The 12-lead ECG is crucial and normally helps in identifying the type of SVT. Things to be checked are heart rate, P waves (morphology, axis, the relationship between P waves and the QRS complex, PR and RP intervals), QRS complex, starting and finishing pattern of tachycardia, and the response to vagal maneuvers and to drugs (Figs. 11.2 and 11.3) [1]. If the baseline ECG with normal sinus rhythm presents a ventricular pre-excitation (short PR interval, delta wave, wide QRS, ST-T changes), the diagnosis of an atrioventricular reentry tachycardia is obvious, while the presence of a long PR interval should suggest a nodal reentry (dual AV nodal pathways and baseline conduction along the slow pathway), and in the case of Mahaim fibers, a normal PR interval, a pseudo-pre-excitation, and a pattern rS in DIII may be present [4].

When a newborn baby arrives in the emergency room in poor clinical conditions and the 12-lead ECG shows sinus rhythm with large P waves, keep in mind the possibility of a just ceased SVT or VT with AV dissociation. STVs are normally characterized by a narrow QRS complex; however, a wide QRS complex could be present in SVT conducted with aberrancy, Mahaim tachycardia, or antidromic AVRT or VT (AV dissociation, fusion beats, capture beats, concordance throughout the chest leads) (Figs. 11.4 and 11.5). Mahaim fibers connect the anterior wall of the right atria to the right bundle branch, to the fascicles, or to the right distal ventricle and possess decremental conduction capacity; they lead to antidromic arrhythmia with an LBBB morphology. Mahaim arrhythmia is very rare, and Mahaim fibers are frequently a bystander of other arrhythmic mechanisms [1].

Recognizing P waves is not easy at a fast heart rate, mostly because they can be hidden in the QRS complex or in the T wave . When they are visible with an RP interval less than the PR interval, the most likely diagnosis is atrioventricular reentry by an accessory pathway (AVRT), since atria activation follows ventricular activation usually with an interval exceeding 70 ms. In this case, the P wave can be hidden in the ST segment or embedded in the T wave (“camel hump,” notch) [5].

On the contrary, in the case of typical atrioventricular nodal reentrant tachycardia (AVNRT) slow-fast type, atrial and ventricular activation is almost simultaneous, and P waves are therefore hidden in QRS waves or may form the final part of the QRS complex, creating pseudo S waves in leads II, III, and aVF or pseudo R waves in V1. To confirm the diagnosis, these appendices should not be evident during sinus rhythm. The relationship between the PR and RP intervals is very important: the RP is virtual or extremely short in the common AVNRT (slow-fast type) and junctional ectopic tachycardia (JET); the RP interval is long in sinus tachycardia, sinoatrial reentrant tachycardia, atrial ectopic tachycardia (AET), atrial flutter, AVNRT (fast-slow or slow-slow), and permanent junctional reciprocating tachycardia (PJRT). In the AVRT using an AP, the RP interval depends on the location of the bypass tract but, in most cases, is >70 ms. If the P wave is visible, in addition to the relationship with the QRS complex, its morphology and axis could be important in order to identify the direction of atrial activation. In the commonest forms of SVT (AVRT and AVNRT), the atria are depolarized from the junction to the top (concentric caudo-cranial activation), leading to negative P waves in the inferior leads. Therefore, in the case of tachycardia and positive P waves in leads II, III, and aVF, the diagnosis of AVNRT and AVNRT can be excluded. PJRT and atypical AVNRT are characterized by a normal or slightly prolonged PR interval with negative P waves in the inferior leads and positive P waves in aVR and aVL. The P wave polarity in lead I has a diagnostic value (positive is typical of AVNRT, negative of PJRT) as well as other clinical and electrophysiological features (AVNRT is usually paroxysmal and starts with ectopic beats, while PJRT is incessant and begins with a critical shortening of the sinusal cycle). Atrial ectopic tachycardia (AET) arises in an ectopic focus which can be single or multiple or congenital or acquired. The P wave axis helps to identify the origin of the arrhythmia either near to the “crista terminalis” or to the right pulmonary veins or parasinusal: in that case, the P is pseudo-normal and an arrhythmia should be suspected due to a heart rate inappropriate for the age or physiological state. In patients with AET, a first-degree AV block could be present; second-degree Mobitz 1 AVB either spontaneous or induced could be helpful for the diagnosis. AETS are normally non-responsive to adenosine; in exceptional cases the drug could slow or block the arrhythmia if the mechanism is the triggered activity.

JET is the only SVT with AV dissociation and sinus P wave. The P wave can be simultaneous, just before or just after the QRS complex. P waves can be negative in the inferior leads as a result of junctional origin and retroconduction to the atria. AV dissociation could cause atrial dilatation which can be inferred from giant P waves. Postoperative JET can be easily diagnosed using the epicardial wires.

Another important diagnostic trick in differentiating SVT is the mode of onset and offset. If the arrhythmia begins with a premature ventricular contraction (PVC), the most likely diagnosis is AVRT since the ventricles are included in the mechanism of the arrhythmia, while AVNRT is less likely considering that the PVC rarely penetrates the AV node retrogradely, stopping the arrhythmia [6–9].

Sometimes, the ECG evaluation cannot lead to the diagnosis, and the response to drugs or to vagal maneuvers can be extremely helpful. Adenosine can stop the vast majority of AV reentrant arrhythmias. On the contrary, in automatic/ectopic tachycardia, adenosine is able to slow the ventricular rate by means of AV node block, but not to terminate the arrhythmia. The same can be said of atrial flutter in which adenosine may have a great diagnostic value [10].

The clinical presentation can help to distinguish reentrant from automatic tachycardia. In a neonate the most likely tachycardia is the reentry form due to an accessory pathway, AVRT, while in an older child, an incessant tachycardia could be automatic (AET or JET) with the exception of PJRT.

A newborn with an SVT usually has a heart rate >220 bpm, and a reentry SVT will often have a heart rate >250 bpm. The diagnosis can be confirmed taking into account the response to drugs or to vagal maneuvers, hence keeping in mind the 220 rule and the 30 rule. If the heart rate is between 220 and 250 bpm, even though the P waves seem sinusal, the most likely mechanism is an automaticity, even more so if the HR changes by a range of 30 bpm during 30 min of observation (reentrant arrhythmias have a quite stable HR and are less sensitive to adrenergic stimulation).

The changing and paroxysmal nature of SVT makes diagnosis with a standard ECG difficult and requires extended monitoring either in the hospital or in the clinic. Several ways of monitoring are available: from the classic Holter ECG to the devices provided by modern cardiology-based telemedicine (event recorder, loop recorder, etc.). The “old” Holte r is rarely able to capture the fleeting SVT and is often poorly tolerated by children. Hence, telemedicine is becoming of paramount importance to make the monitoring of arrhythmias in the pediatric population more and more personalized (Fig. 11.6). Telemedicine is evolving at a very brisk pace; in addition to hospital-provided remote services, the number of apps to monitor heart rate and arrhythmia is growing on an almost daily basis, turning our mobiles into true “smheartphones” [11]. Even social networks are proving valuable in sharing clinical cases in real time (Fig. 11.7).