Influence of phase duration of biphasic waveforms on defibrillation energy requirements with a 70-microF capacitance

The effects of second and third phase duration on defibrillation efficacy of triphasic rectangle waveforms

BACKGROUND

Biphasic waveforms are superior to monophasic waveforms for the termination of ventricular fibrillation (VF). However, whether triphasic waveforms are more effective than biphasic ones is still controversial. In the present study, we investigated the effects of second and third phase duration of triphasic rectangle waveform on defibrillation efficacy in a rabbit model of VF.

METHODS

VF was electrically induced and untreated for 30s in 20 New Zealand rabbits. A defibrillatory shock was applied with one of the 7 waveforms: 6 triphasic rectangle waveforms and a biphasic rectangle waveform. The triphasic waveforms had identical first duration but with different second and third phase durations. A 5 step up-and-down protocol was utilized for determining the defibrillation threshold (DFT). After a 5min interval, the procedure was repeated. A total of 35 cardiac arrest events and defibrillations were investigated for each animal.

RESULTS

Two triphasic waveforms with identical first and second phase duration but shorter third phase duration had significantly lower DFT energy than biphasic waveform (0.57±0.18J vs. 0.80±0.28J, p=0.001; 0.60±0.18J vs. 0.80±0.28J, p=0.003). However, no statistical difference in DFT energy was observed between the two triaphsic waveforms that had identical phase duration but different voltages (0.57±0.18J vs. 0.60±0.18J, p=0.638).

CONCLUSIONS

Phase durations played a main role on defibrillation success for triphasic rectangle waveforms. The optimal triphasic rectangle waveforms that composed of identical second and first phase durations but with shorter third pulse were superior to biphasic rectangle waveform for ventricular defibrillation.

Optimizing defibrillation waveforms for ICDs

While no simple electrical descriptor provides a good measure of defibrillation efficacy, the waveform parameters that most directly influence defibrillation are voltage and duration. Voltage is a critical parameter for defibrillation because its spatial derivative defines the electrical field that interacts with the heart. Similarly, waveform duration is a critical parameter because the shock interacts with the heart for the duration of the waveform. Shock energy is the most often cited metric of shock strength and an ICD's capacity to defibrillate, but it is not a direct measure of shock effectiveness. Despite the physiological complexities of defibrillation, a simple approach in which the heart is modeled as passive resistor-capacitor (RC) network has proved useful for predicting efficient defibrillation waveforms. The model makes two assumptions: (1) The goal of both a monophasic shock and the first phase of a biphasic shock is to maximize the voltage change in the membrane at the end of the shock for a given stored energy. (2) The goal of the second phase of a biphasic shock is to discharge the membrane back to the zero potential, removing the charge deposited by the first phase. This model predicts that the optimal waveform rises in an exponential upward curve, but such an ascending waveform is difficult to generate efficiently. ICDs use electronically efficient capacitive-discharge waveforms, which require truncation for effective defibrillation. Even with optimal truncation, capacitive-discharge waveforms require more voltage and energy to achieve the same membrane voltage than do square waves and ascending waveforms. In ICDs, the value of the shock output capacitance is a key intermediary in establishing the relationship between stored energy-the key determinant of ICD size-and waveform voltage as a function of time, the key determinant of defibrillation efficacy. The RC model predicts that, for capacitive-discharge waveforms, stored energy is minimized when the ICD's system time constant taus equals the cell membrane time constant taum, where taus is the product of the output capacitance and the resistance of the defibrillation pathway. Since the goal of phase two is to reverse the membrane charging effect of phase one, there is no advantage to additional waveform phases. The voltages and capacitances used in commercial ICDs vary widely, resulting in substantial disparities in waveform parameters. The development of present biphasic waveforms in the 1990s resulted in marked improvements in defibrillation efficacy. It is unlikely that substantial improvement in defibrillation efficacy will be achieved without radical changes in waveform design.

Achieving Sufficient Safety Margins with Fixed Duration Waveforms and the Use of Multiple Time Constants

INTRODUCTION

There are several options to achieve a sufficient safety margin in a patient with a high defibrillation threshold (DFT), with varying and typically modest success. Programming fixed (millisecond) durations of both phases of a biphasic waveform in an implantable cardioverter defibrillator (ICD) has demonstrated utility.

