Fusion Beats Starting Runs of VT

Report:

Sinus arrhythmia

Ventricular tachycardia 132-185/min, non-sustained25

1:1 retrograde conduction

Fusion beat

Comment:

The fusion beat starting the paroxysm of VT is diagnostic of the ventricular origin of the latter. Dressler beats are fusion beats often seen during VT and often terminating it For some reason, nobody seems to call fusion beats starting VT Dressler beats. The qR complexes (like monophasic R complexes in the strip below, Fig 33a) also strongly favour the diagnosis VT.

Retrograde conduction is established with the third VT beat; until then the sinus P waves govern the atria. Possibly, the increase in rate improves retrograde conduction and allows it to take over after the first few beats of tachycardia. Retrograde waves often appear spuriously positive, as here, notching a dip in the ST segment before the T wave15.

The retrograde waves imply atrial contraction against the closed AV valves because of their timing; in this case the right atrial systole will produce regular cannon waves in the jugular pulse34.

Below (Fig 33a) is a similar strip, from a 77 year old man 24 hours following acute anterior infarction. It, too, shows a run of VT starting with an obvious fusion beat. There is no manifest retrograde conduction, as in Fig 33, but the first sinus cycle after VT has its PR interval prolonged by concealed retrograde conduction. Otherwise the VT is completely dissociated from the underlying sinus arrhythmia. The cannon waves (if observable, in such a short run, or anyway, in our days of declining clinical skills) would be irregular.