Comparison of monophasic and biphasic defibrillating pulse waveforms for transthoracic cardioversion. Biphasic Waveform Defibrillation Investigators

Biphasic versus monophasic waveforms for transthoracic defibrillation in out-of-hospital cardiac arrest

BACKGROUND

Transthoracic defibrillation is a potentially life-saving treatment for people with ventricular fibrillation (VF) and haemodynamically unstable ventricular tachycardia (VT). In recent years, biphasic waveforms have become more commonly used for defibrillation than monophasic waveforms. Clinical trials of internal defibrillation and transthoracic defibrillation of short-duration arrhythmias of up to 30 seconds have demonstrated the superiority of biphasic waveforms over monophasic waveforms. However, out-of-hospital cardiac arrest (OHCA) involves a duration of VF/VT of several minutes before defibrillation is attempted.

OBJECTIVES

To determine the efficacy and safety of biphasic defibrillation waveforms, compared to monophasic, for resuscitation of people experiencing out-of-hospital cardiac arrest.

SEARCH METHODS

We searched the following electronic databases for potentially relevant studies up to 10 September 2014: the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE. Also we checked the bibliographies of relevant studies and review articles, contacted authors of published reviews and reviewed webpages (including those of device manufacturers) relevant to the review topic. We handsearched the abstracts of conference proceedings for the American Heart Association, American College of Cardiology, European Society of Cardiology, European Resuscitation Council, Society of Critical Care Medicine and European Society of Intensive Care Medicine. Regarding language restrictions, we did not apply any.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) that compared biphasic and monophasic waveform defibrillation in adults with OHCA. Two review authors independently screened the literature search results.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted data from the included trials and performed 'Risk of bias' assessments. We resolved any disagreements by discussion and consensus. The primary outcome was the risk of failure to achieve return of spontaneous circulation (ROSC). Secondary outcomes included risk of failure to revert VF to an organised rhythm following the first shock or up to three shocks, survival to hospital admission and survival to discharge.

MAIN RESULTS

We included four trials (552 participants) that compared biphasic and monophasic waveform defibrillation in people with OHCA. Based on the assessment of five quality domains, we identified two trials that were at high risk of bias, one trial at unclear risk of bias and one trial at low risk of bias. The risk ratio (RR) for failure to achieve ROSC after biphasic compared to monophasic waveform defibrillation was 0.86 (95% CI 0.62 to 1.20; four trials, 552 participants). The RR for failure to defibrillate on the first shock following biphasic defibrillation compared to monophasic was 0.84 (95% CI 0.70 to 1.01; three trials, 450 participants); and 0.81 (95% CI 0.61 to 1.09; two trials, 317 participants) for one to three stacked shocks. The RR for failure to achieve ROSC after the first shock was 0.92 (95% CI 0.81 to 1.04; two trials, 285 participants). Biphasic waveforms did not reduce the risk of death before hospital admission (RR 1.05, 95% CI 0.90 to 1.23; three trials, 383 participants) or before hospital discharge (RR 1.05, 95% CI 0.78 to 1.42; four trials, 550 participants). There was no statistically significant heterogeneity in any of the pooled analyses. None of the included trials reported adverse events.

AUTHORS' CONCLUSIONS

It is uncertain whether biphasic defibrillators have an important effect on defibrillation success in people with OHCA. Further large studies are needed to provide adequate statistical power.

Cardiopulmonary resuscitation: from the beginning to the present day

Cardiac arrest represents a dramatic event that can occur suddenly and often without premonitory signs, characterized by sudden loss of consciousness and breathing after cardiac output ceases and both coronary and cerebral blood flows stop. Restarting of the blood flow by cardiopulmonary resuscitation potentially re-establishes some cardiac output and organ blood flows. This article summarizes the major events that encompass the history of cardiopulmonary resuscitation, beginning with ancient history and evolving into the current American Heart Association's commitment to save hearts.

