ECG and VCG changes in experimental hemiblock and bifascicular block.

ECG and VCG changes in QRS configuration before and after left anterior subdivision block (LASB), isolated and combined with right bundle branch block (LASB combined with RBBB), were studied in canine hearts. In another series left posterior subdivision block (LPSB), isolated and combined with right bundle branch block (LPSB combined with RBBB), were also performed. With LASB, the A QRS axis deviated superiorly to the left and the QRS duration showed no significant prolongation. Additional RBBB to the existing LASB, moreover, rotated the A QRS axis counterclockwise, and prolonged the QRS duration significantly. With LPSB, the axis was deviated inferiorly to the right and the QRS duration showed no significant prolongation. Additional RBBB to the existing LPSB rotated the A QRS axis clockwise. The QRS duration showed significant prolongation. Vectorcardiographically, the initial part of the QRS loop did not show any essential changes with either LASB or LPSB. The main and terminal parts of the QRS loop, however, were deviated superiorly and to the left with LASB and inferiorly and to the right with LPSB. No changes were observed in the direction of rotation of QRS loop with either subdivision block. From these results it appears that the changes in the terminal forces are more of importance than the initial forces in the diagnosis of hemiblock.