Defibrillation Efficacy of Commercially Available Biphasic Impulses in Humans
Importance of Negative-Phase Peak Voltage
Background Recent studies have shown that specifically shaped biphasic waveforms can lower energy requirements for ventricular defibrillation. We prospectively compared the defibrillation efficacy of three different biphasic wave shapes incorporated in three commercially available implantable defibrillators. The results led to the development of a second protocol in which the importance of negative-phase peak voltage and duration was investigated.
Methods and Results Defibrillation threshold (DFT) testing using different biphasic waveforms was performed randomly on 42 patients undergoing implantation of a cardioverter-defibrillator for ventricular arrhythmias. In 23 patients (group 1), 3 waveforms were tested: a CPI waveform with 60% positive-phase (P1) tilt and 50% negative-phase (P2) tilt, a Medtronic waveform with 65% fixed tilt in both P1 and P2, and a Ventritex waveform with 60% P1 tilt and a P2 leading edge voltage equal to half of the P1 trailing edge voltage. In 19 patients (group 2), 3 biphasic waveforms with equal P1 tilt at 65% but shorter P2 duration or smaller P2 peak voltage were tested. The Endotak C 60 series lead system (CPI) was used in 11 patients in group 1 and 10 patients in group 2. A Transvene lead system (Medtronics) was used in the remaining patients. Stored energy required for defibrillation was significantly lower with the CPI waveform compared with the Ventritex waveform. In group 2, energy requirements were significantly increased for the waveform with a smaller P2 peak voltage, whereas a short P2 duration did not influence defibrillation success.
Conclusions Our results suggest that specifically shaped biphasic waveforms delivered from commercially available devices can affect energy requirements for defibrillation. More importantly, the amplitude of the P2 peak voltage may be a more critical determinant than the P2 duration for defibrillation success of biphasic waveforms in humans.
It is known that some biphasic waveforms can decrease the shock strength for ventricular defibrillation compared with monophasic waveforms.1 2 3 4 5 6 7 The shape and efficacy of a biphasic waveform in turn can be altered by many variables including the pulse duration, tilt, and amplitude of each phase. Animal and human studies have shown that specifically shaped biphasic waveforms can further lower energy requirements and improve ventricular defibrillation efficacy.8 9 10 11 12 13 However, the optimal or ideal biphasic waveform remains unknown. In addition, the existence of a single waveform that is universally applicable to all patients is unlikely.
Since commercially available defibrillators utilize different shaped biphasic waveforms, it is important to determine the effect of each specific waveform on ventricular defibrillation. In our first protocol, we prospectively compared the defibrillation efficacy of the specific biphasic wave shapes available in the CPI, Medtronic, and Ventritex implantable defibrillators. The results showed that the Ventritex biphasic waveform with equal duration of both phases but with a smaller negative-phase (P2) peak voltage produced higher defibrillation threshold (DFT) energy requirements than a CPI waveform with higher P2 peak voltage and a shorter P2 than positive-phase (P1) duration. A second protocol was then developed to compare the clinical efficacy of the P2 peak voltage and duration on ventricular defibrillation success.
The study was performed on 42 patients undergoing implantation of a cardioverter-defibrillator for ventricular fibrillation or ventricular tachycardia refractory to antiarrhythmic medications. Before the study, verbal and written consent was obtained according to the guidelines of the Veterans Affair Medical Center Human Research Committee. The study population was divided into two groups undergoing different protocols.
Group 1. Twenty-three patients were enrolled in this part of the study. In each patient, the DFT was determined through the use of three biphasic waveforms (Fig 1⇓). All three waveforms were tested in every patient in a random fashion. A CPI external defibrillator (ED) delivered a waveform with 60% P1 tilt and 50% P2 tilt. A waveform with 65% fixed tilt in both P1 and P2 was delivered by the Medtronic ED. A Ventritex HVS-02 delivered the third waveform that comprised a 60% P1 tilt and a P2 leading edge voltage equal to half of the P1 trailing edge voltage.
