Decline in defibrillation thresholds

Electrical Stimulation for Low-Energy Termination of Cardiac Arrhythmias: a Review

Cardiac arrhythmias are a leading cause of morbidity and mortality in the developed world, estimated to be responsible for hundreds of thousands of deaths annually. Our understanding of the electrophysiological mechanisms of such arrhythmias has grown since they were formally characterized in the late nineteenth century, and this has led to the development of numerous devices and therapies that have markedly improved outcomes for patients affected by such conditions. Despite these advancements, the application of a single large shock remains the clinical standard for treating deadly tachyarrhythmias. Such defibrillating shocks are undoubtedly effective in terminating such arrhythmias; however, they are applied without forewarning, contributing to the patient's stress and anxiety; they can be intensely painful; and they can have adverse psychological and physiological effects on patients. In recent years, there has been interest in developing defibrillation protocols that can terminate arrhythmias without crossing the human pain threshold for energy delivery, generally estimated to be between 0.1 and 1 J. In this article, we review existing literature on the development of such low-energy defibrillation methods and their underlying mechanisms, in an attempt to broadly describe the current landscape of these technologies.

Electrical and pharmacologic cardioversion for atrial fibrillation

In this article, electrical and pharmacologic cardioversion for atrial fibrillation is described in detail. Indications for cardioversion and management of pericardioversion anticoagulation also are discussed. Finally, management strategies for immediate recurrence of atrial fibrillation and cardioversion failure are offered.

A systematic review of systematic reviews and meta-analyses of animal experiments with guidelines for reporting

To maximize the findings of animal experiments to inform likely health effects in humans, a thorough review and evaluation of the animal evidence is required. Systematic reviews and, where appropriate, meta-analyses have great potential in facilitating such an evaluation, making efficient use of the animal evidence while minimizing possible sources of bias. The extent to which systematic review and meta-analysis methods have been applied to evaluate animal experiments to inform human health is unknown. Using systematic review methods, we examine the extent and quality of systematic reviews and meta-analyses of in vivo animal experiments carried out to inform human health. We identified 103 articles meeting the inclusion criteria: 57 reported a systematic review, 29 a systematic review and a meta-analysis, and 17 reported a meta-analysis only. The use of these methods to evaluate animal evidence has increased over time. Although the reporting of systematic reviews is of adequate quality, the reporting of meta-analyses is poor. The inadequate reporting of meta-analyses observed here leads to questions on whether the most appropriate methods were used to maximize the use of the animal evidence to inform policy or decision-making. We recommend that guidelines proposed here be used to help improve the reporting of systematic reviews and meta-analyses of animal experiments. Further consideration of the use and methodological quality and reporting of such studies is needed.

Biphasic and monophasic shocks for transthoracic defibrillation: a meta analysis of randomised controlled trials

INTRODUCTION

Biphasic waveforms are routinely used for implantable defibrillators. These waveforms have been less readily adopted for external defibrillation. This study was performed in order to evaluate the efficacy and harms of biphasic waveforms over monophasic waveforms for the transthoracic defibrillation of patients in ventricular fibrillation (VF) or haemodynamically unstable ventricular tachycardia.

METHODS

Studies included randomised controlled trials comparing monophasic and biphasic external defibrillation for participants with VF or hemodynamically unstable ventricular tachycardia. Seven trials (1129 patients) were included in the analysis. All trials were conducted during electrophysiology procedures or implantable cardioverter/defibrillator testing.

RESULTS

Compared with 200 J monophasic shocks, 200 J biphasic shocks reduced the risk of post-first shock asystole or persistent VF by 81% (relative risk (RR) 0.19; 95% confidence intervals (CI) 0.06-0.60) for the first shock. Reducing the energy of the biphasic waveform to 115-130 J resulted in similar effectiveness compared with the monophasic waveform at 200 J (RR 1.07, CI 0.66-1.74). Low energy biphasic shocks produce less myocardial injury than higher energy monophasic shocks as determined by ST segment deflection after shock.

