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(Circulation. 1995;92:1634-1643.)
© 1995 American Heart Association, Inc.

Articles

Transthoracic Defibrillation of Swine With Monophasic and Biphasic Waveforms

Bradford E. Gliner, MSBME; Thomas E. Lyster, MSEE; Stephen M. Dillion, PhD; Gust H. Bardy, MD

From the Department of Medicine, University of Washington, Seattle, and the Philadelphia Heart Institute (S.M.D.), Presbyterian Medical Center, Philadelphia, Pa.

Correspondence to Bradford E. Gliner, c/o Gust H. Bardy, MD, Electrophysiology Section, Mail Stop RG-22, University Hospital, Seattle, WA 98195.

	Abstract

Top Abstract Introduction Methods Results Discussion References

 
Background Biphasic waveforms have had a favorable impact on internal defibrillation but have seen minimal use in transthoracic defibrillation systems. The purpose of this study was to compare monophasic and biphasic waveforms for transthoracic defibrillation in swine.

Methods and Results Three interrelated studies were performed in 19 swine to establish the relative transthoracic defibrillation efficacy of biphasic shock waveforms. In study 1, we measured voltage (V₅₀) and energy (E₅₀) strength-duration curves for monophasic and biphasic truncated exponential waveforms. We then independently examined the effects of phase duration and tilt on biphasic waveform defibrillation with a total waveform duration from study 1 that provided the minimum V₅₀ (study 2) and the minimum E₅₀ (study 3). At each pulse duration tested in study 1, biphasic waveforms defibrillated with significantly less voltage and energy than monophasic waveforms. At a duration of 12 ms, there was a voltage minimum for biphasic waveform defibrillation. At this duration, V₅₀ was 1378±505 V for the biphasic waveform compared with 2185±361 V for the monophasic waveform, P=.01. For both monophasic and biphasic waveforms, E₅₀ increased with pulse duration. With a total pulse duration of 12 ms, E₅₀ was 169±101 J for the biphasic waveform compared with 414±114 J for the monophasic waveform, P=.003. In study 2, optimization of phase duration and total tilt reduced the defibrillation requirements of the 12-ms "minimum voltage" biphasic waveform to 1284±187 V and 129±36 J. In study 3, the 8-ms "minimum energy" biphasic waveform had an E₅₀ of 115±35 J that was 11% less than the 12-ms biphasic waveform, P=.11; however, voltage requirements of 1476±239 V were 15% higher, P=.005.

Conclusions This study demonstrates the superiority of truncated biphasic waveforms over truncated monophasic waveforms for transthoracic defibrillation of swine. Biphasic waveforms should prove as advantageous at reducing voltage and energy requirements for transthoracic defibrillation as they have for internal defibrillation.

Key Words: fibrillation • defibrillation • waves • death, sudden

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
Biphasic defibrillation waveforms are in widespread use in automatic implantable cardioverter/defibrillators (ICDs). Numerous clinical and experimental internal defibrillation studies have established the superiority of biphasic shock waveforms over comparable monophasic waveforms.^{1 2 3 4 5 6 7 8} In addition to better defibrillation properties, biphasic waveforms produce fewer deleterious effects than monophasic waveforms.^{9 10} Despite the advantages and widespread use of biphasic waveforms in internal defibrillators and the fact that they were first noted to be useful in transthoracic defibrillation studies,¹¹ monophasic waveforms are the only waveforms currently in clinical use in transthoracic defibrillators.

Just as studies of defibrillation waveforms using internal electrodes have promoted advancements in internal defibrillator therapy, eg, lower defibrillation requirements and higher defibrillation safety margins, we anticipate that the results of this study will allow similar advances to be made for external defibrillators. To determine whether this is possible, we compared the performance of biphasic truncated exponential waveforms with that of monophasic truncated exponential waveforms. We also tested a clinically used monophasic damped sine waveform. This waveform gave us the basis for the prediction that two of the biphasic waveforms tested, one chosen for minimum voltage and the other for minimum energy, might be practically applied to human transthoracic defibrillation.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
This investigation consisted of three independent but related experimental studies designed to establish the transthoracic defibrillating efficacy of truncated shock waveforms in swine. This study followed the NIH guidelines for use of laboratory animals in biomedical research and was approved by the Institutional Animal Care and Use Committee of the University of Washington.

Defibrillation efficacy was compared through determination of V₅₀ and E₅₀ for each of the waveforms. V₅₀ and E₅₀ are the peak shock voltage and total delivered energy, respectively, required to produce a 50% likelihood that the shock will defibrillate. In study 1, we measured V₅₀ and E₅₀ strength-duration curves for monophasic and biphasic truncated exponential waveforms. The next two studies were explorations of the relative defibrillation efficacies of two different biphasic waveforms that, on the basis of their total duration, were found in study 1 to require either a minimum V₅₀ or a minimum E₅₀. In study 2, the "minimum voltage" biphasic waveform was examined, whereas study 3 tested the "minimum energy" biphasic waveform. In both of these latter studies, the total biphasic waveform duration was held fixed while the total waveform tilt and the phase duration were independently varied in a randomized and interleaved manner.

