- Email: [email protected]
A comparison of rectilinear and truncated exponential biphasic waveforms in elective cardioversion of atrial fibrillation: A prospective randomized controlled trial
Resuscitation 84 (2013) 286–291
Contents lists available at ScienceDirect
Resuscitation journal homepage: www.elsevier.com/locate/resuscitation
Clinical paper
A comparison of rectilinear and truncated exponential biphasic waveforms in elective cardioversion of atrial fibrillation: A prospective randomized controlled trial夽 Charles D. Deakin a,∗ , Stephanie Connelly a , Rupert Wharton a , Ho Ming Yuen b a b
Department of Anaesthetics, University Hospital Southampton NHS Foundation Trust, Southampton SO16 6YD, UK Public Health Sciences & Medical Statistics, University Hospital Southampton NHS Trust, Southampton SO16 6YD, UK
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 4 July 2012 Accepted 15 July 2012 Keywords: Defibrillation Biphasic Atrial fibrillation Cardioversion Impedance
a b s t r a c t Background: Several different biphasic waveforms are used clinically, but few studies have compared their efficacy. The two main waveforms are the biphasic rectilinear (BR) and biphasic truncated exponential (BTE) waveforms, both of which have important differences, particularly at the extremes of transthoracic impedance. Objective: To compare the efficacy of two commonly used defibrillation waveforms in the elective cardioversion of atrial fibrillation. Methods: In a prospective randomized controlled study, sequential adult patients undergoing elective cardioversion for AF were recruited. Patients were randomized to receive synchronized defibrillation using either a BR or BTE waveform, both using a 50 J, 100 J, 150 J, 200 J, 200 J selected energy escalating protocol. Failure to cardiovert after the fifth shock was classed as failed defibrillation. The power of this study was 80% with 5% significance level to detect a difference of 20% or greater between groups. Survival analysis was used to compare the total energy delivered to achieve successful cardioversion between groups. Results: A total of 202 patients were recruited, of which data are complete for 199 (100 BR; 99 BTE). Median number of shocks to achieve cardioversion was 2 for the BR waveform and 3 for the BTE waveform (P = 0.059). In the BR waveform group, 95/100 (95.0%) achieved sinus rhythm. In the BTE waveform group, 90/99 (90.9%) achieved sinus rhythm and this group required on average 117.1 J more energy to achieve the outcome compared to the BR waveform group (P = 0.838). Conclusions: BR and BTE waveforms show similar high efficacy in the elective cardioversion of atrial fibrillation. © 2012 Elsevier Ireland Ltd. All rights reserved.
1. Introduction The evolution of defibrillation waveforms has progressed significantly since the 1899 studies of Prevost and Batelli demonstrated the ability of electric shocks to reverse ventricular fibrillation in dogs.1 The earliest clinical defibrillators introduced into clinical practice in the 1940s used a crude alternating current delivering 300–1000 V directly onto the myocardium; a technique that was often unsuccessful and associated with significant myocardial injury.2 It was not until 1959, when Lown developed a direct current waveform for external cardioversion, that defibrillation
夽 A Spanish translated version of the summary of this article appears as Appendix in the final online version at http://dx.doi.org/10.1016/j.resuscitation.2012.07.010. ∗ Corresponding author. Tel.: +44 0 2380 796135; fax: +44 0 2380 796135. E-mail address: [email protected] (C.D. Deakin). 0300-9572/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.resuscitation.2012.07.010
efficacy took a further step forward and this monophasic waveform became the standard waveform used in most defibrillators until the 1980s.3 Over the past decade, newer biphasic waveforms have replaced the monophasic waveform. Most studies have demonstrated their superior efficacy and it is these waveforms that are now the standard waveform delivered by most defibrillators. In the quest for greater efficacy, a number of different biphasic waveforms have evolved, the main ones being the biphasic truncated exponential waveform and the biphasic rectilinear waveform. Although similar in principle, these biphasic waveforms differ considerably both in their shape and the response of the waveform to variations in transthoracic impedance (TTI). The biphasic rectilinear (BR) waveform fixes voltage at a maximum output and subsequently varies internal resistance in order to deliver constant current across a broad range of TTI during the first phase of the biphasic waveform. Additionally, the duration of this waveform is fixed at 10 ms, regardless of patient TTI, because some animal
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
studies suggest that waveforms in excess of 10–12 ms are associated with a reduced efficacy4 and an increase in the risk of myocardial dysfunction.5 The biphasic rectilinear waveform is therefore a constant current waveform of fixed duration, which varies little across the range of TTI seen in the clinical setting. The biphasic truncated exponential (BTE) waveform delivers an initial positive phase that decays exponentially but unlike the rectilinear waveform, the voltage and duration of the shock are controlled electronically according to the impedance, resulting in lower peak current and extended waveform duration in patients with higher TTI. There are two main variants of this BTE waveform; the Philips SMART biphasic waveform and the PhysioControl/Medtronic waveforms; the former using a lower energy to deliver the same current. Both BTE waveforms are adjusted electronically in a similar manner to adapt to variations in TTI. Further details on differences in biphasic waveforms and their response to variations in impedance is available at: http://www. resuscitationcentral.com/defibrillation/biphasic-waveform. Several clinical studies have previously compared the efficacy of different biphasic waveforms in the elective cardioversion of atrial fibrillation, but none of the three published studies comparing different biphasic waveforms has reported a significant difference in efficacy.6–8 However, electronic manipulation of the waveform according to the transthoracic impedance results in differences in selected and delivered energy that must be taken into account when comparing the efficiency of different waveforms. Data analysis therefore needs to compare delivered energy to ensure a fair comparison between waveforms; a factor that has been overlooked in some studies.8 Additionally, a relatively large sample size is required to detect clinically significant differences because both waveforms have high cardioversion success rates and this has been a factor limiting conclusions from some previously published comparisons.6–8 We therefore aimed to compare two of the most commonly used biphasic waveforms in a study adequately powered to detect clinically significant differences at low energy levels and analyze the cardioversion efficacy according to total delivered energy.
