Intraventricular Conduction

or by rapid atrial pacing. Activation times were measured with intracardiac electrode catheters positioned at the right ventricular inflow tract (RVIT), right ventricular apex (RVA), right ventricular outflow tract (RVOT), left ventricular apex (LVA) and left ventricular outflow tract (LVOT). The activation after beginning of QRS in milliseconds ± 1 SD and the number of patients studied at each location were: RVIT normal 23 ± 13 (15 patients); RVIT-RBBB 49 ± 16 (15 patients); RVA normal 18 ± 9 (28 patients); RVARBBB 54 ± 16 (30 patients); RVOT normal 40 ± 10 (28 patients); RVOT-RBBB 78 ± 21 (30 patients); LVA normal 9 ± 9 (18 patients); LVA-RBBB 6 ± 10 (10 patients); LVOT normal 45 ± 13 (10 patients); LVOT-RBBB 32 + 9 (7 patients). Significant differences observed were: RVA-normal versus RVARBBB P < 0.001; RVOT-normal versus RVOT-RBBB P < 0.001; RVA-normal versus LVA-normal P < 0.005; LVA-normal versus LVA-RBBB NS, LVOT-normal versus LVOT-RBBB P < 0.05. The LVOT change was unexpected and suggests changes in left ventricular depolarization may occur when right bundle branch block develops. In patients with RBBB the activation of the RVA (r = 0.82) and of the RVOT (r = 0.68) was directly related to the duration of QRS. Changes in activation time when RBBB was induced by rapid atrial pacing or by introduction of atrial premature depolarizations were: RVA (7 patients) 19 ± 11 to 56 ± 16 (P < 0.001); RVOT (9 patients) 41 ± 10 to 77 ± 22 (P < 0.001); LVA (5 patients) and LVOT (2 patients), small insignificant changes. These data indicate that endocardial activation changes can be evaluated in the catheterization laboratory, that right ventricular conduction becomes slower in RBBB as a direct function of total QRS and that left ventricular conduction may be affected when RBBB develops.