Mechanism of idiopathic ventricular tachycardia of the left ventricular origin and the effect of antiarrhythmic agents on it. A study using an entrainment phenomenon.

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The mechanism of idiopathic ventricular tachycardia of the left ventricular origin was studied using a tachycardia entrainment phenomenon. In 4 patients with the tachycardia, rapid ventricular pacing at rates 5-25 beats/minute faster than the tachycardia rate was performed from the right ventricular outflow tract during the tachycardia and resulted in constant fusion beats except for the last captured beat. Moreover, progressive fusion was observed with the increment of the pacing rate. Thus, the mechanism of this type of ventricular tachycardia was best explained by reentry with an excitable gap. During entrainment of the tachycardia, the conduction intervals from the pacing site (right ventricular outflow tract) to the earliest activation site during the tachycardia and to the right ventricular apex were measured: The former interval was 395-430 msec and increased with the increment of the pacing rate, while the latter one was 70-90 msec and was constant even with the increment of th … More e pacing rate. Therefore, an area of slow conduction with a property of decremental conduction was strongly suggested to be present within the reentry circuit of the tachycardia.To examine the effect of verapamil which has been shown to be effective in interrupting the tachycardia, a small dose (1.0 mg) of the drug was administered during the tachycardia, which resulted in slowing the tachycardia rate, and entrainment study was repeated. After administration of verapamil, although the conduction interval from the pacing site to the right ventricular apex remained unchanged, that from the pacing site to the earliest activation site during the tachycardia (i.e., conduction via an area of slow conduction) was increased in al patients. This indicates that verapamil selectively suppressed the conduction through an area of slow conduction and thereby slowed the rate of the tachycardia. Tachycardia entrainment is useful to examine a selective effect of antiarrhythmic agents within the reentry circuit. Less