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Abrupt Slowing of an Escape Pacemaker: What is the Mechanism?

The ECG in Figure 1 shows sinus rhythm with almost complete AV block. The escape rhythm has a left bundle branch block (LBBB) con guration and a rate of about 36/min. The escape cycle length ranges from 1.64 to 1.76 seconds, corresponding to about twice the sinus cycle length of 0.8 seconds. This explains the relative constancy of the intervals between sinus P waves and escape QRS complexes in the top strip. In the bottom strip, two pauses occur: the second and fourth R-R cycles are longer than the basic escape interval and measure 2.32 seconds. What is the mechanism of this irregularity?