Minimal interruption of cardiopulmonary resuscitation for a single shock as mandated by automated external defibrillations does not compromise outcomes in a porcine model of cardiac arrest and resuscitation

OBJECTIVES

Current automated external defibrillations require interruptions in chest compressions to avoid artifacts during electrocardiographic analyses and to minimize the risk of accidental delivery of an electric shock to the rescuer. The earlier three-shock algorithm, with prolonged interruptions of chest compressions, compromised outcomes and increased severity of postresuscitation myocardial dysfunction. In the present study, we investigated the effect of timing of minimal automated external defibrillation-mandated interruptions of chest compressions on cardiopulmonary resuscitation outcomes, using a single-shock algorithm. We hypothesized that an 8-sec interruption of chest compressions for a single shock, as mandated by automated external defibrillations, would not impair initial resuscitation and outcomes of cardiopulmonary resuscitation.

DESIGN

Randomized prospective animal study.

SETTING

University affiliated research laboratory.

SUBJECTS

Domestic pigs.

MEASUREMENTS AND MAIN RESULTS

In 24 domestic male pigs weighing 41 +/- 2 kg, ventricular fibrillation was induced by left anterior descending coronary artery occlusion and untreated for 7 min. Cardiopulmonary resuscitation, including chest compressions and ventilation with oxygen, was then performed for an interval of 2 min before attempted defibrillation. Animals were randomized into three groups: A) interruption immediately before defibrillation; B) interruption after 1 min of cardiopulmonary resuscitation; or C) no interruption. Chest compressions were delivered with the aid of a mechanical chest compressor at a rate of 100 compressions/min and compression/ventilation ratio of 30:2. Defibrillation was attempted with a single biphasic 150-J shock. Each animal was successfully resuscitated and survived for >72 hr. No differences in the number of shocks before return of spontaneous circulation, frequency of recurrent ventricular fibrillation, duration of cardiopulmonary resuscitation, and severity of postresuscitation myocardial dysfunction were observed.

CONCLUSIONS

In this experimental model of cardiac arrest and cardiopulmonary resuscitation, minimal automated external defibrillation-mandated interruption of chest compressions for a single-shock algorithm did not have adverse effects on postresuscitation myocardial or neurologic function. All animals, whether subjected to cardiopulmonary resuscitation interruptions or not, survived.

A prospective, randomized trial of an emergency department observation unit for acute onset atrial fibrillation

STUDY OBJECTIVE

An emergency department (ED) observation unit protocol for the management of acute onset atrial fibrillation is compared with routine hospital admission and management.

METHODS

Adult patients presenting to the ED with atrial fibrillation of less than 48 hours' duration without hemodynamic instability or other comorbid conditions requiring hospitalization were enrolled. Participants were randomized to either ED observation unit care or routine inpatient care. The ED observation unit protocol included pulse rate control, cardiac monitoring, reassessment, and electrical cardioversion if atrial fibrillation persisted. Patients who reverted to sinus rhythm were discharged with a cardiology follow-up within 3 days, whereas those still in atrial fibrillation were admitted. All cases were followed up for 6 months and adverse events recorded.

RESULTS

Of the 153 patients, 75 were randomized to the ED observation unit and 78 to routine inhospital care. Eighty-five percent of ED observation unit patients converted to sinus rhythm versus 73% in the routine care group (difference 12%; 95% confidence interval [CI] -1% to 25%]; P=.06). The median length of stay was 10.1 versus 25.2 hours (difference 15.1 hours; 95% CI 11.2 to 19.6; P<.001) for ED observation unit and inhospital care respectively. Nine ED observation unit patients required inpatient admission. Eleven percent of the ED observation unit group had recurrence of atrial fibrillation during follow-up versus 10% of the routine inpatient care group (difference 1%; 95% CI -9% to 11%; P=.93). There was no significant difference between the groups in the frequency of hospitalization or the number of tests, and the number of adverse events during follow-up was similar in the 2 groups.

