Electrophysiological diagnosis: key role of entrainment pacing
Although a minority pattern, a ”QS” unipolar electrogram was recorded at the breakthrough site of our series of pseudo-focal ATs, confirming previous observation by Pathik et al. Such pattern is theoretically not compatible with an ”epi-endo” breakthrough. However, in clinical practice, unipolar electrograms have been used on the basis of a two-dimensional analysis that considered atrial surface only, without including atrial depth as a third dimension. Thus, the extent to which theory can be applied to the standard recording of a 3-mm thick atrium is unknown, especially if the layers have been previously ablated and are tangentially blending into one another.5 This illustrates a limitation of high-density mapping systems if the operator exclusively relies on activation maps based on automatic electrogram recording and annotation.
From this respect, the present study shows that the strongest evidence for a macroreentry is provided by conventional maneuvers as simple as entrainment pacing. Although mostly close to a collision site, our data show that ”epi-endo” breakthrough sites can sometimes be more than 50 mm remote from a collision site. Hence, entrainment pacing maneuvers should be systematically performed in ATs with centrifugal propagation in order to rule out a macroreentry. Barbhaiya et al. have demonstrated that entrainment pacing with a pacing cycle length of < 20 ms shorter than TCL resulted in an extremely low risk of AT alteration or termination.11 Due to slow conduction at the epicardial bypass, the mean TCL was over 300 ms in our case series. This long TCL might have even reduced the risk.