Spatial Resolution of Atrial Pace Mapping as Determined by Unipolar Atrial Pacing at Adjacent Sites
Background The purpose of this study was to examine the spatial resolution of unipolar atrial pace mapping by pacing at adjacent sites within the coronary sinus and the right atrium.
Methods and Results Unipolar pacing from each pole of a quadripolar catheter was performed in the coronary sinus (n=29) and in the right atrium (n=10). Pacing from the distal electrode was used to simulate the site of origin of an atrial tachycardia. These P waves were compared with the P waves generated by unipolar pacing from each of the three proximal electrodes. The P waves were analyzed for changes in amplitude, duration, and configuration. Pacing within the coronary sinus resulted in significant changes in amplitude and duration at distances of 17 and 21 mm from the distal pole, respectively. Similarly, pacing in the right atrium resulted in significant changes in amplitude and duration at distances of 17 and 32 mm from the distal pole, respectively. No significant changes in configuration were noted in the coronary sinus in any lead at pacing sites ≤32 mm from the distal pole. Configurational changes were noted in the right atrium at pacing sites 17 mm from the distal pole.
Conclusions The spatial resolution of unipolar atrial pace mapping is ≈17 mm. These findings indicate that mapping techniques that depend on the accurate discrimination of P-wave morphology, such as pace mapping or concealed entrainment, are likely to be imprecise when used in the atria.
Mapping techniques for selection of target sites for ablation of atrial tachycardias include atrial pacing in the setting of sinus rhythm (pace mapping) and during tachycardia (concealed entrainment).1 2 3 4 5 The successful application of these techniques depends on the accurate discrimination of P-wave morphology during pacing. However, no prior studies have quantified the spatial resolution of changes in P-wave size or configuration. Therefore, the goal of this study was to quantify the differences in P-wave size and morphology during unipolar pacing within the coronary sinus and the right atrium from the four poles of a quadripolar catheter at known interelectrode distances.
The subjects of this study consisted of 39 patients without structural heart disease who underwent an electrophysiology procedure for the purpose of radiofrequency catheter ablation of supraventricular tachycardia. There were 21 patients with AV nodal reentrant tachycardia, 11 with a concealed accessory pathway, 3 with atrial flutter, and 4 without inducible supraventricular tachycardia. There were 16 men and 23 women, and their mean age was 48±16 years (±SD). The 12-lead ECG did not demonstrate an atrial abnormality in any patient. Inclusion criteria for this study were (1) sinus rate <100 bpm, (2) the ability to perform unipolar cathodal atrial pacing within the coronary sinus and right atrium from each pole of a quadripolar electrode catheter, and (3) catheter stability during cathodal unipolar pacing and recording. Catheter stability was defined by the absence of catheter movement under fluoroscopic inspection.
Informed consent under a protocol approved by the Human Research Committee of the University of Michigan was obtained from all patients. Electrophysiology tests were performed after discontinuation of all antiarrhythmic agents for at least 5 half-lives. The study protocol was performed after completion of the clinically indicated portion of the electrophysiology procedure. Twelve-lead ECGs were recorded during unipolar pacing at a cycle length of 500 ms from each pole of a quadripolar catheter within the coronary sinus or the right atrium. Initially, patients were randomly assigned to undergo the study protocol in the coronary sinus with quadripolar catheters with 5-mm (n=18) or 10-mm (n=11) interelectrode spacing. In 10 additional patients, the study protocol was performed in the right atrium with quadripolar catheters with 5-mm (n=5) or 10-mm (n=5) interelectrode spacing. Via a femoral vein approach, the quadripolar catheter was placed in the coronary sinus with the distal electrode in the region of the posterolateral mitral annulus ≈3 to 4 cm from the coronary sinus ostium or in the right atrium with the distal electrode against the low lateral wall and the proximal electrode against the high right atrium. Pacing from the distal pole was used to simulate the site of origin of atrial tachycardia. Pacing was also performed at poles 2, 3, and 4. The width of the electrodes at poles 2, 3, and 4 of the quadripolar catheter was 1 mm. Relative to the catheter tip, the pacing sites were at distances of 5, 11, and 17 mm with the 5-mm interelectrode quadripolar catheter and 10, 21, and 32 mm with the 10-mm interelectrode quadripolar catheter. The spatial resolution of pace mapping was determined by comparison of the surface ECGs obtained from the distal pole with those obtained from pacing at poles 2, 3, and 4 at diastolic threshold and twice diastolic threshold in the coronary sinus and twice diastolic threshold in the right atrium.
