Effects of Newly Developed Right Versus Left Bundle Branch Block on the QRS Axis, T-wave Axis and Frontal QRS-T Angle in Patients with a Narrow QRS.

Objective The QRS-T angle has been established as a repolarization marker. In the present study, we determined whether or not newly developed bundle branch block (BBB) affected the QRS-T angle in patients with a narrow QRS. Methods Twenty-four patients with newly developed BBB and no adverse cardiac events were retrospectively included. The frontal QRS-T angle was defined as the absolute value of the difference between the frontal plane QRS axis and the T-wave axis. These electrocardiogram parameters were serially measured in the settings of narrow QRS and BBB. Results Twelve patients had newly developed right BBB (RBBB), and 12 had newly developed left BBB (LBBB). The development of RBBB did not affect the QRS axis, T-wave axis of QRS-T angle (41° ±42° to 53° ±65°, p = 0.63). In contrast, the development of LBBB shifted the QRS axis to the left (25° ±29° to -18° ±31°, p = 0.003), resulting in an increased QRS-T angle (72° ±50° to 123° ±39°, p = 0.001). Regarding RBBB, an excellent correlation and agreement were found between the QRS-T angles in the setting of narrow QRS and RBBB (r = 0.88; p <0.001; bias, 2.9° ±20.9°). However, there was a significant bias between the QRS-T angles in the setting of narrow QRS and LBBB (51.9° ±40.4°; p = 0.001). Conclusion Our data suggested that the QRS-T angle in the setting of RBBB reflected the original QRS-T angle in the setting of narrow QRS well, whereas the QRS-T angle in the setting of LBBB did not.