Sample Size
A minimum of six animals were used in each study. With six animals, an approximately 25-J difference in E₅₀ (130-V difference in V₅₀) could be detected with 80% power (ß=.20) and 95% confidence (=.05, two-sided).¹²

Animal Preparations
We studied 19 healthy domestic swine weighing 57±6 kg (range, 49 to 68 kg). We chose the swine model because the thorax and coronary arteries better approximate human anatomy than do canine models.^{13 14 15} General anesthesia was induced with pentobarbital sodium (15 mg/kg IV to effect) after preanesthesia by ketamine (20 mg/kg IM) and xylazine (2 mg/kg IM) injection. We maintained the animals in a surgical plane of anesthesia throughout the study using a combination of ketamine (2 to 3 mg/kg), diazepam (0.1 mg/kg), and pentobarbital sodium (2 to 4 mg/kg) every 20 minutes. The carotid artery was catheterized, and arterial gases and systemic blood pressure were monitored throughout the study. The swine were intubated with a cuffed endotracheal tube and ventilated with oxygen by a respiration pump (model 607, Harvard Apparatus) adjusted as needed to maintain normal arterial blood gases. A jugular vein was cannulated for administration of anesthetic and placement of electrodes for the induction of ventricular fibrillation (VF). Arterial blood pressure and surface ECG lead II were monitored and allowed to return to baseline values between inductions of VF. Normal saline was infused as needed to support baseline arterial blood pressure. Body temperature was maintained at 39°C by a water blanket (model K-20, Baxter Healthcare Corp, Pharmaseal Division). The animals were paralyzed with succinylcholine before induction of VF.

Fibrillation-Defibrillation Episodes
We applied self-adhesive Fast-Patch defibrillation pads with a surface area of 82 cm² (Physio-Control Corp) to the shaved thorax in an anterior-anterior position similar to the position used clinically on human beings. The lower left anterior electrode was always the anode for monophasic defibrillation waveforms and for the first phase of biphasic waveforms. The ECG and blood pressure were allowed to stabilize, and these parameters were taken as baseline values. VF was induced by a 60-Hz current applied for 1 to 2 seconds through a bipolar catheter located in the right ventricle. After induction of VF, ventilation was discontinued at end expiration and fibrillation continued for 15 seconds from the beginning of induction before defibrillation was attempted. Defibrillation waveforms were generated by a custom defibrillator (Bradford E. Gliner, Seattle, Wash) designed to deliver monophasic and biphasic truncated exponential waveforms. If the first defibrillation attempt was not successful, we delivered rescue shocks (see below) until defibrillation occurred. Ventilation was then resumed. We included only the first defibrillation attempt per fibrillation episode in the waveform efficacy analysis.

Defibrillation Waveforms
The monophasic and biphasic truncated waveforms used in this study are similar to those used by others in animal and clinical studies of internal defibrillation^{1 2 3 4 5 6 7 8 16 17 18 19 20 21 22} and are represented in Fig 1A and 1B. In addition, we also evaluated, to a limited extent, the defibrillation efficacy of a damped-sine-wave shock (Fig 1C) in an effort to provide a foundation for future clinical studies. Damped-sine-wave shocks are the most commonly used waveforms in transthoracic defibrillation. This waveform was examined in reference to the clinical standard energy setting of 200 J for monophasic waveforms as recommended by the American Heart Association.²³

View larger version (13K): [in this window] [in a new window]   Figure 1. Representative defibrillation waveforms. A, Monophasic truncated exponential; B, biphasic truncated exponential; and C, damped sine.

Fig 1 plots the time course of the shock voltage for typical monophasic and biphasic truncated exponential waveforms and the damped sine waveform used in study 1. These waveforms were generated by the controlled discharge of a single capacitor. The duration of the monophasic waveform in this example is 8 ms. Another important parameter of interest is the waveform tilt, which for the monophasic waveform is given by the following equation:

The tilt quantifies how steeply the monophasic waveform discharges and also gives an indication of how much of the stored capacitor charge is delivered. The higher the waveform tilt, the greater the electrical efficiency of the shock-generating mechanism, because more of the stored charge is used for defibrillation. However, as we (see "Results") and others^{16 17 18 24 25} have shown, a high waveform tilt results in greater delivered voltage and energy for defibrillation. The waveform tilt is dependent on the product of the device capacitance and the impedance of the transthoracic discharge pathway. Because the impedance will vary from animal to animal and from shock to shock, we adjusted the device capacitance to achieve the desired waveform tilt for each shock.

