Simultaneous biphasic shocks enhance efficacy of endocardial cardioversion defibrillation in man

Multicenter study of principles-based waveforms for external defibrillation

STUDY OBJECTIVE

The efficacy of a shock waveform for external defibrillation depends on the waveform characteristics. Recently, design principles based on cardiac electrophysiology have been developed to determine optimal waveform characteristics. The objective of this clinical trial was to evaluate the efficacy of principles-based monophasic and biphasic waveforms for external defibrillation.

METHODS

A prospective, randomized, blinded, multicenter study of 118 patients undergoing electrophysiologic testing or receiving an implantable defibrillator was conducted. Ventricular fibrillation was induced, and defibrillation was attempted in each patient with a biphasic and a monophasic waveform. Patients were randomly placed into 2 groups: group 1 received shocks of escalating energy, and group 2 received only high-energy shocks.

RESULTS

The biphasic waveform achieved a first-shock success rate of 100% in group 1 (95% confidence interval [CI] 95.1% to 100%) and group 2 (95% CI 94.6% to 100%), with average delivered energies of 201+/-17 J and 295+/-28 J, respectively. The monophasic waveform demonstrated a 96.7% (95% CI 89.1% to 100%) first-shock success rate and average delivered energy of 215+/-12 J for group 1 and a 98.2% (95% CI 91.7% to 100%) first-shock success rate and average delivered energy of 352+/-13 J for group 2.

CONCLUSION

Using principles of electrophysiology, it is possible to design both biphasic and monophasic waveforms for external defibrillation that achieve a high first-shock efficacy.

Preliminary clinical results of a biphasic waveform and an RV lead system

Biphasic defibrillation waveforms have provided a reduction in defibrillation thresholds in transvenous ICD systems. Although a variety of biphasic waveforms have been tested, the optimal pulse durations and tilts have yet to be identified. A multicenter clinical study was conducted to evaluate the performance of a new ICD biphasic waveform and new RV active fixation steroid eluting lead system. Fifty-three patients were entered into the study. Mean age was 63 years with a mean ejection fraction of 36.8%. Primary indication for implantation was monomorphic ventricular tachycardia alone (54.7%). Forty-eight patients (90.6%) were implanted with an RV shocking lead and active can alone as the anodal contact. The ICD can was the cathode. In four cases (7.5%), an additional SVC or CS lead was used due to a high DFT with the RV lead alone. In an additional case, a chronic SVC lead was used although the RV-Can DFT was acceptable. DFT for all cases at implant was 9.8 +/- 3.7 J. Repeat testing at 3 months for a subset of patients showed a reduction in DFT (7.4 +/- 3.0 J), P value = 0.03. Sensing and pacing characteristics of the RV lead system remained excellent during the study period (acute 0.047 +/- 0.005 ms at 5.4 V and 9.9 +/- 6.2 mV R wave; chronic 0.067 +/- 0.11 ms at 5.4 V and 9.3 +/- 5.4 mV R wave). It is concluded that this lead system provides good acute and chronic sensing and pacing characteristics with good DFT values in combination with this waveform.

Influence of polarity reversal on defibrillation success with biphasic shocks and a transvenous/subcutaneous defibrillator system in a porcine animal model

