Europace Advance Access published online on March 3, 2008
Europace, doi:10.1093/europace/eun045
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Frequency analysis of the fibrillatory activity from surface ECG lead V1 and intracardiac recordings: implications for mapping of AF
1 Department of Medicine, Taiwan I-Lan Hospital, I-Lan, Taiwan, Republic of China; 2 Division of Cardiology, Department of Medicine, Taipei Veterans General Hospital, 201, Sec. 2, Shih-Pai Road, Taipei, Taiwan, Republic of China; 3 Institute of Clinical Medicine, and Cardiovascular Research Institute, National Yang-Ming University, Taipei, Taiwan, Republic of China; 4 Institute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan, Republic of China; 5 Wan-Fan Hospital, Taipei Medical University, Taipei, Taiwan, Republic of China; 6 Second Department of Internal Medicine, Faculty of Medicine, University of the Ryukyus, Okinawa, Japan
Manuscript submitted 26 November 2007. Accepted after revision 8 February 2008.
* Corresponding author. Tel: +886 2 2875 7156; fax: +886 2 2873 5656. E-mail address: epsachen{at}ms41.hinet.net
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Abstract |
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Aims: Fibrillatory waves observed in the surface electrograms may be a direct reflection of the electrophysiologic mechanism of the atrial fibrillation (AF). This study compared the fibrillatory waves in the surface ECG and the individual intracardiac mapping sites in different types of paroxysmal AF.
Methods and results: Thirty patients with paroxysmal AF originating from the pulmonary veins (PVs) or superior vena cava (SVC) were enrolled. Frequency analysis was performed on the intracardiac electrograms recorded from various mapping sites in both atria sequentially with simultaneous surface electrogram recordings. The SVC–AF patients had a trend toward a higher DF in ECG lead V1 when compared with the PV–AF patients (7.35 ± 2.09 vs. 5.89 ± 0.79 Hz, P = 0.018). The mean dominant frequency (DF) of the LA mapping sites in the PV–AF patients was higher than that in the SVC–AF patients (7.06 ± 0.66 vs. 6.13 ± 0.96 Hz, P = 0.009), whereas the mean DF of the RA mapping sites was similar between the two groups (5.84 ± 0.80 vs. 6.26 ± 1.11 Hz, P = NS). The intra-class correlation coefficient (ICC) between the mean DF of the RA sites and V1 was higher (r = 0.21, P = 0.02) when compared with the mean DF of the LA sites (r = –0.007, P > 0.05). Furthermore, the maximal ICC was observed in the anterolateral RA free wall (r = 0.84, P < 0.001) and not the other anatomic sites of the RA and LA.
Conclusion: The fibrillatory activity observed in ECG lead V1 correlated primarily with the activity of the anterolateral RA free wall and thus may be useful for detecting the AF source if it is close to that area.
Key Words: Atrial fibrillation, Atrial mapping, Electrogram, Frequency analysis
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Introduction |
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Atrial fibrillation (AF) is characterized by irregular activity on the surface ECG. Many investigators have examined the frequency of the fibrillatory waves from the surface ECG in order to investigate the possible effects of antiarrhythmic drugs, cardioversion, and radiofrequency ablation on AF.1
–4 These results demonstrated that AF with a lower frequency or longer cycle length was likely to be terminated by antiarrhythmic medications or atrial pacing.1–3 In addition, several reports also demonstrated that the fibrillatory waves observed on the surface electrogram are capable of assessing the dynamics of the intracardiac fibrillatory activation and may be a direct reflection of the electrophysiologic mechanism of the AF.3,5–8 Surface electrogram lead V1, which has the largest fibrillatory amplitude for analysis,2,3,9 is frequently applied for frequency analysis. The dominant frequency (DF) of surface ECG lead V1 is considered to represent a summation of the right atrial (RA) activation.2,9,10 However, this observation was based on limited intra-cardiac RA recordings. Considering the regional discrepancy of the fibrillatory activity during AF,7,11 whether surface ECG lead V1 accurately reflects the entire or any part of the RA or LA is not clear. The aim of this study was to compare the DF of surface ECG lead V1 in different types of paroxysmal AF and compare the DF between the spectra simultaneously obtained from surface ECG lead V1 recordings and the individual intracardiac mapping sites throughout the RA and LA. |
Methods |
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Study patients
This study enrolled 30 patients with paroxysmal AF originating from the superior vena cava (SVC) and pulmonary veins (PVs) in the contemporary period.
