Large Change in Voltage at Phase Reversal Improves Biphasic Defibrillation Thresholds
Parallel-Series Mode Switching
Background Multiple factors contribute to an improved defibrillation threshold of biphasic shocks. The leading-edge voltage of the second phase may be an important factor in reducing the defibrillation threshold.
Methods and Results We tested two experimental biphasic waveforms with large voltage changes at phase reversal. The phase 2 leading-edge voltage was twice the phase 1 trailing-edge voltage. This large voltage change was achieved by switching two capacitors from parallel to series mode at phase reversal. Two capacitors were tested (60/15 microfarads [μF] and 90/22.5 μF) and compared with two control biphasic waveforms for which the phase 1 trailing-edge voltage equaled the phase 2 leading-edge voltage. The control waveforms were incorporated into clinical (135/135 μF) or investigational devices (90/90 μF). Defibrillation threshold parameters were evaluated in eight anesthetized pigs by use of a nonthoracotomy transvenous lead to a can electrode system. The stored energy at the defibrillation threshold (in joules) was 8.2±1.5 for 60/15 μF (P<.01 versus 135/135 μF and 90/90 μF), 8.8±2.4 for 90/22.5 μF (P<.01 versus 135/135 μF and 90/90 μF), 12.5±3.4 for 135/135 μF, and 12.6±2.6 for 90/90 μF.
Conclusions The biphasic waveform with large voltage changes at phase reversal caused by parallel-series mode switching appeared to improve the ventricular defibrillation threshold in a pig model compared with a currently available biphasic waveform. The 60/15-μF capacitor performed as well as the 90/22.5-μF capacitor in the experimental waveform. Thus, smaller capacitors may allow reduction in device size without sacrificing defibrillation threshold energy requirements.
Implantable cardioverter-defibrillators have been widely used in the management of patients with malignant ventricular tachyarrhythmias.1 2 3 4 5 6 7 8 9 10 The biphasic waveform has been shown to be more efficient than the monophasic waveform in defibrillating the heart when a capacitive-type exponential discharge is used.11 12 13 14 Several hypotheses have been advanced to explain the greater efficacy of biphasic waveforms. Some experimental data15 16 have suggested that biphasic waveforms can produce cardiac cell excitability at a lower voltage. However, others17 have suggested that such improvement is quite limited. Biphasic waveforms may be more effective in extending cardiac tissue refractory periods that are already activated.18 19 20 Biphasic waveforms may also act to discharge the cellular membrane at the end of a monophasic shock and thus lessen the propensity of the monophasic shock to create additional arrhythmias.21 22 23 Models based on the latter hypothesis are able to explain the improvements seen in single-capacitor biphasic waveforms.24 However, there is no published model that predicts the performance of a capacitance-changing biphasic waveform.
Recently, theories have been developed that suggest that a smaller capacitance value can achieve a lower defibrillation threshold (DFT).25 26 A few studies27 28 have shown results that are consistent with this concept as applied to the biphasic waveform. Additionally, in theory, parallel-series switching has several characteristics that could lead to improved defibrillation efficacy. If a set of capacitors is switched from parallel to series mode at phase reversal, it is possible to create a truncated exponential biphasic waveform in which the first and second phases have different tilts. Furthermore, it is possible to boost the voltage in the middle of the waveform. Walcott et al29 recently reported that in pigs, a parallel-series capacitor system (150/37.5 μF) defibrillated at a significantly lower delivered energy level than did a larger parallel-series capacitor system (300/75 μF) with biphasic waveforms.
The purpose of the present study was to determine whether defibrillation waveforms generated by parallel-series mode switching with two smaller capacitors (60 μF or 90 μF compared with commercially available 135-μF devices) can improve defibrillation efficacy in a transvenous catheter electrode system.