METHODS

We established an informal multisite registry of ICD implanting facilities. Each facility agreed to attempt the use of fixed duration waveforms whenever there was an inadequate safety margin with tilt-based waveforms. A 3.5-ms-based fixed duration shock was tried first. If that failed to achieve a 10-J safety margin then a 2-ms-based shock was used. We also tabulated an HEDFT (high estimate DFT) as precise DFTs were not determined.

RESULTS

Sixteen patients (15 M, 1 F) were entered into the registry (age 58.2 +/- 17.9 years) with ejection fractions of .30 +/-.11. Superior vena cava coils were used in 7 patients according to physician preference. The tilt-based HEDFTs were 35.4 +/- 3.2 J delivered and 35.8 +/- 3.3 J stored energy. The 3.5-ms based shocks were evaluated on 14 patients and the HEDFT fell to 23.4 +/- 6.3 J delivered (P < 0.0001) and 26.2 +/- 6.9 J stored energy (P < 0.0001). The 2-ms-based fixed duration shocks were then evaluated on 6 patients and the delivered energy HEDFT was 22.2 +/- 5.8 J (P = 0.001 vs. tilt-based shocks) while the stored energy HEDFT was 27.9 +/- 6.4 J (P = 0.01 vs. tilt-based shocks). Using the better of the two fixed duration waveforms, the mean safety margin was improved from -1.2 +/- 1.9 J to 9.5 +/- 5.9 J (P < 0.00001). Multivariate predictors of the safety margin improvement were the absence of the Superior Vena Cava (SVC) coil and absence of Ventricular fibrillation (VF) presentation. Four patients still required lead repositioning after the use of the fixed duration waveforms. No additional leads were implanted.

CONCLUSION

The use of a selection of directly programmed fixed duration biphasic shocks had a striking impact on the HEDFT for these difficult patients. Adequate safety margins were obtained for 12 of 16 patients with no lead manipulation or other approaches.

Stepped defibrillation waveform is substantially more efficient than the 50/50% tilt biphasic

BACKGROUND

Even with biphasic waveforms, patients with high defibrillation thresholds (DFTs) still are seen; thus, improved defibrillation waveforms may be of clinical utility. The stepped waveform has three parts: the first portion is positive with two capacitors in parallel, the second is positive with the capacitors in series, and the last portion is negative, also with the capacitors in series.

OBJECTIVES

The purpose of this study was to assess the clinical utility of improved defibrillation waveforms.

METHODS

We measured the delivered energy DFT in 20 patients in a dual-site study using the stepped waveform and a 50/50% tilt biphasic truncated exponential as the control. All shocks were delivered using an arbitrary waveform defibrillator, which was programmed to mimic two 220-microF capacitors (110 microF in series and 440 microF in parallel).

RESULTS

The peak voltage at DFT was reduced in 19 of the 20 patients. The median peak voltage was reduced by 32.0%, from 472 V to 321 V (P <.001). The median energy DFT was reduced by 33%, from 11.7 J to 7.8 J (P = .008). The mean voltage and energy were reduced by 25.3% and 20.2%, respectively. On average, the stepped waveform was able to defibrillate as well as the 50/50% tilt biphasic, with 33% more energy. The benefit was more pronounced in patients with either a lower ejection fraction or a superior vena cava coil. The benefit of the stepped waveform had an inverse quadratic correlation with the resistance (r(2) = 0.47), suggesting that the capacitance values chosen for the stepped waveform were close to optimal for a 35-Omega resistance.

CONCLUSION

The stepped waveform reduced the DFT compared to the 50/50% tilt waveform in this preliminary study.

Present Understanding of Shock Polarity for Internal Defibrillation: The Obvious and Non-Obvious Clinical Implications

BACKGROUND

Uncertainty about the best electrode configuration has combined with the programming flexibility in modern implantable cardioverter-defibrillators (ICDs) to result in routine polarity reversal during an implant to deal with a high defibrillation threshold (DFT). We feel that this practice is not always supported by the clinical data and the present scientific understanding of defibrillation.

METHOD

A meta-analysis of the clinical studies on ICD shock polarity was performed. Subgroup analyses were also performed to test the impact of high DFTs, various tilts, and the use of the hot can electrode. A review of the basic research surrounding the effects of polarity in defibrillation is also presented.

