Normal activation of the ventricles occurs after an impulse, generated in the sinoatrial (SA) node, is conducted through the atrioventricular (AV) node, and travels through the specialized conduction tissues of the His–Purkinje system. This conducted impulse leads to a rapid depolarization of the ventricles. This normal activation pattern results in the surface electrocardiogram (ECG) showing normal (narrow) QRS complex morphology with duration of less than 0.12 s (three small boxes on standard ECG paper recorded at 25 mm/s). A rhythm is considered narrow complex tachycardia (NCT) if the following two features are encountered: a ventricular rate above 100 bpm and a QRS complex width less than 0.12 s. While these criteria apply to adult patients, children have age‐related norms for both rate and QRS duration that are used to make the diagnosis of NCT.

NCTs are further classified as either regular or irregular. Regular NCTs (those having a consistent R–R interval) include sinus tachycardia (ST), atrial tachycardia, AV junctional tachycardia, atrial flutter with fixed AV conduction, and paroxysmal supraventricular tachycardia (PSVT). Rhythms that have consistently irregular R–R intervals throughout are considered irregular NCTs and include atrial fibrillation, atrial flutter with variable AV conduction, and multifocal atrial tachycardia (MAT). Rhythms that have generally consistent, regular R–R intervals but also have occasional irregular beats are likely to be one of the regular rhythms listed above with occasional premature or ectopic beats. Figure 6.1 demonstrates the use of the R–R interval to determine regularity.

Figure 6.2 depicts a decision pathway based on the presence or absence of regular R–R intervals and the presence of ectopic beats. In most cases, a diligent search for the P wave and general knowledge of the cardiac conduction system will lead to appropriate identification of the rhythm. One caveat to the recognition of NCTs is that the presence of a bundle branch block can lead any of the rhythms discussed here to appear as wide complex tachycardia.

Regular Narrow Complex Tachycardia

ST is the most commonly encountered NCT. ST originates from the SA node at a rate greater than 100 bpm (Figure 6.3). Although ST may often be extremely fast in young children, it is rarely seen at rates greater than 200 bpm in older children and adults. Electrical impulses in ST are conducted through the normal cardiac conduction system in the usual manner. The ECG should always include a normal P wave associated with every QRS complex. The QRS complexes are narrow, and the intervals are all normal. Simply put, ST is a normal sinus rhythm at a rapid rate. Importantly, ST should be considered a reactive rhythm, resulting from an abnormal physiologic event; refer Box 6.1 for further information regarding ST.

Atrial tachycardia is a term used to describe an arrhythmia that originates in the atrial myocardium but outside of the SA node with rates greater than 100 bpm. These rhythms may be paroxysmal or sustained, and they may arise from any one of many etiologies including re‐entry pathways and adverse effects related to medications. Atrial tachycardias may be further classified as unifocal or multifocal depending on the number of foci within the atrial myocardium that give rise to conducted beats. MAT can produce an irregular rhythm and is discussed with the irregular NCTs later in this chapter.

Unifocal atrial tachycardia is characterized by atrial depolarizations that are generated by the same atrial ectopic focus for at least three consecutive beats and occur at an atrial rate of 100–240 bpm. Atrial tachycardia may occur intermittently with sudden initiation and termination, and it may be difficult to identify the arrhythmia without a long rhythm strip. Depending on the location of the ectopic atrial focus, the surface ECG or rhythm strip may show P waves that are upright or inverted. These P waves may precede the QRS complex, be buried in the QRS complex (either not visualized, or causing a change in the QRS morphology), or appear immediately after the QRS complex. Symptoms associated with the arrhythmia are also often sudden and intermittent.

In atrial tachycardia, the AV conduction ratio is often 1 : 1. At rates less than 160 bpm, it may be difficult to distinguish atrial tachycardia from ST. However, if a conduction delay exists between the atria and ventricles, the atrial rate will be faster than the ventricular rate. The ratio of atrial contractions to ventricular contractions may be consistent (Figure 6.4) or it may vary (Figure 6.5). Unifocal atrial tachycardia in the presence of AV conduction delay can be difficult to differentiate from atrial flutter. However, unifocal atrial tachycardia should have clear P waves with a distinct isoelectric baseline between P waves rather than the characteristic saw‐tooth waves of atrial flutter.

Supraventricular tachycardia (SVT) is a term that refers to a variety of NCTs with electrical origins proximal to the ventricle including impulses that originate in the SA node, atrial tissue, the AV node, or the His bundle. While the term PSVT includes any of the regular NCTs that occur intermittently as described above, it is commonly and most correctly used to describe atrioventricular nodal re‐entrant tachycardia (AVNRT) and atrioventricular re‐entrant tachycardia (AVRT). AVNRT and AVRT are the most frequently encountered types of PSVT.

Both AVNRT and AVRT are rhythms that rely on conduction pathways in and around the AV node that may transmit electrical impulses, not only anterograde (from the atria to the ventricles) but also retrograde (from the ventricles back up toward the atria). These multiple pathways may form a “loop” or re‐entry circuit in which electrical activity proceeds in a circular manner, leading to rapid depolarization of the atria and the ventricles. These impulses bypass the usual conduction pathway and do not have typical refractory periods. In order for these rhythms to be maintained, a functioning AV node must be present as part of the re‐entry circuit. Therefore, these rhythms are referred to as AV nodal dependent. AVNRT occurs with a “microscopic” loop, with both the anterograde and retrograde pathways lying within the AV node, while AVRT demonstrates a “macroscopic” loop with one electrical pathway lying within the AV node and the other in the adjacent tissues; AVRT is most often seen in patients with ventricular preexcitation syndromes such as the Wolff–Parkinson–White (WPW) syndrome; WPW is discussed in Chapter 21.