Surgical Procedures and Instrumentation
Eight pigs (body weight, 41±6 kg; heart weight, 166±16 g) were anesthetized with sodium pentobarbital (30 mg/kg IV), ketamine (20 mg/kg IM), and morphine (2 mg/kg IM), intubated with a cuffed endotracheal tube, and ventilated through a Drager SAV (North American Drager). Infusion of sodium pentobarbital (1 to 2 mg/kg) and pancuronium bromide (0.1 to 0.2 mg/kg) was repeated as necessary to maintain anesthesia and muscle relaxation, respectively. Body temperature was monitored continuously with a rectal thermistor and maintained between 36.0°C and 37.0°C with a warm-water blanket (Sarns Inc). An IV line was inserted via a cutdown to the left internal jugular vein. An intra-arterial line was inserted via a cutdown to the left carotid artery for continuous systemic blood pressure monitoring as well as blood gas sampling. Leads I, II, aVR, and V3 signals and an intracardiac ECG (in the apex of the right ventricle) signal were displayed continuously on an ECG monitor and stored during defibrillation testing on a computer-based physiological recording system (EP Lab, Quinton Electrophysiology Corp). A transvenous electrode designed for defibrillation (model 4007L, Angeion Corp) having a 4-cm-long distal electrode with a surface area of 1.8 cm2 was inserted via the right internal jugular vein into the apex of the right ventricle under fluoroscopy. A 55.3-cm3 titanium can electrode (Angeion Corp) with a surface area of 91.6 cm2 was implanted subcutaneously over the left pectoral area. Arterial blood samples were drawn every 30 to 60 minutes to determine pH, Po2, Pco2, base excess, total CO2 content, and bicarbonate, sodium, potassium, and calcium concentrations. Any abnormal values were corrected as needed.
The investigation used the Angeion Research Defibrillation System (ARD-9000). The ARD-9000 is an external defibrillator that operates as a high-voltage, linear amplifier for waveforms supplied by its controlling software. This device sampled the current and voltage every 0.1 ms and adjusted the waveform to mimic a capacitive discharge. The continuous voltage adjustment accounted for the impedance changes that occur as a function of voltage change during a discharge.30 Preliminary tests in a saline bath demonstrated that the voltage waveforms generated by this device had <3% variation from waveforms generated by a true capacitor discharge. The waveform tests were conducted with leading-edge voltages ranging from 100 to 750 V.
The right ventricular defibrillation electrode and the subcutaneous can electrode were connected to the ARD-9000. The distal defibrillation electrode of the right ventricle was the anode for the first phase and the cathode for the second phase. Ventricular fibrillation was induced with a 60-Hz alternating current (10 V) for 4 seconds through the defibrillation electrode. Ventricular fibrillation was allowed to persist for 10 seconds after initiation of the alternating current. At 10 seconds, a test shock was delivered. A 15- to 20-J shock was used for the initial defibrillation trial of each waveform in each pig. If ventricular fibrillation did not terminate, a rescue shock (20 to 50 J) that used the same electrode configuration was delivered. A recovery period of at least 3 minutes was allowed between each episode of fibrillation. Fibrillation was not reinitiated until heart rate and blood pressure had returned to preshock levels.
DFT was determined by a “down-up/down-up” technique until three reversals of defibrillation success were completed.27 This technique can be described as follows: After the initial successful shock, the stored energy was decremented by 1 J after each successful defibrillation. When a shock failed to defibrillate, the next trial was performed by incrementing the stored energy by 1 J. This process was repeated until three reversals in decrement or increment occurred. The final shock was always a success that followed the last failure. The DFT was defined as the average of the shock energy and voltage values obtained with all trials starting from the successful shock before the first defibrillation failure to the last successful defibrillation.
We ensured preparation stability by repeating the DFT calculation for the first of the randomized waveforms performed that day. If the second DFT was within 2 J of the first DFT, then the preparation was deemed stable and the experiment was included in the study. If there was a ≥2-J change in the measured DFTs between the first DFT and the second DFT, the DFT was not considered stable, and the data were rejected.
Two clinically available waveforms (wave-1 and wave-2; see below) were tested against two experimental waveforms (wave-3 and wave-4). The experimental waveforms had large voltage changes at phase reversal that were achieved with parallel-series mode switching of the capacitors. The order in which the four waveforms were tested was randomized in each pig.