287
was then drawn in the order of study number for each patient in theatre, immediately prior to each cardioversion procedure to determine which waveform would be used. The corresponding unique study number was then assigned to each patient. The study was conducted at Southampton University Hospital NHS Trust with patients who were under the care of consultant cardiologists at this institute. 2.2. Study population Following Regional Ethics Committee approval (REC Ref: 06/Q1704/109) and informed written consent, we sequentially recruited adult patients who were scheduled to undergoing elective day case cardioversion for atrial fibrillation under general anesthesia. Prospective patient enrollment was carried out from May 2007 to November 2011, but was non-consecutive because of availability of study physicians and refusal to participate in the study by patients. Exclusion criteria were age <18 years, inability to give informed consent, prisoners, young offenders and residents of care homes, refusal to participate in the study, previous participation in the study, atrial flutter, hemodynamically unstable atrial fibrillation, atrial dimension >60 mm, duration of arrhythmia >2 years (or of unknown duration) and untreated hyperthyroidism. 2.3. Intervention Patients were admitted to the anesthetic room, where intravenous access was obtained and routine monitoring including ECG, non-invasive blood pressure and pulse oximetry established. Under supervision of a consultant anesthesiologist, general anesthesia was induced following pre-oxygenation, using a sleep dose of propofol (2–3 mg/kg). Cardioversion was deemed to be successful if the patient converted to sinus rhythm immediately following a shock and remained in sinus rhythm at 60 s. If the patient remained in atrial fibrillation after the fifth shock (200 J), the defibrillation attempt was deemed to have been unsuccessful. 2.4. Outcome measures
2. Materials and methods 2.1. Study design and setting We conducted a prospective single-blinded (i.e. blinded to patients), randomized controlled trial with two biphasic waveform defibrillation study groups: a biphasic truncated exponential (BTE) waveform group delivered using a Philips Heartstart XL defibrillator (Philips Healthcare, 3000 Minuteman Road, Andover, MA 01810-1099, USA) and a biphasic rectilinear (BR) waveform group delivered using a Zoll R Series defibrillator (Zoll Medical Corporation, 269 Mill Road, Chelmsford, MA 01824-4105, USA). Device specific wet polymer self-adhesive gel pads (PhysioControl ‘Quik-Combo’ for the BTE waveform and Zoll ‘Pro-Padz’ for the BR waveform) were applied to the patients’ skin following shaving of chest hair if necessary, according to the current resuscitation guidelines9 and relevant manufacturer’s instructions. An identical sequential selected energy protocol was used for both waveforms, with a synchronized shock initially delivered at 50 J and then at 100 J, 150 J, 200 J, and 200 J again as necessary, until cardioversion was achieved. Patients who agreed to participate to this study were randomized to either group using a random number generator (http://stattrek.com/Tables/Random.aspx; Accessed August 2011) which generated a series of binary numbers that were then placed in separate sealed randomization envelopes labeled with a study number and stored in a secure cabinet. A randomization envelope
The main objective of the study was to compare the efficacy of the two waveforms in cardioversion success, according to the total energy delivered. The secondary objective of the study was to investigate the efficacy of the waveform in relation to the transthoracic impedance. 2.5. Data collection Patient records were reviewed by researchers to record gender, age, body weight, duration of oral amiodarone medication (if prescribed), and duration of atrial fibrillation. Energy delivered and TTI were recorded from the printed data on the ensuing rhythm strip following each shock. Non-patient identifiable data was transcribed on to an Excel database (Microsoft Excel 2008 v 12.3.0; Microsoft Corporation, Redmond, WA 98052-6399) and checked by a second researcher for accuracy. 2.6. Data and statistical analysis First shock success rates for elective cardioversion of atrial fibrillation using biphasic waveforms have been reported at this center as 33% at 70 J,10 and elsewhere as 54–61% at 50 J,7 52–62% at 50 J8 , 57% at 50 J,11 approximately 25% at 50 J,12 and 60% at 70 J.13 Sample size calculations were performed using Sample Size Tables for Clinical Studies Program Software v1.0; (S-H Tan, National Cancer
288
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Fig. 1. CONSORT flow diagram showing patient recruitment and progression through to data analysis.
Centre Singapore, July 2008). We therefore assumed that at 50 J, and based on this information, a clinically important difference of 20% in success rate between the 2 waveform groups (40% and 60%) with a power of 80% and a significance level of 5%, would require a total of 194 patients (97 patients per group). Descriptive statistics were used to describe demographic data and transthoracic impedance at first shock data. For measures that were continuous, distributions were assessed first. A Mann–Whitney U test was used to compare initial TTI values between groups. Kaplan–Meier survival curves and log-rank tests were used to compare the total energy delivered and the number of shocks needed to achieve cardioversion between the BR and the BTE waveform groups. Chi-square test or Fisher’s exact test was used to compare the success rate at each shock and up to the fifth shock. Cox proportional-hazards regression model was used to investigate the effect of TTI on waveform efficacy between the BR and the BTE waveform groups. Statistical significance was taken as P < 0.05. Statistical software SPSS (v 18.0 www.ibm.com/software/ analytics/spss) was used to perform the analyses. Confidence intervals were presented for all estimates and were produced by software Confidence Interval Analysis (CIA) if it was not available in SPSS.14
3. Results Between April 2007 and November 2010, a total of 205 patients were approached to participate in the study. A CONSORT flow diagram15,16 summarizes patient allocation (Fig. 1). The demographic details of the patients are shown in Table 1; these indicate the 2 groups were relatively balanced.
3.1. Transthoracic impedance Median (lower quartile, upper quartile) initial TTI at first shock for the BR waveform was 106.9 (91.9, 122.1) and for the BTE waveform, it was 92.0 (80.0, 106.0). The median difference between groups of 13.7 (95%CI 8.5, 19.4) was statistically significant (P < 0.001). For all patients with 5 consecutive shocks, TTI decreased progressively by approximately 9% from the initial shock. 3.2. Primary endpoint Of the 100 patients in the BR waveform group, 95 (95.0%) achieved sinus rhythm after a maximum of 5 shocks, with a median total energy delivered of 182.9 J (95%CI from 180.0 to 185.8 J). Of the 99 patients in the BTE waveform group, 90 (90.9%) achieved sinus rhythm with a median total energy delivered of 300.0 J (95%CI from 166.2 to 433.8 J), requiring an average of 117.1 J more energy to achieve cardioversion compared to the BR waveform group. The
Table 1 Demographic details of patients according to waveform allocation (BR, biphasic rectilinear; BTE, biphasic truncated exponential; LQ, lower quartile; UQ, upper quartile). Variable
BR (N = 101)
BTE (N = 99)
Age – years Median (LQ, UQ) Weight – kg Median (LQ, UQ) Duration of AF – months Median (LQ, UQ) Gender – male, N (%) Amiodarone, N (%)
65.5 (60.0, 72.0)
68.0(61.5, 72.0)
90.2(80.0, 103.0)
88.9(80.2, 98.5)
7.0
6.0 (4.0, 10.0)
(5.0, 9.0)
77 (76.2%) 24 (23.8%)
69 (69.7%) 18 (18.2%)
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
289
Table 2 Number (%) of patients achieved sinus rhythm at each of the 5 available energy levels according to waveform group (Fisher’s exact test was used due to small expected cell counts; Chi-square test was used in the rest of the analyses) (BR, biphasic rectilinear; BTE, biphasic truncated exponential; AF, atrial fibrillation; SR, sinus rhythm). Shock number and energy level
