Pacemaker Rhythm and LBBB
Report:
Electronic demand pacemaker rhythm 70/min 2
Atrial fibrillation 2
Left bundle branch block (last 3 beats) 6
Comment:
If it were not for the rhythm strip, few would notice the transition from pacemaker to supraventricular rhythm at the end of the trace. The third last beat has a pacemaker spike at its onset, but there is no evidence of fusion in either the ensuing QRS contour or its repolarisation: it is a pseudofusion beat. Lead 2 is quite good discriminator between the LBBB and the paced complexes in this case, since the former have normal frontal plane axis and the latter, as expected, have marked LAD. Approximately 50% of LBBBs have normal axis.
The very broad (almost 0.20”) complexes in both the paced rhythm and the LBBB conduction suggest myocardial disease.
More of the conducted AF in other leads is shown below (Fig 15a).
See also Cases 42 and 94.
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