SVT (probably AVNRT) with spontaneous normalization

Figure 1 - ECG case

No clinical history. 

Is this ventricular tachycardia (VT) that converted to supraventricular tachycardia (SVT)?

Figure 2 - Initiation

Figure 3 - RBBB morphology

Figure 4 - Spontaneous normalization of the QRS

Figure 5 - SVT with normal QRs duration

Figure 6 - Termination of SVT (adenosine)

The tachyarrhythmia was initiated by a premature atrial beat (PAC) with a long PR interval (Figure 2). After a few cycles, the morphology changed to right bundle branch block (RBBB). After a few minutes, there is spontaneous normalization of the morphology (Figure 4) .

This is regular narrow complex short RP tachycardia. A short RP SVT can be any of the following:

Table 1 - SVT based on the relation of the P wave to the R wave

A tachycardia that is initiated with a PAC is most likely an AV nodal reentry tachycardia. How an AVNRT is initiated can be seen on this video.

Figure 7 - Pseudo R' and pseudo S 

A SVT with a pseudo S, a pseuo R' or both is 90-100% specific for typical AVNRT and an 81% positive predictive value for typical AVNRT. These criteria are only 42% sensitive for typical AVNRT. 

There was also an initial aberration or right bundle branch block (RBBB) morphology at the start but the QRS normalized later.

According to Dr. Fisch, the most plausible explanation for the normalization of intraventricular conduction is due to the gradual shortening of the bundle branch refractory period in response to the tachycardia which is a physiologic phenomenon.

References:

Fisch C and Knoebel SB. 2000. Electrocardiography of Clinical Arrhythmia. New York. Futura 

Kumar UN et al. 2006. The 12L Electrocardiogram in Supraventricular Tachycardia. Cardiology Clinics ;24: 427-437

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