Slow-Slow AVNRT (AtypicalAVNRT) 1-5% AVNRT; Associated with Slow AV nodal pathway for anterograde conduction and Slow left atrial fibres approaching the AV node as the pathway for retrograde conduction. ECG: Tachaycardia with a P-wave seen in mid-diastole… effectively appearing ‘before the QRS complex’…

Atypical AVNRT (fast-slow): 10% of all cases of AVNRT. In atypical AVNRT the fast pathway conducts the impulse in antegrade direction while the slow pathway conducts it in the retrograde direction. The P-wave will be visible before the QRS complex. The P-wave will be retrograde in lead II, III and aVF and positive in lead V1. Refer to Figure 2 ...

Atypical ANVRT (also described as uncommon AVNRT or fast-slow AVNRT): The impulse travels via the fast pathway towards the ventricles and returns via the slow pathway to the atria. The retrograde P wave appears far behind the QRS. Only about 6% of all ANVRT patients are diagnosed with atypical AVNRT.

The difference in the AH interval between atrial pacing and the tachycardia may allow differentiation of atypical AVNRT from other types of long RP′ tachycardias. A ΔAH >40 ms has been reported to indicate AVNRT, whereas in patients with AVRT due to septal pathways or atrial tachycardia, these differences were <20 and 10 ms, respectively. 18

The diagnosis of AVNRT requires visualization on an electrocardiogram (ECG). In most cases, an ECG will show heart rate between 140 and 280 beats per minute (bpm), and in the absence of aberrant conduction, a QRS complex of fewer than 120 milliseconds. ECG criteria based on the reentrant circuits are as follows: Slow-Fast AVNRT