Group 2. Nineteen patients were enrolled in this part of the study. In each patient, the DFT was determined using a second set of three biphasic waveforms (Fig 2⇓). Again, all three waveforms were tested in every patient randomly. All three waveforms had a 65% P1 tilt delivered from a 120-μF capacitor. The P2 of waveform I had a 65% fixed tilt and a peak voltage equal to the trailing edge voltage of the P1. Waveform II also had a 65% P2 tilt but with a peak voltage equal to half of the trailing edge voltage of the P1.
Waveform III P2 had a peak voltage equal to the trailing edge voltage of the positive phase with a reduction of the P2 duration to 2 ms (≈20% tilt).
Refer to Table 1⇓ for clinical and demographic data of the two groups. No patient was taking amiodarone at the time of device implantation. In addition, all other antiarrhythmic medications were discontinued at least five half-lives before the implantation procedure.
Defibrillation Lead System
In 11 patients in group 1 and 10 patients in group 2, the transvenous lead system used for defibrillation comprised a 12F tripolar endocardial catheter (60 series Endotak C, Cardiac Pacemakers Inc). The lead consisted of a proximal spring electrode with a surface area of 617 mm2 and a distal spring electrode with a surface area of 295 mm2. In the remaining 12 patients in group 1 and 9 patients in group 2, defibrillation testing was performed with the use of a two-lead system (Transvene, Medtronics Inc). A 10.5F tripolar lead with a defibrillating surface area of 426 mm2 was positioned in the right ventricular apex, and a 7F transvenous defibrillation catheter with a surface area of 90 mm2 was positioned in the superior vena cava. With both electrode systems, the right ventricular coil was the common cathode.
Before defibrillation testing with each different waveform, a low-energy test shock of ≈1 J was delivered to assess pathway integrity and impedance. In group 1, the CPI waveform was delivered through the CPI ED model 2815, the Medtronic waveform delivered through the Medtronic ED model 2394, and the Ventritex waveform was delivered through the Ventritex HVS-02 (Ventritex Inc). In group 2, all biphasic waveforms were delivered through a Medtronic ED model 2394. Tilt was defined by the amount of voltage decay at the time of truncation. For each waveform, stored energy, peak voltage, pulse duration, and impedance were recorded through a Medtronic breakout box model 2394004 that was connected between the defibrillator and the patient.
All patients were sedated with intravenous midazolam and fentanyl. Ventricular fibrillation was induced by 60-Hz alternating current delivered through the right ventricular coil. Defibrillation was then attempted after 10 seconds of stable ventricular fibrillation by delivering a biphasic shock. The first shock energy was ≈10 J. If successful, the shock energy of subsequent fibrillation episodes was decremented by 2 J until failure was obtained. If the 10-J shock was unsuccessful, a rescue shock was delivered. The shock energy was then increased by 2.5 J until success occurred. All fibrillation-defibrillation episodes were separated by at least 5 minutes. ECG and hemodynamic changes were allowed to return to baseline before inducing the next episode of ventricular fibrillation. Defibrillation threshold was defined as the lowest energy shock that successfully terminated ventricular fibrillation. At the end of the testing, all patients underwent permanent implantation of a cardioverter-defibrillator.
The data are expressed as mean±SD. Repeated-measures ANOVA was performed to compare measured threshold values among multiple waveforms. Multiple comparisons between waveforms were made with the Student-Neuman-Keuls test. Defibrillation efficacy of the biphasic waveforms was compared for stored energy, peak voltage, pulse duration, and impedance at defibrillation threshold. The power of the tests was approximated through the use of power curves. Differences were considered statistically significant at P<.05.
The protocol was successfully completed in all 23 patients. Table 2⇓ shows the mean and SD values of the DFT energy, voltage, impedance, and pulse width for each wave shape. The CPI waveform provided lower energy requirements for ventricular defibrillation compared with the Ventritex waveform. Similar trends were observed with comparison of the peak voltages. There was no significant difference in impedance between the three waveforms. The combination of the CPI waveform/Endotak lead provided lower energy and voltage requirements than the Ventritex waveform/Endotak lead system (Table 3⇓). In the Transvene lead system, DFT energy was significantly reduced by both the CPI and Medtronic waveforms. In addition, the difference between the three waveforms was more pronounced using the Transvene system.
