This is similar to the z score with

This is similar to the z score, with the mean intra-sample correlation providing a “population mean”. A score greater than −1 was considered a good match (1 point), a score greater than −2 was a partial match (0.5 points), and a score less than −2 was no match (0 points). Spatial resolution was then calculated in a similar fashion to manual matching. In the example shown in Fig. 2, the inter-sample correlation between the template and a location 2mm away was high for ECG, suggesting a good match. On the other hand, the correlation was low for the bipolar EGM recorded simultaneously, suggesting no match. Continuous variables were described as mean and standard deviation and discrete variables as proportions. Spatial resolution of 12-lead ECG and bipolar EGM was compared using Student׳s paired t-test. All tests were two tailed, and a P-value <0.05 was considered statistically significant.
Results With manual matching, the pace match score at the same location as the template was 11.5±0.6 out of 12 for the 12-lead ECG, and 3.0±0.1 for the bipolar EGM. With cutoff values of 10 for the ECG and 2.5 for bipolar EGM, resolution of pace mapping with bipolar EGM (6.1±3.8mm) was significantly better than that for ECG (9.9±5.2mm, P=0.012). With automated matching, the resolution of pace mapping was again better using bipolar EGM compared to that for ECG (4±0mm vs. 9.9±4mm, P<0.001). Fig. 3 shows representative P waves from the template and from pacing sites 0, 11, and 33mm from the focus. At 11mm, no significant change in ECG P-wave morphology was seen, and the match score was 11.5/12. However, for bipolar EGM, the difference in morphology was clearly seen, and the match score was only 0.5. Tables 1 and 2 summarize the pace match scores at each electrode for manual and automated matching, respectively.
Discussion Pace mapping is useful for the management of focal atrial tachycardia [5]. Assessment of P-wave morphology is also useful to identify fusion during entrainment of macro-reentrant atrial arrhythmias [2,3]. However, comparing P-wave morphology to assess pace match is inaccurate. Resolution of pace mapping is usually measured as the shortest distance from the tachycardia focus at which the paced morphology is different. In a previous study using unipolar pacing, Man et al. [4] found that the resolution of pace mapping in the right atrium was more than 17mm. This is in clozapine-n-oxide to the better spatial resolution of 5–10mm in the ventricle [6]. Lower amplitude of the P waves and, sometimes, difficulty in identifying morphology when obscured by the preceding T waves [5] have been implicated as causes of this poor resolution. Paced activation sequence mapping can be used as an alternative [7] to P-wave morphology, but requires the use of multiple electrodes to assess activation. We found that the resolution of pace mapping in the atrium using P waves in the 12-lead ECG was about 10mm, which is better than previously reported. This could be partly attributable to using printed ECGs recorded at a speed of 100mm/s and variable gain, instead of 50mm/s and fixed gain in the previous study. We used changes in morphology only, and did not measure P-wave amplitude and duration as in the previous study [4]. When only P-wave morphology is considered, Mann et al. [4] found differences in morphology in a mean of one lead at a distance of about 10mm. In the study by Mann et al., inter-electrode spacing was also larger than that in our study, 5mm in half of the patients, and 10mm in the other half. This could also have contributed to a larger spatial resolution in their study. The systematic assessment of morphology using the scoring system described by us may have helped improve the resolution of pace mapping. We present an unusual configuration of bipolar EGMs to assess P-wave morphology. Traditionally, bipolar EGMs are recorded between two closely spaced electrodes in the heart. In some situations, polarity of bipolar EGMs may provide information on the direction of activation [8,9]. With these exceptions, EGM morphology in bipolar EGMs does not provide useful information because the short distance between the recording electrodes and their variable spatial relationships make the information unreliable. While unipolar EGMs present more information in their morphology, the directional information relates only to activation moving towards or away from one point. Therefore, they provide no vectorial information about the activation wavefront. We used bipolar EGMs recorded from two widely spaced electrodes because of the more consistent spatial relationship between the recording electrodes. The three electrode pairs provide relatively orthogonal vectorial information on the activation wavefront. Increasing spacing between bipolar electrodes increases the signal amplitude, with more contribution of far-field electrical activity and a resultant increase in far-field to near-field ratio [10]. By increasing the inter-electrode distance significantly, we obtained bipolar EGMs that were large in amplitude and provided a view of global activation. Using intracardiac bipolar EGMs should also alleviate obscuration by T waves, although this has not been specifically tested and does not occur at slower pacing rates we used in this study. We used unipolar pacing to simulate the tachycardia focus because bipolar pacing may have some variation in the site of origin of depolarization resulting from variable contribution of the anode [11,12].