Rate Hysteresis

Report:

Sinus rhythm 1

VEBs, multiform 2

Demand ventricular pacemaker 3

Rate hysteresis 3

Fusion beat (top panel) 1

Comment:

The VVI pacemaker takes over after the VEBs; the long cycle it terminates corresponds to a rate 50/min, while the subsequent three paced cycles (in the lower panel) have a rate of 60/min. The escape cycle rate is programmed slower than that of subsequent pacing in order to preserve the native sinus rhythm at lower rates. This feature – the rate hysteresis - is most commonly seen in VVI pacemakers.

The last paced beat is preceded by a conductable sinus P wave and its contour is modified by the supraventricular contribution – it is a fusion beat.

Below, the pacemaker escapes (in the lower panel) after a blocked P wave and the four paced cycles are, again, faster than the escape cycle.

Rate hysteresis or fusion are not routinely reported: they are a “normal” feature of many paced rhythms. Another recording from the same patient is seen in Fig 7a.

Fig 7a. The VEB makes less of a pause (with faster sinus rate) than in Fig 7 in the upper pannel. In the lower pannel, the pacemaker takes over after a blocked P wave. Its escape cycle is obviously longer than the subsequent paced cycles.

Fig 8. 62 year old man paced for bradycardia and syncope.