RESULTS

A total of 224 patients were studied. The use of an anodal right ventricular (RV) coil lowers the mean DFT by 14.8% (P = 0.00001). It provides thresholds equal to or lower than cathodal defibrillation in 83% of patients. The fraction of patients with lower anodal DFTs was 94/224 versus 38/224 for cathodal polarity. This phenomenon may be explained by virtual electrode effects. In particular, anodal electrodes tend to produce collapsing wavefronts while cathodal electrodes tend to produce expanding proarrhythmic wavefronts.

CONCLUSION

In an ICD implant, the RV coil should be the anode. Furthermore, DFT testing beginning with cathodal defibrillation is most likely unnecessary and needlessly extends the procedure's duration and increases the risks for the patient.

Benefit of millisecond waveform durations for patients with high defibrillation thresholds

BACKGROUND

Patients with a high defibrillation threshold (DFT) present an atypical but vexing problem with regard to implantable cardioverter-defibrillator (ICD) therapy. Their implant procedures are lengthy and involve more risk of complications. These patients often sustain a reduced safety margin that may compromise their survival.

OBJECTIVES

The purpose of this study was to evaluate the use of fixed millisecond duration model-optimized biphasic waveforms compared with conventional tilt-based waveforms in patients having a high DFT.

METHODS

We compared a 65%/65% tilt biphasic waveform to a millisecond duration biphasic waveform based on the biphasic burping theory using a 90-microF shock capacitor.

RESULTS

Fifty-four patients were evaluated. Mean DFT with tilt was reduced from 11.0 +/- 5.5 J to 8.8 +/- 4.1 J, for a mean reduction of 20% (P < .0001). For the 13 patients with tilt-based DFTs > or = 15 J, DFT was reduced from 18.7 +/- 4.1 J to 13.4 +/- 3.5 J, for a mean DFT reduction of 28% (P = .009). The population peak DFT was reduced from 29.0 J to 17.5 J, for a 41% reduction (P = .03).

CONCLUSION

Use of simple millisecond biphasic waveforms instead of conventional tilt-based waveforms can lead to substantial reductions in DFT, especially in patients with high DFT.

Triphasic waveforms are superior to biphasic waveforms for transthoracic defibrillation: experimental studies

OBJECTIVES

Our objective was to evaluate the efficacy of triphasic waveforms for transthoracic defibrillation in a swine model.

BACKGROUND

Triphasic shocks have been found to cause less post-shock dysfunction than biphasic shocks in chick embryo studies.

METHODS

After 30 s of electrically induced ventricular fibrillation (VF), each pig in part I (n = 32) received truncated exponential biphasic (7.2/7.2 ms) and triphasic (4.8/4.8/4.8 ms) transthoracic shocks. Each pig in part II (n = 14) received biphasic (5/5 ms) and triphasic shocks (5/5/5 ms). Three selected energy levels (50, 100, and 150 J) were tested for parts I and II. Pigs in part III (n = 13) received biphasic (5/5 ms) and triphasic (5/5/5 ms) shocks at a higher energy (200 and 300 J). Although the individual pulse durations of these shocks were equal, the energy of each pulse varied. Nine pigs in part I also received shocks where each individual pulse contained equal energy but was of a different duration (biphasic 3.3/11.1 ms; triphasic 2.0/3.2/9.2 ms).

RESULTS

Triphasic shocks of equal duration pulses achieved higher success than biphasic shocks at delivered low energies: <40 J: 38 +/- 5% triphasic vs. 19 +/- 4% biphasic (p < 0.01); 40 to <50 J: 66 +/- 7% vs. 42 +/- 7% (p < 0.01); and 50 to <65 J: 78 +/- 4% vs. 54 +/- 5% (p < 0.05). Shocks of equal energy but different duration pulses achieved relatively poor success for both triphasic and biphasic waveforms. Shock-induced ventricular tachycardia (VT) and asystole occurred less often after triphasic shocks.

CONCLUSIONS

Triphasic transthoracic shocks composed of equal duration pulses were superior to biphasic shocks for VF termination at low energies and caused less VT and asystole.

