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Figure 5 Measurement of the refractory period (RP_AF) and temporal excitable gap (EG_AF) during atrial fibrillation by slow fixed-rate pacing (8). After four weeks of atrioventricular block and two days of electrical remodeling, 600 stimuli with an interval of 1 s were applied to the fibrillating atria. From a bipolar electrogram close to the pacing site, the random intervals between the fibrillation waves and the stimuli (AF-S intervals) were measured together with the associated atrial fibrillation cycle length (AFCL). Capture was defined as atrial activation within 15 ms after the stimulus. The probability of local capture of the fibrillating atria followed an S-shaped curve (lower right panel). At coupling intervals of <117 ms, capture did not occur, whereas at AF-S intervals of >197 ms the atria were captured 100% of the time. The hatched columns in the lower panels indicate the range of the measured refractory periods. The AF-S interval resulting in 50% capture was taken as the RP_AF (163 ms). The average temporal excitable gap was calculated by subtracting the RP_AF from the median AFCL (218 – 163 ms = 55 ms).