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............... shorter than one-half the tachycardia RR interval, whereas long-RP SVTs display RP>_PR (Figure 1). Rarely, recording of U waves during typical AVNRT may simulate a long-RP tachycardia.53

On EPS, a very short VA interval (<_70 ms) usually indicates typical AVNRT, or less commonly focal AT, but has also been reported in AVRT.54 For surface ECG measurements, a cut-off interval of 90 ms has been shown to be useful and can be used if P waves are visible,55 but data on actual RP measurements during various types of SVT are scarce. P waves similar to those in normal sinus rhythm suggest appropriate or inappropriate sinus nodal tachycardia, sinus nodal re-entrant tachycardia, or focal AT arising close to the sinus node. P waves different from those in sinus rhythm, and conducted with a PR interval equal to or longer than the PR in sinus rhythm, are typically seen in focal AT. In AT, the conduction to the ventricles may be fast (1:1) or slow (3:1 or 4:1). The possibility of atrial flutter with 2:1 conduction should also be considered if the ventricular rate during SVT is 150 b.p.m., as the atrial activity is usually 250 - 330 b.p.m. In the presence of antiarrhythmic medication in this setting, lowering the atrial rate may result in a higher ventricular rate in the absence of AV nodal blockade. In the case of relatively delayed retrograde conduction that allows the identification of retrograde P waves, a pseudo r deflection in lead V1 and a pseudo S wave in the inferior leads are more common in typical AVNRT than in AVRT or AT.56,57 These criteria are specific (91100%) but modestly sensitive (58 and 14%, respectively).56 A difference in RP intervals in leads V1 and III >20 ms is also indicative of AVNRT rather than AVRT due to a posteroseptal pathway.57 The presence of a QRS notch in lead aVL has also been found to be a reliable criterion suggesting AVNRT,58 while a pseudo r in aVR has been shown to have greater sensitivity and specificity than a pseudo r in V1 for the diagnosis of typical AVNRT.59 However, in all referenced studies, cases of AT or atypical AVNRT were limited or entirely absent. AV block or dissociation during narrow QRS tachycardia is not often seen, but it rules out AVRT as both atria and ventricles are parts of the circuit. The development of bundle branch block (BBB) during SVT may also be helpful in the diagnosis of AVRT. BBB ipsilateral to the AP can result in CL prolongation due to VA prolongation, as the ventricular arm of the circuit is prolonged by conduction through the interventricular septum from the conducting bundle branch.60 However, it should be noted that lengthening of the VA interval may not necessarily result in CL prolongation, due to a potential switch of antegrade conduction from the slow to the fast AV nodal pathway.

9.1.2 Vagal manoeuvres and adenosine Vagal manoeuvres (such as carotid sinus massage) and adenosine injection may help in clinical diagnosis, particularly in situations in which the ECG during tachycardia is unclear. Possible responses to vagal manoeuvres and adenosine are shown in Table 8 and Figure 2. Termination of the arrhythmia with a P wave after the last QRS complex is very unlikely in AT, and most common in AVRT and typical AVNRT. Termination with a QRS complex is often seen in AT, and possibly in atypical AVNRT. Adenosine does not interrupt macro-re-entrant ATs (MRATs).61 Fascicular VTs, in particular, are verapamil- but not adenosine-sensitive. Most VTs, as opposed to SVTs, do not respond to carotid sinus massage, but a narrow QRS VT originating at the left bundle branch and terminated with carotid sinus massage has been reported.62

Table 8 Possible responses of narrow QRS tachycardia to vagal manoeuvres and adenosine (1) Slowing of AVN conduction and induction of intermittent AV block.

Atrial electrical activity can thus be unmasked, revealing dissociated P waves (focal AT, atrial flutter, or AF waves). (2) Temporary decrease in the atrial rate of automatic tachycardias (focal AT, sinus tachycardia, and JET). (3) Tachycardia termination. This can happen by interrupting the re-entry

circuit in AVNRT and AVRT by acting on the AVN that is part of the circuit. More rarely, sinus nodal re-entry and ATs due to triggered activity can slow down and terminate. (4) No effect is observed in some cases.

AF = atrial fibrillation; AT = atrial tachycardia; AV = atrioventricular; AVN = atrioventricular node; AVNRT = atrioventricular nodal re-entrant tachycardia; AVRT = atrioventricular re-entrant tachycardia; JET = junctional ectopic tachycardia.

©ESC 2019

Adenosine in regular, narrow QRS tachycardia

Response Diagnosis

No effect Inadequate dose/

delivery High septal VT

Gradual slowing then reacceleration Sinus tachycardia Automatic focal AT

Junctional ectopic

tachycardia

Sudden termination AVNRT

AVRT Sinus nodal re-entry Triggered focal AT

(DADs)

Persisting atrial tachycardia with transient high-grade

AV block Atrial flutter Micro-re-entrant

focal AT

Figure 2 Responses of narrow-complex tachycardias to adenosine. AT = atrial tachycardia; AV = atrioventricular; AVNRT = atrioventricular nodal re-entrant tachycardia; AVRT = atrioventricular re-entrant tachycardia; DADs = delayed after-depolarizations; VT = ventricular tachycardia. Downloaded from academic.oup.com/eurheartj/article/41/5/655/5556821 by guest on 07 October 2026