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Figure 2 An example of spontaneous arrhythmia progression over time in a susceptible (A) and resistant (B) dog with ischemic heart disease and moderate left ventricular dysfunction (EF approximately 35%). Note the rapid progression to sustained, incessant ventricular tachycardia in the susceptible dog within three months. In contrast, the resistant dog had isolated premature ventricular contractions after six months of follow-up. Left ventricular ejection fractions in the two dogs were similar. EF = ejection fraction; MI = myocardial infarction.