Outcome
Waveform group RBW
Total
P
BTE
Shock 1 at 50 J
AF SR Total
82 (81.2%) 19 (18.8%) 101 (100.0%)
84 (84.8%) 15 (15.2%) 99 (100.0%)
166 (83.0%) 34 (17.0%) 200 (100.0%)
0.491
Shock 2 at 100 J
AF SR Total
42 (51.2%) 40 (48.8%) 82 (100.0%)
52 (61.9%) 32 (38.1%) 84 (100.0%)
94 (56.6%) 72 (43.4%) 166 (100.0%)
0.165
Shock 3 at 150 J
AF SR Total
18 (42.9%) 24 (57.1%) 42 (100.0%)
31 (59.6%) 21 (40.4%) 52 (100.0%)
49 (52.1%) 45 (47.9%) 94 (100.0%)
0.106
Shock 4 at 200 J
AF SR Total
9 (50.0%) 9 (50.0%) 18 (100.0%)
12 (38.7%) 19 (61.3%) 31 (100.0%)
21 (42.9%) 28 (57.1%) 49 (100.0%)
0.441
Shock 5 at 200 J
AF SR Total
5 (55.6%) 4 (44.4%) 9 (100.0%)
9 (75.0%) 3 (25.0%) 12 (100.0%)
14 (66.7%) 7 (33.3%) 21 (100.0%)
0.397*
log-rank test indicated that this difference was not statistically significant (P = 0.838). Since the total amount of energy delivered was related to the number of shocks delivered, similar analysis was performed on the number of shocks delivered. Patients in the BR group required a median of two shocks to achieve cardioversion, compared with three shocks in the BTE waveform group. Moreover, there were more patients in the BTE waveform group not achieving sinus rhythm at the end of the 5th shock (n = 9; 9.1%) compared to the BR waveform group (n = 5; 5.0%). However, the log-rank test suggested that these differences were not statistically significant (P = 0.059). Kaplan–Meier survival curves in Fig. 2 illustrate these results. Although not a specific endpoint, we also investigated any difference in the proportion of patients achieving sinus rhythm at each shock level (up to the 5th shock) between the two waveforms. The results are shown in Table 2. Chi-square tests or Fisher’s exact tests indicated that there was no statistically significant difference in the proportion of patients achieving sinus rhythm at any of the 5 shocks used. However, there were a constantly higher proportion
of patients who had achieved the outcome from the BR waveform group at each shock, except in shock 4. 3.3. Secondary endpoint To investigate if there was any difference in shock success between the two types of waveform according to transthoracic impedance, TTI at 50 J (first shock) was dichotomized in such a way that the TTI values in the top quartile (≥115 ) were classified as high TTI, while the remaining TTI values (<115 ) were classified as low TTI. One patient had missing TTI value at 50 J. Table 3 shows the division of TTI values according to waveform type. Four groups were created according to the four combinations between the two types of waveform and the two TTI levels. The shock success was then compared between these four waveform-TTI groups. The number of shock success, median number of shocks and total energy delivered in these 4 groups are presented in Table 4. Our first Cox proportional-hazards regression model was used to investigate the two main effects, which are the waveform types and TTI groups separately as independent variables. Overall, patients in the BR waveform group were 1.044 times more likely with 95%CI (0.776, 1.404) to achieve the outcome than those in the BTE waveform group after adjusting for the TTI groups, however this was not statistically significant (P = 0.777). The low TTI group were 1.675 times more likely to achieve sinus rhythm compared with the high TTI group (95%CI: 1.183–2.371; P = 0.004) after adjusting for the types of waveform. Adjusting for the difference in TTI groups, the Cox proportionalhazards regression model shows the effect in each of the four waveform-TTI groups with respect to the outcome measure. By setting the BTE waveform-high TTI group as the reference level, Table 3 Frequency (percentage) of patients by transthoracic impedance (TTI) group and waveform type (BR, biphasic rectilinear; BTE, biphasic truncated exponential). TTI groupa
Fig. 2. Kaplan–Meier survival curves showing the number of shocks required to achieve sinus rhythm (up to shock 5) against the proportion of patients not achieving sinus rhythm in the two waveform groups (BR, biphasic rectilinear; BTE, biphasic truncated exponential).
Waveform
Total
BR
BTE
Low High
65 (65.0%) 35 (35.0%)
84 (84.8%) 15 (15.2%)
149 (74.9%) 50 (25.1%)
Total
100 (100%)b
99 (100%)
199 (100%)
a b
Based on TTI at 50 J. Data incomplete for one patient in the BR group.
290
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Table 4 Basic descriptive statistics of the total amount energy and number of shocks delivered in each of the four waveform-TTI groups and the proportion of patients achieving sinus rhythm up to maximum of 5 shocks (N = 199) (TTI, transthoracic impedance; BR, biphasic rectilinear; BTE, biphasic truncated exponential). TTI group
Waveform
N (%) achieving sinus rhythm
Median no. of shocks delivered
Median (95%CI) total energy delivered (J)
[3,0]High TTI
BTE (N = 15) BR (N = 35)
10 (66.7%) 34 (97.1%)
4 3
516.0 (239.5, 792.5) 357.9 (154.6, 561.2)
[3,0]Low TTI
BTE (N = 84) BR (N = 65)
80 (95.2%) 61 (93.8%)
2 2
155.0 (72.5, 237.5) 179.2 (177.5, 180.9)
patients in the BR waveform group with high TTI, were approximately twice as likely to achieve cardioversion (hazard ratio (HR) = 1.930), but this difference was not statistically significant (P = 0.070). The two low TTI groups were statistically significantly different compared to the reference group (BTE: P = 0.002, HR = 2.783; BR: P = 0.008, HR = 2.484). 4. Discussion This is the largest published study comparing the efficacy of BR and BTE waveforms in elective cardioversion of atrial fibrillation. Both waveforms achieved >90% cardioversion efficacy after an escalating protocol to 200 J selected energy. No statistically significant difference was shown between the waveforms in either cumulative or step-wise energy delivered or the number of shocks required to achieve cardioversion. These results are in keeping with previous smaller studies that have demonstrated similar efficacy between BR and BTE waveforms for cardioversion of supraventricular arrhythmias. Several studies have compared different biphasic waveforms; all reporting outcome data with patients requiring cardioversion for atrial tachyarrhythmias. Most have recruited patients with atrial fibrillation, but some have also included patients with atrial flutter who are known to have lower energy requirements for cardioversion, making a comparison between groups less clear. Only three studies have compared patients with atrial fibrillation alone. Alatawi et al. studied 141 patients who were randomized to either a Medtronic/PhysioControl BTE waveform or a Zoll BR waveform.6 The study was powered to detect a >16% difference in cumulative efficacy between the two waveforms. Selected energy levels were not matched between groups, but there was no significant difference in cardioversion success rate using BR waveform energy levels up to 200 J and BTE waveform energy levels up to 360 J.6 Although Alatawi et al. found cumulative selected and delivered energy was less in the BTE group,6 we found no statistically significant difference between the two groups. The Alatawi study was not powered to detect differences in lower energy efficacy. Another study compared the Medtronic/PhysioControl BTE waveform with a Zoll BR waveform.7 In 145 patients randomized to matched selected energy levels, there was no significant difference between waveform efficacy at any energy level. The authors acknowledged that the study may have been underpowered leading to a type II statistical error. A smaller study powered to detect differences of >20% between groups, enrolled 101 patients who were randomized to matched selected energy levels using either a Zoll M-series BR waveform or a PhysioControl BTE waveform.8 There were no differences in efficacy at any selected energy level or in cumulative efficacy, but results were not analyzed according to delivered energy. Patients in both waveform groups with higher TTI were more likely to require more shocks and a higher cumulative energy for successful cardioversion. Patients with a high TTI were 40% less likely to achieve sinus rhythm. Although this phenomenon has been
