Delayed initial QRS forces of intraventricular pacing: unreliability of the scalar ECG in their diagnosis.

Delta waves, similar to those observed in the Wolff-Parkinson-White syndrome, are currently observed in the ECG tracings of patients with endocardial pacing. With the aid of standard ECG and vectorcardiography (VCG), we studied the initial QRS forces during right endocardial pacing in 20 patients. More details were obtained by VCG concerning the amplitude duration, electrical axis and shape of the delta waves of paced ventricular beats, than by ECG. It is remarkable that delayed initial QRS forces, diagnosed by VCG analysis, were present in only 9 of 20 patients with delta waves in their ECG. In other patients, the slurring or thickening at the onset of the R waves in the ECG resulted from the perpendicular direction of the initial QRS vectors to the lead axis, or from multiple changes in the direction of the early, nondelayed QRS vectors. The superiority of VCG in the study of the initial QRS forces, particularly in the diagnosis of delayed initial QRS forces, is discussed.