Part (a) Ten millisecond isochrones of the simulated normal ventricular activation sequence is shown in cross sections through the mid regions of the apical, middle, and basal myocardial subdivisions defined in Fig . The darkened regions on the left ventricular endocardial surface represent the initial 10 ms isochrone. The computer propagation program generates each time step of the activation as it spreads out from these starting regions. Each line on the diagram represents a 10 ms isochrone. The fourth heavy shaded line is the 40 ms isochrone about midway in the activation. In part 2 (b), left anterior (superior) and inferior (posterior) fascicular blocks (LAFB, LIFB) are simulated by removing the starting points fed by the fan of Purkinje fascicles of the left bundle from one outside subdivision while the starting points of the other remains active as does those fed by the middle fascicles. The early activation of one wall and the late unapposed activation on the side of the blocked fascicle accounts for the significant shift of the frontal QRS axis toward the blocked fascicle, i.e. left anterior superior shift in LAFB and right inferior in LIFB.
16.8 illustrates the graphic output of this propagating excitation
wave in this fine grid heart simulation of normal and fascicular
Figure 16.8 shows a simulation of the sequence of activation seen
in these patients.
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