808-4 The Presence and Duration of an Excitable Gap During Ventricular Fibrillation in a Canine Model of Myocardial Infarction

Abstract

An excitable gap (EG) occupying 50% of the cycle length has been reported in a non-infarcted canine model of ventricular fibrillation (VF). To evaluate the presence and duration of an EG in VF post infarction, we studied 9 mongrel dogs at 5 days (n = 5) and 8 weeks (n = 4) post LAO ligation. VF induced by programmed stimulation was recorded using an 8 by 14 electrode array (2.5 mm interelectrode distance) over the infarcted area. The mean VF cycle length (VFCL) was determined at each site. EG was determined by a wavefront analysis propagation method. Sites at which both conduction and block occurred on different cycles were analyzed. At each, the interval from each activation to the subsequent blocked (R-B) or conducted (R-C) impulse was recorded, along with the R-C interval immediately preceding. The recovery period at each site was estimated as the range bounded by the maximal R-B and minimal R-C. The EG was determined by the difference of the recovery period and the mean R-C at each site Fifty eight sites were suitable for analysis. Results of analysis (Mean or mean ± standard deviation): Group Mean R-C Max R-B Min R-C EG Mean EG as %CL 5 Day 154 ± 29 ms 104 ± 31 ms 124 ± 35 ms 29–49 ms 22–31% 8 Week 132 ± 23 ms 97 ± 21 ms 101 ± 25 ms 30–35 ms 23–30% p = 0.002 p = 0.28 p = 0.008 The mean R-C at these sites was longer than the mean VFCL for all sites in the 5 day group (mean VFCL 133 ± 22 ms at 5 days, 130 ± 15 ms at 8 weeks). A trend toward a longer upper limit of EG was seen in the 5 day group (p = 0.08). In 29% of all sites (36% at 5 days, 20% at 8 weeks) a significant EG (≥ 10 ms) was not present. An excitable gap exists at most sites during VF in a canine model of myocardial infarction, but is absent in a minority of sites. The gap may be shorter in absolute duration in chronic infarction (8 weeks) than in subacute infarction (5 days). although the proportion of cycle length occupied by the EG is similar in both groups. The site to site heterogeneity in the magnitude and presence of EG in infarcted myocardium has implications for the use of pacing techniques to modulate VE