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Figure 2 Pacing from an outer loop site in the computer simulation is shown. (A) Capturing stimuli produce wavefronts that propagate directly away from the circuit altering the QRS complex and producing QRS fusion. The S-QRS_n is the short interval from the stimulus to the fused QRS after the stimulus. (B) S-QRS_n+1 is the interval required for the stimulated wavefront to travel orthodromically through the circuit to the exit (arrow) to produce QRS_n+1. (C) The stimulated orthodromic wavefront continues through the circuit, returning to the stimulus site. The PPI is the revolution time around the circuit (423 ms). (D) The S-QRS_n+1 interval is 369 ms, equaling the Eg_n+1-QRS_n+2 and also Eg_n+2 - QRS_n+3 interval. Thus, the N + 1 difference is 0. The PPI of 423 ms equals the tachycardia cycle length. See text for further discussion. Eg = electrogram; PPI = postpacing interval; S = stimulus.