Impedance compensated biphasic waveforms for transthoracic cardioversion of atrial fibrillation: a multi-centre comparison of antero-apical and antero-posterior pad positions
Simon J. Walsh1,
David McCarty1,
Anthony J.J. McClelland1,
Colum G. Owens1,
Tom G. Trouton2,
Mark T. Harbinson2,
Siobhan O'Mullan2,
Andrew McAllister3,
Brian M. McClements3,
Mike Stevenson4,
Gavin W.N. Dalzell1 and
A.A. Jennifer Adgey1,*
1Regional Medical Cardiology Centre, Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, Northern Ireland, UK
2Cardiology Department, Antrim Area Hospital, Bush Road, Antrim, Northern Ireland, UK
3Cardiology Department, Mater Infirmorum Hospital, Crumlin Road, Belfast, Northern Ireland, UK
4Department of Epidemiology and Public Health, Queen's University, University Road, Belfast, Northern Ireland, UK
Received 16 July 2004; revised 26 January 2005; accepted 3 February 2005; online publish-ahead-of-print 11 April 2005.
* Corresponding author. Tel: +44 28 90240503; fax: +44 28 90312907. E-mail address: jennifer.adgey{at}royalhospitals.n-i.nhs.uk
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Abstract
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Aims To compare the success rate for transthoracic direct current cardioversion (DCC) of atrial fibrillation (AF) with antero-posterior (AP) and antero-apical (AA) electrode positions using an impedance compensated biphasic (ICB) waveform.
Methods and results Three-hundred and seven patients [mean age 66 (SD±13), 195 male] with AF were recruited in three centres. Patients were randomized to an AA (n=150) or AP (n=144) pad position. Thirteen patients with implanted pacemakers were defaulted to the AP pad position. Cardioversion was performed using an ICB waveform with a 70, 100, 150, and 200 J energy selection protocol. If the fourth shock was unsuccessful, the pads were crossed over to the alternative position for a final 200 J shock. Shock 1 was successful in 54/150 (36%) AA and 45/144 (31%) AP patients, whereas success was achieved by shock 2 in 99/150 (66%) AA and 74/144 (51%) AP, by shock 3 in 123/150 (82%) AA and 109/144 (76%) AP, and by shock 4 in 143/150 (95%) AA and 127/144 (88%) AP and after cross-over in 144/150 (96%) AA and 135/144 (94%) AP. Overall success rate was higher than expected at 95%. Pad position was not associated significantly with success. There was a trend towards an improved outcome with the AA configuration (P=0.05).
Conclusion The influence of pad position for DCC of AF may be less pertinent with ICB waveforms than with monophasic waveforms.
Key Words: Biphasic waveform Atrial fibrillation Direct current cardioversion Pad position
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Introduction
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Electrical cardioversion is a commonly performed procedure for the treatment of atrial fibrillation (AF). Direct current cardioversion (DCC) has been successfully performed since the 1960s, mainly using damped sine wave monophasic waveforms.1 However, advances have occurred in defibrillation over the last two decades. Biphasic waveforms have been shown to be more efficient in terms of peak voltage and delivered energy than monophasic waveforms of equivalent duration both for endocardial atrial2 and for ventricular defibrillation.36 Transthoracic impedance (TTI) has been shown to be a determinant of atrial7 and ventricular defibrillation8 and is now widely accepted as an important predictor of success.9 The new generation of external defibrillators have therefore been developed to deliver impedance compensated biphasic (ICB) waveforms.
External defibrillators that deliver biphasic waveforms have recently been shown to be more efficacious for the treatment of AF, where lower energy ICB waveforms are equivalent or superior to their higher energy monophasic counterparts.1014 Biphasic waveforms also cause less skin burns during DCC.11 Biphasic cardioversion is also associated with less skeletal muscle damage13 and less pain post-cardioversion.14 In addition, fewer shocks are needed with a biphasic device,11 potentially leading to shorter cardioversion procedures.
AF is a common arrhythmia that can cause unpleasant symptoms and haemodynamic sequelae. Although the rationale of rhythm control has been challenged by recent studies,15,16 electrical cardioversion is likely to remain an important therapeutic option for the treatment of symptomatic AF. Therefore, optimization of this technique remains important.