documented for monophasic waveforms, it had previously been thought that biphasic waveforms were more efficient in patients with high TTI and that this was not a significant phenomenon.17 Patients with high TTI such as those who are obese, or patients with air trapping secondary to asthma,18 may therefore benefit from higher defibrillation energy levels when treating acute ventricular arrhythmias. Both waveforms were compared to establish whether either was more effective at cardioversion of patients with high TTI. Comparison of cardioversion efficacy in the highest TTI quartile of patients was compared between waveform groups. Although this is an artificial divide, it was chosen to be representative of patients who are approximately 100 kg or more in weight and also allowed sufficient numbers to be compared between groups. There was a trend towards more patients in the BR waveform group achieving sinus rhythm, but this did not reach statistical significance (P = 0.070). Overall TTI was significantly higher in the BR group than the BTE group. Because both groups had very similar body mass, it is likely that this is an artificial difference resulting from technical differences in measurement of TTI by the respective defibrillators. Both the Zoll (BR waveform) and Philips (BTE waveform) defibrillators display the TTI associated with each shock, which is measured using a technique called large signal estimation. Because TTI changes according to magnitude (current density) and current frequency, TTI values vary at differing stages of the waveform and differing techniques in large signal estimation measurements may result in discrepancies between defibrillators in measured values. As a consequence, although patients in the BR group were measured as having a higher TTI, the true values may have been distributed similarly to the BTE group. As a result, analysis of waveform efficacy in patients with high TTI may have been biased towards greater BR waveform efficacy. A larger study is needed to establish whether the apparent greater efficacy of the BR waveform at higher TTI is a true phenomenon. TTI showed a small decline with successive shocks, decreasing progressively by approximately 9% in both waveform groups following a total of 5 shocks. This phenomenon has been reported previously with monophasic waveforms19 and with BTE waveforms20,21 but as far as we are aware, has not been reported with BR waveforms. The magnitude of change is similar for both BTE and BR waveforms although the exact mechanism is unknown; theories including an inflammatory mechanism, cell membrane ionization and the effect of decreasing TTI as current increases have all been proposed.20 The small decrease in TTI is unlikely to be of any clinical significance for repeated shocks in the same patient. Although this study compared patients undergoing elective procedures, future comparison between the two waveforms could usefully focus on emergency defibrillation of ventricular arrhythmias; a more challenging population to study. Additional studies of patients that have arrhythmias resistant to defibrillation, for example patients with high TTI and those with long duration ventricular fibrillation are also of particular interest as they represent groups that have proved resistant to even the newer biphasic technology.22
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Funding This work was supported by a research grant from the Resuscitation Council UK.
8.
9.
Conflict of interest 10.
All authors declare no conflict of interest in relation to this study. Acknowledgements We thank Zoll Medical Corporation and Philips Medical Systems for loan of their defibrillators and donation of associated self-adhesive pads for this study.
11.
12.
13.
Appendix A. Supplementary data 14.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.resuscitation. 2012.07.010.
15.
References
16.
1. Prevost J-L, Battelli F. La mort par les courants électriques – courants alternatifs a haute tension. J Physiol Pathol Gen 1899;1:427–42. 2. Beck CS, Pritchard WH, Feil HS. Ventricular fibrillation of long duration abolished by electric shock. J Am Med Assoc 1947;135:985–6. 3. Lown B, Amarasingham R, Neuman J. New method for terminating cardiac arrhythmias: use of synchronized capacitor discharge. J Am Med Assoc 1962;182:548–55. 4. Gliner BE, Lyster TE, Dillion SM, Bardy GH. Transthoracic defibrillation of swine with monophasic and biphasic waveforms. Circulation 1995;92:1634–43. 5. Tang W, Weil MH, Sun S, et al. The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function. J Am Coll Cardiol 1999;34:815–22. 6. Alatawi F, Gurevitz O, White RD, et al. Prospective, randomized comparison of two biphasic waveforms for the efficacy and safety of transthoracic biphasic cardioversion of atrial fibrillation. Heart Rhythm 2005;2:382–7. 7. Kim ML, Kim SG, Park DS, et al. Comparison of rectilinear biphasic waveform energy versus truncated exponential biphasic waveform energy for
17. 18.
19.
20. 21.
22.
291
transthoracic cardioversion of atrial fibrillation. Am J Cardiol 2004;94: 1438–40. Neal S, Ngarmukos T, Lessard D, Rosenthal L. Comparison of the efficacy and safety of two biphasic defibrillator waveforms for the conversion of atrial fibrillation to sinus rhythm. Am J Cardiol 2003;92:810–4. Deakin CD, Nolan JP. European Resuscitation Council guidelines for resuscitation 2005. Section 3. Electrical therapies: automated external defibrillators, defibrillation, cardioversion and pacing. Resuscitation 2005;67:S25–37. Ambler JJ, Deakin CD. A randomized controlled trial of efficacy and ST change following use of the Welch-Allyn MRL PIC biphasic waveform versus damped sine monophasic waveform for external DC cardioversion. Resuscitation 2006;71:146–51. Kawabata VS, Vianna CB, Moretti MA, et al. Monophasic versus biphasic waveform shocks for atrial fibrillation cardioversion in patients with concomitant amiodarone therapy. Europace 2007;9:143–6. Reisinger J, Gstrein C, Winter T, et al. Optimization of initial energy for cardioversion of atrial tachyarrhythmias with biphasic shocks. Am J Emerg Med 2010;28:159–65. Koster RW, Dorian P, Chapman FW, et al. A randomized trial comparing monophasic and biphasic waveform shocks for external cardioversion of atrial fibrillation. Am Heart J 2004;147:e20. Software accompanying Bryant TN. Computer Software for Calculating Confidence Intervals (CIA). In: Altman DG, Machin D, Bryant TN, Gardner MJ, editors. Statistics with confidence. 2nd ed. London: BMJ Books; 2000. p. 208–13. Moher D, Hopewell S, Schulz KF, et al. CONSORT 2010 Explanation and Elaboration: updated guidelines for reporting parallel group randomised trial. J Clin Epidemiol 2010;63:e1–37. Schulz KF, Altman DG, Moher D, for the CONSORT Group. CONSORT 2010 Statement: updated guidelines for reporting parallel group randomized trials. Open Med 2010;4:60–8. White RD. New concepts in transthoracic defibrillation. Emerg Med Clin North Am 2002;20:785–807. Deakin CD, McLaren RM, Petley GW, et al. Effects of positive end-expiratory pressure on transthoracic impedance—implications for defibrillation. Resuscitation 1998;37:9–12. Dahl CF, Ewy GA, Ewy MD, Thomas ED. Transthoracic impedance to direct current discharge: effect of repeated countershocks. Med Instrum 1976;10: 151–4. Deakin CD, Ambler JJ, Shaw S. Changes in transthoracic impedance during sequential biphasic defibrillation. Resuscitation 2008;78:141–5. Walker RG, Koster RW, Sun C, et al. Defibrillation probability and impedance change between shocks during resuscitation from out-of-hospital cardiac arrest. Resuscitation 2009;80:773–7. Koster RW, Walker RG, Chapman FW. Recurrent ventricular fibrillation during advanced life support care of patients with prehospital cardiac arrest. Resuscitation 2008;78:252–7.
