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Ectopic atrial, A-V junctional, and ventricular tachycardias in man have been associated with digitalis medication. Recently it has become possible to distinguish various locations of pacemakers within the specialized conduction system of the ventricles on the basis of the form of the QRS complexes in the standard electrocardiogram. Tachyeardias originating in the left bundle branch and documented by right, left, and His bundle recordings have been produced in animals given excessive digitalis. We have noted a similar tachycardia in a patient with ischemic heart disease receiving digitalis during hypokalemia. The QRS complexes were 0.10 sec in duration and by contour suggested an ectopic focus located in the posterior fascicle of the left bundle; His bundle recordings were consistent with this diagnosis. As the ectopic rhythm became synchronized with a slightly irregular sinus rhythm, bidirectional depolarization of the His bundle with fusion His potentials could be demonstrated.

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