The effect of an entrainment protocol on ouabain-induced ventricular tachycardia

Overdrive stimulation of reentrant ventricular tachycardias (VT) may result in entrainment and/or termination of these arrhythmias. We investigated whether surface ECG criteria of entrainment can also be observed in nonreentrant VT. For this purpose ouabain-induced tachycardias were used that are considered to be based on delayed afterdepolarizations. In nine conscious dogs, having surgically induced complete AV block, pacing was performed using trains of 20 stimuli from a site distant to the origin of the VT. The pacing intervals were shortened in steps of 5-10 msec, until complete capture from the pacing site or termination of the VT was obtained. During stimulation variable fusion was seen and complete capture of the ventricles from the pacing site occurred just after a slight decrease in pacing cycle length (20 +/- 10 msec). Overdrive stimulation resulted only in 1 out of 58 stimulation trains in termination of VT. Following stimulation it was observed that: (1) The length of the first postpacing interval was significantly longer (P less than 0.001) than both the mean prepacing VT cycle length and interstimulus interval; (2) A change in QRS configuration occurred after 57% of the stimulation trains; (3) The VT accelerated slightly in comparison to the prepacing rate (P less than 0.05); and (4) The length of the first postpacing interval and the mean R-R interval of the VT postpacing were directly related to the interstimulus interval (r = 0.82 and 0.97, respectively). In conclusion, overdrive stimulation of ouabain-induced arrhythmias did not result in entrainment or in termination of the tachycardia. Instead, other responses were seen that may be of help in differentiating between arrhythmias caused by delayed afterdepolarizations and reentry.