Clinical studies show that polarity reversal affects defibrillation success in transvenous monophasic defibrillators. Current devices use biphasic shocks for defibrillation. We investigated in a porcine animal model whether polarity reversal influences defibrillation success with biphasic shocks. In nine anesthetized, ventilated pigs, the defibrillation efficacy of biphasic shocks (14.3 ms and 10.8 ms pulse duration) with "initial polarity" (IP, distal electrode = cathode) and "reversed polarity" (RP, distal electrode = anode) delivered via a transvenous/subcutaneous lead system was compared. Voltage and current of each defibrillating pulse were recorded on an oscilloscope and impedance calculated as voltage divided by current. Cumulative defibrillation success was significantly higher for RP than for IP for both pulse durations (55% vs 44%, P = 0.019) for 14.3 ms (57% vs 45%, P < 0.05) and insignificantly higher for 10.8 ms (52% vs 42%, P = ns). Impedance was significantly lower with RP at the trailing edge of pulse 1 (IP: 44 +/- 8.4 vs RP: 37 +/- 9.3 with 14.3 ms, P < 0.001 and IP: 44 +/- 6.2 vs RP: 41 +/- 7.6 omega with 10.8 ms, P < 0.001) and the leading edge of pulse 2 (IP: 37 +/- 5 vs RP: 35 +/- 4.2 omega with 14.3 ms, P = 0.05 and IP: 37.5 +/- 3.7 vs RP: 36 +/- 5 omega with 10.8 ms, P = 0.02). In conclusion, in this animal model, internal defibrillation using the distal coil as anode results in higher defibrillation efficacy than using the distal coil as cathode. Calculated impedances show different courses throughout the shock pulses suggesting differences in current flow during the shock.

Sawtooth first phase biphasic defibrillation waveform: a comparison with standard waveform in clinical devices

INTRODUCTION

A major limitation in a conventional truncated exponential waveform is the rapid drop in current that results in short duration of high current or longer duration with a lower average current. We hypothesized that increasing the first phase average current by boosting the decaying waveform prior to phase reversal may improve defibrillation efficacy.

METHODS AND RESULTS

To better simulate a "rectangular" waveform during the first phase, a "sawtooth" defibrillation waveform was constructed using "parallel-series" switching of capacitances (each 30 microF) during the first phase. This permitted a boost in the voltage late in the first phase. This sawtooth biphasic waveform (sawtooth) was compared to two clinical waveforms: a 135-microF capacitance (control-1) and a 90-microF capacitance (control-2) waveform. Defibrillation threshold (DFT) parameters were evaluated in 13 anesthetized pig models using a system consisting of a transvenous right ventricular apex lead (anode) and a left pectoral "hot can" electrode (cathode) system. DFT was determined by a "down-up down-up" protocol. The stored energy for sawtooth, control-1, and control-2 was 10.5 +/- 2.8 J, 12.3 +/- 3.7 J*, and 12.2 +/- 2.8 J*, respectively (*P < or = 0.01 vs sawtooth). The average current of the first phase for sawtooth, control-1, and control-2 was 7.6 +/- 1.3 A, 4.7 +/- 0.9 A*, and 6.2 +/- 0.9 A*, respectively (*P = 0.0001 vs sawtooth).

CONCLUSION

A sawtooth biphasic waveform utilizing a "parallel-series" switching system of smaller capacitors can improve defibrillation efficacy. A higher average current in the first phase generated by such a waveform may contribute to more efficient defibrillation by facilitating myocyte capture.

[Influence of waveform and configuration of electrodes on the defibrillation threshold of implantable cardioverter-defibrillators]

The defibrillation threshold (DFT) is no threshold in the true sense. Between energy levels which defibrillate in all cases and energy levels which never defibrillate, a broad range of energies exists which might or might not defibrillate. Thus, the value of the DFT is dependant on the protocol used for its determination. Usually the DFT presents an energy at which the implantable cardioverter-defibrillator (ICD) will defibrillate successfully at a rate of approximately 75%. To achieve a 100% success rate the energy has to be programmed 15 J above the DFT or twice the DFT.Using DFT measurements the energy needed for internal defibrillation could be gradually reduced in the last years. Major break throughs have been the introduction of the biphasic defibrillation waveform and the use of pectorally implanted ICD shells as defibrillation electrodes. The shortening of the defibrillation impulse by the use of lower capacitances could not improve DFTs but allowed to construct ICDs of smaller volume. Addition of a superior vena cava electrode or a subcutaneous array electrode at the left lateral chest to the standard bipolar electrode system (right ventricle, pectoral ICD can) allowed for tri- and quadripolar lead configurations which reduced DFTs on average only slightly but reduced the standard deviation of DFTs significantly and thus helped to avoid high DFTs. Besides building smaller ICDs, reduction of DFTs and thus programming of lower defibrillation ICD energies allows for improved battery longevities and reduced capacitor charging times and thus a lower incidence of syncopes.