Wave-1 (135/135 μF)
A 135-μF capacitor was discharged to 35% of initial voltage (a phase 1 tilt of 65%). At that point, the waveform was inverted and the capacitor discharged to 35% of the remaining voltage (a phase 2 tilt of 65%).
Wave-2 (90/90 μF)
A 90-μF capacitor was discharged to 56% of initial voltage (a phase 1 tilt of 44%). The capacitor was further discharged for 1.6 ms. At that point, the waveform was inverted and the capacitor discharged for 2.5 ms.
Wave-3 (60/15 μF)
A 60-μF capacitor equal to two 30-μF capacitors in parallel mode was discharged to 50% of initial voltage (a phase 1 tilt of 50%). At that point, the waveform was inverted and the capacitor was switched to series mode (equal to a single 15-μF capacitor) and discharged to 1% of initial voltage (a phase 2 tilt of 99%).
Wave-4 (90/22.5 μF)
A 90-μF capacitor equal to two 45-μF capacitors in parallel mode was discharged to 50% of initial voltage (a phase 1 tilt of 50%). At that point, the waveform was inverted and the capacitor was switched to series mode (equal to a single 22.5-μF capacitor) and discharged to 1% of initial voltage (a phase 2 tilt of 99%). (See Figure.)⇓
After the above-described protocols were completed, the pigs were euthanatized by electrically induced fibrillation. The hearts were then excised and weighed.
Group data are expressed as mean±SD. Repeated measures one-way ANOVA was used to compare stored energy, delivered energy, leading-edge voltage, peak current, and median impedance among the four defibrillation waveforms. The null hypothesis was rejected for values of P≤.05. Multiple comparisons were made with use of the Bonferroni correction.31
Complete DFT data sets were obtained from eight pigs. Test data from two pigs were rejected because of unstable defibrillation environments as defined in “Methods.” The DFT parameters and pulse width for each waveform are given in detail in the Table⇓.
The mean stored energy at DFT was 12.5±3.4 J (range, 6.5 to 16.9 J) for wave-1 (135/135-μF capacitance waveform), 12.6±2.6 J (range, 9.2 to 16.7 J) for wave-2 (90/90-μF capacitance waveform), 8.2±1.5 J (range, 6.6 to 10.9 J) for wave-3 (60/15-μF capacitance parallel-series mode-switching waveform), and 8.8±2.4 J (range, 6.1 to 12.5 J) for wave-4 (90/22.5-μF capacitance parallel-series mode-switching waveform). The stored energies for wave-3 and wave-4 were significantly less than for wave-1 (34% and 30% less, respectively) and wave-2 (35% and 30% less, respectively).
The mean delivered energy at DFT was 12.3±3.3 J (range, 6.4 to 16.6 J) for wave-1, 11.9±2.5 J (range, 8.6 to 15.8 J) for wave-2, 8.2±1.5 J (range, 6.6 to 10.9 J) for wave-3, and 8.8±2.4 J (range, 6.1 to 12.5 J) for wave-4. The delivered energy for wave-3 was significantly lower than that for wave-1 (P=.003) and wave-2 (P=.008). There was no statistically significant difference between wave-1 and wave-4 or between wave-2 and wave-4.
The mean leading-edge voltage in the first phase at DFT was 426±62 V for wave-1, 526±55 V for wave-2, 522±47 V for wave-3, and 439±61 V for wave-4. The mean leading-edge voltage in the first phase for wave-2 was significantly higher than for wave-1 (P=.001). The mean first-phase leading-edge voltage for wave-3 was significantly higher than for wave-4 (P=.007). There was no statistically significant difference between wave-2 and wave-3 or between wave-1 and wave-4. The mean leading-edge voltage in the second phase at DFT was 149±22 V for wave-1, 213±22 V for wave-2, 510±47 V for wave-3, and 433±61 V for wave-4. Leading-edge voltages in the first and second phases were a reflection of the stored energies and capacitor sizes. The leading-edge voltage in the second phase for wave-3 and wave-4 was equal to the leading-edge voltage in the first phase. This was because the first-phase tilt of these waveforms was 50%, and voltage doubling occurred during phase reversal. The second-phase leading-edge voltage was significantly higher in the parallel-series switching modes (waves 3 and 4) than in the conventional modes (waves 1 and 2).