CONCLUSIONS

Biphasic waveforms defibrillate with similar efficacy at lower energies than standard 200 J monophasic waveforms, and greater efficacy than monophasic shocks of the same energy. Available data suggests that lower delivered energy and voltage result in less post-shock myocardial injury.

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.

Ventricular fibrillation and defibrillation thresholds in sheep and dogs

Ventricular defibrillation studies normally use dogs rather than other large species. To investigate the suitability of sheep, which are often cheaper and more readily available, we compared ventricular fibrillation and defibrillation thresholds (VFT, DFT) in sheep and dogs. A total of 12 sheep (31 +/- 5 kg) and six dogs (19 +/- 1 kg) were anesthetised with halothane. Fibrillation was induced via epicardial pacing leads, using a 1 s 50 Hz pulse. Biphasic defibrillation shocks were delivered across epicardial patches. Voltage-response curves for both fibrillation and defibrillation were generated. Logistic regression analysis was used to determine 50 and 90% probability of success for fibrillation induction and defibrillation. VFT was similar in sheep and dogs. DFT at 50% probability of success was significantly higher in sheep (369 +/- 14 V) than in dogs (299 +/- 31 V, P < 0.04) but within each species there was no correlation between heart weight and DFT. After defibrillation sheep took longer to return to sinus rhythm than dogs and electro-mechanical dissociation was observed in sheep, but not in dogs. Thus, sheep may not be an ideal model for ventricular defibrillation research but further studies of the intrinsic differences between sheep and dogs may provide insights into basic mechanisms of defibrillation.

Effects of polarity on defibrillation thresholds using a biphasic waveform in a hot can electrode system

The polarity of a monophasic and biphasic shocks have been reported to influence DFTs in some studies. The purpose of this study was to evaluate the effect of the first phase polarity on the DFT of a biphasic shock utilizing a nonthoracotomy "hot can" electrode configuration which had a 90-microF capacitance. We tested the hypothesis that anodal first phase was more effective than cathodal ones for defibrillation using biphasic shocks in ten anesthetized pigs weighing 38.9 +/- 3.9 kg. The lead system consisted of a right ventricular catheter electrode with a surface area of 2.7 cm2 and a left pectoral "hot can" electrode with 92.9 cm2 surface area. DFT was determined using a repeated "down-up" technique. A shock was tested 10 seconds after initiation of ventricular fibrillation. The mean delivered energy at DFT was 11.2 +/- 1.7 J when using the right ventricular apex electrode as the cathode and 11.3 +/- 1.2 J (P = NS) when using it as the anode. The peak voltage at DFT was also not significantly different (529.0 +/- 41.3 and 531.8 +/- 28.6 V, respectively). We concluded that the first phase polarity of a biphasic shock used with a nonthroracotomy "hot can" electrode configuration did not affect DFT.

Pinacidil's Effects on Defibrillation Outcomes: Role of Increased Potassium Conductance Via the K(ATP) Channel