CONCLUSION

An ED observation unit protocol that includes electrical cardioversion is a feasible alternative to routine hospital admission for acute onset of atrial fibrillation and results in a shorter initial length of stay.

Transthoracic Incremental Monophasic Versus Biphasic Defibrillation by Emergency Responders (TIMBER)

Background— Although biphasic, as compared with monophasic, waveform defibrillation for cardiac arrest is increasing in use and popularity, whether it is truly a more lifesaving waveform is unproven.

Methods and Results— Consecutive adults with nontraumatic out-of-hospital ventricular fibrillation cardiac arrest were randomly allocated to defibrillation according to the waveform from automated external defibrillators administered by prehospital medical providers. The primary event of interest was admission alive to the hospital. Secondary events included return of rhythm and circulation, survival, and neurological outcome. Providers were blinded to automated defibrillator waveform. Of 168 randomized patients, 80 (48%) and 68 (40%) consistently received only monophasic or biphasic waveform shocks, respectively, throughout resuscitation. The prevalence of ventricular fibrillation, asystole, or organized rhythms at 5, 10, or 20 seconds after each shock did not differ significantly between treatment groups. The proportion of patients admitted alive to the hospital was relatively high: 73% in monophasic and 76% in biphasic treatment groups ( P =0.58). Several favorable trends were consistently associated with receipt of biphasic waveform shock, none of which reached statistical significance. Notably, 27 of 80 monophasic shock recipients (34%), compared with 28 of 68 biphasic shock recipients (41%), survived ( P =0.35). Neurological outcome was similar in both treatment groups ( P =0.4). Earlier administration of shock did not significantly alter the performance of one waveform relative to the other, nor did shock waveform predict any clinical outcome after multivariate adjustment.

Conclusions— No statistically significant differences in outcome could be ascribed to use of one waveform over another when out-of-hospital ventricular fibrillation was treated.

New cardiopulmonary resuscitation guidelines 2005: importance of uninterrupted chest compression

The evidence supports quality controlled chest compression as the initial intervention after "sudden death" before attempted defibrillation, if the duration of cardiac arrest is more than 5 minutes. The new guidelines mandate lesser interruptions for ventilation, before and following electrical shocks, and single rather than multiple electrical shocks before resuming chest compression. The new guidelines refocus on uninterrupted chest compression after cardiac arrest of nonasphyxial cause and modifications in practices that reduce the need for interruptions.

A retrospective evaluation of transthoracic biphasic electrical cardioversion for atrial fibrillation in dogs

OBJECTIVES

To evaluate safety, efficacy, and clinical usefulness of biphasic transthoracic cardioversion for management of dogs with atrial fibrillation (AF).

BACKGROUND

In dogs AF is usually managed with heart rate control rather than by restoration of sinus rhythm (SR). However, restoration of SR has potential advantages of improving cardiac output and reducing ventricular filling pressures, and biphasic cardioversion provides an improved benefit/risk ratio compared to traditional monophasic cardioversion.

ANIMALS, MATERIALS AND METHODS

Retrospective analysis of data from 39 dogs with spontaneous AF managed with biphasic transthoracic cardioversion was done. Conversion characteristics, adverse effects, and duration of SR were evaluated. Effects of heart disease and pretreatment with amiodarone on success of cardioversion and on duration of SR were also evaluated.

RESULTS

Restoration of SR was achieved in 36 of 39 dogs (92.3%). Presence of heart disease or atrial enlargement had no effect on cardioversion characteristics or ability to restore SR. Median duration of SR following cardioversion and treatment with amiodarone was 120 days. Dogs with lone AF remained in SR longer than those with heart disease.

CONCLUSIONS

Biphasic cardioversion is safe and effective. Although duration of SR varied, a majority of dogs remained in SR long enough to benefit.