The late diastolic stimulation threshold was determined in incremental steps of 0.1 mA. Unipolar cathodal pacing was performed with a pulse duration of 2 ms with a programmable stimulator (Bloom Associates, Ltd). An electrode catheter placed at the junction of the inferior vena cava and the right atrium was used as the indifferent electrode.6 The mean unipolar atrial pacing threshold at electrodes 1, 2, 3, and 4 were not significantly different with either the 5- or the 10-mm interelectrode spacing catheters, ranging from 1.1±0.3 to 1.8±1.6 and 0.9±0.3 to 1.4±1.0 mV, respectively, in the coronary sinus and from 0.8±0.5 to 2.3±1.9 and 0.6±0.5 to 1.2±1.4 mV, respectively, in the right atrium. Twelve-lead ECGs were recorded at a paper speed of 50 mm/s and at a gain setting of 2 cm/mV on a Mingograph 7 recorder (Siemens-Elema, Inc).
Analysis of P Waves
In each ECG lead, three consecutive P waves were examined for amplitude, duration, and configuration differences according to the following criteria.
The amplitude of P waves was measured to the nearest 0.025 mV (0.5 mm). The isoelectric portion of the PR interval served as the baseline for the measurement of amplitude. The TP interval could not be used as the baseline because the pacing stimulus often obscured the terminal portion of the preceding T wave.
The P-wave duration was measured at a paper speed of 50 mm/s. The first 20 ms after the pacing stimulus was deleted to exclude the pacing artifact from the measurement. Onset of P wave was defined as the first rapid deflection from the isoelectric line.
Changes in P-wave configuration were defined as specific morphological variations, eg, axis shift and notching, from the P wave generated by pacing at the distal electrode.
All ECGs were analyzed by one of the authors. The measurements then were repeated by two other authors to assess for intraobserver variability. A total of 9792 P waves were analyzed. The intraobserver agreement was within 0.05 mV and 2 ms in 65% of the P waves and within 0.1 mV and 5 ms in 95% of the P waves. There was 80% intraobserver agreement in the determination of configurational changes in the P waves. Differences were resolved by consensus.
Data are expressed as mean±SD. Amplitude and duration comparisons were performed by ANOVA with repeated measures. A value of P<.05 was considered significant.
Pacing at threshold from the coronary sinus did not result in P-wave amplitude differences in any of the ECG leads at pacing distances that were <11 mm apart. When the pacing sites were 17 mm apart, a significant difference in mean amplitude of 0.014±0.015 mV was present in only lead I (P<.05, Fig 1⇓). At pacing sites 21 mm apart, 6 of 12 leads demonstrated a significant amplitude difference, ranging from 0.006±0.056 mV in lead V4 to 0.059±0.038 mV in lead aVL (mean, 0.037±0.037 mV; P<.05). At pacing sites 32 mm apart, 9 of 12 leads demonstrated a significant amplitude difference, ranging from 0.009±0.056 mV in lead V5 to 0.072±0.042 mV in lead aVL (mean, 0.032±0.041 mV; P<.05) (Table 1⇓).
Pacing in the coronary sinus at a current strength of twice threshold did not change the proximity at which the amplitude differences were detected. A mean amplitude difference of 0.022±0.029 mV (P<.05) was noted only in lead aVL at pacing sites 17 mm apart. At pacing sites 21 mm apart, 9 of 12 leads demonstrated a significant amplitude difference, ranging from 0.008±0.011 mV in lead V6 to 0.044±0.027 mV in lead III (mean, 0.021±0.024 mV; P<.05). At pacing sites 32 mm apart, 8 of 12 leads demonstrated a significant amplitude difference, ranging from 0.009±0.010 mV in lead V6 to 0.058±0.040 mV in lead III (mean, 0.021±0.023 mV; P<.05) (Table 2⇓).