BACKGROUND: It has been shown that the inhibition of potassium ion conductance decreases defibrillation threshold. We postulated that if potassium conductance is a primary mechanism affecting defibrillation threshold values, then increasing potassium ion conductance will increase defibrillation values. The primary objective of this study was to determine if the ATP-dependent potassium (K(ATP)) channel opener pinacidil would increase defibrillation threshold values. The second objective was to prove that the observed changes were due to potassium conductance by using the K(ATP) inhibitor, glyburide, to reverse the electrophysiologic actions of pinacidil. The third objective was to determine if the electrophysiology action sof pinacidil correlate with changes in defibrillation threshold value. METHODS AND RESULTS: Domestic farm swine (n = 14) were anesthetized and intubated. Subsequently, they were instrumented with monophasic action potential catheters and epicardial defibrillation patches. Defibrillation threshold values, action potential duration, effective refractory period, and ventricular fibrillation cycle length were determined at baseline and during treatment phase 1 and treatment phase 2. Pigs were randomized into 2 groups: group 1 (n = 6) received D(5)W in treatment phase one followed by D(5)W in treatment phase 2 and group 2 (n = 8) received pinacidil in treatment phase one followed by the addition of glyburide in treatment phase two. DFT(ED50) did not change at baseline, treatment phase one or treatment phase two for group 1 (10.5 +/- 2, 11.1 +/- 1.7, 10.5 +/- 1.0 J) or for group 2 (10.1 +/- 2.2, 11.4 +/- 4.2, 11.4 +/- 3.0 J). Electrophysiologic parameters )QRS, effective refractory period, action potential duration(90), and ventricular fibrillation cycle length) were not significantly changed from baseline in group 1. In contrast, effective refractory period, action potential duration(90), and ventricular fibrillation cycle length significantly decreased at all recorded sites after the administration of pinacidil in group 2 (range of 7-13%, 6-9%, and 12-17%, respectively). However, pinacidil did not change the basal level of dispersion in effective refractory period, action potential duration, and ventricular fibrillation cycle length during paced rhythm or ventricular fibrillation. Glyburide reversed pinacidil's electrophysiologic actions. CONCLUSIONS: Pinacidil does not alter defibrillation threshold, but it reduces effective refractory period, action potential duration, and ventricular fibrillation cycle length and does not increase electrical heterogeneity. Therefore, changes in potassium channel conductance as well as shortening repolarization are unlikely primary mechanisms for elevating defibrillation threshold.

Charge-burping theory correctly predicts optimal ratios of phase duration for biphasic defibrillation waveforms

BACKGROUND

For biphasic waveforms, it is accepted that the ratio of the duration of phase 2 to the duration of phase 1 (phase-duration ratio) should be < or = 1. The charge-burping theory postulates that the beneficial effects of phase 2 are maximal when it completely removes the charge delivered by phase 1. It predicts that the phase-duration ratio should be < 1 when the time constant of the defibrillation system (tau s) exceeds the time constant of the cell membrane (tau m) but > 1 when tau s < tau m. This study tested the hypothesis that the optimal phase-duration ratio depends on tau s (the product of the defibrillator capacitance and pathway resistance).

METHODS AND RESULTS

In a canine model of transvenous defibrillation (n = 8), we determined stored-energy defibrillation thresholds (DFTs) for biphasic waveforms from conventional capacitors (140 microF. tau s = 7.1 +/- 0.8 ms) and very small capacitors (40 microF. tau s = 2.0 +/- 0.2 ms). Each capacitance was tested with phase-duration ratios of 0.5, 1, 2, and 3. The duration of phase 1 approximated the optimal monophasic waveform, 6.3 +/- 0.7 ms for 140-microF waveforms and 2.8 +/- 0.2 ms for 40-microF waveforms. For 140-microF waveforms, the DFT was lower for phase-duration ratios < or = 1 than for phase-duration ratios > 1 (P = .0003). The reverse was true for 40-microF capacitors (P = .0008). There was a significant interaction between the effects of capacitance and phase-duration ratio on DFT (P = .0002). The lowest DFT for 40-microF waveforms was less than the lowest DFT for 140-microF waveforms (4.9 +/- 2.5 versus 6.4 +/- 2.4 J, P < .05).

CONCLUSIONS

The optimal phase-duration ratio is < or = 1 for conventional capacitors and > 1 for small capacitors. This supports the predictions of the charge-burping theory.

