Post-pacing T loop abnormalities.

Vectorcardiograms of paced and unpaced beats were recorded from 18 patients with implanted pacemakers to investigate the characteristics of post-pacing T inversions. The intraventricular conduction during sinus rhythm of unpaced beats was normal in 13 of 18 patients and abnormal in the other 5 (RBBB in 3, LBBB in 2). The directions of the maximum QRS vector in paced beats (max QRSp.V) and maximum T vector in unpaced beats (max T unp.V) were studied. In 8 of 13 patients with normal intraventricular conduction (group A) and in 3 patients with right bundle branch block (RBBB), the differences between the directions of max QRSp.V and max T unp.V in both frontal and horizontal planes were less than 30 degrees, and in 5 of 13 patients with normal intraventricular conduction (group B) and 2 patients with left bundle branch block (LBBB), differences exceeded 30 degrees. The direction of max T unp.V tended to be superiorly, posteriorly and to the right regardless of max QRSp.V direction. The similarity in direction of max QRSp.V and max T unp.V supports the hypothesis by Rosenbaum et al that post-pacing T inversions may be, in part, explained on the basis of 'cardiac memory'. However the absence of correlation between the max QRSp.V and max T unp.V suggests that the post-pacing T inversions may be caused by other unknown mechanisms.