Electrophysiologic study
Each patient underwent an electrophysiological study and catheter ablation in the fasting, non-sedated state after informed consent was obtained. As described previously,7,12–14 all antiarrhythmic drugs except for amiodarone were discontinued for at least five half-lives before the procedure. In brief, an atrial transseptal procedure and selective PV angiography were performed, and RA and LA electroanatomic maps were constructed using an EnSite®/NavXTM (AF Division, St Jude Medical, Inc., St Paul, MN, USA) electroanatomic mapping system during AF. A duodecapolar catheter (electrode width: 1 mm; inter-electrode spacing: 2 mm, AF Division, St Jude Medical, Inc., Minnetonka, MN, USA) was placed along the posterolateral RA and advanced to the SVC with the proximal five bipolar pairs in the RA. The SVC to RA junction was determined fluoroscopically during SVC angiography. A 7-French deflectable decapolar catheter with a 2 mm inter-electrode distance and 5 mm spacing between each electrode pair (AF Division, St Jude Medical, Inc.) was inserted into the coronary sinus (CS) via the internal jugular vein. A 4 mm tipped ablation catheter (EPT, Boston Scientific Corp., Natick, MA, USA) or circular catheter (Spiral-SC, AF Division, St Jude Medical, Inc., Minnetonka, MN, USA) was used for sequential recording of the bi-atrial mapping sites. The circular decapolar catheter (for SVC and PV recordings) had an electrode width of 1 mm, and 3 mm distance between each bipolar recording pair. The 4 mm tipped ablation catheter (for all other bi-atrial sites) had an inter-electrode bipolar spacing of 2 mm (electrode width 1 mm), but the distance between each bipolar recording was 5 mm.
We attempted to find the spontaneous onset of atrial ectopic beats or repetitive episodes of short runs or sustained AF and predict the location of the initiating foci.12,14 When the AF patients presented to the electrophysiology laboratory in sinus rhythm, we first attempted to observe the spontaneous activation during baseline or after an isoproterenol infusion. If spontaneous initiation of AF did not appear, burst atrial pacing was used to induce AF. After AF was sustained for more than 5 min, external cardioversion was attempted to convert the AF to SR and observe for the spontaneous initiation of AF. The methods used to provoke spontaneous AF were attempted at least twice to ensure the reproducibility in all patients. If the AF patients presented to the electrophysiology laboratory in sustained AF, all the bipolar recordings for the frequency analysis were made immediately after the transseptal puncture. Patients with induced or spontaneous initiation of AF had at least a 2 min waiting period before the bipolar recordings were made. If the patients required isoproterenol to induce the AF, the drug was discontinued after the AF occurred and a 5 min waiting period was carried out before any intracardiac recordings were made for the frequency analysis.