Influence of hypertonic saline solution infusion on defibrillation efficacy

Hypertonic saline solution may enhance cardiac conduction via the fast inward sodium channel and alter transmembrane Ca+2 conductance via the sodium-calcium exchanger. Evidence suggests that both Ca+2 conductance and myocardial conduction velocity may affect ventricular defibrillation. Since hypertonic saline solution solutions (ie, sodium bicarbonate) may be administered to patients who have conditions that often require ventricular defibrillation (ie, cardiac arrest or hypovolemic shock), we studied the effect of hypertonic saline solution on the defibrillation threshold (DFT) in 16 pentobarbital-anesthetized domestic farm swine (20 to 30 kg). Defibrillation was performed using two interfaced epicardial electrode patches. DFTs were determined at baseline and during treatment phase. Pigs were randomly assigned to treatment consisting of either hypertonic saline solution (6 mmol/kg load, 2.0 to 3.0 mmol/kg infusion) to maintain serum sodium concentrations 10 to 15 mmol/L above baseline or control (D5W given in equal volume). DFT values (joules) that predicted 50% success were modeled from a best-fit histogram. Hypertonic saline solution did not change DFT values from baseline values (10.2 +/- 4.3 vs 10.8 +/- 7.0, respectively). Likewise, placebo (D5W) did not change DFT values from baseline values (10.1 +/- 4.5 vs 11.3 +/- 4.3). During treatment phase, DFT values were 99 +/- 28% of baseline values in the hypertonic saline solution group and 116 +/- 23% of baseline values in the D5W groups (p = 0.21). The administration of hypertonic saline solution also did not affect ventricular conduction velocity, right ventricular action potential duration, or right ventricular effective refractory period. These data indicate that hypertonic saline solution does not appreciably affect defibrillation efficacy or electrical treatment of ventricular fibrillation.

Mechanism of antiarrhythmic drug-induced changes in defibrillation threshold: role of potassium and sodium channel conductance

OBJECTIVES

We sought to determine which ion current predominantly affects defibrillation outcomes by using specific pharmacologic probes (lidocaine [a sodium channel blocking agent] and cesium [an outward potassium channel blocking agent]) in 26 swine.

BACKGROUND

The effect of a drug on sodium or potassium channel conductance, or both, may affect defibrillation threshold values. However, it is unknown which ion channel predominates.

METHODS

Each pig was randomly assigned to one of four treatment groups with two treatment phases: group 1 = placebo (D5W) in treatment phase I followed by placebo plus cesium in treatment phase II (n = 6); group 2 = lidocaine followed by lidocaine plus placebo (n = 7); group 3 = lidocaine followed by lidocaine plus cesium (n = 7); group 4 = placebo followed by placebo plus placebo (n = 6). Defibrillation threshold values and electrocardiographic measurements were obtained at baseline and at treatment phases I and II.

RESULTS

Lidocaine increased defibrillation threshold values from baseline by 71% in group 2 (p = 0.02) and by 92% in group 3 (p < 0.01). There were no changes in defibrillation threshold values from baseline to D5W in groups 1 and 4. When D5W was added to lidocaine in group 2 and D5W in group 4, there were no significant changes in defibrillation threshold values. However, when cesium was added to lidocaine in group 3, the elevated defibrillation threshold values (mean +/- SD) returned to baseline values (from 15.7 +/- 3.46 to 7.55 +/- 3.19 J, p < 0.01). Cesium added to D5W in group 1 also significantly reduced defibrillation threshold values from 7.10 +/- 1.27 to 4.14 +/- 1.75 J (p < 0.01). The effect of cesium on defibrillation threshold values was similar between groups 1 and 3, regardless of lidocaine, such that these values were reduced by 40 +/- 14% and 51 +/- 18%, respectively (p = 0.28).

CONCLUSIONS

Cesium, through potassium blockade, reverses lidocaine-induced elevation in defibrillation threshold values. The magnitude of defibrillation threshold reduction when cesium was added to lidocaine was similar to the defibrillation threshold reduction when cesium was added to placebo. Thus, inhibiting outward potassium conductance and prolonging repolarization decreases defibrillation threshold values independent of sodium channel blockade.