Contents lists available at ScienceDirect
Resuscitation journal homepage: www.elsevier.com/locate/resuscitation
Clinical paper
A comparison of rectilinear and truncated exponential biphasic waveforms in elective cardioversion of atrial fibrillation: A prospective randomized controlled trial夽 Charles D. Deakin a,∗ , Stephanie Connelly a , Rupert Wharton a , Ho Ming Yuen b a b
Department of Anaesthetics, University Hospital Southampton NHS Foundation Trust, Southampton SO16 6YD, UK Public Health Sciences & Medical Statistics, University Hospital Southampton NHS Trust, Southampton SO16 6YD, UK
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 4 July 2012 Accepted 15 July 2012 Keywords: Defibrillation Biphasic Atrial fibrillation Cardioversion Impedance
a b s t r a c t Background: Several different biphasic waveforms are used clinically, but few studies have compared their efficacy. The two main waveforms are the biphasic rectilinear (BR) and biphasic truncated exponential (BTE) waveforms, both of which have important differences, particularly at the extremes of transthoracic impedance. Objective: To compare the efficacy of two commonly used defibrillation waveforms in the elective cardioversion of atrial fibrillation. Methods: In a prospective randomized controlled study, sequential adult patients undergoing elective cardioversion for AF were recruited. Patients were randomized to receive synchronized defibrillation using either a BR or BTE waveform, both using a 50 J, 100 J, 150 J, 200 J, 200 J selected energy escalating protocol. Failure to cardiovert after the fifth shock was classed as failed defibrillation. The power of this study was 80% with 5% significance level to detect a difference of 20% or greater between groups. Survival analysis was used to compare the total energy delivered to achieve successful cardioversion between groups. Results: A total of 202 patients were recruited, of which data are complete for 199 (100 BR; 99 BTE). Median number of shocks to achieve cardioversion was 2 for the BR waveform and 3 for the BTE waveform (P = 0.059). In the BR waveform group, 95/100 (95.0%) achieved sinus rhythm. In the BTE waveform group, 90/99 (90.9%) achieved sinus rhythm and this group required on average 117.1 J more energy to achieve the outcome compared to the BR waveform group (P = 0.838). Conclusions: BR and BTE waveforms show similar high efficacy in the elective cardioversion of atrial fibrillation. © 2012 Elsevier Ireland Ltd. All rights reserved.
1. Introduction The evolution of defibrillation waveforms has progressed significantly since the 1899 studies of Prevost and Batelli demonstrated the ability of electric shocks to reverse ventricular fibrillation in dogs.1 The earliest clinical defibrillators introduced into clinical practice in the 1940s used a crude alternating current delivering 300–1000 V directly onto the myocardium; a technique that was often unsuccessful and associated with significant myocardial injury.2 It was not until 1959, when Lown developed a direct current waveform for external cardioversion, that defibrillation
夽 A Spanish translated version of the summary of this article appears as Appendix in the final online version at http://dx.doi.org/10.1016/j.resuscitation.2012.07.010. ∗ Corresponding author. Tel.: +44 0 2380 796135; fax: +44 0 2380 796135. E-mail address: [email protected] (C.D. Deakin). 0300-9572/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.resuscitation.2012.07.010
efficacy took a further step forward and this monophasic waveform became the standard waveform used in most defibrillators until the 1980s.3 Over the past decade, newer biphasic waveforms have replaced the monophasic waveform. Most studies have demonstrated their superior efficacy and it is these waveforms that are now the standard waveform delivered by most defibrillators. In the quest for greater efficacy, a number of different biphasic waveforms have evolved, the main ones being the biphasic truncated exponential waveform and the biphasic rectilinear waveform. Although similar in principle, these biphasic waveforms differ considerably both in their shape and the response of the waveform to variations in transthoracic impedance (TTI). The biphasic rectilinear (BR) waveform fixes voltage at a maximum output and subsequently varies internal resistance in order to deliver constant current across a broad range of TTI during the first phase of the biphasic waveform. Additionally, the duration of this waveform is fixed at 10 ms, regardless of patient TTI, because some animal
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
studies suggest that waveforms in excess of 10–12 ms are associated with a reduced efficacy4 and an increase in the risk of myocardial dysfunction.5 The biphasic rectilinear waveform is therefore a constant current waveform of fixed duration, which varies little across the range of TTI seen in the clinical setting. The biphasic truncated exponential (BTE) waveform delivers an initial positive phase that decays exponentially but unlike the rectilinear waveform, the voltage and duration of the shock are controlled electronically according to the impedance, resulting in lower peak current and extended waveform duration in patients with higher TTI. There are two main variants of this BTE waveform; the Philips SMART biphasic waveform and the PhysioControl/Medtronic waveforms; the former using a lower energy to deliver the same current. Both BTE waveforms are adjusted electronically in a similar manner to adapt to variations in TTI. Further details on differences in biphasic waveforms and their response to variations in impedance is available at: http://www. resuscitationcentral.com/defibrillation/biphasic-waveform. Several clinical studies have previously compared the efficacy of different biphasic waveforms in the elective cardioversion of atrial fibrillation, but none of the three published studies comparing different biphasic waveforms has reported a significant difference in efficacy.6–8 However, electronic manipulation of the waveform according to the transthoracic impedance results in differences in selected and delivered energy that must be taken into account when comparing the efficiency of different waveforms. Data analysis therefore needs to compare delivered energy to ensure a fair comparison between waveforms; a factor that has been overlooked in some studies.8 Additionally, a relatively large sample size is required to detect clinically significant differences because both waveforms have high cardioversion success rates and this has been a factor limiting conclusions from some previously published comparisons.6–8 We therefore aimed to compare two of the most commonly used biphasic waveforms in a study adequately powered to detect clinically significant differences at low energy levels and analyze the cardioversion efficacy according to total delivered energy.