The mean peak current in the first phase at DFT was 7.4±1.0 A (range, 5.9 to 8.6 A) for wave-1, 9.5±1.2 A (range, 7.7 to 11.4 A) for wave-2, 9.3±1.4 A (range, 8.4 to 12.2 A) for wave-3, and 7.9±1.1 A (range, 6.3 to 9.8 A) for wave-4. The peak current for wave-2 and wave-3 was significantly higher than for wave-1 (P=.002 and P=.004, respectively).
The mean median impedance at DFT was 57.4±3.5 Ω for wave-1, 55.9±2.1 Ω for wave-2, 56.6±3.2 Ω for wave-3, and 56.0±3.2 Ω for wave-4. There were no statistically significant differences in impedance among the four waveforms.
The mean pulse width in the first phase at DFT was 8.2±0.5 ms for wave-1, 4.6±0.2 ms for wave-2, 2.5±0.1 ms for wave-3, and 3.5±0.2 ms for wave-4. The mean pulse width in the second phase at DFT was 8.4±0.6 ms for wave-1, 2.5±0.0 ms for wave-2, 4.1±0.5 ms for wave-3, and 5.9±0.5 ms for wave-4. Interestingly, although wave-1 and wave-2 had first-phase pulse widths that were equal to or longer than the second-phase pulse width, wave-3 and wave-4 had first-phase pulse widths that were shorter than those in the second phase.
Large Change of Voltage at Phase Reversal
Previous studies investigating the effect of large changes in voltage at phase reversal have reported mixed findings. Feeser et al32 determined defibrillation percent success curves for several biphasic waveforms in which the first phase was held constant while the second-phase leading-edge voltage was 21%, 62%, 94%, and 141% of the first-phase leading-edge voltage. As the second-phase leading-edge voltage increased, defibrillation efficacy first improved, then declined, and improved again. On the other hand, in the study by Dixon et al,12 DFTs were determined for several biphasic waveforms in which the duration of each of the two phases was different but the total duration of the waveform was held at 10 ms. Waveforms in which the duration of the first phase was greater than or equal to that of the second phase produced a lower DFT. This occurred even though the phase-reversal voltages for these waveforms were smaller than the phase-reversal voltages of waveforms with shorter first-phase than second-phase durations. These findings indicate the importance of longer first-phase durations with respect to the second phase and downplay the role of phase-reversal voltages. However, in the present study, the biphasic waveforms (wave-3, wave-4) with a shorter first-phase than second-phase duration had lower DFT energy than the biphasic waveforms (wave-1, wave-2) with a longer first-phase than second-phase duration. This finding suggested that the large change in voltage at phase reversal was an important factor in improving DFT. Two differences between the present study and those of Dixon et al12 and Feeser et al32 are the smaller capacitance during the first phase and a further reduction of capacitance during the second phase in our experimental waveforms.
Theoretically, a smaller capacitance value can achieve a lower DFT.25 26 However, smaller capacitor values require higher leading-edge voltages than do larger capacitors. Recently, Rist et al27 compared an 85-μF capacitor with a 140-μF capacitor and showed a 20% decrease in stored DFT energy for the smaller capacitor using a 65% tilt in the biphasic waveform. The mean leading-edge voltage was higher for the 85-μF capacitor. In a study in humans, Bardy et al28 also reported that stored DFT energy for a small capacitor (60 μF) decreased 14% compared with a large capacitor (120 μF) that used biphasic pulses delivered between transvenous and subcutaneous can electrodes. In another study in humans, Swerdlow et al33 recently showed that DFT energy for a 60-μF capacitor was moderately higher than DFT energy for a 120-μF capacitor in low-resistance pathways, comparable to DFT energy for a 120-μF capacitor in intermediate-resistance pathways, but substantially lower than DFT energy for a 120-μF capacitor in high-resistance pathways. This is consistent with theoretical models25 26 that hold that the RC (resistance-capacitance) time constant of the shock should be close to the chronaxie time of the heart. For these reasons, the smaller capacitor used in the present study is considered to be more efficient than a larger capacitor for defibrillation with high-resistance pathways.