Data acquisition and signal analysis
The bipolar atrial electrograms from the multiple recording sites in the LA and RA during AF were recorded using a Cardiolab system (Prucka Engineering Inc., Houston, TX, USA). Seven-second recordings were sampled at 984 Hz and stored on a removable hard disc for offline analysis. The frequency resolution was 0.14 Hz. Each intra-atrial recording was filtered with a second-order, zero-phase Butterworth filter at 40–250 Hz. A second-order, zero-phase low-pass filter at 20 Hz was then applied to the absolute value of the resulting signal. As shown in Figure 1, an attenuation of the QRS-T complex of bipolar signals was performed by an adaptive cancellation technique before the frequency analysis, which has been described previously.3 The final step of the process involved frequency analysis. A fast Fourier transform (FFT) with a Hamming window was performed for each 7 s segment from the multiple recording sites in the LA and RA. The largest peak frequency of the resulting spectrum was identified as the DF. Concerning the duration for the FFT analysis, a longer analysis interval (over 7 s) may have resulted in spectral noise, which may have interfered with the identification of the DF.15 On the other hand, a temporal variation in the DF may have been observed with a short-duration analysis (2 s).16 The stability of the DF over time has been confirmed in previous studies.7,16,17
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DF analysis and individual intracardiac sites and surface ECG lead V1
In the preliminary study, analysing the fibrillatory activity from a complete 12-lead ECG showed that a low fibrillatory amplitude in the other surface ECG leads may inhibit the accurate identification of the DF value of the atrial fibrillatory activity. Therefore, surface electrogram lead V1, which has the largest fibrillatory amplitude for analysis, was applied for the frequency analysis.2
,10 During sustained AF, a FFT was performed on simultaneous recordings (7 s, 984 Hz/channel, frequency resolution 0.14 Hz) from surface ECG lead V1 and multiple intracardiac mapping sites throughout the bi-atria. Regional DF analyses were obtained from individual mapping sites throughout the RA and LA. The RA mapping sites included the SVC, crista terminalis, posterior wall, anterolateral RA free wall, septum, appendage, and cavotricuspid isthmus. The LA mapping sites included the four PVs and their ostia, roof, posterior wall, anterior wall, septum, and mitral annulus area (including the lateral mitral isthmus and medial mitral isthmus regions). At each of these regions, an average of three to five mapping points was obtained to confirm the consistency of the frequency spectra and DF of each region. An LA-to-RA DF gradient was obtained from the averaged DF of the entire LA sites and averaged DF of the entire RA sites.
Statistical analysis
All continuous data were presented as the mean value±standard deviation (SD). A 
2 test with Fisher’s exact test was used for the categorical data. Comparisons of the continuous data were performed with a one-way ANOVA. The difference in the DF between the atrial sites and surface ECG lead V1 was evaluated individually using an intra-class correlation coefficient (ICC). Statistical significance was considered when the two-sided P-value was < 0.05.
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Results |
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Patient characteristics
This study consisted of 15 patients with paroxysmal AF originating from the PVs (male = 11, age = 52 ± 7.6, range = 32–68), and 15 age-matched controlled patients with AF originating from the SVC (male = 2, age = 53 ± 11, range = 33–77) in the contemporary period. There was no difference in the age, underlying disease, LA dimension, RA dimension, or LV ejection fraction between the two groups of patients (Table 1). Only a predominance of females in the SVC–AF patients was noted (P = 0.001). This was compatible with our previous observations.18
In all patients, no anti-arrhythmic drugs were used during the procedure, and all bipolar signals were acquired before the radiofrequency ablation. Overall 15 patients (50%) were in AF during the baseline procedure [paroxysmal atrial fibrillation (PAF) from the PVs = 7, PAF from the SVC = 8]. In the other 15 patients (50%), AF was induced in the laboratory by atrial pacing or an isoproterenol infusion (PV–PAF = 8, SVC–PAF = 7).
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DF measurements of the intracardiac mapping sites
As shown in Table 2, the value of the highest DF for the various intracardiac recordings was similar between the two groups of patients. The mean DF of the LA mapping sites was higher in the PV–AF patients than in the SVC–AF patients (7.06 ± 0.66 vs. 6.13 ± 0.96 Hz, in PV–AF and SVC–AF patients, respectively, P = 0.009); however, the mean DF of the RA mapping was similar between the two groups of patients (5.84 ± 0.80 vs. 6.26 ± 1.11 Hz, P = 0.248). An LA-to-RA DF gradient was evident in the PV–AF patients with an average DF gradient of 1.22 ± 0.91 Hz. Conversely, no LA-to-RA DF gradient was evident in the patients with SVC–AF with an average DF gradient of 0.22 ± 0.85 Hz.
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Comparison of the DF measurements between surface ECG lead V1 and the intracardiac recordings
The DFs from surface ECG lead V1 in both groups of patients are shown in Table 2. The SVC–AF patients exhibited a higher DF in ECG lead V1 when compared to the patients with PV–AF (7.35 ± 2.09 vs. 5.89 ± 0.79 Hz, P = 0.018, Table 2). When comparing the DF in ECG lead V1 and averaged DF for each chamber, the DF in V1 significantly correlated with the averaged DF of all RA recording sites in each patient with a correlation coefficient of 0.42 (P < 0.001). However, it did not correlate with the averaged DF in the LA recordings (P > 0.05). The variation in the DFs in surface ECG lead V1 was not consistent with the fibrillatory activity in the LA.
