The measurement of the PPI of
The measurement of the PPI of DP following entrainment pacing usually consists of measuring the interval between the last capturing stimulus and the next DP. This technique is generally based on the assumption that, in the presence of 1:1 capture, pacing depolarizes the zone of slow conduction in the reentry circuit that generates the DP [1,2]. In the present case, the interval between the last stimulus and the following DP might be misidentified as the PPI, if one believes that entrainment pacing directly captured the zone of the slow conduction in a 1:1 manner despite the lack of capture of the ventricular myocardium by the last stimulus.
The 2:1 captures were comparable to single extrastimuli delivered at a fixed coupling interval in a bigeminal pattern during VT. When the extrastimulus penetrates the zone of slow conduction, its return dna stain is identical to the PPI after entrainment pacing [3,4]. Because 2:1 pacing captured the ventricular myocardium only outside the zone of slow conduction, the interval between the pacing stimulus and the ventricular deflection following the subsequent DP should be measured as the return cycle. However, the ventricular deflections during entrainment pacing were superimposed upon the pacing artifact from the non-captured stimulus. While one might assume that the ventricular deflections in timing have the same interval between the DP and the ventricular deflection as that during VT, the true PPI measured between the captured spike and the presumed ventricular deflection would be longer than the TCL, suggesting a remote bystander as the captured ventricular myocardium outside the isthmus, which might be located beneath the sheet of “higher threshold scar tissue” .
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