The ability to use smaller capacitors can provide an important benefit in the design of implantable defibrillators. Capacitors occupy ≈30% of the generator volume in current implantable defibrillators.34 If an estimated peak energy density of 1.3 J/cm3 were used for these capacitors,35 a 135-μF capacitance generator charged to 750 V would require 29 cm3 of volume for the capacitors, a 90-μF generator would require 19 cm3, and a 60-μF system would occupy 13 cm3. A reduction from a 135-μF system to a 60-μF system would result in capacitor volumes that are more than halved. Furthermore, reducing the stored energy in the system can reduce the battery size or, alternatively, battery longevity can be increased.
With reduction of capacitor sizes, stored energy will be reduced. This could impact the implantability of such a device in a patient with a high DFT. In a 90-μF defibrillator, the peak stored energy is 25.3 J, which is approximately twice the DFT noted for this capacitance in wave-2. In a 60-μF defibrillator, the peak stored energy would be 16.9 J, which is also approximately twice the DFT noted for this capacitance in wave-3. Thus, the lower stored energy in the 60-μF capacitance of wave-3 could still be quite appropriate if lower DFTs could be achieved in patients.
Parallel-Series Mode Switching
A single capacitor can be used to create a biphasic waveform in which the trailing-edge voltage of the first phase equals the leading-edge voltage of the second phase. Two capacitors must be used if the leading-edge voltage of the second phase is equal to or larger than that of the first phase. Preliminary reports29 36 37 also showed that a smaller parallel-series capacitor system improved defibrillation efficacy in the heart. In one report,29 two parallel-series capacitor systems were used that produced a biphasic waveform in which the second-phase leading-edge voltage was twice the first-phase trailing-edge voltage. The smaller parallel-series capacitor system (150/37.5 μF) defibrillated at a significantly lower delivered energy level than did the larger parallel-series capacitor system (300/75 μF). Our results are consistent with those preliminary reports in that the biphasic waveforms that had an increased voltage at phase reversal when a parallel-series capacitor system (90/22.5 μF or 60/15 μF) was used were associated with lower DFTs. However, our study used smaller capacitors than those used in previous studies.
Our labeling of the capacitance switching as “parallel to series,” although conceptually accurate to describe the process, may be confusing in view of current capacitor technology used in implantable defibrillators. Because the maximum voltage limitation in the “photo flash” type of aluminum electrolytic capacitors available today is ≈375 V, the typical maximum voltage of 750 V in implantable defibrillators is achieved by connecting two capacitors in series. In other words, a quoted 135-μF capacitor is in reality two 270-μF capacitors connected in series. To achieve similar voltages in our proposed parallel-series switching system, the first-phase parallel connection would consist of parallel connection of two sets of capacitors, each set incorporating two capacitors in series. In the second series phase, all four capacitors would be connected in series. Thus, in application, the parallel-series switching system would require four capacitors. Future capacitor technologies may allow larger stored voltages, thus eliminating the need to connect capacitors in series to achieve adequate peak voltages at the leading edge of the shock.
Shorter Pulse Width in Phase 1 Than in Phase 2
Previous reports11 12 13 32 indicated that waveforms in which the first-phase pulse width was greater than or equal to the second-phase pulse width produced a lower DFT. Jones et al18 suggested that the first-phase pulse width of the biphasic waveform must have a minimum duration of 3 to 5 ms to be effective. In their modeling study, shorter duration of the first-phase pulse width did not enhance refractory period prolongation. In the present study, waveforms with 50% first-phase tilt that used 60-μF (wave-3) and 90-μF (wave-4) capacitors gave first-phase pulse widths of ≈2.5 and 3.5 ms, respectively, at DFT (Table⇑). Furthermore, the first-phase pulse width was shorter than the second-phase pulse width (4.1 or 5.9 ms, respectively) in these waveforms. However, these waveforms (wave-3 and wave-4) were more energy efficient than wave-1 and wave-2. Our finding that a relatively shorter pulse width in the first phase produced lower DFTs was in contrast to the findings of previous investigations.11 12 13 18 32 This difference may be due to the low capacitance of the second phase used in the present study. This low capacitance (22.5 and 15 μF) caused voltage to drop quickly during the second phase, thus shortening the duration when an effective voltage was present. Hence, the effective pulse width for the second phase may have been considerably shorter than the full-discharge pulse width.