Two studies have suggested that an antero-posterior (AP) electrode configuration is superior to an antero-apical (AA) electrode configuration when monophasic waveforms are used for DCC of AF.17,18 However, it has been suggested that these results are not applicable when ICB waveforms are employed during the procedure.19 In addition, selection of an appropriate initial energy can reduce the number of shocks required for the cardioversion procedure. This has implications for the overall duration of the procedure (including exposure to sedation or anaesthesia), the total energy delivered, and the likelihood of post-cardioversion skin burns. The optimal initial energy for biphasic cardioversion of AF has not been defined.
We compared the energy requirements and success rate for cardioversion of AF using AP and AA electrode positions, utilizing an ICB waveform (Heartstream XL defibrillator).
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Methods
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We hypothesized that the AP electrode configuration would be superior to the AA configuration. We estimated that a sample size of 150 patients per group would detect an absolute difference in efficacy between the electrode configurations of 12%, with a power of 80% (at a significance level of 0.05). The main endpoint of the study was the stage in the protocol at which the patient was successfully cardioverted. We estimated that the AP configuration would lead to a success rate of 90% (based on the BiCard study results11), and that the absolute difference between the AA and the AP configurations would be
12% based on the findings of Botto et al.17 Ethical approval was obtained from the Queen's University Belfast Research Ethics Committee and the study was compliant with the Declaration of Helsinki. Three-hundred and seven patients undergoing elective cardioversion for AF were recruited for the study, after giving informed consent, in three centres between May 2002 and June 2003. Two-hundred and ninety four patients were randomized prospectively to an AP or AA pad position. Pad position was assigned according to a prepared schedule and was based on the order of the patient's arrival on the ward on the day of the procedure. Thirteen patients with right-sided pacemakers were also included in the study; these patients were defaulted into the AP pad position. Patients were excluded if they were <18 years old, unable to provide informed consent or had any contraindication to the procedure (inadequate anticoagulation, electrolyte disturbance, digoxin toxicity, known intracardiac thrombus). Patients with atrial flutter were also excluded. The duration of AF was determined as the time from first ECG documentation. The arrhythmia was classified according to the current guidelines on the management of patients with AF (persistent, paroxysmal, or recurrent).20 None of the patients with recurrent AF had failed an electrical cardioversion in the past.
All medications taken at the time of the cardioversion were recorded, the patients' height and weight were measured before the procedure and body mass index (BMI) calculated. Other cardiac disease was determined by a review of the patients' hospital records. Transthoracic echocardiograms had been recorded within 24 months of the procedure in 183 patients, and these were used to obtain left atrial dimensions and left ventricular systolic function. TTI was calculated by the defibrillator for each shock and recorded on the printed rhythm strip.
Patients were fasted overnight prior to the procedure and digoxin was omitted for 24 h. Sedation was administered with either midazolam or propofol at the physician's discretion. All shocks were R-wave synchronized.
All cardioversions were performed using a Heartstream XL defibrillator (formerly Agilent Technologies, now Philips Medical Systems, Andover, MA, USA) and self-adhesive electrode pads (Agilent Adult Plus Electrode pads ref: M3713A). The defibrillator employs an ICB truncated exponential waveform that has been described previously.11 For the AP pad positions, electrodes were applied to the right upper parasternal area and left lower scapular region. The AP configuration was identical to that described by Botto et al.17 and has previously been demonstrated to be superior to an AA configuration when a monophasic waveform was employed for DCC. For the AA positions, electrodes were applied to the right infraclavicular region and over the cardiac apex.
Cardioversion was attempted with escalating shock energies until success was achieved or the protocol was completed. Shocks were delivered at 70, 100, 150, and 200 J. If the fourth shock was unsuccessful, the pads were switched to the alternative position (cross-over) and a further 200 J shock was attempted. Success was defined as the restoration of sinus rhythm for at least 30 s.
