Is there longitudinal dissociation in the undamaged his bundle? In vitro studies in the normal canine heart

We investigated the concept of longitudinal dissociation in the His-Purkinj e system in vitro. Hearts were excised from the eleven anesthetized dogs an d a septal preparation containing the exposed His bundle and the entire rig ht bundle branch and left bundle branch were displayed in a two-dimensional arrangement pinned to the bottom of a superfusion chamber. Tyrodes solutio n, gassed with 95% O-2 and 5% CO2, at 37 degreesC was continuously passed o ver the preparation. Pacing was performed over a wide range of heart rates (30 to 180/min) from the proximal His bundle and by moving bipolar electrod es we monitored activation at various sites along the right and left bundle branch. The earliest site of muscle activation (27 +/- 2 msec) on the left septum was a relatively large area in the midposterior septal region; wher eas, on the right septum the earliest site of activation (27 +/- 3 msec) wa s a relatively small zone at the base of the anterior papillary muscle, (p, N.S). The larger area of early activation on the left compared to the righ t is consonant with a left to right septal vector accounting for the Q wave in the standard bipolar leads in the normal heart and the loss of Q waves in left bundle branch block. We conclude that His-Purkinje and ventricular muscle activation is remarkably synchronous on both sides of the heart and accounts for optimal contractile function during His bundle or biventricula r pacing compared to standard site ventricular pacing, particularly in pati ents with left ventricular dysfunction.