Lower Capacitance in Phase 2
Truncation of the second-phase waveform in standard biphasic shocks inevitably wastes a certain amount of energy. However, the low-voltage tail that is truncated does not add to defibrillation efficacy and may even interfere with the process. The parallel-series switching system may have the additional advantage of using the energy in this low-voltage tail of the second phase by converting it to a higher voltage with the series connection. Thus, the switch to a series connection not only generates a higher second-phase leading-edge voltage but also effectively shortens the second-phase pulse width by decreasing the capacitance and uses all the stored energy in a more efficient manner. These may be additional reasons why this waveform demonstrated a lower DFT.
Ventricular fibrillation has different characteristics depending on the size of the heart. Therefore, to obtain data that can be applied to the human heart, the experiments must be performed in animals with comparably sized hearts. The pig is commonly used by many investigators for this purpose. The main limitation of the present study is that ventricular fibrillation was induced in normal pig hearts. It remains to be seen whether diseased hearts with ventricular fibrillation or flutter seen in the clinical setting will respond as the pig hearts did. In addition, many patients with ventricular fibrillation or flutter take medication and have severe cardiac dysfunction or ischemia. These factors may modify the results reported here.
Another limitation is the material of which a defibrillation electrode is made. Two of the most important considerations in designing a defibrillation electrode are biocompatibility and biostability. Titanium is considered a material of choice because of its biocompatibility, high conductivity, and good fatigue life.38 In the present study, we used a titanium can electrode. However, certain aspects of titanium chemistry need to be understood when it is used for delivery of high-voltage shocks. Titanium oxide (TiO2) can be formed by the delivery of a charge. The can electrode used for defibrillation in the present study gained a dark-blue hue after many shocks because of titanium oxide buildup that can theoretically act as a semiconductor and reduce the efficiency of the shock. In the present acute, experimental study, despite the formation of titanium oxide, the can electrode did not affect DFT parameters. However, further studies may be needed to test whether titanium oxide could affect DFT during chronic implantation.
Although parallel-series switching waveforms (wave-3 and wave-4) demonstrated improved DFTs in the present report, one should be cautious in attributing this improvement directly to the boost in second-phase voltage achieved by this switching. Both wave-4 and wave-2 had 90-μF capacitances during the first phase. However, the first-phase pulse widths and thus the tilts of these two waveforms were not identical, as evidenced in the Figure⇑. It is possible that at least some of the improvement in DFT could be attributed to the difference in first-phase pulse width. The improved DFT seen with wave-3 (60/15 μF) compared with wave-1 and wave-2 could also be attributed to the lower capacitance. Thus, further studies comparing identical capacitances and first-phase pulse widths need to be performed to confidently attribute these improved DFTs to the parallel-series switching mechanism.
The major findings of the present study are as follows: (1) The large change in voltage at the phase reversal may improve the efficacy of defibrillation. The exact mechanism of such improvement is unclear but may relate to facilitation of myocardial capture or conduction. The larger second-phase voltage was achieved by switching from a parallel discharge to a series discharge at phase reversal. (2) The 60/15-μF capacitor performed as well as the 90/22.5-μF capacitor in the experimental waveform. Therefore, small capacitors may permit reduction in device size without increasing DFT energy requirements. (3) Relatively short pulse widths in the first phase of a biphasic waveform can achieve excellent defibrillation efficacy.
This work was supported in part by a grant from the Angeion Corporation, Minneapolis, Minn.
- Received December 19, 1995.
- Revision received April 11, 1996.
- Accepted April 16, 1996.
- Copyright © 1996 by American Heart Association
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