Figure 94-35c

Exceptions to the rule for compensatory pauses.
top: Ventricular extrasystole with less than compensatory pause. Retrograde conduction to the atria (retrograde P waves deforms first part of St segment) discharges the sinus pacemaker early and thus shortens the postextrasystolic cycle.
Middle: Atrial premature contraction followed by fully compensatory pause. The third and the eighth beats are atrial extra systoles;but presumably because they suppress the sinus pacemaker,they are folowed by compensatory pauses.
Bottom: Ventricular extrasystoles with less than compensatory pauses. Each postextrasystolic cycle ends in an escape beat and so is slightly less than compensatory.