Similarly, P-wave amplitude differences were not observed in any of the ECG leads at pacing distances <11 mm apart when performed at a current strength of twice threshold in the right atrium. At pacing sites 17 mm apart, a significant difference in mean amplitude of 0.045±0.078 mV was present in only lead V3 (P<.05) (Fig 2⇓). At pacing sites 21 mm apart, 2 of 12 leads demonstrated a significant amplitude difference, ranging from 0.065±0.068 mV in lead aVF to 0.110±0.076 mV in lead II (mean, 0.034±0.072 mV; P<.05). At pacing sites 32 mm apart, 5 of 12 leads demonstrated a significant amplitude difference, ranging from 0.001±0.102 mV in lead V1 to 0.090±0.058 mV in lead V5 (mean, 0.056±0.076 mV; P<.05) (Table 3⇓).
The number of leads demonstrating a significant difference in P-wave duration increased as the distance from the pacing site increased. Pacing at threshold in the coronary sinus did not result in any significant changes in P-wave duration in any surface ECG lead at pacing distances <17 mm. At pacing sites 21 mm apart, 2 of 12 leads demonstrated a significant change in duration: 9 ms in lead aVR and 13 ms in lead I (mean, 11 ms; P<.05) (Fig 3⇓). At pacing sites 32 mm apart, 3 of 12 leads demonstrated a significant change in duration, ranging from 12 ms in lead II to 15 ms in lead I (mean, 14 ms; P<.05) (Table 4⇓).
Pacing at a current strength of twice threshold in the coronary sinus did not change the proximity at which significant duration changes were detected. At a pacing distance of 21 mm apart, 5 of 12 leads demonstrated a significant duration difference, ranging from 4 ms in lead V2 to 11 ms in lead V4 (mean, 8 ms; P<.05). At a pacing distance of 32 mm, 11 of 12 leads demonstrated a significant duration difference, ranging from 5 ms in lead aVL to 14 ms in lead V4 (mean, 8 ms; P<.05) (Table 5⇓).
No significant duration changes were detected at a current strength of twice threshold in the right atrium at pacing distances <21 mm apart. At a pacing distance of 32 mm apart, a significant difference in duration was noted in only lead aVL (P<.05) (Fig 4⇓, Table 6⇓).
Only minor and infrequent differences in P-wave configuration were noted in the coronary sinus among the 12 ECG leads as distance between the pacing sites increased (Figs 5⇓ and 6).⇓ Pacing at twice the current strength in the coronary sinus had no effect on P-wave configuration.
When pacing was performed at twice threshold in the right atrium, there was a progressive increase in configurational changes as the distance between the pacing sites increased. When the catheter with 5-mm interelectrode spacing was used, pacing sites of 5, 11, and 17 mm from the tip demonstrated a change in configuration in a mean of 0.2±1.2, 0.8±3.6, and 1.2±3.6 leads per ECG, respectively (Fig 7⇓). When the catheter with 10-mm interelectrode spacing was used, pacing sites of 10, 21, and 32 mm from the tip demonstrated a change in configuration in a mean of 1.0±3.6, 4.8±6.0, and 4.8±6.0 leads per ECG, respectively (Fig 8⇓).
The results of this study demonstrate that pacing sites as far apart as 32 mm in the coronary sinus and 17 mm in the right atrium can result in P waves that are very similar or identical in appearance. Although changes in P-wave amplitude and/or duration may be detected at pacing sites 17 mm apart, these changes are minor in magnitude and not apparent by visual inspection. These findings indicate that atrial mapping techniques that rely on discrimination of changes in P-wave configuration, such as pace mapping or concealed entrainment, are likely to be inaccurate by at least 17 mm and therefore not clinically useful for identifying potential target sites for catheter ablation of atrial tachycardia or atrial flutter.
