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............................................................................................................................................................................. tend to last longer than AT episodes, which may occur in a series of repetitive runs.32 Clear descriptions of pounding in the neck (the socalled ‘frog sign’) or ‘shirt flapping’ would point to the possible competing influences of atrial and ventricular contraction on the tricuspid valve, and to AVNRT as a likely cause.15,38,39

SVT may be unrecognized at initial medical evaluation and the clinical characteristics can mimic panic disorder.40 In patients with possible sinus tachycardia in association with anxiety and postural orthostatic tachycardia syndrome (POTS), it is important to rule out the possibility of a re-entrant tachycardia.

8 Initial evaluation of patients with

supraventricular tachycardia

Complete history taking, including family history, and physical examination are essential. As discussed, clinical history is especially relevant in cases of palpitations without ECG documentation of the tachycardia. Initial evaluation points to a re-entrant arrhythmia when the mode of onset/termination is sudden, is often associated with a change of position, and the tachycardia is perceived as regular. Initiation of the episode(s), frequency, and conditions of triggering the arrhythmia are important clues for a specific diagnosis.3 There may be evidence that specific actions lead to arrhythmia termination. These could include standard vagal manoeuvres,41 but also abortive measures such as drinking a glass of iced water. Also, responses to the administration of drugs (e.g. adenosine or verapamil), where termination has been observed but ECGs are missing, may provide diagnostic help. Full blood counts and a biochemistry profile—including renal function, electrolytes, and thyroid function tests—can be useful in specific cases (Table 7). An ECG recorded during tachycardia is ideal, and patients should be encouraged to seek medical assistance and recording of their ECG during episodes. A 12 lead resting ECG and baseline echocardiographic assessment are necessary. It may be useful to do 24 h ECG recordings, but tachycardia episodes are usually sporadic and may not be frequent enough to be recorded on ambulatory monitoring. Transtelephonic monitoring, mobile recording devices, or, very rarely, an implantable loop recorder may be required. Wrist-worn, optically based heart rate monitors are user-friendly, but appropriate validation of the device used is imperative.42 A tolerance exercise test may also be useful in patients with apparent preexcitation and in cases of catecholamine-dependent arrhythmias. Myocardial ischaemia testing is also needed in patients with angina or significant risk factors for coronary artery disease.43 An electrophysiology study (EPS) is usually necessary to establish the diagnosis, particularly when catheter ablation is anticipated.

9 Differential diagnosis of

tachycardias

9.1 Narrow QRS (120 ms) tachycardias

Narrow QRS complexes are due to rapid activation of the ventricles via the HisPurkinje system (HPS), which suggests that the origin of the arrhythmia is above or within the His bundle. However, early activation of the His bundle can also occur in high septal VTs, thus resulting in relatively narrow QRS complexes (110140 ms).44

9.1.1 Electrocardiographic differential diagnosis In the absence of an ECG recorded during the tachycardia, a 12 lead ECG in sinus rhythm may provide clues for the diagnosis of SVT and should be scrutinized for any abnormality. The presence of preexcitation in a patient with a history of regular paroxysmal palpitations is generally suggestive of AVRT. The absence of apparent pre-excitation does not rule out the diagnosis of AVRT, as it may be due to a concealed accessory pathway (AP) that conducts only retrogradely or to an atypical (Mahaim) pathway, which is latent at sinus rhythm. An ECG taken during tachycardia is very useful in the efficient diagnosis of SVT, although it may fail to lead to a specific diagnosis.45 It may not be available in patients with very short or infrequent periods of palpitations.

9.1.1.1 Initiation and termination of the tachycardia Sudden prolongation of the PR interval occurs in typical AVNRT after an atrial ectopic beat. An AT may also be initiated by an atrial ectopic beat, but is not dependent on marked PR prolongation. Automatic, focal ATs are characterized by gradual acceleration (warm-up phenomenon) followed by deceleration (cool-down phenomenon),46 and may also be incessant with short interruption by sinus beats. Premature atrial or ventricular beats may trigger AVRT. Premature ventricular complexes are a common trigger of atypical AVNRT, but rarely induce typical AVNRT, and only exceptionally AT.

9.1.1.2 Regularity of tachycardia cycle length The regularity of the RR interval should be assessed (Figure 1). Irregular tachycardias may represent focal or multifocal AT, focal AF, and atrial flutter with varying AV conduction. Patterns of irregularity can sometimes be found, such as in atrial flutter conducted with Wenckebach periodicity. Irregular arrhythmias, such as multifocal AT, typically display variable P-wave morphologies, and varying PP, RR, and PR intervals. Atrial flutter can have fixed AV conduction and present as a regular tachycardia, and even AF may appear almost regular when very fast. Re-entrant tachycardias, whether micro or macro-re-entries, are usually regular. Incessant tachycardias may be the so-called permanent junctional reciprocating tachycardia (PJRT), focal AT, or, rarely, atypical AVNRT. Cycle length (CL) alternans (also called RR alternans) may be seen in AVNRT, but these changes are <15% of the tachycardia CL.47 If the irregularity exceeds 15% of the CL, a focal arrhythmia is much more likely.48 QRS alternans is a rare phenomenon in slow SVTs, which may not be related to CL alternans, and has been initially described with AVRT.49,50 However, this may be seen in any fast SVT.51 A change in ventricular CL preceded by a change in atrial CL is seen in AT or atypical AVNRT. A change in ventricular CL preceding a change in subsequent atrial CL favours typical AVNRT or AVRT.47,52 A fixed ventriculoatrial (VA) interval in the presence of variable RR intervals excludes AT.45

9.1.1.3 P/QRS relationship According to their P/QRS relationships, SVTs are classified as having short or long RP intervals. Short-RP SVTs are those with RP intervals Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026