A model of chronic ischemic arrhythmias: the relation between electrically inducible ventricular tachycardia, ventricular fibrillation threshold and myocardial infarct size.

To study the relation between inducible ventricular tachycardia and ventricular vulnerability, myocardial infarction was created in 22 closed chest mongrel dogs by inflating a balloon catheter in the left anterior descending coronary artery for 2 hours. The presence of inducible ventricular tachycardia was determined by programmed electrical stimulation of the right ventricle in each dog before and 4 days after infarction, using a transvenous electrode catheter and a "clinical" stimulation protocol. In each dog the repetitive ventricular response threshold and the ventricular fibrillation threshold were measured before and 4 days after infarction. Ventricular tachycardia was not inducible in any dog before infarction. After infarction, sustained ventricular tachycardia was inducible in 10 (45 percent) of 22 dogs and nonsustained tachycardia in an additional 4 dogs (18 percent). Ventricular fibrillation threshold was greatly reduced 4 days after infarction in dogs with inducible sustained tachycardia (mean +/- standard deviation 29 +/- 11 to 10 +/- 5 mA, p less than 0.001); the mean threshold did not change significantly in dogs without inducible sustained tachycardia. Both the ventricular fibrillation threshold and mean ventricular repetitive response threshold were reduced in the dogs with sustained ventricular tachycardia; neither was significantly altered in the dogs without sustained tachycardia. The magnitude of change in the two thresholds frequently differed; hence, a correlation was weak between the control and postinfarction repetitive response/fibrillation threshold ratio (r = 0.41). Postmortem measurement of infarct size demonstrated an association between this measurement and the presence of inducible ventricular tachycardia. Sustained ventricular tachycardia was not inducible in the presence of a small infarct. It is concluded that: (1) inducible ventricular tachycardia on the 4th day after myocardial infarction is associated with a considerable decrease in the ventricular fibrillation threshold; (2) changes in the repetitive response and fibrillation thresholds after myocardial infarction may not be parallel, complicating the use of the repetitive ventricular response threshold as a substitute for the ventricular fibrillation threshold in the postinfarction state; (3) a large infarct predisposes the heart to electrically inducible sustained ventricular tachycardia.