Effect of Current Strength
Unipolar pacing from the distal electrode was used to simulate an atrial tachycardia. An increase in current strength might increase the amount of myocardial tissue captured by the pacing stimulus, thereby decreasing the ability to discriminate changes in P-wave morphology. Traditionally, pacing has been performed at twice the diastolic threshold. In this study, pacing also was performed at threshold in the coronary sinus, the minimal energy required for cellular depolarization, to determine whether the resolution of atrial pace mapping could be improved. However, pacing at threshold did not improve the proximity at which amplitude or duration differences were detected, nor did it have an effect on P-wave configuration. The spatial resolution of atrial pace mapping remains poor even when the smallest possible area of atrial myocardium is captured.
Prior studies demonstrated that P-wave morphology may be used only as a rough guide in localizing a region of the atrium.7 8 9 In the present study, the results indicate that within certain regions of the atrium, eg, the posterolateral portion of the left atrium and the lateral wall of the right atrium, inspection of the P wave provides no useful information regarding site of origin. Although the spatial resolution in the right atrium is improved on the basis of configurational changes, no significant changes could be observed within 1.7 cm.
Previous studies have compared differences in QRS configuration during unipolar pacing from adjacent sites in the ventricle to determine the resolution of ventricular pace mapping.10 11 When analysis of the QRS complexes was based on major configurational changes, pacing sites as far apart as 15 mm were similar. However, when minor differences in configuration and amplitude were considered, pacing sites 5 mm apart usually could be distinguished. Therefore, the spatial resolution of unipolar ventricular pace mapping was within 5 mm. The lower resolution power of atrial pace mapping compared with ventricular pace mapping may be attributable to the smaller amplitude of the P wave compared with the QRS complex, to obscuration of the P wave by the preceding T wave,12 and to possible differences in activation patterns between the atrium and ventricle.
Concealed entrainment has been described as being useful in identifying appropriate sites for ablation of atrial tachycardia and atrial flutter.1 2 3 4 5 During classic entrainment, which is not helpful in identifying specific target sites for ablation, there is fusion of the paced P-wave morphology with the P wave generated by the atrial tachycardia or flutter. In contrast, during concealed entrainment, there is no fusion of the P wave, and the P wave during pacing maintains the same configuration as during atrial tachycardia or flutter. Therefore, the differentiation of classic from concealed entrainment depends on the ability to discriminate whether or not there is P-wave fusion during pacing.
In the present study, P waves generated at sites that were up to 32 mm apart in the coronary sinus and 17 mm apart in the right atrium usually could not be distinguished by visual inspection. This implies that the phenomenon of classic entrainment at sites up to ≈1.7 cm away from where concealed entrainment occurs could easily be mistaken to be concealed entrainment. Therefore, although concealed entrainment may be a sensitive indicator of critical sites within the reentry circuit of atrial tachycardia or flutter, limitations in the ability to identify P-wave fusion indicate that in practice, its specificity in identifying appropriate target sites will be low.
A limitation of this study is that the pacing sites were limited to the coronary sinus and the lateral wall of the right atrium. Therefore, the possibility that different results might be obtained in other parts of the atria cannot be ruled out. This study was limited to the coronary sinus and the right atrium because these were convenient, stable, and reproducible locations for pacing.
A second possible limitation is that the analysis of P waves was performed only at a pacing cycle length of 500 ms. This may not accurately reflect the rate-related changes that might occur during atrial tachycardias with a cycle length <500 ms. However, when pacing was performed at shorter cycle lengths, the P waves often were obscured by the preceding T wave.
This study demonstrates that only subtle changes in P-wave morphology occur when atrial pacing sites are between 11 and 17 mm apart. Although significant changes in P-wave amplitude and duration can be detected, the magnitude of the differences are small and usually indiscernible by visual inspection. Even at pacing sites 17 mm apart, the P waves generated by pacing may be virtually indistinguishable. Therefore, mapping techniques that depend on the accurate detection of changes in P-wave morphology, ie, pace mapping and concealed entrainment, are unlikely to be clinically useful in patients with atrial tachycardia or atrial flutter.
Reprint requests to K. Ching Man, DO, University of Michigan Medical Center, 1500 E Medical Center Dr, Ann Arbor, MI 48109-0022.
- Received January 22, 1996.
- Revision received March 13, 1996.
- Accepted March 26, 1996.
- Copyright © 1996 by American Heart Association
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