Statistics
The
2 test was used to assess differences in categorical variables, and the independent sample t-test was used to assess differences in continuous variables between the two groups. A crude analysis of the effect of pad position on the probability of success at each stage was carried out using ordinal regression analysis. A test of parallel lines was carried out to verify the assumption of proportional odds. This model was subsequently adjusted for potential predictors of successful cardioversion (gender, BMI, and duration of AF and TTI). The main endpoint of the study was the stage in the protocol at which the patient was successfully cardioverted. For the purposes of this analysis, all patients who failed to defibrillate at the fourth shock were considered to have failed in the cardioversion protocol. Therefore, the outcome of the cross-over shock was not included in the analysis. All statistical tests were two-sided. All data were processed using the SPSS 12 for Windows software package. A P-value of less than 0.05 was taken as significant.
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Results
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A total of 322 patients were screened for the study (Figure 1). Of the 307 patients included in the study, 195 (64%) were male. For the whole group, mean age was 66 (SD±13), mean duration of AF was 23±41 weeks, mean BMI was 28±5 kg/m2, and mean TTI measured at the time of the first shock was 87±20
. A total of 150 patients were included in the AA group and 144 patients in the AP group (totalled 157 when pacemaker patients were included).
The baseline characteristics for all AA and AP patients are summarized in Tables 1 and 2. The groups were well matched for baseline characteristics, although, there were significantly more hypertensive patients in the AP group (P=0.02). The TTI for the first shock was significantly lower in the AP group (P=0.02).
The main results of the study are presented in Figure 1. Shock 1 was successful in 54/150 (36%) AA and 45/144 (31%) AP patients, success was achieved by shock 2 in 99/150 (66%) AA and 74/144 (51%) AP, by shock 3 in 123/150 (82%) AA and 109/144 (76%) AP, and by shock 4 in 143/150 (95%) AA and 127/144 (88%) AP. The success rate for the total of 294 by the fourth shock was 270/294 (92%). After cross-over, this rose to 279/294 (95%).
There was a progressive decrease in TTI for the first four shocks in both pad positions. For the AA position, TTI fell by a mean of 4.3±1.6
between shocks 1 and 2, 2.1±1.6
between shocks 2 and 3, and 2.0±0.8
between shocks 3 and 4. The corresponding decreases for AP patients were 3.9±4.5, 1.9±2.0, and 1.5±0.9
, respectively. However at cross-over, the TTI rose by a mean of 12.1±7.8
when changing from AP to AA position and fell by 11.9±9.6
when changing from AA to AP position.
The main analysis was performed by means of ordinal regression. Excluding the patients with pacemakers, there was borderline evidence to suggest that the AA pad position was associated with an improved outcome (P=0.05). The unadjusted estimated odds [common odds ratio (OR) from ordinal regression] of having success are 0.66 for those patients in the AP pad position when compared with those in the AA pad position (95% CI: 0.440.99). After adjusting for known predictors of successful cardioversion (gender, BMI, and duration of AF and TTI), pad position was not found to significantly influence success (P=0.06; OR=0.64; 95% CI: 0.401.02). We did not find evidence that the AP configuration was superior to the AA configuration. In addition, there was a trend towards an improved outcome with the AA configuration.
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Discussion
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Historically, AP configurations have been considered to provide a superior shock vector through the atria when compared with the AA configuration.21,22 However, the actual path taken by current through human tissue is complex, with a small per cent (
4%) of current delivered by the transthoracic route reaching the heart in an AA configuration.23 The majority of current is shunted around the heart (around the thoracic cage or through the lungs). The route taken by current during AP defibrillation has not been described. Several AP pad positions have been described. Some authors feel that a right anterior to left posterior configuration is better where the underlying pathology involves both atria, whereas a left anterior to posterior configuration is better when the left atrium is primarily affected,22 although Kerber24 indicated that there was no difference among apex-anterior, apex-posterior, and anteriorposterior configurations. The right anterior to left posterior configuration (as used in our study) is described,10,17,25 as are the left anterior to right posterior11 and AP midline configurations.18,24 No data have been published showing superiority of one AP configuration over the other. Nevertheless, the exact electrode position may influence cardioversion outcome with biphasic waveforms. At the time the study protocol was written, the AP configuration employed in our study was the only one that was shown to be superior to an AA configuration.17
In addition, the per cent success obtained with our AP configuration is very similar to that obtained in the BiCard study (where an identical biphasic waveform was used),11 where the left anterior to right posterior configuration was used in contrast to our investigation. At 100 J, in our study 51% success was achieved when compared with 60% in the BiCard study. Comparable efficacy is also seen at 150 J (76 vs. 77%, respectively) and at 200 J (88 vs. 90%). However, this comparison must be treated with some caution, as the study populations were different. Direct comparison of our results with other defibrillators that employ ICB waveforms is difficult owing to the different methodology, varying technology employed, and different patient populations.
