Methods for determining the refractory period and excitable gap during persistent atrial fibrillation in the goat

Background-Recently, the temporal excitable gap during atrial fibrillation (AF) has been identified as a vulnerable parameter for cardioversion of AF. In this study, we evaluated 5 methods to measure the refractory period (RPAF) and the excitable period (EPAF) during persistent AF. Methods and Results-In I I goats instrumented with 83 epicardial atrial electrodes, persistent A-F (43 +/- 34 days) was induced with a median AF cycle length (CL) of 98 +/- 14 ms. To measure RPAF, premature stimuli were applied to the center of the electrode array on the right or left atrium. The RPAF measured by mapping of premature stimuli was 70 +/- 12 ms ("gold standard"). The RPAF determined during entrainment of AF was 77 +/- 17 ms (R-2 = 0.88, P < 0.01). Statistical analysis of the effects of synchronized stimuli (each coupling interval X 100) on the AFCL histogram yielded an RPAF of 70 +/- 13 ms (R-2 = 0.94, P < 0.01). A further simplification was to apply slow fixed-rate pacing (I Hz) during A-F. For each stimulus (n = 250 to 500), the paced AFCL was plotted against its coupling interval, and capture was determined by statistical shortening of the AFCL (RPAF 71 +/- 17 ms, R-2 = 0.84, P < 0.01). The 5th percentile of the AFCL histogram as an index of RPAF was 77 +/- 2 ms (R-2 = 0.90, P < 0.01). Conclusions-During persistent AF with an AFCL of 98 14 ins, the RPAF determined by mapping of synchronized premature stimuli (gold standard) was 70 +/- 12 ms, with an excitable period of 28 +/- 8 ins. Although the indirect methods to measure RPAF all correlated well with the gold standard, slow fixed-rate pacing seems to be the most attractive technique because of the ease of acquiring the data and the clear graphic result.