Focal atrial tachycardia: increased electrogram fractionation in the vicinity of the earliest activation site

Fractionated electrograms are often noted during mapping of focal atrial tachycardia (FAT). This finding suggests poor cell-to-cell coupling, which is thought to be an important prerequisite in the process of ectopic impulse initiation and propagation. The purpose of the present study was to assess the electrogram fractionation in the vicinity of the earliest activation site and in the remaining atrium in these patients.

Thirteen patients with FAT (age 48 +/- 17 years) who underwent catheter ablation were investigated. Mapping was performed with the CARTO system. Electrogram fractionation was assessed on the basis of the number of negative deflections, both in the region surrounding the earliest activation site and in the remaining atrium. Unipolar and bipolar peak-to-peak voltage and bipolar electrogram duration were also studied. All patients underwent successful radiofrequency ablation. A higher degree of electrogram fractionation existed in the region surrounding the earliest activation site and activated within the first 15 ms when compared with the remaining atrium (incidence of bipolar electrograms with multiple negative deflections: 88 vs. 79%, P < 0.0001; incidence of unipolar electrograms with multiple negative deflections: 56 vs. 43%, P = 0.0001). The peak-to-peak voltage in the region activated within the first 15 ms was less than that in the remaining atrium (bipolar voltage: 1.33 +/- 0.99 vs. 1.61 +/- 1.11 mV, P < 0.001; unipolar voltage: 1.75 +/- 0.92 vs. 1.95 +/- 1.11 mV, P = 0.0188). There were no significant differences in bipolar electrogram duration. Within the region activated during the first 15 ms, from the periphery to the earliest activation site, there was a gradual increase in electrogram fractionation (incidence of bipolar electrograms with multiple negative deflections gradually increasing from 82 to 100% and incidence of unipolar electrograms with multiple negative deflections increasing from 56 to 90%), as well as a gradual decrease in peak-to-peak voltage (bipolar voltage gradually decreasing from 1.47 +/- 1.06 to 0.89 +/- 0.54 mV, P < 0.0001; unipolar voltage gradually decreasing from 1.89 +/- 0.94 to 1.30 +/- 0.63 mV, P < 0.0001). Irregular, closely spaced isochrones were also noted in the region activated during the first 15 ms. The area of this region was 4.88 +/- 3.59 cm(2).

Increased electrogram fractionation exists within a relatively wide region around the tachycardia origin when compared with the remaining atrium. Moreover, this region is electrically heterogeneous, as suggested by the fact that the degree of electrogram fractionation increases gradually whereas the electrogram voltage decreases gradually towards the earliest activation site. These findings suggest that a non-discrete atrial region with gradually changing electrophysiological properties may underlie the substrate of FAT.