Controlling the Ventricular Rate: PVARP

Report:

Atrial tachycardia 222/min (? slow flutter) with 2:1 block 4

LVH with ST/T changes (incomplete LBBB) 1

Atrial-sensing electronic pacemaker firing 111/min 1

Pseudofusion beats 4

Comment:

The atrial tachycardia is somewhat discreet, but two atrial waves per QRS can be made out once looked for. The pacemaker tracks the alternate atrial waves at 110/min, well below its own upper rate limit (known to be, as usual, 120/min). If there was no post-ventricular atrial refractory period (PVARP), the full atrial rate of 222/min would be sensed and the pacemaker, depending on its specifications, would develop a form of AV block (which would not help here, as the patient is not pacemaker-dependent) or a mode switch – become a VVI unit or equivalent.

The PVARP in this case was 0.30”, masking the atrial waves in the wake of each QRS complex. If there was no native conduction, the ventricle would be fully paced by the ventricular electrode at 110/min. Paradoxically, it is those masked (by the PVARP) atrial waves that, in this case, activate the ventricles: the expected PR interval in atrial tachycardia or flutter is often over 0.40” and over the top conduction58 is the rule rather than an exception.

As the patients own complexes resemble the paced ones, one cannot be completely sure that the QRSs are pseudofusion rather than fusion beats. A “tad of fusion” remains a possibility. Below (Fig 113a) is an example of an atrial capture with short PR interval resulting in a normal QRS, surrounded by atrial-triggered but fully and almost-fully paced beats.