Our data confirm that there was a statistically significant difference in the first shock TTI among patients randomized to the two electrode configurations. The most recent data that suggested an important effect of pad position on DCC success were reported in 2002 by Kirchhof et al.18 This group reported that a midline AP electrode configuration18,24 was superior in achieving success in patients (n=108) receiving monophasic defibrillation. Our use of ICB waveforms better reflects contemporary practice and will, we believe, become the gold standard in the future. It has been demonstrated that TTI is an important predictor of success in defibrillation.79 Although Kirchhof et al.18 did not report TTI in their patient group, we have shown that the AP pad position is associated with a lower TTI in a large number of patients. Kirchhof et al. used hand-held paddles (application of pressure on paddles is known to alter TTI) and acknowledged that this may have contributed to cardioversion success in their study. A lower TTI will certainly facilitate the delivery of more current to the heart. Furthermore, it is known that membrane polarization during defibrillation is strongly influenced by tissue anisotropy and the curvature of myofibres.26 Although the AP configuration may result in greater current delivery than the AA positions (via lower TTI with monophasic waveforms), the shock vector may be a less important determinant of successful cardioversion as such a small per cent of delivered current is likely to reach the heart from any given electrode configuration. We have shown that when the influence of TTI is diminished with a modern defibrillator that delivers a biphasic waveform, the AA configuration is at least as efficacious as the AP configuration (and indeed may be more favourable in terms of atrial anatomy and tissue anisotropy). Our results were similar in the AP group to the BiCard study at equivalent energy settings, despite the theoretically different AP shock vectors. Therefore, the significance of electrode positioning may be less pertinent than thought previously when machines that compensate for TTI deliver biphasic waveforms. Other investigators have recently drawn similar conclusions with regards to TTI.27 The improved efficacy of biphasic waveforms may further reduce the influence of shock vectors, thus explaining the unexpectedly high success rate in the AA position.
Our results show that a 150 J ICB shock is necessary to give a 82% success rate (AA pad position). This finding is in keeping with data published by Page et al.11 and by Marinsek et al.,13 where the same defibrillator was used. A comparison of cumulative successes with other biphasic trials of DCC is presented in Table 3. In general, shocks of
150 J are associated with a success rate of at least 80%. Some devices are more successful at lower energies. However, direct comparison of results across the different studies is very difficult and potentially misleading in view of the different methodologies, devices employed, and patient populations. Thus, recognizing that cardioversion success is likely to be lower in clinical practice than in randomized trials, we would suggest a first shock of at least 150 J delivered via pads in the AA position as a starting energy for biphasic DCC of AF. This should minimize the number of shocks given to patients.
Starting the procedure at the highest effective energy is a manoeuvre favoured by some physicians in order to minimize the number of shocks and total procedure time. It is also known that a high cumulative energy exposure (median 960 J) during DCC does not lead to troponin elevation.29 Therefore, some physicians may prefer an initial 200 J ICB shock.
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Conclusions
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The use of an AP pad configuration is associated with a lower TTI. When an ICB waveform was employed for cardioversion, we did not find the AP electrode configuration to be superior to the AA configuration. There was a trend towards an improved outcome with the AA configuration. The influence of pad position for DCC of AF may be less pertinent with ICB waveforms than with monophasic waveforms. Initial energy selection should be at least 150 J to provide
80% success with an ICB waveform.
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Acknowledgements
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S.J.W. is the recipient of a Research and Development Fellowship from the Northern Ireland Health and Personal Social Services Office. Shervin Ayati of Philips Medical provided defibrillators and pads to carry out this study.
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