Upper Rate Limit

Report:

Sinus tachycardia 120/min 0.5

VEBs, one couplet 1

SVEB 1

AV block, probably complete 1

Atrial-sensing ventricular-pacing pacemaker 1

Pauses due to upper rate limit 120/min 4

Fusion beats 1

Left atrial abnormality (LAA) 0.5

Comment:

The patient has sinus tachycardia from congestive cardiac failure. Slight accelerations result in pacemaker inhibition. The resultant pauses are filled by succeeding P waves and VEBs. The first pause is terminated by a beat with pacing spike following a P wave, but looks intermediate in morphology between “pure” paced beats and “pure” VEBs. A similar beat, first in a couplet, terminates the second pause. These are pacemaker – ventricular fusion beats; even though there is a preceding P wave, there cannot be any supraventricular contribution. The P wave is connected to the paced QRSs only through the pacemaker. The P waves not followed by a pacing spike are blocked.

Three paced beats in the middle of the trace do not have a “blocked” P wave; the pause is almost certainly due to an invisible SVEB, buried somewhere in the T wave and, of course, also blocked.

The appearance is reminiscent of endless loop pacing tachycardia, except that the sinus P waves can be mapped out and are seen to be positive in lead 2 – not retrograde.

The tachycardia resolved with therapy of heart failure and the pacemaker resumed its 1:1 response to P waves (Fig 87a).