The protocol was successfully completed in all 19 patients. The stored energy at DFT for waveform I and waveform III were lower compared with waveform II (Table 4⇓). Among the three waveforms, the peak voltage was the highest for waveform II. There was no statistically significant difference between waveforms I and III for DFT energy and peak voltage. Again, the impedances for all three waveforms were similar. In the Endotak lead system, waveforms I and III lowered the DFT energy compared with waveform II (Table 5⇓). A similar trend was seen in the Transvene lead system. Both lead systems provided similar energy and voltage requirements for all three waveforms.
Biphasic waveforms can defibrillate with lower energies compared with monophasic waveforms of similar durations.1 2 3 4 5 6 7 Animal studies have also shown that biphasic waveforms with a P2 of shorter duration and lower amplitude than the P1 are more efficacious than a biphasic waveform with a P2 longer or larger in amplitude than the P1.5 8 Unfortunately, the design of an optimal biphasic waveform includes many variables such as capacitance, tilt, duration, and amplitude of each phase. In our first protocol, we compared the defibrillation efficacy of specifically shaped biphasic waveforms typically used in commercially available defibrillators. The Ventritex waveform in which both the P1 and P2 durations were the same but the P2 peak voltage was one-half the P1 trailing edge voltage provided a significantly higher stored energy at DFT. Despite small differences in the capacitance, tilt, and duration, the CPI and Medtronic waveforms in which the P2 peak voltage was equal to the P1 trailing edge voltage with an overall shorter duration provided lower energies for successful defibrillation. A second protocol was developed to further explore the importance of P2 peak voltage in biphasic waveforms with the same tilt, capacitance, and P2 duration (waveforms I and II). In addition, the importance of decreasing the P2 tilt and duration in biphasic waveforms with equal P2 peak voltages was investigated (waveforms I and III). The findings of this second protocol indicate that in biphasic waveforms, a large peak voltage of P2 may be a more important determinant of defibrillation efficacy than a short P2 duration.
Effect of Specifically Shaped Biphasic Waveforms
In humans, Swartz et al12 and Natale et al13 showed that low-tilt biphasic waveforms with a smaller difference between the leading and trailing edge voltages appear to have a superior defibrillation efficacy compared with high-tilt biphasic waveforms with a larger difference between the leading and trailing edge voltages. Although the mechanism explaining this difference is unknown, short-duration biphasic waveforms appear to lower the DFT by decreasing the degree of shock-induced dysfunction in the high-current density regions of the heart,14 15 whereas the defibrillation efficacy of longer-duration biphasic waveforms may depend more on the shape and amplitude of the negative phase.16
In our results, the Ventritex waveform with equal P1 and P2 durations required higher energy levels for defibrillation. Compared with the CPI and Medtronic waveforms, the P2 leading edge voltage was one half the P1 trailing edge voltage and the overall duration was longer. Thus, both a longer P2 duration and a relatively small P2 amplitude reflect a less efficient waveform. By decreasing the P2 duration and increasing the P2 peak voltage, the CPI waveform demonstrated superior defibrillation efficacy. Comparison of the CPI and Medtronic waveforms revealed no significant difference in stored energy levels despite a longer P2 duration in the Medtronic waveform. These results suggested that in biphasic waveforms, the P2 peak voltage may have affected the overall defibrillation efficacy more than the P2 duration.
Effect of the Negative-Phase Peak Voltage and Duration
Feeser et al10 showed a bimodal effect on defibrillation success when the P2 phase duration was increased. The defibrillation efficacy first improved, then declined, and then improved again. In addition, a similar bimodal effect was seen when the P2 amplitude was increased, thus suggesting that factors other than the P2 duration may effect defibrillation efficacy. In fact, the amplitude of the P2 was reported to be an important determinant of defibrillation efficacy in earlier animal studies.1 9
Our results confirm the importance of the P2 peak voltage on defibrillation efficacy in humans. Despite decreasing the P2 duration and tilt while keeping the P2 peak voltage the same, there was no significant difference in energy requirements for defibrillation. The effect of a larger P2 peak voltage on improved defibrillation could not be explained by the pathway impedances. There was no significant difference in impedances between the waveforms utilized in either protocol.