Nonpharmacological therapy for malignant ventricular arrhythmias: implantable defibrillator trials

Implantable cardioverter-defibrillators (ICDs) are an important nonpharmacological option in the treatment of malignant ventricular arrhythmias. Technological advances in current devices permit nonthoracotomy implantation with transvenous lead systems using biphasic shocks. Decreasing device size has resulted in pectoral implantation. Battery longevity is still short in comparison with that of pacemakers. Lead failure rates as well as pacing thresholds are significantly higher than those for cardiac pacing lead systems. Other complications of ICD systems include infection, perforation, and thrombosis. The long-term performance of nonthoracotomy lead systems for ICD devices has now been extensively studied. Sudden death recurrence rates for these systems are less than 2% in 3 years and less than 5% at 5 years. Clinical trials with both monophasic and biphasic systems show a high degree of prevention of sudden death. Comparison of ICD outcome with that of drug therapy in three large retrospective studies and two small prospective randomized trials favors improved survival and sudden death prevention with device therapy. However, these studies need corroboration from large prospective trials. Two large prospective trials, CIDS and the AVID study, are now in progress to address this issue.

Epicardial sock mapping following monophasic and biphasic shocks of equal voltage with an endocardial lead system

INTRODUCTION

The reason for the increased defibrillation efficacy of biphasic shocks over monophasic shock is not definitely known.

METHODS AND RESULTS

In six anesthetized pigs, we mapped the epicardium after transvenous defibrillation shocks to compare the activation patterns following successful biphasic shocks with unsuccessful monophasic shocks of the same voltage. The heart was exposed and a 510-electrode sock with approximately 4-mm interelectrode spacing was pulled over the entire ventricular epicardium and sutured to the pericardium. Defibrillation catheters were placed in the right ventricular apex and in the superior vena cava. Paired monophasic 12 msec and biphasic 6/6 msec defibrillation shocks were given using an up-down protocol to keep shock strength between the defibrillation thresholds for the two waveforms so that the biphasic shock was successful while the monophasic shock was not. Activation fronts immediately following 60 paired shocks were recorded and analyzed by animated maps of the first derivative of the electrograms. The ventricles were divided into apical (I), middle (II), and basal (III) thirds, and early sites, i.e., the sites from which activation fronts first appeared on the epicardium following the shock, were grouped according to their location. Postshock intervals, i.e., the time from the shock until earliest epicardial activation occurred, were also determined. No ectopic activation fronts followed the shock in 20 biphasic episodes. In the other 40 paired episodes, the number of early sites was smaller after biphasic shocks than after monophasic shocks [monophasic: 198 (total), 3.3 +/- 0.9 (mean +/- SD) per shock episode; biphasic: 67, 1.1 +/- 1.0, P < 0.05]. For biphasic but not monophasic shocks, early sites were less likely to arise from the middle (II) and basal (III) thirds than from the apical third (I) [monophasic: I: 84 (42%), II: 68 (34%), III: 46 (23%); biphasic: I: 49 (73%), II: 10 (15%), III: 8 (12%), P < 0.05]. Postshock intervals were significantly shorter for monophasic shocks (54 +/- 14 msec) than for biphasic shocks (75 +/- 23 msec, P < 0.05).

CONCLUSION

The decreased number of activation fronts and the longer delay following the shock for the earliest epicardial appearance of those activation fronts that do occur may be responsible for the increased defibrillation efficacy for biphasic shocks.

