Alternating Paced and Fusion Beats

Report:

Slow atrial or junctional rhythm 38/min 2

Ventricular pacemaker rhythm 79/min 2

Superior electrode placement 2

Fusion in alternate beats 2

Presumptive RBBB 1

ST/T changes of uncertain significance 1

Comment:

The narrower (0.12 sec) complexes are preceded by retrograde P waves with PR (P-to-spike) interval 0.16 sec. In V1 these P waves have a down-up instead of up-down orientation; in V6 they are negative. Various names have been used for this morphology: junctional, atrial, left atrial and coronary sinus rhythm. It really matters little: all we have to note is that there exists a slow supraventricular escape rhythm and ensure that there are no extra P waves between those counted (there aren’t any, at least not half-way in-between, where they would be easily spotted). Their QRS complexes look unusual, but supraventricular in origin; if they occurred a little earlier there would be a typical escape-capture bigeminy between them and the broad, obviously paced, complexes. And they contain a pacemaker spike either at the onset or very shortly after it. The pacemaker spikes are perfectly regular at 79/sec.

This calls – for some of us, at least - for a magnifying glass, best applied to V1 rhythm strip. There is little doubt that the narrower QRS complexes start with a positive deflection just before the pacemaker spikes. The rhythm is almost imperceptibly bigeminal – an extreme case of escape-capture bigeminy. The narrower QRSs are either fusion or pseudofusion beats: partially paced or just distorted by the pacemaker spikes.

Could there still be fusion after the inscription of the native QRS onset? There can be if the native QRS is a broad complex itself, with slowly rising R wave unable to inhibit the pacemaker, like the one recorded later (Fig 10a): a RBBB complex, expected to normalise the LBBB-like paced beats in true fusion. The sinus rhythm trace, shown below, also shows evidence of a possible old posterolateral infarction and many obvious pseudofusion beats (all except the two shortest-cycle QRSs contain a pacemaker spike but are otherwise identical).

The pacemaker tip is not in its usual place at the “floor” of the right ventricle (its apex) and the frontal plane axis is moved to the right, with positive L2; this does not matter provided the position is stable and there are no arrhythmias.