The voltage time course of a typical biphasic waveform is plotted in Fig 1B. Like the monophasic waveform, it is generated by the discharge of a single capacitor. The total tilt of the biphasic waveform is given by the following equation:

The duration of this biphasic waveform is given by a convention adopted in this study, which ignores the dead time caused by switching between the shock phases. The biphasic waveform duration in Fig 1B is thus 8 ms, with 4 ms per phase, even though a total duration of 8.5 ms elapses from beginning to end of the waveform. We do not think that the 0.5-ms delay between biphasic shock phases affected these experiments, because a study of shock phase separation for internal defibrillation showed no effect for delays <8 to 10 ms.^{19 20} An additional parameter, phase duration, is needed to characterize the biphasic waveform. Phase duration is the percentage of the total waveform duration occupied by the first shock phase. For example, the biphasic waveform in Fig 1B has a phase duration of 50%, because the duration of the first phase is 4 ms and the total duration of the waveform is 8 ms, as measured in this report.

We also tested a monophasic damped sine waveform to reproduce a waveform widely used in clinical transthoracic defibrillation. Fig 1C shows an example of a typical damped-sine-wave shock used in this study. This waveform was generated by the discharge of a 50-µF capacitor through a 30-mH, 10- inductor. As in standard defibrillation, only the shock voltage of this waveform was changed, with other parameters that affected its wave shape held constant. However, peak delivered voltage, delivered energy, and waveform duration changed with load impedance because of the passive nature of the wave-shaping elements. No commercially available damped-sine-wave transthoracic defibrillator is available to vary the shock strength according to the protocol required in this study. Consequently, the custom device built for this study was used to test defibrillation efficacy of the monophasic damped sine waveform as well. However, the charge voltage of this device was limited to 3200 V (200 J delivered to a 50- load) because of the circuit design requirements needed to generate truncated waveforms.

The shock strengths recorded in this study were given in terms of both the peak applied voltage and the delivered energy. To do this, the voltage and current waveforms of the applied shock were recorded at 10 000 samples per second (model Lab-NB I/O board, National Instruments). The peak shock voltage was measured from the recording, and the total energy delivered was computed on the basis of the peak delivered voltage and the capacitance used to generate the shock. The energy computation was done separately for each of the two phases of the biphasic waveform. The defibrillation impedance was also reported as the mean of the ratio of the shock voltage to the current for voltage and current computed point by point throughout the discharge.

V₅₀ and E₅₀ Estimation
For each of the waveforms used in studies 1, 2, and 3, we estimated V₅₀ using an up-and-down protocol with three reversals.^{26 27 28 29} In this protocol, only the results of the first shocks were considered for the V₅₀ estimate; subsequent rescue shocks delivered to defibrillate the animal were not included in the analysis. The protocol began by attempting defibrillation with a shock strength estimated to be equal to V₅₀ for the waveform under study. Subsequent waveforms studied used the cumulative average V₅₀ value from preceding experiments for the first defibrillation attempt. The succeeding shock voltages were incremented or decremented by 20%, depending on failure or success of the previous episode, until a reversal from failure to success or from success to failure occurred. We then changed the voltage by 10% until we obtained a total of three reversals for each waveform. We estimated V₅₀ by taking the average of the voltages delivered, excluding the first attempt and including the next step after the third reversal. We defined this average as the V₅₀ if it was within 20% of the voltage used for the first attempt; otherwise, we defined V₅₀ as the average of the voltages at each reversal. The first definition is more accurate when the initial estimate of V₅₀ is close to the true V₅₀, whereas the alternative definition was used to reduce bias when the initial estimate was not close. We also expressed defibrillation requirements in terms of E₅₀.

We allowed respiration, arterial pressure, and surface ECG to return to baseline values, with a minimum of 2 minutes between the end and the start of successive episodes. Defibrillation was defined as conversion from ventricular fibrillation within 3 seconds after the shock without any additional intervention. Studies 1 through 3 were conducted independently with different animals. Also, all of the tested waveforms were randomly interleaved during the defibrillation threshold determination within each study to mitigate the effects of spontaneous shifts in defibrillation threshold.

Experimental Protocols
The experimental protocols were designed to accomplish two goals: first, to determine whether transthoracic defibrillation could be accomplished by use of a biphasic shock waveform with less voltage and energy than with monophasic waveforms and second, to explore the sensitivity of biphasic defibrillation efficacy to variations in waveform tilt and phase duration. The first goal was pursued not only to determine whether biphasic waveform defibrillation is as efficacious when applied externally as when applied internally but also to compare biphasic waveform defibrillation with the performance of a standard clinical damped sine waveform. These goals were achieved by use of the protocols outlined in study 1 (below).

