Peculiar P-QRS Interaction

 Another great case from Dave Richley, @DaveRichley

 

 

My hypothesis:

 

Orange arrows: Sinus P waves (mostly stable PP CL)

Solid black line: Fast Pathway (FP). Dashed black line: Slow Pathway (SP)

 

Fundamentally, dual AVN conduction. The basic, recurring them (demonstrated in QRS 2 and 3) Sinus conduction down FP. Then, preferential utilization of SP because FP is refractory to antegrade travels, however subsequently recovers enough to permit momentary retrograde conduction into the Atria (blue upright arrow). This would otherwise reset the PP CL, but the assumption here is that the SN is protected via entrance block. Thus, it fires, and the surrounding Atrial tissue is receptive to its impulse, however fails to effectively enter the AVN.

 

QRS 9 is ventricular extrasystole that invades the AVN and stifles what should have been SP conduction at this predictable moment in time. I initially considered the very next PR-interval (associated with QRS 10) to be the consequence of concealed conduction into FP. –And this remains a possibility. However, given that the PR-interval here shares similar distance to all previous SP conduction, I think it might be preferential use of SP because FP remains refractory from the extrastystolic penetration.

 

From Dave Richley:

 

In his words:

 

Beat 2: conducts down fast AVN p/way 

 

Beat 3: block in fast p/w. Conducts down slow p/w, then retrograde conduction up fast p/w & thru atria causing pseudo r’ and scooped ST seg. Retrograde concealed conduction from PVC blocks conduction in both p/ws.