287
was then drawn in the order of study number for each patient in theatre, immediately prior to each cardioversion procedure to determine which waveform would be used. The corresponding unique study number was then assigned to each patient. The study was conducted at Southampton University Hospital NHS Trust with patients who were under the care of consultant cardiologists at this institute. 2.2. Study population Following Regional Ethics Committee approval (REC Ref: 06/Q1704/109) and informed written consent, we sequentially recruited adult patients who were scheduled to undergoing elective day case cardioversion for atrial fibrillation under general anesthesia. Prospective patient enrollment was carried out from May 2007 to November 2011, but was non-consecutive because of availability of study physicians and refusal to participate in the study by patients. Exclusion criteria were age <18 years, inability to give informed consent, prisoners, young offenders and residents of care homes, refusal to participate in the study, previous participation in the study, atrial flutter, hemodynamically unstable atrial fibrillation, atrial dimension >60 mm, duration of arrhythmia >2 years (or of unknown duration) and untreated hyperthyroidism. 2.3. Intervention Patients were admitted to the anesthetic room, where intravenous access was obtained and routine monitoring including ECG, non-invasive blood pressure and pulse oximetry established. Under supervision of a consultant anesthesiologist, general anesthesia was induced following pre-oxygenation, using a sleep dose of propofol (2–3 mg/kg). Cardioversion was deemed to be successful if the patient converted to sinus rhythm immediately following a shock and remained in sinus rhythm at 60 s. If the patient remained in atrial fibrillation after the fifth shock (200 J), the defibrillation attempt was deemed to have been unsuccessful. 2.4. Outcome measures
2. Materials and methods 2.1. Study design and setting We conducted a prospective single-blinded (i.e. blinded to patients), randomized controlled trial with two biphasic waveform defibrillation study groups: a biphasic truncated exponential (BTE) waveform group delivered using a Philips Heartstart XL defibrillator (Philips Healthcare, 3000 Minuteman Road, Andover, MA 01810-1099, USA) and a biphasic rectilinear (BR) waveform group delivered using a Zoll R Series defibrillator (Zoll Medical Corporation, 269 Mill Road, Chelmsford, MA 01824-4105, USA). Device specific wet polymer self-adhesive gel pads (PhysioControl ‘Quik-Combo’ for the BTE waveform and Zoll ‘Pro-Padz’ for the BR waveform) were applied to the patients’ skin following shaving of chest hair if necessary, according to the current resuscitation guidelines9 and relevant manufacturer’s instructions. An identical sequential selected energy protocol was used for both waveforms, with a synchronized shock initially delivered at 50 J and then at 100 J, 150 J, 200 J, and 200 J again as necessary, until cardioversion was achieved. Patients who agreed to participate to this study were randomized to either group using a random number generator (http://stattrek.com/Tables/Random.aspx; Accessed August 2011) which generated a series of binary numbers that were then placed in separate sealed randomization envelopes labeled with a study number and stored in a secure cabinet. A randomization envelope
The main objective of the study was to compare the efficacy of the two waveforms in cardioversion success, according to the total energy delivered. The secondary objective of the study was to investigate the efficacy of the waveform in relation to the transthoracic impedance. 2.5. Data collection Patient records were reviewed by researchers to record gender, age, body weight, duration of oral amiodarone medication (if prescribed), and duration of atrial fibrillation. Energy delivered and TTI were recorded from the printed data on the ensuing rhythm strip following each shock. Non-patient identifiable data was transcribed on to an Excel database (Microsoft Excel 2008 v 12.3.0; Microsoft Corporation, Redmond, WA 98052-6399) and checked by a second researcher for accuracy. 2.6. Data and statistical analysis First shock success rates for elective cardioversion of atrial fibrillation using biphasic waveforms have been reported at this center as 33% at 70 J,10 and elsewhere as 54–61% at 50 J,7 52–62% at 50 J8 , 57% at 50 J,11 approximately 25% at 50 J,12 and 60% at 70 J.13 Sample size calculations were performed using Sample Size Tables for Clinical Studies Program Software v1.0; (S-H Tan, National Cancer
288
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Fig. 1. CONSORT flow diagram showing patient recruitment and progression through to data analysis.
Centre Singapore, July 2008). We therefore assumed that at 50 J, and based on this information, a clinically important difference of 20% in success rate between the 2 waveform groups (40% and 60%) with a power of 80% and a significance level of 5%, would require a total of 194 patients (97 patients per group). Descriptive statistics were used to describe demographic data and transthoracic impedance at first shock data. For measures that were continuous, distributions were assessed first. A Mann–Whitney U test was used to compare initial TTI values between groups. Kaplan–Meier survival curves and log-rank tests were used to compare the total energy delivered and the number of shocks needed to achieve cardioversion between the BR and the BTE waveform groups. Chi-square test or Fisher’s exact test was used to compare the success rate at each shock and up to the fifth shock. Cox proportional-hazards regression model was used to investigate the effect of TTI on waveform efficacy between the BR and the BTE waveform groups. Statistical significance was taken as P < 0.05. Statistical software SPSS (v 18.0 www.ibm.com/software/ analytics/spss) was used to perform the analyses. Confidence intervals were presented for all estimates and were produced by software Confidence Interval Analysis (CIA) if it was not available in SPSS.14
3. Results Between April 2007 and November 2010, a total of 205 patients were approached to participate in the study. A CONSORT flow diagram15,16 summarizes patient allocation (Fig. 1). The demographic details of the patients are shown in Table 1; these indicate the 2 groups were relatively balanced.
3.1. Transthoracic impedance Median (lower quartile, upper quartile) initial TTI at first shock for the BR waveform was 106.9 (91.9, 122.1) and for the BTE waveform, it was 92.0 (80.0, 106.0). The median difference between groups of 13.7 (95%CI 8.5, 19.4) was statistically significant (P < 0.001). For all patients with 5 consecutive shocks, TTI decreased progressively by approximately 9% from the initial shock. 3.2. Primary endpoint Of the 100 patients in the BR waveform group, 95 (95.0%) achieved sinus rhythm after a maximum of 5 shocks, with a median total energy delivered of 182.9 J (95%CI from 180.0 to 185.8 J). Of the 99 patients in the BTE waveform group, 90 (90.9%) achieved sinus rhythm with a median total energy delivered of 300.0 J (95%CI from 166.2 to 433.8 J), requiring an average of 117.1 J more energy to achieve cardioversion compared to the BR waveform group. The
Table 1 Demographic details of patients according to waveform allocation (BR, biphasic rectilinear; BTE, biphasic truncated exponential; LQ, lower quartile; UQ, upper quartile). Variable
BR (N = 101)
BTE (N = 99)
Age – years Median (LQ, UQ) Weight – kg Median (LQ, UQ) Duration of AF – months Median (LQ, UQ) Gender – male, N (%) Amiodarone, N (%)
65.5 (60.0, 72.0)
68.0(61.5, 72.0)
90.2(80.0, 103.0)
88.9(80.2, 98.5)
7.0
6.0 (4.0, 10.0)
(5.0, 9.0)
77 (76.2%) 24 (23.8%)
69 (69.7%) 18 (18.2%)
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
289
Table 2 Number (%) of patients achieved sinus rhythm at each of the 5 available energy levels according to waveform group (Fisher’s exact test was used due to small expected cell counts; Chi-square test was used in the rest of the analyses) (BR, biphasic rectilinear; BTE, biphasic truncated exponential; AF, atrial fibrillation; SR, sinus rhythm). Shock number and energy level
