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Figure 3 Drawing explaining why antiarrhythmic drug studies during programmed stimulation of the heart result in more dependable information in supraventricular re-entrant tachycardia (SVT) than in ventricular tachycardia (VT) occurring in a scar after myocardial infarction. In the tachycardia on the left, a fixed tachycardia circuit is present consisting of atrial tissue, the atrioventricular node-His bundle and bundle branches, ventricular tissue, and an accessory atrioventricular connection. This allows the selection of a drug that blocks conduction in the "weakest" part of the circuit. In contrast, in the scar after myocardial infarction, VT usually has several possible reentry circuits that, because of differences in size and electrophysiologic properties, are affected differently by antiarrhythmic drugs.