Figure 23 shows a patient with salvos of nonsustained ventricular tachycardia. Notice that although a P wave precedes the first QRS complex of the tachycardia, it occurs at the expected time and the PR interval is short. This rules out intermittent right bundle branch block aberrancy, since in this case the PR interval would be the same as the narrow QRS beat. The rate of the wide QRS rhythm is faster than the sinus rate (AV dissociation), so that a P wave does not precede the third or fourth beats of the wide QRS salvos. Finally, the mechanism is probably an automatic site, since the first and subsequent beats of the salvos have the same morphology. By evaluating the QRS morphlogy, the general location of the automatic site can be determined. The right bundle branch block morphology suggests that the site is within the left ventricle, the relative narrowness of the QRS complex suggests that at least a portion of the Purkinje system is used (site must be septal), and finally the superior axis suggests that the ventricle is activated from inferior to superior. Taken together, analysis of the QRS morphology suggests that the automatic site arises near the inferior and midportion of the left side of the interventricular septum. Specialized cardiologists called electrophysiologists use this information to ablate or destroy the abnormal focus if clinically indicated.