, 14–21, 33–36].
