Profusion of Pseudofusion Beats

Report:

Sinus rhythm 70/min 1

Normal trace 2

Demand pacemaker 1

Fusion beat 3

Multiple pseudofusion beats 3

Comment:

It just happened that the patient’s sinus rate was 70/min – the rate usually pre-set for permanent pacemakers. There is no pacemaker contribution to the QRS complex until the very last beat, when sinus arrhythmia prolongs the sinus cycle and rewards the persistent pacemaker by a partial capture. That’s what fusion beats are: incomplete capture or incomplete dissociation.

In pseudofusion beats, the pacing spike usually falls after the onset of the QRS complex. This is obvious in the rhythm strip’s fourth and fifth complexes, but not in others, where the spike and the QRS onsets appear simultaneous. Even then, the natural pathway’s speed outstrips its electronic counterpart and the QRS is entirely supraventricular. We can be sure of it since all the QRSs and their T waves look the same until true fusion occurs, at the end.

Fig 30a shows the patient’s sinus rhythm without any electronic interference.

Fig 30a.

Fig 30b. Two fusion beats (arrows) precede a fully paced beat. Two pseudofusion beats precede them.

Fig 30c. There is a subtle penultimate beat’s “tad of fusion”. And obvious fusion in the last beat.

Fig 31. 77 year old lady with a permanent pacemaker, implanted many years previously for sick sinus syndrome.