Rate Hysteresis

Report:

Demand pacemaker rhythm 69/min 1

Posterior electrode placement 1

Rate hysteresis 3

Sinus rhythm 53/min 1

Sinus captures 1

AV dissociation with interference 1

Right bundle branch block 1

Possible old anteroseptal infarction 1

Comment:

The fourth complex is a QR RBBB one, preceded by a P wave and premature with respect to the three preceding paced beats. It is a ventricular capture by the slower sinus rhythm – a break in the prevailing AV dissociation. This is why the dissociation is called dissociation with interference – because of the interfering capture beats.

The next paced beat comes after a delay of one second, corresponding to a rate of 60/min; subsequent ones are at 69/min. The delay (hysteresis) in the onset of paced rhythm is programmed to allow the patient more time in sinus rhythm, even though at a slower rate.

There is no hysteresis after the second sinus capture, because it is in fact a fusion beat, with pacemaker as well as sinus contribution to its QRS (it could also be a pseudofusion beat – a “tad of fusion” is often hard to determine). In either case, the pacemaker spike occurs, and the next beat is timed from it at the normal pacing rate.

The rate of 69/min is a little slower than the usual 70/min; if true slowing can be demonstrated it indicates impending battery depletion. It would be of value to see the fixed mode magnet rate here.

Below is an earlier trace, with a magnet over the pulse generator. The first three beats show the typical magnet rate of 100/min, succeeded by beats with pacing rate of 70/min. The battery was full then. Transition to conducted sinus rhythm occurs through two fusion beats. There is a marked increase in P-terminal force (PTF) in lead V1 and T wave inversion suggestive of ischæmia.