Effect of the Defibrillation Lead Systems
In both the Endotak and the Transvene systems, energy requirements were reduced with the CPI waveform. On the other hand, the variance of the energy data and the difference among waveforms in the first protocol were higher with the use of the Transvene system. It is possible that a lead system with a higher impedance is more susceptible to the effect of various biphasic impulses. In comparing the CPI, Medtronic, and Ventritex waveforms, the Endotak lead did provide a lower DFT than the Transvene system. A direct comparison of the two lead systems was not performed because the two leads were not tested in the same patient. However, preliminary results from our institution in which the Transvene and the Endotak 70 lead series were directly compared in the same patient did show a significant reduction in energy for the Endotak system.17
Proposed Mechanisms of Ventricular Defibrillation
The mechanisms explaining why certain biphasic waveforms defibrillate better than monophasic and other biphasic waveforms have not been elucidated. Two recent hypotheses based on the upper limit of vulnerability theory18 19 state that the positive phase of the biphasic waveform defibrillates, whereas the negative phase prevents the heart from refibrillating.20 21 Jones et al,22 however, suggest that membrane hyperpolarization during the positive or conditioning phase improves sodium channel availability during the negative or defibrillating phase, thus prolonging the refractory period and protecting the cells from approaching wave fronts. Although the bases for these theories are different, there are some common denominators. First, the large sudden change in voltage and the charge ratio at phase reversal improves the defibrillation efficacy of biphasic waveforms compared with monophasic waveforms. Second, a defibrillatory shock that is unsuccessful results in reinitiation of fibrillation in the area of the lowest potential field gradient.
Our study was not designed to explain the mechanisms involved in biphasic defibrillation. It is possible, however, that a higher voltage at the time of phase reversal can prevent the generation of new propagating wave fronts and reduce the incidence of post-shock recurrence of fibrillation by providing a more uniform polarization and voltage gradient across the cells. In addition, the higher negative phase peak voltage may enhance sodium channel recovery, prolong the refractory period response of the myocardium, and protect the heart from refibrillating.
The major limitations of this study relate to the determination of the DFT and the small sample size. Although the reliability of using the single lowest value in the measurement of the DFT is not as predictable as the generation of dose-response curves,23 24 the process is not applicable for repetitive determinations in humans, especially in the setting of determining three separate thresholds in each patient. In addition, our conclusion that a larger P2 peak voltage may be more efficient than the P2 duration is based on finding a significant difference (P<.05) between waveforms I and II and not demonstrating a difference between waveforms I and III. Due to the population size, it is possible that the negative findings may become significantly different if a larger number of patients were studied. However, because the measure of DFT included relatively low energy steps (2 to 2.5 J) and the power of the second study was ≈0.77, we believe that these findings are a reasonable representation of a larger population and in the very least, lead to the conclusion that a small P2 peak voltage has a greater negative impact on the DFT than a short P2 duration.
Since future-generation implantable cardioverter-defibrillators may use a variety of biphasic wave shapes including programmable tilt, duration, and peak voltages of each phase, it is important to determine which variables are most critical for ventricular defibrillation success. Our study shows that specifically shaped biphasic waveforms used in present devices can lower energy requirements at DFT in patients. In addition, the amplitude of the P2 peak voltage and not the duration is a more important determinant in effecting defibrillation success of biphasic waveforms in humans. Since it is unlikely that a single ideal biphasic waveform exists for all patients, the use of our information may improve defibrillation efficacy and minimize the need for alternative pulsing methods.
- Received August 21, 1996.
- Revision received October 21, 1996.
- Accepted November 18, 1996.
- Copyright © 1997 by American Heart Association
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