Outcome
Waveform group RBW
Total
P
BTE
Shock 1 at 50 J
AF SR Total
82 (81.2%) 19 (18.8%) 101 (100.0%)
84 (84.8%) 15 (15.2%) 99 (100.0%)
166 (83.0%) 34 (17.0%) 200 (100.0%)
0.491
Shock 2 at 100 J
AF SR Total
42 (51.2%) 40 (48.8%) 82 (100.0%)
52 (61.9%) 32 (38.1%) 84 (100.0%)
94 (56.6%) 72 (43.4%) 166 (100.0%)
0.165
Shock 3 at 150 J
AF SR Total
18 (42.9%) 24 (57.1%) 42 (100.0%)
31 (59.6%) 21 (40.4%) 52 (100.0%)
49 (52.1%) 45 (47.9%) 94 (100.0%)
0.106
Shock 4 at 200 J
AF SR Total
9 (50.0%) 9 (50.0%) 18 (100.0%)
12 (38.7%) 19 (61.3%) 31 (100.0%)
21 (42.9%) 28 (57.1%) 49 (100.0%)
0.441
Shock 5 at 200 J
AF SR Total
5 (55.6%) 4 (44.4%) 9 (100.0%)
9 (75.0%) 3 (25.0%) 12 (100.0%)
14 (66.7%) 7 (33.3%) 21 (100.0%)
0.397*
log-rank test indicated that this difference was not statistically significant (P = 0.838). Since the total amount of energy delivered was related to the number of shocks delivered, similar analysis was performed on the number of shocks delivered. Patients in the BR group required a median of two shocks to achieve cardioversion, compared with three shocks in the BTE waveform group. Moreover, there were more patients in the BTE waveform group not achieving sinus rhythm at the end of the 5th shock (n = 9; 9.1%) compared to the BR waveform group (n = 5; 5.0%). However, the log-rank test suggested that these differences were not statistically significant (P = 0.059). Kaplan–Meier survival curves in Fig. 2 illustrate these results. Although not a specific endpoint, we also investigated any difference in the proportion of patients achieving sinus rhythm at each shock level (up to the 5th shock) between the two waveforms. The results are shown in Table 2. Chi-square tests or Fisher’s exact tests indicated that there was no statistically significant difference in the proportion of patients achieving sinus rhythm at any of the 5 shocks used. However, there were a constantly higher proportion
of patients who had achieved the outcome from the BR waveform group at each shock, except in shock 4. 3.3. Secondary endpoint To investigate if there was any difference in shock success between the two types of waveform according to transthoracic impedance, TTI at 50 J (first shock) was dichotomized in such a way that the TTI values in the top quartile (≥115 ) were classified as high TTI, while the remaining TTI values (<115 ) were classified as low TTI. One patient had missing TTI value at 50 J. Table 3 shows the division of TTI values according to waveform type. Four groups were created according to the four combinations between the two types of waveform and the two TTI levels. The shock success was then compared between these four waveform-TTI groups. The number of shock success, median number of shocks and total energy delivered in these 4 groups are presented in Table 4. Our first Cox proportional-hazards regression model was used to investigate the two main effects, which are the waveform types and TTI groups separately as independent variables. Overall, patients in the BR waveform group were 1.044 times more likely with 95%CI (0.776, 1.404) to achieve the outcome than those in the BTE waveform group after adjusting for the TTI groups, however this was not statistically significant (P = 0.777). The low TTI group were 1.675 times more likely to achieve sinus rhythm compared with the high TTI group (95%CI: 1.183–2.371; P = 0.004) after adjusting for the types of waveform. Adjusting for the difference in TTI groups, the Cox proportionalhazards regression model shows the effect in each of the four waveform-TTI groups with respect to the outcome measure. By setting the BTE waveform-high TTI group as the reference level, Table 3 Frequency (percentage) of patients by transthoracic impedance (TTI) group and waveform type (BR, biphasic rectilinear; BTE, biphasic truncated exponential). TTI groupa
Fig. 2. Kaplan–Meier survival curves showing the number of shocks required to achieve sinus rhythm (up to shock 5) against the proportion of patients not achieving sinus rhythm in the two waveform groups (BR, biphasic rectilinear; BTE, biphasic truncated exponential).
Waveform
Total
BR
BTE
Low High
65 (65.0%) 35 (35.0%)
84 (84.8%) 15 (15.2%)
149 (74.9%) 50 (25.1%)
Total
100 (100%)b
99 (100%)
199 (100%)
a b
Based on TTI at 50 J. Data incomplete for one patient in the BR group.
290
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Table 4 Basic descriptive statistics of the total amount energy and number of shocks delivered in each of the four waveform-TTI groups and the proportion of patients achieving sinus rhythm up to maximum of 5 shocks (N = 199) (TTI, transthoracic impedance; BR, biphasic rectilinear; BTE, biphasic truncated exponential). TTI group
Waveform
N (%) achieving sinus rhythm
Median no. of shocks delivered
Median (95%CI) total energy delivered (J)
[3,0]High TTI
BTE (N = 15) BR (N = 35)
10 (66.7%) 34 (97.1%)
4 3
516.0 (239.5, 792.5) 357.9 (154.6, 561.2)
[3,0]Low TTI
BTE (N = 84) BR (N = 65)
80 (95.2%) 61 (93.8%)
2 2
155.0 (72.5, 237.5) 179.2 (177.5, 180.9)
patients in the BR waveform group with high TTI, were approximately twice as likely to achieve cardioversion (hazard ratio (HR) = 1.930), but this difference was not statistically significant (P = 0.070). The two low TTI groups were statistically significantly different compared to the reference group (BTE: P = 0.002, HR = 2.783; BR: P = 0.008, HR = 2.484). 4. Discussion This is the largest published study comparing the efficacy of BR and BTE waveforms in elective cardioversion of atrial fibrillation. Both waveforms achieved >90% cardioversion efficacy after an escalating protocol to 200 J selected energy. No statistically significant difference was shown between the waveforms in either cumulative or step-wise energy delivered or the number of shocks required to achieve cardioversion. These results are in keeping with previous smaller studies that have demonstrated similar efficacy between BR and BTE waveforms for cardioversion of supraventricular arrhythmias. Several studies have compared different biphasic waveforms; all reporting outcome data with patients requiring cardioversion for atrial tachyarrhythmias. Most have recruited patients with atrial fibrillation, but some have also included patients with atrial flutter who are known to have lower energy requirements for cardioversion, making a comparison between groups less clear. Only three studies have compared patients with atrial fibrillation alone. Alatawi et al. studied 141 patients who were randomized to either a Medtronic/PhysioControl BTE waveform or a Zoll BR waveform.6 The study was powered to detect a >16% difference in cumulative efficacy between the two waveforms. Selected energy levels were not matched between groups, but there was no significant difference in cardioversion success rate using BR waveform energy levels up to 200 J and BTE waveform energy levels up to 360 J.6 Although Alatawi et al. found cumulative selected and delivered energy was less in the BTE group,6 we found no statistically significant difference between the two groups. The Alatawi study was not powered to detect differences in lower energy efficacy. Another study compared the Medtronic/PhysioControl BTE waveform with a Zoll BR waveform.7 In 145 patients randomized to matched selected energy levels, there was no significant difference between waveform efficacy at any energy level. The authors acknowledged that the study may have been underpowered leading to a type II statistical error. A smaller study powered to detect differences of >20% between groups, enrolled 101 patients who were randomized to matched selected energy levels using either a Zoll M-series BR waveform or a PhysioControl BTE waveform.8 There were no differences in efficacy at any selected energy level or in cumulative efficacy, but results were not analyzed according to delivered energy. Patients in both waveform groups with higher TTI were more likely to require more shocks and a higher cumulative energy for successful cardioversion. Patients with a high TTI were 40% less likely to achieve sinus rhythm. Although this phenomenon has been