Differential effects of lidocaine on defibrillation threshold with monophasic versus biphasic shock waveforms

BACKGROUND

Defibrillation waveforms and antiarrhythmic drugs have disparate effects on myocardial excitability and refractoriness, making it likely that antiarrhythmic drugs will interact with one waveform differently than with another. The aim of the present study was to determine if the increase in defibrillation threshold (DFT) induced by lidocaine is similar for electrical shocks with monophasic and biphasic waveforms.

METHODS AND RESULTS

Twenty-six pentobarbital-anesthetized farm-raised pigs were instrumented with pacing catheters and epicardial defibrillation electrodes. Each pig was assigned to one of four groups: (1) monophasic shock waveform and placebo (5% dextrose in water [D5W]) (n = 7), (2) monophasic shock waveform and lidocaine (n = 7), (3) biphasic shock waveform and placebo (D5W) (n = 5), or (4) biphasic shock waveform and lidocaine (n = 7). DFT was measured at baseline and subsequently during treatment (D5W or lidocaine). In the monophasic waveform groups, DFT increased from baseline in response to lidocaine by 92% (P < .0001), whereas DFT values in response to D5W did not change. In the biphasic waveform groups, DFT values did not change from baseline in response to lidocaine (P = NS), whereas DFT values from baseline in response to D5W significantly decreased by 29% (P = .04). In the monophasic waveform groups, the change in DFT from baseline in response to lidocaine was significantly different than the change from baseline in response to D5W (92 +/- 29% versus -0.5 +/- 29%, respectively) (P < .0002). In the biphasic waveform groups, however, the change in DFT from baseline in response to lidocaine was similar to the change from baseline in response to D5W (-5.66 +/- 15% versus -29 +/- 17%, respectively) (P = .48). Furthermore, the change in DFT from baseline in response to lidocaine differed significantly between monophasic and biphasic waveform groups (92 +/- 29% versus -5.66 +/- 15%) (P < .0002), whereas the change from baseline in response to D5W did not differ between monophasic and biphasic waveforms (-0.5 +/- 29% versus -29 +/- 17%) (P = .34).

CONCLUSIONS

Compared with placebo groups, DFT values increased during lidocaine treatment to a much greater degree in the monophasic waveform group than in the biphasic waveform group receiving lidocaine. These data support our hypothesis that antiarrhythmic drugs can affect the defibrillation efficacy of monophasic waveforms differently than that of biphasic waveforms.

Defibrillation thresholds are lower with smaller storage capacitors

Present implantable cardioverter defibrillators use a wide range of capacitance values for the storage capacitor. However, the optimal capacitance value is unknown. We hypothesized that a smaller capacitor, by delivering its charge in a time closer to the heart chronaxie, should lower the defibrillation threshold (DFT). We compared the energy required to defibrillate 10 open-chest dogs, after 15 seconds of ventricular fibrillation, with a monophasic, time-truncated waveform delivered from either a 85-microF or a 140-microF capacitor. Shocks were delivered through a pair of 14-cm2 epicardial patch electrodes: The two capacitors were randomly tested twice with each dog using a modified 3-reversal method for each DFT determination. The average stored and delivered DFT energies for the 85-microF capacitor were 6.0 +/- 1.7 joules and 5.2 +/- 1.5 joules, respectively, compared to 6.7 +/- 1.7 joules and 6.0 +/- 1.5 joules for the 140-microF capacitor (P = 0.01 and P = 0.004, respectively). The mean leading edge voltages were higher, the pulse duration shorter, and the mean impedance lower for the 85-microF capacitor. The impedance was inversely related to the pulse duration and the voltage decay suggesting that, at least in part, the mechanism of improved defibrillation could be accounted for by the waveform electrical characteristics. There was an equal number of episodes of postshock bradyarrhythmias and tachyarrhythmias following discharges from each capacitor. Moreover, there was no relationship between the likelihood of these arrhythmias and either the initial voltage or the delivered current nor there was a higher number of episodes of postshock hypotension following the smaller capacitor discharges.(ABSTRACT TRUNCATED AT 250 WORDS)