The second goal of this work was also twofold: to further characterize transthoracic biphasic waveform defibrillation and to identify which combination of waveform parameters resulted in either a minimum voltage or a minimum energy defibrillation waveform. Studies 2 and 3 (below) accomplish the second goal by testing the shock durations yielding the minimum voltage and minimum energy biphasic waveforms found in study 1, respectively. Table 1 summarizes the waveform parameters investigated in each study.

View this table: [in this window] [in a new window]   Table 1. Waveform Parameters Investigated in Studies 1, 2, and 3

Study 1
The first study determined the V₅₀ and E₅₀ strength-duration curves of truncated monophasic and biphasic waveforms. The total tilt of both waveforms was set to 80%, and the phase duration of the biphasic waveforms was set to 50%. The phase duration was chosen on the basis of reports indicating that the first phase should be at least 50% of the total duration.^{4 21} The total tilt of the biphasic and monophasic waveforms was selected to achieve electrical efficiency, ie, utilization of stored electrical charge, without incurring low-voltage tails at the end of the waveform, which have been reported to decrease defibrillation efficacy.^{16 17 18 19 24 25} If we required the shock generation process to be at least 95% efficient, then an 80% tilt waveform would be an appropriate choice, since it results in 96% of the capacitor charge being delivered. We examined total waveform durations of 5.3, 8, 12, 18, and 27 ms (each duration increased by 50%). Fig 2 illustrates these monophasic and biphasic waveforms in which, for the sake of clarity, the switching delay between phases has been eliminated. The damped sine waveform (see Fig 1C) was also tested in this series of experiments. The shocks were randomly selected and interleaved. A 2200-V, 8-ms biphasic waveform was typically used for rescue defibrillation. Six swine were studied in this portion of the investigation.

View larger version (13K): [in this window] [in a new window]   Figure 2. Illustration of study 1 waveforms. A, Monophasic and B, biphasic truncated exponential waveforms with 80% total tilt and 50% phase duration. Total duration varies as 5.3, 8, 12, 18, and 27 ms.

Study 2
In this study, two series of experiments were conducted with the total biphasic waveform duration fixed while either the phase duration (part A) or the total tilt (part B) was changed. The waveform duration was set to 12 ms, which required the least voltage for defibrillation in study 1 (see "Results"). In part A, the phase duration was varied from 30% to 70% in 10% increments while the total tilt was kept at 80%. In part B, the total tilt was varied as 60%, 70%, 80%, 90%, 95%, and 99% while phase duration was kept at 50%. These waveforms are illustrated in Fig 3. The shocks for parts A and B were randomly selected and interleaved. Rescue shocks in study 2 were typically 12-ms, 1800-V biphasic waveforms. Six swine were studied in this portion of the investigation.

View larger version (13K): [in this window] [in a new window]   Figure 3. Illustration of study 2 waveforms: 12-ms biphasic truncated exponential waveforms with (A) 80% total tilt and phase duration varying as 30%, 40%, 50%, 60%, and 70% and (B) total tilt varying as 60%, 70%, 80%, 90%, and 95% with 50% phase duration.

Study 3
As in study 2, in this study the total biphasic waveform duration was fixed while either the phase duration (part A) or the total tilt (part B) was changed. The waveform duration was set to 8 ms, which required the minimum energy for defibrillation in study 1 (see "Results"). In part A, phase duration was varied from 40% to 70% in 10% increments while the total tilt was kept at 80%. In part B, the total tilt was varied as 70%, 80%, 90%, and 95% while phase duration was kept at 50%. These waveforms are also illustrated in Fig 3, the difference from those used in study 2 being the shorter total duration. Also included in study 3 was the minimum voltage 12-ms biphasic waveform having a 60% phase duration and an 80% total tilt. The results from study 2 indicated that this waveform will defibrillate with minimum delivered voltage (see "Results"). Defibrillation by a monophasic damped sine waveform, the same as that used in study 1, was also examined. This allowed an overall comparison of the minimum voltage and minimum energy biphasic defibrillation waveforms with this traditional defibrillation waveform to be made. Again, shocks from parts A and B were randomly selected and interleaved. Rescue shocks in study 3 were typically 12-ms, 1800-V biphasic waveforms. Seven swine were studied in this portion of the investigation.