Bipolar transvenous defibrillation: efficacy of two different positions of the anode

For most nonthoracotomy defibrillation lead systems, the transvenous anode can positioned independently of the right ventricular (RV) cathode. Usually a vertical position in the superior vena cava (SVC) is chosen. However, it is unknown if this position yields the optimal defibrillation threshold (DFT). Therefore, in 15 patients undergoing defibrillator implantation the SVC position was compared in a crossover study design with a horizontal position in the left brachiocephalic vein (BCV). Mean DFT was not different for SVC and BCV (19.2 +/- 9.6 J vs 18.5 +/- 9.1 J) but DFT of individual patients differed by up to 12 joules. A positive correlation between impedance and DFT in the BCV position (r = 0.6; P < or = 0.05) indicated that the improved geometry of the defibrillation field with the BCV position is opposed by a higher impedance found for this position (63 +/- 15 omega vs 52 +/- 7 omega). Thus, defibrillation is not improved in general although individual patients might benefit.

Influence of malpositioned transvenous leads on defibrillation efficacy with and without a subcutaneous array electrode

Some patients cannot receive a transvenous lead system because of high defibrillation thresholds (DFTs). We hypothesized that a right ventricular (RV) catheter electrode not extending as far as possible into the RV apex could cause high DFTs. Recently, a subcutaneous array (SQA) electrode has been shown to lower DFTs substantially. We compared the influence of a malpositioned RV catheter electrode on defibrillation efficacy for endocardial lead systems with and without a SQA. In eight anesthetized pigs, defibrillation catheters were placed in the RV apex and near the junction of the superior vena cava (SVC) and right atrium. SQA, formed by three elements, each 20 cm in length, was placed in the left thorax. DFTs were determined for a biphasic waveform using an up/down protocol with the RV catheter at the apex and with it repositioned 1-cm and 2-cm proximal to the apex. The mean DFT energies for the configurations with a SQA were less than those without a SQA for every catheter position. The placement of the RV catheter away from the apex caused an increase in defibrillation energy for the configurations without a SQA (apex: 17.1 +/- 3.8 J [mean +/- SD]; 1 cm: 20.1 +/- 4.6 J; 2 cm: 27.6 +/- 9.5 J; P < 0.05), but not for the configurations with a SQA (apex: 12.2 +/- 2.2 J; 1 cm: 12.3 +/- 2.9 J; 2 cm: 12.1 +/- 0.9 J: P = NS). These results suggest that a malpositioned RV catheter electrode, at the time of implantation or by late dislodgment, significantly elevates DFTs for a total endocardial system but not for a system that includes a SQA.

Endocardial carbon-braid electrodes. A new concept for lower defibrillation thresholds

BACKGROUND

In the treatment of patients with life-threatening ventricular arrhythmia, transvenous implantable cardioverter/defibrillators provide significant advantages over devices requiring a thoracotomy. This study tested the hypothesis that a new carbon-fiber electrode, designed at the Technische Universität in Munich, Germany, has a lower defibrillation threshold (DFT) than standard transvenous defibrillation electrodes.

METHODS AND RESULTS

In 8 mongrel dogs (weight, 25.2 +/- 0.8 kg; heart weight, 192 +/- 19 g), we examined the efficacy and electrical characteristics of a right ventricular endocardial carbon prototype defibrillation electrode (9.5F, 4.4-cm2 surface) compared with a standard CPI 0062 Endotak electrode and a Medtronic 6966 Transvene endocardial right ventricular defibrillation electrode. The new electrode consists of 24 braided, tubular carbon filaments, each containing 1000 highly isotropic carbon fibers of 7-microns diameter, yielding a theoretical electrical surface of 480 cm2. The DFTs were determined in random order between each of the three right ventricular electrodes and a subcutaneous wire array anode placed on the left thorax. A standard step-down/up DFT protocol of 20-V shock steps was applied. Two different biphasic waveforms with a 1-ms delay between phases were tested: 3.2-ms first phase/2.0-ms second phase, and 6.0-ms first phase/6.0-ms second phase. For the 3.2/2.0-ms waveform, we found a significantly lower DFT for the carbon lead (4.96 +/- 1.58 J) compared with the CPI 0062 (6.93 +/- 1.67 J) and the Medtronic 6966 (7.49 +/- 0.99 J) leads. For the 6.0/6.0-ms waveform, the DFT for the carbon electrode (5.97 +/- 2.09 J) was significantly lower than for the Medtronic 6966 lead (8.55 +/- 1.93 J) but not for the CPI 0062 lead (6.30 +/- 1.41 J). The impedance with carbon was lower than with the other two leads for the 6.0/6.0-ms waveform but not for the 3.2/2.0-ms waveform. For the carbon electrode, the 3.2/2.0-ms waveform had a lower DFT than the 6.0/6.0-ms waveform.

