Intermittent Ventricular Pacing

1. Sinus bradycardia (<50 bpm)
2. Ventricular (escape) pacing at 50 bpm
3. Variable morphology of the ventricular paced beats.
That much is clear.
The grouped beats with variable morphology of the paced impulses almost suggests a fusion mechanism (or rate related changes). The short-coupled appearance of the final v-paced complex and sinus complex in each group suggests an escape-capture mechanism.
I have three thoughts:
1. Sino-atrial block with a ventricular pacemaker leading to escape-capture trigeminy (bigeminy? is it only the last two that participate?).
2. Sinus bradycardia, ventricular (escape) pacing, (hidden) retrograde conduction to the atria, setting up escape-capture bigeminy at the end of the group.
3. Sinus bradycardia, fusion of a junctional escape and ventricular (escape) pacing, setting up escape-capture bigeminy at the end of the group.
This is a great strip.