Statistical Analysis
Values are expressed as mean±SD. Statistical comparisons were made with ANOVA and paired t tests with JMP 3.0.2 software (SAS Institute). Comparisons having values of P<.05 were considered statistically significant.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Study 1
Fig 4A shows that biphasic waveforms defibrillated with significantly less voltage than monophasic waveforms at each pulse duration (P<.0001). For many of the monophasic waveforms, we were not able to defibrillate even at maximum device voltage. For these cases, we reported the maximum voltage tested, thus underestimating V₅₀. Similarly, we were not able to estimate V₅₀ for the American Heart Association–specified monophasic damped sine waveform with a maximum capacitor voltage of 3200 V available in this custom-built device. The biphasic waveform strength-duration curve did not follow the classic hyperbolic stimulation curve; there is a V₅₀ minimum at 12 ms. With a total duration of 12 ms, V₅₀ was 1378±505 V for the biphasic waveform compared with 2185±361 V for the monophasic waveform (P=.01) and >2500 V for the damped sine waveform (P=.0003).

View larger version (14K): [in this window] [in a new window]   Figure 4. Study 1 means and SDs of (A) V50 and (B) E50 for monophasic and biphasic waveforms vs total duration from six pigs. indicates monophasic; , biphasic.

Fig 4B shows that biphasic waveforms defibrillated with significantly less energy than monophasic waveforms at each duration (P=.0007). Because we were not able to defibrillate with many of the monophasic waveforms at maximum device voltage, we reported the maximum energy tested as E₅₀. The maximum energy tested for the damped sine waveform was 180 J, which was not high enough for an E₅₀ estimation. Thus, the E₅₀ for some monophasic waveforms and the damped sine waveform was underestimated in this study. The defibrillation requirements for swine proved higher than anticipated. We were not able to predict the higher requirements from the literature because previous relevant work has been done only in dogs or small pigs. For both monophasic and biphasic waveforms, E₅₀ increased with duration. With a total duration of 12 ms, E₅₀ was 169±101 J for the biphasic waveform compared with 414±114 J for the monophasic waveform (P=.003) and >180 J for the damped sine waveform. Mean defibrillation impedance was 42±6 for study 1 animals.

Study 2
Figs 5 and 6 show the characteristic curves for the 12-ms biphasic waveform. Fig 5 shows a voltage minimum and energy minimum with a phase duration of 60%. However, defibrillation voltage and energy requirements with a phase duration of 50% were not statistically different from the requirements with a phase duration of 60% (P=.3 for V₅₀, P=.4 for E₅₀). Defibrillation requirements with a 40% phase duration were significantly higher than those with a 60% phase duration (P=.005 for V₅₀, P=.005 for E₅₀).

View larger version (14K): [in this window] [in a new window]   Figure 5. Study 2 means and SDs of (A) V50 and (B) E50 for 12-ms, 80% total tilt biphasic waveforms vs phase duration from six pigs.

View larger version (13K): [in this window] [in a new window]   Figure 6. Study 2 means and SDs of (A) V50 and (B) E50 for 12-ms, 50% phase duration biphasic waveforms vs total tilt from six pigs.

As total tilt increased from 60% to 90%, defibrillation voltage requirements increased gradually, as Fig 6A shows. For tilts >90%, V₅₀ increased sharply. A total tilt of 95% had significantly higher voltage requirements than 90% total tilt (P=.02). We were not able to estimate V₅₀ for a 99% tilt waveform with a capacitor voltage limit of 3.2 kV. Because of the smaller capacitor values, as total tilt increases from 60% to 90%, defibrillation energy requirements decreased gradually, as Fig 6B shows. For tilts >90%, E₅₀ increased because of the sharply increased voltage requirements; however, the energy difference for 90% and 95% tilt did not reach statistical significance (P=.3). Mean defibrillation impedance was 41±3 for study 2 animals.

Study 3
Figs 7 and 8 show the characteristic curves for the 8-ms biphasic waveform. Although Fig 4B shows the 5.2-ms waveform to be the minimum energy biphasic waveform from study 1, the 8-ms waveform was chosen for this study for practical reasons. This is because voltage requirements of the 5.2-ms waveform were higher than deemed suitable for a clinically usable biphasic transthoracic defibrillator. The effect of phase duration on the voltage and energy was not as great for the 8-ms waveform as it was for the 12-ms waveform, as Fig 7 shows. However, there appears to be a trend for a voltage and energy minimum at a phase duration of 50% (V₅₀, P=.11 for 40% versus 50%, P=.08 for 50% versus 60% phase duration; E₅₀, P=.09 for 40% versus 50%, P=.06 for 50% versus 60% phase duration).

View larger version (12K): [in this window] [in a new window]   Figure 7. Study 3 means and SDs of (A) V50 and (B) E50 for 8-ms, 80% total tilt biphasic waveforms vs phase duration from seven pigs.

View larger version (11K): [in this window] [in a new window]   Figure 8. Study 3 means and SDs of (A) V50 and (B) E50 for 8-ms, 50% phase duration biphasic waveforms vs total tilt from seven pigs.