CONCLUSIONS

The present canine study found a lower DFT for a new carbon electrode compared with DFTs for endocardial defibrillation electrodes made of standard metal. Further long-term animal studies and clinical studies are needed to determine whether carbon materials and braided-lead technology are practical and beneficial in patients.

Optimizing defibrillation through improved waveforms

Defibrillation of the heart is achieved if an electrical current depolarizes the majority of the unsynchronized fibrillating myocardial cells. The applied current or the corresponding voltage described as a function of time is called the waveform. In pacing, to stimulate myocardial cells close to the electrode, a relatively low voltage is needed for a relatively brief duration. However, in defibrillation, approximately a 100-fold higher voltage is needed and achieved by the use of capacitors. The exponential voltage decay of a capacitor during its discharge determines the basic waveform for defibrillation. In an attempt to lower the energy needed for defibrillation, the steepness of the decay (different capacitances), the duration (fixed duration waveforms) or tilt (fixed tilt waveforms), or the initial polarity can be changed. Additionally, the polarity of the electrodes can be reversed during the discharge of the capacitor once (biphasic waveform) or twice (triphasic waveform). If two capacitors and defibrillation pathways are available, bidirectional defibrillation pulses can be delivered sequentially. In humans, the original standard waveform used with endocardial leads was a single monophasic pulse delivered by a 125-microF capacitor using the endocardial right ventricular electrode as cathode. It is now known that a reversal of the initial polarity and a reversal of polarity during capacitor discharge may significantly lower the energy needed for defibrillation, thereby preventing formerly frequent failures of defibrillation with endocardial lead systems. The use of sequential pulses showed no or only slight reductions of energy requirements and was abandoned due to the additional electrode needed. The use of a smaller capacitance (60-90 microF reduced maximum energy output but generally did not reduce energy requirements for defibrillation. However, with more efficient electrodes, smaller capacitances that will help to reduce the size of the defibrillator might be used. Thus, today defibrillation is optimized with respect to energy, capacitor size, and ease of implantation if an approximately 90-microF capacitor is used to deliver a biphasic pulse via a bipolar lead system using the right ventricular electrode as anode.

Transvenous-subcutaneous defibrillation leads: effect of transvenous electrode polarity on defibrillation threshold

INTRODUCTION

The defibrillation threshold (DFT) of a transvenous-subcutaneous electrode configuration is sometimes unacceptably high. To obtain a DFT with a sufficient safety margin, the defibrillation field can be modified by repositioning the electrodes or more easily by a change of electrode polarity. In a prospective randomized cross-over study, the effect of transvenous electrode polarity on DFT was evaluated.

METHODS AND RESULTS

In 21 patients receiving transvenous-subcutaneous defibrillation leads, the DFT was determined intraoperatively for two electrode configurations. Two monophasic defibrillation pulses were delivered in sequential mode between either the right ventricular (RV) electrode as common cathode and the superior vena cava (SVC) and subcutaneous electrodes as anodes (configuration I) or the SVC electrode as common cathode and the RV and subcutaneous electrodes as anodes (configuration II). In each patient, both electrode configurations were used alternately with declining energies (25, 15, 10, 5, 2 J) until failure of defibrillation occurred. The DFT did not differ between both configurations (18.3 +/- 8.2 J vs 18.9 +/- 8.9 J; P = 0.72). Eleven patients had the same DFT with both electrode configurations, 5 patients a lower DFT with the RV electrode as cathode, and 5 patients a lower DFT with the SVC as cathode. Four patients had a sufficiently low DFT (< or = 25 J) with only 1 of the 2 configurations.

