Relationship of Ventricular Excitability Characteristics to Ventricular Arrhythmias in Dogs

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Abstract
The relationship of the form of ventricular anodal strength-interval curves to the types of arrhythmias induced by trains of low-intensity stimuli was studied in 16 dogs. Strength-interval curves were determined after basic atrial driven beats and after induced premature ventricular depolarizations. The test stimuli for these determinations were 2-msec anodal square waves delivered in 1-msec decrements during the first half of the cardiac cycle and in 10-msec decrements during the last half of the cycle. At each time step, the stimulus intensity was increased in 5-µamp steps until a ventricular response occurred or a level of 1.2 mamp was reached. Three types of strength-interval curves were observed. Each form of the strength-interval curve was associated with a specific type of arrhythmia induced by a train of 50-Hz, 2-msec anodal stimuli of minimum threshold intensity which was applied for periods of up to several minutes. When the minimum threshold in the strength-interval curve following the basic driven beat was lower than the minimum threshold following the induced premature ventricular depolarization, the train of stimuli induced stable ventricular bigeminy. Accelerating ventricular tachycardia followed by ventricular fibrillation resulted if there were deep dips in the strength-interval curves following both the basic driven beats and the premature ventricular depolarizations. When the strength-interval curves following both the basic driven beats and the premature ventricular depolarizations smoothly approached diastolic levels without deep dips or supernormal periods, the train of stimuli caused either occasional premature ventricular depolarizations with long coupling intervals or slow ventricular tachycardia. These relationships may participate in the initiation and the maintenance of arrhythmias.