Importance of using standard rather than torso surface electrocardiographic leads for pacemapping at the right ventricular outflow tract

Although pacemapping has been used to localize the origin of ventricular tachycardia, the effect of changes in the position of ECG electrodes during ventricular pacing remains unknown. To clarify the relationship between the position of ECG limb electrodes and QRS configuration during pacemapping at the right ventricular outflow tract (RVOT), RVOT pacing was performed on 12 patients at eight pacing sites located in the anterior, septal, lateral, and posterior portions each in the high and low RVOT. Standard and torso ECGs were recorded simultaneously during each pacing protocol, and the QRS axis and amplitude were compared between the two ECGs. Differences between sites in the horizontal plane and in the longitudinal direction were also compared. The QRS axis on the torso ECG was significantly more rightward than that on the standard ECG at all eight pacing sites (72.1+/-17.4 vs 64.0+/-21.9 degrees). The magnitude of differences in the QRS axis and amplitude between the anterior and other sites at the same height was significantly greater in the standard ECG in all locations and in 7 of 18 comparable leads, respectively. The magnitude of differences between high and low sites was significantly greater in the standard ECG in three of four locations and in 5 of 12 comparable leads, respectively. In conclusion, the torso ECG is less sensitive to changes in pacing site at the RVOT than the standard ECG. The torso ECG is, therefore, not proper for pacemapping in attempts to ablate ventricular tachycardia arising from the RVOT.