CONCLUSION

A change of electrode polarity of transvenous-subcutaneous defibrillation electrodes may result in effective defibrillation if the first electrode polarity tested fails to defibrillate. In general, neither the RV electrode nor the SVC electrode is superior if used as a common cathode in combination with a subcutaneous anodal chest patch.

A prospective randomized cross-over comparison of mono- and biphasic defibrillation using nonthoracotomy lead configurations in humans

INTRODUCTION

For current implantable defibrillators, the nonthoracotomy approach to implantation fails in a substantial number of patients. In a prospective randomized cross-over study the defibrillation efficacy of a standard monophasic and a new biphasic waveform was compared for different lead configurations.

METHODS AND RESULTS

Intraoperatively, in 79 patients receiving nonthoracotomy defibrillation leads, the defibrillation threshold was determined in the initial lead configuration for the mono- and biphasic waveform. In each patient, both waveforms were used alternately with declining energies (20, 15, 10, 5 J) until failure of defibrillation occurred. Three different initial lead configurations were tested in different, consecutive, nonrandomized patients using a bipolar endocardial defibrillation lead alone (A; n = 36) or in combination with a subcutaneous defibrillation patch (B; n = 24) or array (C; n = 19) lead. The lowest successful defibrillation energy with the biphasic waveform was less than, equal to, or higher than with the monophasic waveform in 64%, 28%, and 8% of patients, respectively, and on average significantly lower with the biphasic waveform for all three lead configurations (A: 11.3 +/- 4.4 J vs 14.5 +/- 4.5 J; B: 9.7 +/- 4.7 J vs 15.1 +/- 4.5 J; C: 7.9 +/- 4.5 J vs 12.4 +/- 4.9 J). Defibrillation efficacy at 20 J was significantly improved by the biphasic waveform (91% vs 76%).

CONCLUSION

In combination with nonthoracotomy defibrillation leads, the biphasic waveform of a new implantable cardioverter defibrillator showed superior defibrillation efficacy in comparison to the standard monophasic waveform. Defibrillation thresholds were improved for lead systems with and without a subcutaneous patch or array lead.

Mechanisms of electrical defibrillation: impact of new experimental defibrillator waveforms

Six possible explanations for why some biphasic waveforms have lower defibrillation thresholds than monophasic waveforms of the same duration are as follows: (1) the impedance for the second phase of the biphasic shock is very low because electrode polarization develops during the first phase; (2) the large change in voltage between the first and second phases of a biphasic waveform is responsible for the increased defibrillation efficacy; (3) biphasic waveforms cause less severe detrimental effects in regions of high potential gradient; (4) the first phase of the biphasic waveform restores activity of the sodium channels, which makes defibrillation easier for the second phase; (5) the potential gradient required for defibrillation is less for biphasic waveforms than for monophasic waveforms; and (6) biphasic waveforms are better able to stimulate the myocardium to induce new action potentials or to cause refractory period prolongation. Evidence shows that, while a few of these proposed mechanisms are incorrect, several of the others may together contribute to the general superiority of biphasic waveforms.

