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 (RP(AF)) and the excitable period (EP(AF)) during persistent AF. METHODS AND RESULTS: In 11 goats instrumented with 83 epicardial atrial electrodes, persistent AF (43+/-34 days) was induced with a median AF cycle length (CL) of 98+/-14 ms. To measure RP(AF), premature stimuli were applied to the center of the electrode array on the right or left atrium. The RP(AF) measured by mapping of premature stimuli was 70+/-12 ms ("gold standard"). The RP(AF) 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 x100) on the AFCL histogram yielded an RP(AF) of 70+/-13 ms (R(2)=0.94, P<0.01). A further simplification was to apply slow fixed-rate pacing (1 Hz) during AF. 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 (RP(AF) 71+/-17 ms, R(2)=0.84, P<0.01). The 5th percentile of the AFCL histogram as an index of RP(AF) was 77+/-12 ms (R(2)=0.90, P<0.01). CONCLUSIONS: During persistent AF with an AFCL of 98+/-14 ms, the RP(AF) determined by mapping of synchronized premature stimuli (gold standard) was 70+/-12 ms, with an excitable period of 28+/-8 ms. Although the indirect methods to measure RP(AF) 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.