As total tilt increased from 70% to 95%, defibrillation voltage requirements of the 8-ms biphasic waveform increased sharply, as Fig 8A shows. Because of the smaller capacitor values, as total tilt increased from 60% to 90%, defibrillation energy requirements did not change (Fig 8B). For 95% total tilt, energy requirements increased significantly because of the sharply increased voltage requirements (P=.01 versus 90% total tilt). Mean defibrillation impedance was 44±3 for study 3 animals.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Comparison of Results
With a waveform duration of 12 ms, study 1 biphasic defibrillation voltage requirements were 63% and energy requirements were 41% of those needed for the monophasic waveform (see Table 2). The 12-ms biphasic waveform required <44% of the maximum capacitor voltage tested for the damped sine waveform. Fig 4 shows that the lowest-voltage waveform was not the lowest-energy waveform. The 8-ms biphasic waveform had an E₅₀ that was 15% less than the 12-ms biphasic waveform; however, voltage requirements were 14% higher. For monophasic truncated waveforms, increasing total duration reduced V₅₀ slightly but increased E₅₀ significantly.

View this table: [in this window] [in a new window]   Table 2. Comparison of 12-ms Monophasic and Biphasic Waveforms and the Damped Sine Waveform

From study 3, the 8-ms minimum energy biphasic waveform with 50% phase duration and 80% total tilt required 15% more voltage and 11% less energy than the 12-ms minimum voltage biphasic waveform with 60% phase duration and 80% total tilt (see Table 3 for comparison). The minimum voltage biphasic waveform required <60% of the capacitor voltage of the monophasic damped sine waveform, and the minimum energy biphasic waveform required <50% of the stored energy of the monophasic damped sine waveform.

View this table: [in this window] [in a new window]   Table 3. Comparison of the 8-ms Minimum Energy and the 12-ms Minimum Voltage Biphasic Waveforms and the Damped Sine Waveform

Other Biphasic Waveform Transthoracic Defibrillation Studies
The first study of transthoracic defibrillation with a biphasic waveform was conducted on dogs by Gurvich and Markarychev.¹¹ They showed that lower peak currents were needed for defibrillation with a biphasic damped sine waveform than with a monophasic damped sine waveform. The investigators also demonstrated a higher therapeutic index for biphasic over monophasic damped sine waveforms by showing a greater ratio between the energy required to produce cardiac damage and the energy required for defibrillation. Schuder et al^{30 31} first compared monophasic and biphasic rectangular waveforms for transthoracic defibrillation. Consistent with our findings, they found that in calves, biphasic rectangular waveforms defibrillated with greater success and at lower energies than monophasic rectangular waveforms. Biphasic waveforms were also associated with a more rapid return to a normal perfusing rhythm after the shock than monophasic waveforms.

Two different animal studies failed to find an advantage of biphasic shocks over monophasic shocks for use in transthoracic defibrillation. Kerber et al³² found that dual-pathway sequential (biphasic) shocks did not improve shock success or reduce energy requirements compared with standard single-shock transthoracic defibrillation. Their study differs from ours in the animal model, the defibrillation pad position, and the type of defibrillation waveforms, which may affect the results. The conical shape of the canine thorax compared with the rounder swine thorax alters the current pathway and may affect the results. They used three or four defibrillation pads to deliver sequential shocks; thus, as the authors suggested, a more bidirectional approach may be more effective. Also, as acknowledged by the authors, the long intershock separation of 100 ms may not be optimum, especially compared with the 0.5-ms shock separation used in this study.^{19 20}

Also in contrast to our results, Scott et al³³ did not find an improvement in defibrillation success or lower defibrillation energy requirements with biphasic transthoracic shocks in dogs. Differences from our study again include the animal model, the defibrillation pad location, and the delivered waveform. The conical shape of the canine thorax compared with the rounder swine thorax and a left chest and right chest lateral defibrillation pad position compared with a lower left and upper right pad position may affect the results. In addition, we controlled the total tilt of the waveforms by adapting capacitance, whereas they used two 150-µF capacitors and controlled the duration of the pulses. It is well known that effective biphasic defibrillation can be achieved with the second-phase leading-edge voltage set to only a fraction of that of the first phase.^{6 7}

Walcott et al³⁴ compared monophasic and biphasic truncated waveforms and a monophasic damped sine waveform for external defibrillation of beagles. Like our study, theirs also found that the biphasic waveforms required less peak voltage and total energy than monophasic waveforms. However, in contrast to our study, they found a relatively low delivered energy requirement for the damped sine waveform. The size and anatomic geometry of the animal models may have contributed to the differences observed. The ratio of defibrillation pad area to animal size is much greater for the Walcott study and may affect the defibrillation current pathway. Also, the capacitance and inductance of the damped sine waveform were not reported by Walcott. Differences in these circuit parameters may produce damped sine waveforms of different efficacies.^{35 36}

