Pseudo-Wenckebach due to Low Upper Rate Limit

Report:

Sinus rhythm 92-96/min 0.5

Conducted sinus capture beat (penultimate QRS) 0.5

First degree AV block (PR 0.24”) 0.5

Second degree AV block (last cycle) with ventricular pacemaker escape beat 0.5

Atrial-sensing ventricular pacemaker 1

4:3 pseudo-Wenckebach sequences 3

Fusion beats 2

Failure to sense (last two P waves) - atrial undersensing 2

Comment:

The overall ventricular rate is about 73/min, due to Wenckebach “conduction” of sinus rhythm 92 – 96/min, engendered by the pacemaker’s upper rate limit of 80/min. This is uncommonly low limit, but may be useful if the patient develops angina in faster paced (but not necessarily native) rhythm. This was the case here; she was in fact on thyroxin 100 mcg daily for myxœdema. Another situation where rate limitation may be useful is mitral stenosis.

The only pacemaker malfunction eventually discovered was on this ECG: failure to sense a two P waves, one conducted with AV interval just over 0.20” and the other blocked. The sensitivity was increased from 1.0 to 0.5 mV. Later 24-hour Holter monitor showed normal sinus rhythm, mostly about 90/min and no pacemaker malfunction.

This patient’s problem were elsewhere: she was dehydrated and on Persantin SR 200 mg BD, with marked postural hypotension. This was addressed and the symptoms resolved.

Fig 121. Preoperative ECG on a 82 year old man with DDDR pacemaker and critical left main CA stenosis. The settings include rate 59/min, paced AV interval 0.20”, sensed AV interval 0.17” and no rate hysteresis.