Pediatric defibrillation doses often fail to terminate prolonged out-of-hospital ventricular fibrillation in children

BACKGROUND

The recommended dose for pediatric defibrillation is 2 J/kg, based on animal studies of brief duration ventricular fibrillation (VF) and a single pediatric study of short duration in-hospital VF. In a piglet model of out-of-hospital (prolonged) cardiac arrest, this recommended dose was usually ineffective at terminating VF. We, therefore, hypothesized that pediatric dose defibrillation may be less effective for prolonged out-of-hospital pediatric VF.

METHODS

We evaluated retrospectively all cardiac arrests in children less than 13 years old in Tucson from November 1998 to April 2003, with special attention to all children in ventricular fibrillation. We determined the rate of ventricular fibrillation termination after pediatric dose shocks in this cohort, and compared this rate with a published historical pediatric in-hospital defibrillation control group. A pediatric dose shock was defined as 2 J/kg (+/-10 J). All shocks in both groups were provided as monophasic damped sinusoidal waveforms.

RESULTS

Thirteen of 151 (9%) children with out-of-hospital cardiac arrest had documented VF. Eleven children received a total of 14 pediatric dose shocks. The median minimum untreated dispatch-to-shock time in unwitnessed arrest or collapse-to-shock in witnessed arrest for those 11 children was 11 min (interquartile range 25-75%; 9-15.5 min). Seven of the 14 pediatric dose shocks terminated the VF (six to asystole, one to pulseless electrical activity). Nine children (68%) died in the emergency department and four (31%) in the pediatric intensive care unit; none survived to hospital discharge. Failure to terminate VF after a pediatric dose shock in this study group with prolonged out-of-hospital ventricular fibrillation was substantially more common than the previously reported in-hospital data (7/14 versus 5/57; OR 10.4; 95% CI 2.6-42; P=0.001).

CONCLUSIONS

Termination of VF after a pediatric defibrillation dose is substantially worse for prolonged pediatric out-of-hospital VF cardiac arrest compared with in-hospital (short duration) ventricular fibrillation. The optimal pediatric defibrillation dose for prolonged VF is not known.

Out-of-hospital cardiac arrest rectilinear biphasic to monophasic damped sine defibrillation waveforms with advanced life support intervention trial (ORBIT)

BACKGROUND

Although biphasic defibrillation waveforms appear to be superior to monophasic waveforms in terminating VF, their relative benefits in out-of-hospital resuscitation are incompletely understood. Prior comparisons of defibrillation waveform efficacy in out-of-hospital cardiac arrest (OHCA) are confined to patients presenting in a shockable rhythm and resuscitated by first responder (basic life support). This effectiveness study compared monophasic and biphasic defibrillation waveform for conversion of ventricular arrhythmias in all OHCA treated with advance life support (ALS).

METHODS AND RESULTS

This prospective randomized controlled trial compared the rectilinear biphasic (RLB) waveform with the monophasic damped sine (MDS) waveform, using step-up energy levels. The study enrolled OHCA patients requiring at least one shock delivered by ALS providers, regardless of initial presenting rhythm. Shock success was defined as conversion at 5s to organized rhythm after one to three escalating shocks. We report efficacy results for the cohort of patients treated by ALS paramedics who presented with an initially shockable rhythm who had not received a shock from a first responder (MDS: n=83; RLB: n=86). Shock success within the first three ascending energy shocks for RLB (120, 150, 200J) was superior to MDS (200, 300, 360J) for patients initially presenting in a shockable rhythm (52% versus 34%, p=0.01). First shock conversion was 23% and12%, for RLB and MDS, respectively (p=0.07). There were no significant differences in return of spontaneous circulation (47% versus 47%), survival to 24h (31% versus 27%), and survival to discharge (9% versus 7%). Mean 24h survival rates of bystander witnessed events showed differences between waveforms in the early circulatory phase at 4-10 min post event (mean (S.D.) RLB 0.45 (0.07) versus MDS 0.31 (0.06), p=0.0002) and demonstrated decline as time to first shock increased to 20 min.

