The effect of a spontaneous PVC in a supraventricular tachycardia: It's significance

Image 1

A 65yo pt c/o of shortness of breath.

As most will call it, this is a supraventricular tachycardia (SVT). A SVT is a narrow complex  (unless there is aberrant conduction) tachycardia that requires the atrial tissue or the atrioventricular (AV) node as an integral part of the arrhythmia.

SVT is classified as short RP' or long RP' tachycardia depending on the RP or PR interval. If the interval from the R wave to the next P wave exceeds the  interval from that same P wave to the next R wave, then the SVT is called long RP' tachycardia. If the interval form the R wave to next P wave is shorter than the interval from the same P wave to the next R wave, then the SVT is called short RP tachycardia.

Image 2 - Short RP' and long RP' tachycardia

Image 2A is a short RP' tachycardia and image 2B is a long RP tachycardia. 

These are the SVT's based on this classification

Image 3 - Short RP' and long RP' tachycardia

About 90% of AV nodal reentry tachycardia (AVNRT) and 87% of AV reentry tachycardia (AVRT) are short RP' tachycardia. Only 11% of atrial tachycardias (AT) are short RP' tachycardia. 

AVNRT is the the most common form of paroxysmal SVT.

Image 4  - ECG case highlighted

The ECG case  is a regular short RP narrow complex tachycardia (~190 bpm) . There is a pseudo-R' in V1 (red arrows) and pseudo-S in II, III and aVF (blue arrows).  There is lengthening of the RR interval before the termination of the tachycardia. The arrhythmia is terminated with a P wave (black arrow). A PVC can be seen that did not disturb the rate of the tachycardia. All this points to AVNRT.

The presence of a pseudo-S, a pseudo-R', or both is 90-100% specific for typical AVNRT and has an 81% positive predictive value for typical AVNRT. AVNRT terminates with a P wave. The change in the cycle length or the change in the RR interval prior to the termination is not diagnostic for AVNRT because it can also occur during AVRT.

The tachycardia RATE CANNOT BE USED TO DIFFERENTIATE BETWEEN SVT's. AVNRT can vary between 100-280 bpm (200-250). AVRT and AT can also have similar heart rates.

A short review on what creates the pseudo-S and pseudo-R and the concept of RP/PR classification

In some individuals, there are 2 atrionodal connections (dual AV node physiology). One connection (pathway) conducts fast but recovers slow (longer refractory period) and the other pathway has a slower conduction but recovers fast (shorter refractory period). 

Image 5 - Dual AV node physiology

In a short RP tachycardia, the retrograde conduction is via the FAST PATHWAY. This creates a short RP interval. The anterograde conduction is via the SLOW PATHWAY. This creates the long PR interval. The retrograde impulse also reaches the atria and creates an inverted P wave in II, III and aVF  (pseudo-S) and a small R wave in V1 (pseudo-R). 

In long RP tachycardia, retrograde conduction is via the SLOW PATHWAY and the antegrade conduction is via the FAST PATHWAY. Thus, on the surface ECG, there is a a long RP and shorter PR interval.

Effect of spontaneous PVC during a short RP tachycardia

Image 6 - PVC during AVNRT when HIS is refractory

During AVNRT, the retrograde conduction after a spontaneous PVC cannot reach the atrium because there is no alternated pathway. So, the AVNRT cycle is not changed and the PP interval is not change after the PVC. This means that the ventricles are not part of the circuit.

Image 7 - PVC during AVRT when HIS is refractory

During AVRT,retrograde conduction after a PVC cannot reach the atium (via the AV node) when the His bundle is refractory but can reach the atrium via the AP. So, the subsequent the atrial depolarization will occur earlier or the P wave on the surface ECG will occur earlier. This means the an AP is present. 

Image 8 - PP interval and the PVC

In this ECG case, the PP interval (red arrows) did not change as revealed in the simultaneous comparison of leads.

This termination of the tachycardia was due to adenosine.

Reference:

Bonnow et al. 2011. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 9th Edition. PA.Saunders 

Das and Zipes. 2012. Electrocardiography of arrhythmias : a comprehensive review. Elsevier PA

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

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