Premature escape beats. A model for triggered activity in the intact heart?
In conscious dogs with complete atrioventricular block, overdrive pacing of the idioventricular rhythm normally results in overdrive suppression (OS). Frequently, however, we observed another response to overdrive, that is, QRS complex or complexes with unexpectedly short coupling intervals followed by normal OS. We have named such a QRS complex a "premature escape beat" (PEB). Based on the response of PEBs to electrical stimulation, we postulate that PEBs are based on triggered activity
... red activity resulting from delayed afterdepolarizations. This hypothesis was tested in 82 experiments by 1) stimulation under control conditions and in combination with 2) subtoxic and toxic amounts of ouabain 20-50 jig/kg, 3) lidocaine 3 mg/kg, and 4) doxorubicin 16-24 mg/M2. The stimulation protocol, which was repeated at random five to 10 times, consisted of 10 and 50 stimuli using interstimulus intervals of 200, 400, 600, and 800 msec. This protocol was not only performed during spontaneous idioventricular rhythm but also during a continuously paced rhythm with interstimulus intervals of 800 msec. It was found that 1) the chance to induce a PEB or PEBs increased and 2) their first postpacing interval significantly decreased using short or fast drives, or both. Ouabain increased significantly and in a dose-dependent manner 1) the ability to induce PEBs and 2) the number of PEBs per stimulation-train, and also shortened their first postpacing interval. Opposite effects were seen after lidocaine, doxorubicin, and continuous pacing as follows: 1) a lower incidence of PEBs and 2) lengthening of their first postpacing interval. These results support our hypothesis that PEBs are based on triggered activity resulting from delayed afterdepolarizations. (Circulation 1990;82:213-224) W hile studying the response of ouabaininduced ventricular tachycardia (VT) to electrical stimulation in conscious dogs,1 we noticed that initiation of "triggered" beats by pacing was possible in the absence of ouabain (Figure 1) . We have termed these QRS complexes with unexpectedly short coupling intervals "premature escape beats" (PEBs). Their occurrence is in contrast to the phenomenon of overdrive suppression, which is normally seen when an idioventricular rhythm is overpaced.2 The amount of overdrive suppression is dependent on rate and duration of stimulation34 and is followed by slow acceleration of the ventricular rate until prepacing values are attained. Based on the response of PEBs to electrical stimulation, we considered the possibility that PEBs are based on triggered activity resulting from delayed afterdepolarizations (DADs). This arrhythmogenic From the mechanism has been most extensively investigated in the setting of digitalis intoxication.5-8 Other interventions inducing DADs have also been described, like catecholamines,9,10 hypopotassemia,1""2 after myocardial infarction, '1314 during reperfusion,15,16 and hypertrophy.17 DADs have been demonstrated to result from intracellular calcium overload.8'8 Resulting arrhythmias have the following characteristics in common ,5-01,19-22: 1) their induction is promoted by fast pacing rates and 2) their coupling interval shows a concordant relation to the interstimulus interval. In the present study, we have tested our hypothesis by using the following different protocols: 1) programmed electrical stimulation under control conditions, and in combination with 2) subtoxic and toxic amounts of ouabain, 3) lidocaine, and 4) doxorubicin. We expected that under control conditions, faster pacing rates would 1) increase the incidence of PEBs and 2) shorten their first postpacing interval. Ouabain administration was expected to increase the inducibility, to increase the number, and to shorten in a doserelated way the coupling interval of the PEBs.202' Lidocaine and doxorubicin were chosen for their ability by guest on August 16, 2017 http://circ.ahajournals.org/ Downloaded from