Fusion Terminating AIVR

Report:

Sinus rhythm 74 – 80/min

Accelerated idioventricular rhythm 69 – 74/min

Ventricular fusion beat

Comment:

The AIVR accelerates, but to no avail: the faster sinus rhythm takes over by the middle of the recording. Between the two is a typical fusion beat, intermediate in morphology between pure sinus and pure AIVR. Were it not for fusion, the AIVR beats would pass for LBBB of supraventricular origin, as an unusual (deceleration-dependent) LBBB.

Below (Fig 221a) is a trace taken in CCU day later, showing retrograde conduction in the AIVR. In the middle, again, there is fusion: the 6th complex has a smaller inverted P wave in its wake and the 7th one has an almost flat one. Atrial fusion. Upright sinus P waves dominate the atria for the next three cycles, dissociated, and then effect ventricular capture.

Fusion is by no means always present in AIVR (or, for that matter, parasystole).

Further below (Figs 221b, 221c) are two tracings from a healthy teenage girl: the first one shows runs of AIVR separated by a single sinus beat without any visible atrial activity during the AIVR. Judging from a longish sinus escape interval (projecting a rate of 44/min) the AIVR beats are retrogradely conducted, their P waves hidden within the QRS.

The next tracing, taken a month earlier and before β-blocker therapy, show a faster AIVR (technically a VT at 110 – 120/min) and faster sinus rhythm at 95/min. Here retrograde P waves deflect the ST-T junction of the last three beats in each paroxysm. Again, sinus escape is prolonged beyond its normal cycle length, projecting a rate of 84/min. But there is no fusion, either in the atria or the ventricles: the chambers belong to only one master at a time.

I was shown her ECGs by another Cardiologist worried about the “lack of response to metoprolol”. I told him (a free opinion, for whatever its worth) that she had responded enough. AIVR is best left alone; it is very different condition from exercise-induced, adrenergic VT, let alone the lethal CPVT145, where beta-blockers are mandatory