Please click here to obtain permission to reproduce this image.
Click on image to view larger version.
Figure 1 (Top) A flow chart of the mapping scheme is shown. If pacing entrains tachycardia with concealed fusion the postpacing interval (PPI) or S-QRS interval is assessed to determine if the site is in the reentry circuit. If the site is not in the circuit it is classified as an adjacent bystander. If the site is in the circuit the S-QRS interval expressed as a percentage of the ventricular tachycardia cycle length (VTCL) is used to classify the site relative to the circuit exit. Sites with S-QRS <70% of the VTCL (exit, central and proximal sites) have the highest incidence of tachycardia termination and are referred to collectively as isthmus sites. If pacing entrains tachycardia with QRS fusion and the PPI is consistent with a reentry circuit site, the site is classified as an outer loop site. Sites where pacing entrains VT with QRS fusion and the PPI exceeds the VTCL are remote bystanders. EG-QRS = the interval from the electrogram recorded at the pacing site during VT to the QRS onset. See text for discussion. (Bottom) A theoretical reentry circuit is shown. The reentry wave fronts are indicated by the black arrows. The gray arrows indicate excitation wave fronts from the circuit that depolarize tissue that is not in the circuit (bystanders). The gray stippled areas represent inexcitable regions in the chronic infarct. The circuit contains two loops and a common pathway through which conduction is slowed. The common pathway is a relatively small mass of tissue in the chronic infarct, depolarization of which generates low amplitude signals that are not detectable in the standard body surface electrocardiogram. The QRS complex is inscribed after the excitation wave front leaves the common pathway at the exit, and begins propagating around the border of the scar through the outer loop. The excitation wave fronts then return back to the entrance of the common pathway. Several regions that are in the chronic infarct but do not participate in the circuit are labeled as bystanders.
|
