Time-dependence of inducible ventricular tachycardia in a chronic canine model

The natural history of inducible ventricular tachycardia in post-infarction dogs was followed with serial programmed electrical stimulation (PES: 1-3 extrastimuli, 4 msec duration, 2 x diastolic threshold). Arrhythmia inducibility was defined as a minimum of four unstimulated ventricular ectopic beats. Of 119 dogs prepared for chronic electrophysiological study, 87 (73.1%) were ambulant 24 hours after surgery. Mean infarct size was 11.1 +/- 1.5% of left ventricular (LV) mass for animals dying in the first week, before stimulation. 92.4% of 66 animals were inducible when stimulated at one week, 66.7% at two weeks and 64.3% and 55.6% at the third and fourth weeks respectively (p less than 0.01, Chi-square analysis). Infarct sizes fell from 7.0 +/- 0.5% LV mass at first stimulation to 4.6 +/- 0.8% at third stimulation and could not be visualized thereafter (p less than 0.01,ANOVA). There was no statistical difference between infarct sizes for inducible and non-inducible animals, but in both cases infarcts were smaller (p less than 0.01) than for those animals which died suddenly during the first week. This time-dependent decrease in arrhythmia inducibility, which may be related to infarct size, should be considered when similar models are employed for chronic electrophysiological studies.