The only clinical comparative study of transthoracic monophasic and biphasic waveforms was conducted by Echt et al.³⁷ They compared the efficacy of a 200-J hybrid biphasic damped sine waveform (Gurvich waveform) with that of a 200-J monophasic damped sine waveform (Edmark waveform) for cardioversion and defibrillation of induced ventricular arrhythmias during electrophysiology studies. They demonstrated a higher probability of cardioversion/defibrillation with the biphasic Gurvich waveform (98.3% versus 85.5%, P<.02). Although the biphasic Gurvich waveform had a higher efficacy, the capacitor voltage (5900 V) and stored energy requirements (280 J) are high, and the waveform circuit requires a large wave-shaping inductor (80 mH) that is energy-inefficient; ie, 28% of the stored energy is lost in the wave-shaping. However, the greater efficacy of the biphasic Gurvich waveform over the monophasic Edmark waveform in human transthoracic defibrillation suggests that biphasic truncated exponential waveforms might also be applied beneficially in the clinical setting.

Limitations
In study 1, which demonstrated the superiority of biphasic waveforms, we compared a single type of biphasic shock waveform with a single type of monophasic shock waveform. The tested monophasic waveforms were chosen to have the same duration, energy content, and electrical efficiency as the comparable biphasic waveform. Although scientifically rigorous, this experimental design did not ensure that we used the best possible monophasic waveform for testing against our candidate biphasic waveforms. For example, there is reason to believe that modest (about 13%) reductions in delivered voltage thresholds for the monophasic waveforms could be attained. This indication comes from the work of Bourland et al,³⁸ who empirically derived an equation describing the relation between peak shock current and the shock duration and tilt for externally applied monophasic shocks. This equation predicts that truncating the 12-ms monophasic waveform early, after 6 ms, would lead to a 13.2% reduction in the delivered voltage threshold. Alternatively, a 13% reduction could be predicted to occur if a larger capacitor were used to produce a 55% tilt, 12-ms monophasic waveform. Even against these predictions, however, the 12-ms biphasic waveform would still require significantly less voltage, 27% less than the monophasic.

Another consideration is that our exploration of biphasic waveform parameter sensitivities independently tested the effects of phase duration, total tilt, and total duration. Independent parameter variation allowed us to characterize first-order effects in a practical, yet comprehensive, study. A more exhaustive study would be required to determine the interdependence of the waveform parameters (second-order effects). However, because changes in the waveform parameters gradually affected defibrillation efficacy, second-order effects are probably negligible.

Hypothetical Biphasic Waveforms for Clinical Transthoracic Defibrillation
One of the benefits of this study is that our comparison of biphasic and damped sine waveforms makes it possible to estimate the defibrillation energy requirements of these biphasic waveforms in humans. Furthermore, the results from our exploration of waveform parameter sensitivity could be used to tailor biphasic waveforms for use in the clinical setting.

The results of study 1 show that the minimum voltage and energy biphasic waveforms could, at first approximation, be selected on the basis of shock duration. The choices for waveform tilt can be made on the basis of a pragmatic consideration, the transthoracic impedance, which can be expected to vary from patient to patient and from shock to shock. The typical patient impedance is 75 to 80 , with 50 at the 5th percentile and 120 at the 95th percentile.^{39 40 41} Although this study compensated for the variations in transthoracic impedance by capacitor selection to maintain a given tilt, a clinical device will use a single capacitance; therefore, the effects of tilt variation must be anticipated.

There are some practical guidelines for choosing the nominal tilt of the hypothetical biphasic waveform. Our study suggests that we do not want a total tilt >90%, because defibrillation voltage requirements increased sharply. If we are constrained to a maximum tilt of 90%, then accommodating a relatively low patient impedance of 50 would require a 104-µF capacitor for the minimum voltage biphasic waveform. For the typical patient impedance of 80 , a nominal 76% tilt waveform would result; the high-impedance patient of 120 would receive a 62% tilt waveform. A 69-µF capacitor would be appropriate for the 8-ms minimum energy biphasic waveform. In this example of a waveform of fixed capacitance and duration, tilt is typically 76% and varies between 62% and 90%, depending on patient impedance.

Another interesting consequence of differing transthoracic impedance is that peak current, in addition to tilt, varies with patient impedance. For the low-impedance patient of 50 , peak current would increase by 60% over the typical 80- patient. This might be of additional benefit and compensate for any loss in efficacy due to a higher tilt. At the other extreme, the high-impedance patient of 120 would have a peak current 33% less than the typical 80- patient; however, the total tilt would be low at 62%, which might compensate for the lower peak current. The net effect may be that there is peak current compensation for the low-impedance patient and total tilt compensation for the high-impedance patient.