Frequency analysis of the intracardiac mapping sites showed that the mean DF of the LA mapping sites in the PV–AF patients was higher than that in the SVC–AF patients (7.06 ± 0.66 vs. 6.13 ± 0.96 Hz, P = 0.001), whereas the mean DF of the RA mapping sites was similar between the two groups (5.84 ± 0.80 vs. 6.26 ± 1.11 Hz, P = 0.248, as shown in Table 2).
When comparing the simultaneously obtained DFs in ECG lead V1 and the individual atrial sites, the ICC was higher in all the RA mapping sites (N = 190, r = 0.21, P = 0.02) than the LA mapping sites (N = 190, r = –0.007, P = 0.52), indicating that surface ECG V1 is more likely to represent the fibrillatory activity of the RA. However, there was a difference between the DF observed in the recordings from the individual RA regions and that from surface ECG lead V1 (Figures 2 and 3). There was a mean difference in the DF between the individual RA regions and surface ECG lead V1 and it correlated with the DF value of the individual sites (r = 0.86, P < 0.001, Figure 3).
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Compared to the individual mapping sites in the RA and LA, the mean DF value of the RA anterolateral free wall was similar to the simultaneous lead V1 recordings (6.74 ± 1.07 vs. 6.43 ± 1.21 Hz, paired t-test, P > 0.05). The maximal ICC was observed in the RA anterolateral free wall (r = 0.84, P < 0.001, Figure 4), indicating that the fibrillatory activity of surface ECG lead V1 was consistent with the activity of the RA anterolateral free wall (Figure 2). On the other hand, the ICC for the other intracardiac mapping sites ranged from –0.341 to 0.405, indicating that the fibrillatory activity did not represent the activity at those sites. In Figure 3, in comparison to the other sites, the difference in the DF between the RA anterolateral free wall and V1 approached zero.
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Discussion |
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Main findings
This study demonstrated that the fibrillatory activity in the RA, especially in the area of the RA anterolateral free wall, was revealed by surface ECG lead V1. On the other hand, the fibrillatory activity in surface ECG lead V1 did not represent the fibrillatory activity of the other parts of the LA, RA, PVs, or SVC. This finding may have an implication for the identification of the location of the AF driver as a non-invasive assessment. This study also showed that the DF in surface ECG lead V1 was higher in the patients with paroxysmal AF originating from the SVC, than in the patients with paroxysmal AF originating from the PVs.
Compared with previous studies
Bollmann et al.2 proposed that the fibrillatory activities from RA and CS recordings were revealed as multiple DF peaks on the frequency spectra obtained. Holm et al.10 demonstrated that the surface ECG lead V1 correlated with the mean RA recordings rather than the CS activity. Considering the important role of the PVs and LA in AF, the assessment of the LA and PV activity is mandatory. However, the correlation between lead V1 and the LA recordings has not previously been well defined. Although esophageal recordings may be better for assessing the CS activity when compared with the V1 recordings,10 Prystowsky et al.19 demonstrated that only the distal esophageal recordings revealed only the paraseptal recordings, rather than all parts of the LA. Further, the CS activity did not reflect the LA activity.7,20 This study demonstrated that the rapid activity of the LA/PVs could not be observed in the frequency spectra of surface ECG lead V1. On the other hand, a recent study revealed that the surface ECG lead V1 highly correlated with the RA recordings, when compared with the CS and LA recordings.2,9,10 The question is whether or not the fibrillatory activity of surface ECG lead V1 represents the activity of all parts of the RA chamber or even any of the RA sites. In this study, the frequency analysis of surface ECG lead V1 constantly revealed single DF peaks in all patients, and the fibrillatory activity observed in surface ECG lead V1 may represent the activity of a specific atrial site, rather than the summation of the entire RA chamber.2
Regarding the correlation between V1 and the intracardiac RA recordings, this study further showed that the fibrillatory activity of surface ECG lead V1 correlated with the mean RA recordings with an r value of 0.42. As shown in Figure 3, a difference in the DF between the individual atrial sites and surface ECG existed, and it had not been clarified in previous studies. This study demonstrated the discrepancy in the DF between the individual RA sites and V1, and the discrepancy in the DF depended on the DF value of the individual RA sites. This study showed that there was a minimal difference in the DF between surface ECG V1 and the right anterior free wall recordings, indicating that surface ECG lead V1 did represent only that region. Roithinger et al.21 also demonstrated that the fibrillatory cycle length was highly correlated with the RA free wall recordings with an r valve of 0.97, and the organized activation of that area also influenced the voltage observed on surface ECG lead V1. This could be explained by the fact that ECG lead V1 was anatomically close to the RA anterolateral free wall region. Therefore, we believed that the highest DF would be demonstrated in surface ECG lead V1 only if the AF source was at the RA anterolateral free wall.