ICD waveform optimization: a randomized, prospective, pair-sampled multicenter study

The theoretical tissue model-based estimates of phase 1 and phase 2 duration of biphasic waveforms are considerably shorter than the pulse widths currently used in ICDs with standard tilt. This study used a tissue resistance/capacitance (RC) model to identify optimal biphasic pulse widths. By paired step-down defibrillation threshold (DFT) testing, the efficacy of standard versus "tuned" biphasic waveforms was evaluated in 91 patients. Standard waveforms consisted of a phase 1 set to 65% tilt and phase 2 = phase 1. The tuned waveform was based on an RC model of membrane characteristics with a time constant of 3.5 ms. The optimal phase 1 truncation point is at the peak of membrane response. The optimal phase 2 duration ends with a membrane response near or just below 0. In paired analysis, no significant differences were found in DFT or impedance between standard and tuned waveforms. In patients with DFTs > 400 V, the tuned waveform lowered the DFT by an average of 38 V (P < 0.05). Multivariate analyses showed a significant inverse relationship between DFT and impedance (P < 0.001). As impedance increased, the tuned waveform was associated with DFTs comparable to the standard waveform with shorter pulse duration and lower delivered energy. No single tilt value allowing an easy calculation of delivered energy was related to ICD waveform efficacy. The use of ICDs with tuned optimal pulse durations offer a greater flexibility of choice for patients with high DFTs.

Optimized pulse durations minimize the effect of polarity reversal on defibrillation efficacy with biphasic shocks

There are conflicting results on the effect of polarity change on the defibrillation efficacy of biphasic shocks possibly caused by different shock durations. The goal of the present study was to investigate the influence of polarity reversal on defibrillation efficacy for different biphasic shock durations in a porcine animal model. In eight anesthesized pigs using a transvenous/submuscular lead system DFTs for 4 phase 1 durations were determined: 8.1 ms, 6 ms, 3.8 ms and 1.7 ms. The phase 1/phase 2 ratio was constant at 60%/40%. For cathodal shocks, the defibrillation coil in the right ventricular apex was the cathode during phase 1 and for anodal shocks it was the anode. For both polarities, the strength-duration curve revealed a DFT minimum at 3.8 ms (cathodal shocks: 21.3 +/- 6.4 J, P < 0.001; anodal shocks: 21.9 +/- 8 J, P = 0.05). For anodal shocks and phase 1 durations of 1.7, 3.8, and 6 ms there was no significant difference of the stored energy at the DFT compared to cathodal shocks. In contrast, significantly lower DFTs were observed for anodal shocks with a phase 1 duration of 8.1 ms (28.8 +/- 6.4 J compared to 33.1 +/- 5.9 J for cathodal shocks, P = 0.006). The effect of lower defibrillation energy requirements with polarity reversal depends on the total biphasic shock duration; for the pulse duration with the lowest DFT, polarity reversal does not increase defibrillation efficacy of biphasic shocks.

Influence of body position on defibrillation thresholds of nonthoracotomy implantable defibrillators: a prospective randomized evaluation

INTRODUCTION

Defibrillation thresholds (DFTs) usually are determined with the patient in the supine position. However, patients may be in the upright position when a shock is delivered during follow-up, which may explain some first shock failures observed clinically. This study investigated whether body posture affects defibrillation energy requirements of nonthoracotomy implantable cardioverter defibrillators with biphasic shocks.

METHODS AND RESULTS

Using a step up-down protocol, DFTs were compared intraindividually in 52 patients ("active-can" sytems in 41 patients, two-lead systems in 11 patients) for the supine and upright positions as achieved by a tilt table. The mean DFT was 7.3 +/- 4.2 J in the supine versus 9.2 +/- 4.8 J in the upright position (P = 0.002). Repeated comparison in reversed order 3 months after implantation in 22 patients revealed thresholds of 6.2 +/- 2.5 J (supine) versus 8.4 +/- 3.7 J (upright; P < 0.03) 1 week and 4.4 +/- 2.4 J (supine) versus 6.2 +/- 4.1 J (upright; P < 0.04) 3 months after implantation. DFTs decreased significantly for both body positions from 1 week to 3 months after implantation (P < 0.04).

CONCLUSION

(1) DFTs for biphasic shocks delivered by nonthoracotomy defibrillators are higher in the upright compared to the supine body position. (2) Differences remain significant 3 months after implantation. For both body positions, DFT decreases significantly from 1 week to 3 months after implantation. These findings have important implications for programming first shock energy to lower than maximal values or for development of devices with lower maximal stored energy.