Clinical efficacy of shock waveforms and lead configurations for defibrillation

A randomized, prospective comparison of the defibrillation efficacy of various shock waveforms and nonthoracotomy lead configurations was performed in five distinct patient groups undergoing implantation of a cardioverter defibrillator. In the first group using a bidirectional lead configuration, there was no significant difference in the mean defibrillation threshold (DFT) between simultaneous and sequential monophasic shocks (17.8 +/- 5.8 joules versus 17.3 +/- 2.7 joules). In the second group using a bidirectional lead configuration, the mean DFT was 21.9 +/- 7.3 joules with monophasic shocks and 14.9 +/- 5.0 joules with biphasic shocks (p < 0.001). In the third group using a unidirectional lead configuration, the mean DFT was significantly higher (p < 0.001) with monophasic shocks (22.1 +/- 4.2 joules) compared with biphasic shocks (15.0 +/- 5.4 joules). In the fourth group, an intraindividual comparison with monophasic shock waveforms showed no significant differences in DFT using either a bidirectional (21.3 +/- 5.8 joules) or a unidirectional (21.7 +/- 2.6 joules) lead configuration. In the fifth group, a simplified unipolar transvenous defibrillation lead system ("active can") demonstrated significant lower DFTs (9.7 +/- 3.8 joules) compared with a standardized unidirectional lead configuration (18.0 +/- 6.8 joules). It is concluded that: (1) there seems to be no significant difference in the DFT between simultaneous and sequential monophasic shocks; (2) biphasic waveforms require significantly less energy for defibrillation than their corresponding monophasic waveforms; and (3) the unipolar single-electrode defibrillation system is easy to implant and provides DFTs at energies comparable with epicardial lead systems.

Combination biphasic waveform plus sequential pulse defibrillation improves defibrillation efficacy of a nonthoracotomy lead system

OBJECTIVES

We hypothesized that combining biphasic waveform and sequential pulse defibrillation techniques would lower the defibrillation threshold of a nonthoracotomy lead system in humans below that obtained with biphasic or sequential pulse defibrillation alone.

BACKGROUND

Previous studies have shown that sequential pulse monophasic shocks and biphasic waveform shocks are more effective than single monophasic shocks for ventricular defibrillation.

METHODS

Thirteen patients aged 48 to 71 years undergoing nonthoracotomy defibrillation lead testing participated in the study. Transvenous electrodes were positioned in the right ventricular apex, superior vena cava and coronary sinus. A cutaneous patch electrode was placed on the left chest wall. All electrodes were connected to an external defibrillator. In random order, defibrillation threshold measurements were made for biphasic defibrillation alone, sequential defibrillation alone and combined biphasic plus sequential defibrillation.

RESULTS

The mean defibrillation threshold-delivered energy was 18.0 +/- 11.9 J for biphasic defibrillation and 16.3 +/- 9.0 J for sequential defibrillation. Biphasic plus sequential defibrillation significantly reduced the threshold energy to 10.2 +/- 5.3 J (p < 0.001). Threshold peak voltage and current values showed corresponding reductions. The combined waveform resulted in a greater reduction in defibrillation threshold in patients with threshold energies > 18 J versus those with threshold values < or = 18 J for sequential (p = 0.001) or biphasic (p < 0.01) waveform alone. The nonthoracotomy lead implantation rate was improved from 62% with each of the single techniques (biphasic waveform or sequential pulse defibrillation) to 85% with the combined waveform.

CONCLUSIONS

Adding biphasic waveform to sequential pulse defibrillation significantly reduced the defibrillation threshold compared with either technique alone, and nonthoracotomy lead system implantation can be enhanced by this combined technique.

Why do some patients have high defibrillation thresholds at defibrillator implantation? Answers from basic research

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.

Optimization of biphasic waveforms for human nonthoracotomy defibrillation

BACKGROUND

Biphasic waveforms reduce defibrillation threshold (DFT) in a wide variety of models. Although there are several human studies of long-duration, high-tilt biphasic waveform defibrillation, the specific biphasic waveform shape required to achieve optimal DFT reduction is unknown.

