Figure 23.

Programmed left bundle branch (LBB) area stimulation. (A) ‘Myocardial’ and selective LBB responses in the same patient. Top panel: fast drive shortens the refractoriness of septal myocardium; then, long pause (S2) prolongs refractoriness of the LBB; consequently, S3 finds myocardium excitable but LBB refractory (myocardial response). Bottom panel: slow drive prolongs refractoriness of the myocardium; then, short-coupled S2 shortens the refractoriness of the LBB; consequently, S3 finds myocardium refractory but LBB excitable (selective response). Reproduced with permission from Jastrzebski.⁸² (B) Double extra-stimuli delivered during intrinsic rhythm. Top panel: no change in QRS morphology with S2 coupling interval of 320 ms. Bottom panel: with S2 of 270 ms, a selective response is observed (note: also splitting of the signal in the endocardial channel).