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.