The characteristics of the defibrillation waveform may also be actively adjusted in response to a real-time measurement (ie, during delivery of the waveform) of a patient-dependent electrical parameter. For example, on the basis of the impedance of each shock, the total duration, phase duration, and total tilt of the waveform may be adjusted for each shock delivered. For example, biphasic waveforms with relatively longer first phases may have better conversion properties than waveforms with equal or shorter first phases, provided that the total duration exceeds a critical minimum. Therefore, in the case of high-impedance patients, it may be desirable to increase not only the total duration but also the phase duration of the biphasic waveform to increase the overall defibrillation efficacy. Because impedance varies among patients and from shock to shock, such a patient-specific technique may have a further advantage of being able to deliver the most efficacious waveform for a given capacitor size charged to a predetermined voltage.

Several caveats must be considered in the estimation of human defibrillation requirements from the experimental data obtained in this study. First, the swine in our study required more energy to defibrillate than people typically require. For example, with 10 to 15 seconds of VF, a 200-J monophasic damped sine waveform has a probability of defibrillation of 86% in people³⁷ and <50% in our swine. The reason for the higher energy requirement is not known. Although there is evidence that the efficacy of the monophasic damped sine waveform decreases at higher impedances,^{39 40 41} the mean defibrillation impedance of the animals was 42 compared with a typical human defibrillation impedance of 80 . If impedance were the cause of the difference, defibrillation efficacy would be higher in the animals than in human beings, which is not the case. Perhaps energy differences can be attributed to physiological and pathophysiological differences of the heart itself or to differences in the interface between defibrillation electrode and skin.

Finally, we used V₅₀ and E₅₀ to describe defibrillation requirements, which, while useful in comparing many different waveforms, is not representative of the high probability of defibrillation required for clinical application. The scaling of the V₅₀ and E₅₀ to the high probabilities needed for human transthoracic defibrillation remains to be studied. However, estimates can be made from our data with the following equation, which assumes that the probabilities of defibrillation for damped sine and biphasic waveforms scale equally as a function of energy.

This equation estimates the energy required for clinical application of a biphasic waveform, E_clin,bi, by scaling the recommended first-shock energy of the monophasic damped sine waveform, E_clin,ds, by the ratio of E₅₀ estimates for the biphasic waveform, E_swin,bi, and the monophasic damped sine waveform, E_swine,ds, from our animal model. In our study, we were not able to estimate E₅₀ for E_swine,ds because of the high voltage requirements; however, we know that it is greater than the maximum energy tested, 180 J. We conservatively use 200 J for E_swine,ds. Because the clinically recommended first-shock energy is 200 J, E_swine,ds and E_clin,ds cancel, and we are left with the following identity.

This identity is valid only within the context of this study, ie, using relatively large swine and estimating the energy of a waveform that has the same clinical efficacy as a 200-J damped sine waveform.

The above equation, in conjunction with the data presented in Table 3 and Figs 5 through 8, enables us to estimate the clinical defibrillation voltage and energy requirements of the hypothetical minimum voltage and minimum energy transthoracic biphasic shock waveforms. These values, along with the waveform parameters, are given in Table 4. Note that both of these waveforms are predicted to defibrillate with markedly less energy than the 200 J recommended for human defibrillation with the monophasic damped sine waveform and that the peak delivered voltages are much less than those developed by the damped sine waveform. These differences in delivered energy and voltage not only would be expected to benefit the patient but also portend the possibility of constructing small, lightweight external defibrillators. This goal is important because an essential element to increasing resuscitation rates for victims of sudden cardiac arrest will be the institution of a wider network of defibrillator-equipped first responders capable of reaching victims within the first few minutes after collapse. The miniaturization and cost reduction of automatic external defibrillators are critical to realizing this goal of reducing response times to <6 minutes from time of collapse, which is typically required for survival.^{42 43 44 45 46 47} If biphasic waveforms prove to be as advantageous at reducing energy and voltage requirements in transthoracic defibrillation as they have been in internal defibrillation, the size, weight, and cost of present transthoracic defibrillators could be reduced substantially. If automatic external defibrillators can be made small, lightweight, and less costly, they will become a more appropriate lifesaving tool for a much broader group of emergency first responders.

View this table: [in this window] [in a new window]   Table 4. Estimated Waveform Parameters for Hypothetical Clinically Applied Biphasic Waveforms With Efficacy Equal to a 200-J Damped Sine Waveform

	Acknowledgments

 
The authors thank Richard Kerber, MD, for his review and comments on this manuscript and Susan Bernard, DVM, for assistance in animal care.

	Footnotes

 
Messrs Gliner and Lyster are employees of Heartstream, the defibrillator company that provided the technology necessary for this study; Drs Dillon and Bardy perform consulting work for Heartstream.

Received January 18, 1995; accepted March 17, 1995.

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