Regional discrepancy in the fibrillatory activity in AF patients
It is well known that paroxysmal AF originates from the PVs and SVC with the highest DF in those arrhythmogenic veins.7 Therefore, there is a frequency gradient between the AF source (PVs and SVC) and the substrate (LA and RA, respectively).11,16 The DF in surface ECG lead V1 was higher in the patients with paroxysmal AF originating from the SVC, possibly because it was closer to the AF maintaining source in the SVC.7 However, the DF in ECG lead V1 in the SVC–AF patients was not always higher than that in the PV–AF patients. This could be explained by the variable SVC exit sites to the RA in the individual SVC–AF patients. The frequency breakdown in the wave propagation across the pectinate muscle may further decrease the DF in the RA anterolateral free wall.22
In the patients with paroxysmal AF originating from the PVs, the fibrillatory activity from surface ECG lead V1 did not reflect the activity of the AF source in the LA/PV regions (Table 2). However, previous reports demonstrated that the fibrillatory waves observed on the surface electrogram may be a direct reflection of the electrophysiologic mechanism of the AF.3,5,6 Considering the importance of PV-initiated AF and the regional discrepancy of the AF activation, analysing the regional fibrillatory waves would be more helpful for studying AF patients.
Clinical implications
Considering the regional discrepancy in the AF activation, analysing the regional fibrillatory waves from the intracardiac recordings is mandatory for the mapping and ablation of AF. This study showed that surface ECG lead V1 was not able to assess the fibrillatory activity of the SVC, PVs, and LA regions, which was where most of the AF sources may be located.7,16,23 In the patients with paroxysmal AF originating from the SVC, there was a trend toward recording a higher DF in surface ECG lead V1, than in those with paroxysmal AF originating from the PVs. This non-invasive method was capable of assessing the dynamics of the fibrillatory waves in the RA anterolateral free wall region, and may be useful for detecting the AF sources if they are close to that region.
Limitations
First, the results of this study were based on a retrospective analysis and limited by the patient number. However, in these paroxysmal AF patients, PV and SVC tachycardias may imitate AF in the contemporary period and can represent the fibrillatory activity in most patients with paroxysmal AF. Second, intracardiac signals from the superior PVs and inferior PVs were obtained sequentially. In a subset of patients, the recording time may have taken longer in order to obtain an optimal recording of each PV for the frequency analysis. However, the accuracy of the frequency analysis was based on the temporal stability of the AF activity in a previous study and this study.7,16 Last, the importance of these high DF sites during AF was not confirmed by catheter ablation in this study.
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Conclusion |
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The fibrillatory activity observed in lead V1 only correlated with the RA recordings and reflected the activity of the RA anterolateral free wall. It did not represent the activation of the other atrial sites. This non-invasive method was capable of assessing the dynamics of the fibrillatory waves in the RA anterolateral free wall region, and may be useful for detecting the AF sources if they are close to that area. Patients with SVC–AF had a slightly higher DF in ECG lead V1 than did those with PV–AF because the RA free wall was closer to the driving source within the SVC.
Conflict of interest: none declared.
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Acknowledgements |
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The present work was supported by the Research Foundation of Cardiovascular Medicine, Taiwan (96-02-018).
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