Implantation and follow-up of a third-generation cardioverter defibrillator: comparison of epicardial and nonthoracotomy defibrillation lead system

OBJECTIVE

The intraoperative and follow-up results were compared in 67 patients with ventricular tachyarrhythmias who underwent implantation of the Ventritex Cadence defibrillator with either epicardial patch (EPI, 25 patients) or nonthoracotomy CPI Endotak (ENDO, 42 patients) defibrillation lead systems.

RESULTS

There was no significant difference between groups in age, sex, structural heart disease, ejection fraction, arrhythmia history, or drug therapy. Successful implantation was accomplished in all patients using either lead system. In the ENDO group, 35 patients (83%) had a defibrillation threshold < or = 550 V and did not require a subcutaneous patch. Intraoperatively, the defibrillation threshold was 453 +/- 139 V (13 +/- 9 J) for EPI and 490 +/- 113 V (15 +/- 8 J) for ENDO (P = NS). There were no perioperative deaths in either group. At predischarge testing, the defibrillation threshold was 445 +/- 183 V (14 +/- 12 J) for EPI and 439 +/- 133 V (13 +/- 7 J) for ENDO (P = NS). During a mean follow-up of 16 +/- 8 months, there were no sudden deaths, and four patients died from congestive heart failure (3 EPI, 1 ENDO). During follow-up, 916 spontaneous arrhythmia episodes occurred in 16 of 25 EPI patients (64%) and 967 episodes occurred in 31 of 42 ENDO patients (74%) (P = NS). The number of episodes detected as ventricular fibrillation were 192 for EPI (21%) and 232 for ENDO (24%), with first shock success in 76% and 75%, respectively; all episodes were successfully terminated by the device. In the remaining episodes detected as ventricular tachycardia, antitachycardia pacing was attempted and was successful in 672 of 724 episodes (93%) with EPI and 666 of 735 episodes (91%) with ENDO lead systems (P = NS). Acceleration of ventricular tachycardia with antitachycardia pacing occurred in 21 episodes (3%) with EPI and in 37 episodes (5%) with ENDO leads (P = NS).

CONCLUSIONS

A nonthoracotomy approach using the third generation cardioverter defibrillator Cadence V-100 is safe and effective and has clinical results that are not significantly different from epicardial defibrillation lead systems.

A minimal model of the single capacitor biphasic defibrillation waveform

A quantitative model of the single capacitor biphasic defibrillation waveform is proposed. The primary hypothesis of this model is that the first phase leaves a residual charge on the membranes of the unsynchronized cells, which can then reinitiate fibrillation. The second phase diminishes this charge, reducing the potential for refibrillation. To suppress this potential refibrillation, a monophasic shock must be strong enough to synchronize a critical mass of nearly 100% of the myocytes. Since the biphasic waveform performs this protection function by removing the residual charge (with its second phase), its first phase may be of a lower strength than a monophasic shock of equivalent performance. A quantitative model was developed to calculate the residual membrane voltage, Vm, assuming a capacitive membrane being alternately charged and discharged by the first and second phases, respectively. It was further assumed that the amplitude of the first phase would be predicted by a minimum value plus a term proportional to Vm2. The model was evaluated on the pooled data of three relevant published studies comparing biphasic waveforms. The model explained 79% of the variance in the first phase amplitude and predicted optimal durations for various defibrillator capacitances and electrode resistances. Assuming a first phase of optimal duration, the optimal second phase duration appears to be about 2.5 msec for all capacitances and resistances now seen clinically.

CONCLUSION

The effectiveness of the single capacitor biphasic waveform may be explained by the second phase "burping" of the deleterious residual charge of the first phase that, in turn, reduces the synchronization requirement and the amplitude requirements of the first phase.