Abstract
The spatial vectorcardiograms of 20 patients with implanted pacemakers were studied. Ten had left ventricular and 10 right ventricular units. The orientation of the stimulus artifacts did not prove to be an important parameter in the distinction between each type of instrument. The spikes appeared as a sharp deflection which lasted more than the accepted value of 2.5 msec. Initial delays, attributed to conduction through ordinary cardiac muscle, was seen in all patients with left ventricular and in 8 of the patients with right ventricular pacemakers. The morphology of the loops elicited by left ventricular stimulation resembled vectorcardiograms considered diagnostic of right bundle branch block with or without right ventricular hypertrophy. Beats originating in the right ventricle yielded vectorcardiograms similar to those seen in cases of left bundle branch block. One patient had an independent sinus rhythm with left bundle branch block: both natural and artificial QRS loops were similar, but not identical. This case showed that artificially induced apical beats, but not necessarily left bundle branch block, can display abnormal left axis deviation. One patient with an implanted right ventricular pacemaker showed an electrical axis oriented to the right. Since the same phenomenon was seen in another case during transient, intracardiac, pacing, it seems that stimulation of the anterior region of the heart, either slightly to the left or to the right of the interventricular septum can yield QRS complexes oriented to the right and posteriorly. Hence, the morphology of the QRS loop is not an infallible criterion for locating the position of the stimulating electrodes.
