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majority of cases, are indiscernible or very close to the QRS complex. Thus, P waves are either masked by the QRS complex or seen as a small terminal P’ wave that is not present during sinus rhythm.323 In the atypical form of AVNRT, P waves are clearly visible before the QRS, i.e. RP>PR, denoting a long RP tachycardia, and are negative or shallow in leads II, III, aVF, and V6, but positive in V1.317 Tachycardia-related ST-segment depression, RR-interval variation, as well as QRS alternans may be seen. Specific, although modestly sensitive, ECG criteria for AVNRT, as opposed to AT and AVRT, are a pseudo R deflection in lead V1 and a pseudo S wave in the inferior leads, a notch in lead aVL, and a pseudo R in aVR.45 If the tachycardia is initiated by atrial ectopic beats, the initial (ectopic) P wave usually differs from the subsequent (retrograde) P waves.

11.2.1.1.2 Electrophysiology study. Heterogeneity of both fast and slow conduction patterns has been well described, and all forms of AVNRT may display anterior, posterior, and middle, or even LA retrograde, activation patterns.322324 Thus, specific electrophysiological manoeuvres may be required for differential diagnosis of typical and, especially, atypical AVNRT from focal AT or AVRT due to a concealed septal pathway.45 The rare form of verapamil-sensitive AT is due to re-entry in the atrial tissue close to the AVN, but not the AV nodal conducting system.325

11.2.1.1.3 Typical atrioventricular nodal re-entrant tachycardia. In the slowfast form of AVNRT, the onset of atrial activation appears before, at the onset of, or just after the QRS complex, thus maintaining an AH/Hisatrial (HA) ratio >1. The VA interval measured from the onset of ventricular activation on surface ECG to the earliest deflection of the atrial activation in the His bundle electrogram is <_60 ms. Although, the earliest retrograde atrial activation is typically recorded at the His bundle electrogram, careful mapping studies have demonstrated that posterior or even left septal fast pathways may occur in <_7.6% in patients with typical AVNRT.326328

11.2.1.1.4 Atypical atrioventricular nodal re-entrant tachycardia. Atypical AVNRT is seen in 6% of all AVNRT cases,317 and in some patients may coexist with the typical form.329 A higher incidence of atypical AVNRT has been documented in athletes.330 In the socalled ‘fastslow form of AVNRT, retrograde atrial electrograms begin well after ventricular activation with an AH/HA ratio <1, indicating that retrograde conduction is slower than antegrade conduction. The AH interval is <185200 ms. The VA interval measured from the onset of ventricular activation on surface ECG to the earliest deflection of the atrial activation in the His bundle electrogram is >60 ms. Earliest retrograde atrial activation has generally been reported at the base of the triangle of Koch, near the coronary sinus ostium, but it can be variable, with eccentric atrial activation at the lower septum or even the distal coronary sinus.328,331,332 In the ‘slowslow’ form, the AH/HA ratio is >1 and the AH interval >200 ms, but the VA interval is >60 ms, suggesting that two slow pathways are utilized for both anterograde and retrograde activation. Earliest retrograde atrial activation is usually at the coronary sinus ostium, but variants of left-sided atrial retrograde activation have also been published.333,334 The distinction between ‘fastslow’ and ‘slowslow’ forms is of no practical significance, and certain cases of atypical AVNRT cannot be classified according to described criteria.324 There is also evidence that the ‘fast’ pathway during slowfast AVNRT is not identical to the ‘fast’ component of so-called fastslow AVNRT.329 Therefore, AVNRT can be classified as typical or atypical according to the HA interval, or—when a His bundle electrogram is not reliably recorded—according to the VA interval measured on the His bundle recording electrode.322 Table 12 presents a conventional classification system. Other approaches have also been published.335

11.2.1.2 Therapy

Table 12 Classification of atrioventricular nodal reentrant tachycardia types324

HA VA (His) AH/HA

Typical AVNRT <_70 ms <_60 ms >1 Atypical AVNRT >70 ms >60 ms Variable Atypical atrioventricular nodal re-entrant tachycardia has been traditionally classified as fastslow (Hisatrial >70 ms, ventriculoatrial >60, atrialHis/Hisatrial <1, and atrialHis <200 ms) or slowslow (Hisatrial >70 ms, ventriculoatrial interval >60 ms, atrialHis/Hisatrial >1, and atrialHis >200 ms). Intermediate, unclassified types may also exist. AH = atrialHis interval; AVNRT = atrioventricular nodal re-entrant tachycardia; HA = Hisatrial interval; VA = ventriculoatrial interval measured from the onset of ventricular activation on surface ECG to the earliest deflection of the atrial activation on the His bundle electrogram.

Recommendations for the management of atrioventricular nodal re-entrant tachycardia (AVNRT)

Recommendation Classa Levelb

Acute therapy Haemodynamically unstable patients Synchronized DC cardioversion is recommended for haemodynamically unstable patients.8688 I B Haemodynamically stable patients Vagal manoeuvres, preferably in the supine position with leg elevation, are recommended.41,8991 I B Adenosine (618 mg i.v. bolus) is recommended if vagal manoeuvres fail.9294 I B Verapamil or diltiazem i.v. should be considered if vagal manoeuvres and adenosine fail.92,9498 IIa B Beta-blockers (i.v. esmolol or metoprolol) should be considered if vagal manoeuvres and adenosine fail.97,99,100 IIa C Synchronized DC cardioversion is recommended when drug therapy fails to convert or control the tachycardia.87,88 I B

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