documented for monophasic waveforms, it had previously been thought that biphasic waveforms were more efficient in patients with high TTI and that this was not a significant phenomenon.17 Patients with high TTI such as those who are obese, or patients with air trapping secondary to asthma,18 may therefore benefit from higher defibrillation energy levels when treating acute ventricular arrhythmias. Both waveforms were compared to establish whether either was more effective at cardioversion of patients with high TTI. Comparison of cardioversion efficacy in the highest TTI quartile of patients was compared between waveform groups. Although this is an artificial divide, it was chosen to be representative of patients who are approximately 100 kg or more in weight and also allowed sufficient numbers to be compared between groups. There was a trend towards more patients in the BR waveform group achieving sinus rhythm, but this did not reach statistical significance (P = 0.070). Overall TTI was significantly higher in the BR group than the BTE group. Because both groups had very similar body mass, it is likely that this is an artificial difference resulting from technical differences in measurement of TTI by the respective defibrillators. Both the Zoll (BR waveform) and Philips (BTE waveform) defibrillators display the TTI associated with each shock, which is measured using a technique called large signal estimation. Because TTI changes according to magnitude (current density) and current frequency, TTI values vary at differing stages of the waveform and differing techniques in large signal estimation measurements may result in discrepancies between defibrillators in measured values. As a consequence, although patients in the BR group were measured as having a higher TTI, the true values may have been distributed similarly to the BTE group. As a result, analysis of waveform efficacy in patients with high TTI may have been biased towards greater BR waveform efficacy. A larger study is needed to establish whether the apparent greater efficacy of the BR waveform at higher TTI is a true phenomenon. TTI showed a small decline with successive shocks, decreasing progressively by approximately 9% in both waveform groups following a total of 5 shocks. This phenomenon has been reported previously with monophasic waveforms19 and with BTE waveforms20,21 but as far as we are aware, has not been reported with BR waveforms. The magnitude of change is similar for both BTE and BR waveforms although the exact mechanism is unknown; theories including an inflammatory mechanism, cell membrane ionization and the effect of decreasing TTI as current increases have all been proposed.20 The small decrease in TTI is unlikely to be of any clinical significance for repeated shocks in the same patient. Although this study compared patients undergoing elective procedures, future comparison between the two waveforms could usefully focus on emergency defibrillation of ventricular arrhythmias; a more challenging population to study. Additional studies of patients that have arrhythmias resistant to defibrillation, for example patients with high TTI and those with long duration ventricular fibrillation are also of particular interest as they represent groups that have proved resistant to even the newer biphasic technology.22
C.D. Deakin et al. / Resuscitation 84 (2013) 286–291
Funding This work was supported by a research grant from the Resuscitation Council UK.
8.
9.
Conflict of interest 10.
All authors declare no conflict of interest in relation to this study. Acknowledgements We thank Zoll Medical Corporation and Philips Medical Systems for loan of their defibrillators and donation of associated self-adhesive pads for this study.
11.
12.
13.
Appendix A. Supplementary data 14.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.resuscitation. 2012.07.010.
15.
References
16.
1. Prevost J-L, Battelli F. La mort par les courants électriques – courants alternatifs a haute tension. J Physiol Pathol Gen 1899;1:427–42. 2. Beck CS, Pritchard WH, Feil HS. Ventricular fibrillation of long duration abolished by electric shock. J Am Med Assoc 1947;135:985–6. 3. Lown B, Amarasingham R, Neuman J. New method for terminating cardiac arrhythmias: use of synchronized capacitor discharge. J Am Med Assoc 1962;182:548–55. 4. Gliner BE, Lyster TE, Dillion SM, Bardy GH. Transthoracic defibrillation of swine with monophasic and biphasic waveforms. Circulation 1995;92:1634–43. 5. Tang W, Weil MH, Sun S, et al. The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function. J Am Coll Cardiol 1999;34:815–22. 6. Alatawi F, Gurevitz O, White RD, et al. Prospective, randomized comparison of two biphasic waveforms for the efficacy and safety of transthoracic biphasic cardioversion of atrial fibrillation. Heart Rhythm 2005;2:382–7. 7. Kim ML, Kim SG, Park DS, et al. Comparison of rectilinear biphasic waveform energy versus truncated exponential biphasic waveform energy for
17. 18.
19.
20. 21.
22.
291
transthoracic cardioversion of atrial fibrillation. Am J Cardiol 2004;94: 1438–40. Neal S, Ngarmukos T, Lessard D, Rosenthal L. Comparison of the efficacy and safety of two biphasic defibrillator waveforms for the conversion of atrial fibrillation to sinus rhythm. Am J Cardiol 2003;92:810–4. Deakin CD, Nolan JP. European Resuscitation Council guidelines for resuscitation 2005. Section 3. Electrical therapies: automated external defibrillators, defibrillation, cardioversion and pacing. Resuscitation 2005;67:S25–37. Ambler JJ, Deakin CD. A randomized controlled trial of efficacy and ST change following use of the Welch-Allyn MRL PIC biphasic waveform versus damped sine monophasic waveform for external DC cardioversion. Resuscitation 2006;71:146–51. Kawabata VS, Vianna CB, Moretti MA, et al. Monophasic versus biphasic waveform shocks for atrial fibrillation cardioversion in patients with concomitant amiodarone therapy. Europace 2007;9:143–6. Reisinger J, Gstrein C, Winter T, et al. Optimization of initial energy for cardioversion of atrial tachyarrhythmias with biphasic shocks. Am J Emerg Med 2010;28:159–65. Koster RW, Dorian P, Chapman FW, et al. A randomized trial comparing monophasic and biphasic waveform shocks for external cardioversion of atrial fibrillation. Am Heart J 2004;147:e20. Software accompanying Bryant TN. Computer Software for Calculating Confidence Intervals (CIA). In: Altman DG, Machin D, Bryant TN, Gardner MJ, editors. Statistics with confidence. 2nd ed. London: BMJ Books; 2000. p. 208–13. Moher D, Hopewell S, Schulz KF, et al. CONSORT 2010 Explanation and Elaboration: updated guidelines for reporting parallel group randomised trial. J Clin Epidemiol 2010;63:e1–37. Schulz KF, Altman DG, Moher D, for the CONSORT Group. CONSORT 2010 Statement: updated guidelines for reporting parallel group randomized trials. Open Med 2010;4:60–8. White RD. New concepts in transthoracic defibrillation. Emerg Med Clin North Am 2002;20:785–807. Deakin CD, McLaren RM, Petley GW, et al. Effects of positive end-expiratory pressure on transthoracic impedance—implications for defibrillation. Resuscitation 1998;37:9–12. Dahl CF, Ewy GA, Ewy MD, Thomas ED. Transthoracic impedance to direct current discharge: effect of repeated countershocks. Med Instrum 1976;10: 151–4. Deakin CD, Ambler JJ, Shaw S. Changes in transthoracic impedance during sequential biphasic defibrillation. Resuscitation 2008;78:141–5. Walker RG, Koster RW, Sun C, et al. Defibrillation probability and impedance change between shocks during resuscitation from out-of-hospital cardiac arrest. Resuscitation 2009;80:773–7. Koster RW, Walker RG, Chapman FW. Recurrent ventricular fibrillation during advanced life support care of patients with prehospital cardiac arrest. Resuscitation 2008;78:252–7.