METHODS AND RESULTS

This study tested the effect of single capacitor biphasic waveform tilt modification on DFT using a paired study design in 18 patients undergoing nonthoracotomy defibrillator implantation. Baseline DFT was obtained using a 65% tilt, simultaneous pulse, bidirectional monophasic shock from a right ventricular cathode to a coronary sinus or superior vena cava lead and a subscapular patch. The single-capacitor biphasic waveform shocks, delivered over the same pathways, consisted of either both phases at 65% tilt (65/65 biphasic waveform) to produce an overall tilt of 88% and a delivered energy 11% greater than monophasic shock or both phases at 42% tilt (42/42 biphasic waveform) to produce an overall tilt of 66% and delivered energy equal to monophasic shock. The 65/65 biphasic waveform reduced stored energy DFT 25%, from 16.2 +/- 4.4 J with monophasic shock to 12.1 +/- 5.3 J (P < .02); however, it did not significantly reduce the delivered energy DFT. In contrast, the 42/42 biphasic waveform required 49% less stored energy (16.2 +/- 4.4 J, monophasic shock, vs 8.3 +/- 3.3 J, biphasic waveform; P < .001) and 49% less delivered energy (14.2 +/- 3.8 J, monophasic shock, vs 7.3 +/- 2.9 J, biphasic waveform; P < .001) than monophasic shock for successful defibrillation. The 42/42 biphasic waveform delivered energy DFT was 4.6 +/- 5.2 J (39%) less than 65/65 biphasic waveform DFT (P < .002).

CONCLUSIONS

DFT reduction is an inherent electrophysiological property of biphasic waveforms that is independent of delivered energy. Overall biphasic waveform tilt and the relative amplitudes of the waveform phases are important factors in defibrillation efficacy. Defibrillation with a 42/42 biphasic waveform is more efficacious than 65/65 biphasic waveform defibrillation; however, the optimal biphasic waveform remains unknown.

Defibrillation electrode configurations developed from cardiac mapping that combine biphasic shocks with sequential timing

Previous canine mapping studies of the transvenous defibrillation lead configuration of right ventricle (RV) to left R2 patch (P) revealed regions of low potential gradient in the left ventricular apex (A) and the right ventricular outflow tract (O). Thus 16 new lead configurations were tested in eight dogs, which incorporated electrodes in A and O to raise the gradient. When used in conjunction with two sequential biphasic shocks, the average defibrillation threshold energy from these configurations was 57% lower than that produced by a single biphasic shock delivered through RV-->P (phase 1 cathode-->anode, p < 0.001). Of the 16 configurations tested, the most effective was RV-->P followed by A-->O. When the shocking order of this configuration was reversed in another eight dogs, no difference in defibrillation efficacy was noted. In individual configurations of RV-->P and A-->O that used a single biphasic shock, defibrillation was not effective. Finally, when two sequential biphasic shocks were delivered to the same two electrodes in seven other dogs, the defibrillation efficacy was low. Thus configurations that use two sequential biphasic shocks can produce low defibrillation thresholds when the shocks are delivered to two different sets of electrodes. The high efficacy may be caused by one shock increasing the potential gradient in regions of low potential gradient that are produced by the other shock.

Effects of chronic amiodarone therapy on defibrillation threshold

In a prospective and parallel, randomized study, the long-term stability of epicardial defibrillation threshold was evaluated in 22 patients, using a patch-patch lead configuration at the time of implantation and generator replacement. The concomitant antiarrhythmic drug treatment consisted of either mexiletine (720 mg/day) or amiodarone (400 mg/day) and was administered to patients in a randomized and parallel manner. During a mean follow-up of 24 +/- 6 months, the defibrillation threshold increased significantly from 14.3 +/- 2.8 to 17.9 +/- 5.3 J (p < 0.05) for the entire patient group. The increase in the chronic defibrillation threshold was due to a marked increase in defibrillation energy needs in the subgroup of patients receiving amiodarone. Whereas no significant change in the defibrillation threshold was documented in the subgroup of patients receiving mexiletine, the mean defibrillation threshold increased from 14.1 +/- 3.0 to 20.9 +/- 5.4 J (p < 0.001) in those receiving amiodarone. In all patients with increased defibrillation thresholds, reevaluation showed a reduction in the defibrillation threshold after discontinuation of antiarrhythmic drug therapy. The only variable associated with an increase in the chronic defibrillation threshold was amiodarone treatment. These findings suggest that the defibrillation threshold should be measured at each generator replacement and in case of a change in antiarrhythmic drug treatment. In particular, if amiodarone treatment is initiated, it is recommended that the defibrillation threshold should be reevaluated to ensure an adequate margin of safety.