CONCLUSION

Shock success to an organized rhythm comparing step-up protocol for energy settings demonstrated the RLB waveform was superior to MDS in ALS treatment of OHCA. Survival rates for both waveforms are consistent with current theories on the circulatory and metabolic phases of out-of-hospital cardiac arrest.

The Gurvich waveform has lower defibrillation threshold than the rectilinear waveform and the truncated exponential waveform in the rabbit heart

Implantable cardioverter defibrillator studies have established the superiority of biphasic waveforms over monophasic waveforms. However, external defibrillator studies of biphasic waveforms are not as widespread. Our objective was to compare the defibrillation efficacy of clinically used biphasic waveforms, i.e., truncated exponential, rectilinear, and quasi-sinusoidal (Gurvich) waveforms in a fibrillating heart model. Langendorff-perfused rabbit hearts (n = 10) were stained with a voltage-sensitive fluorescent dye, Di-4-ANEPPS. Transmembrane action potentials were optically mapped from the anterior epicardium. We found that the Gurvich waveform was significantly superior (p < 0.05) to the rectilinear and truncated exponential waveforms. The defibrillation thresholds (mean +/- SE) were as follows: Gurvich, 0.25 +/- 0.01 J; rectilinear-1, 0.34 +/- 0.01 J; rectilinear-2, 0.33 +/- 0.01 J; and truncated exponential, 0.32 +/- 0.02 J. Using optically recorded transmembrane responses, we determined the shock-response transfer function, which allowed us to predict the cellular response to waveforms at high accuracy. The passive parallel resistor-capacitor model (RC-model) predicted polarization superiority of the Gurvich waveform in the myocardium with a membrane time constant (taum) of less than 2 ms. The finding of a lower defibrillation threshold with the Gurvich waveform in an in vitro model of external defibrillation suggests that the Gurvich waveform may be important for future external defibrillator designs.

Mechanisms of superiority of ascending ramp waveforms: new insights into mechanisms of shock-induced vulnerability and defibrillation

Monophasic ascending ramp (AR) and descending ramp (DR) waveforms are known to have significantly different defibrillation thresholds. We hypothesized that this difference arises due to differences in mechanisms of arrhythmia induction for the two waveforms. Rabbit hearts (n = 10) were Langendorff perfused, and AR and DR waveforms (7, 20, and 40 ms) were randomly delivered from two line electrodes placed 10 mm apart on the anterior ventricular epicardium. We optically mapped cellular responses to shocks of various strengths (5, 10, and 20 V/cm) and coupling intervals (CIs; 120, 180, and 300 ms). Optical mapping revealed that maximum virtual electrode polarization (VEP) was reached at significantly different times for AR and DR of the same duration (P < 0.05) for all tested CIs. As a result, VEP for AR were stronger than for DR at the end of the shock. Postshock break excitation resulting from AR generated faster propagation and typically could not form reentry. In contrast, partially dissipated VEP resulting from DR generated slower propagation; the wavefront was able to propagate into deexcited tissue and thus formed a shock-induced reentry circuit. Therefore, for the same delivered energy, AR was less proarrhythmic compared with DR. An active bidomain model was used to confirm the electrophysiological results. The VEP hypothesis explains differences in vulnerability associated with monophasic AR and DR waveforms and, by extension, the superior defibrillation efficacy of the AR waveform compared with the DR waveform.

Better outcome after pediatric defibrillation dosage than adult dosage in a swine model of pediatric ventricular fibrillation

OBJECTIVES

This study was designed to compare outcome after adult defibrillation dosing versus pediatric dosing in a piglet model of prolonged prehospital ventricular fibrillation (VF).

BACKGROUND

Weight-based 2 to 4 J/kg monophasic defibrillation dosing is recommended for children in VF, but impractical for automated external defibrillator (AED) use. Present AEDs can only provide adult shock doses or newly developed attenuated adult doses intended for children. A single escalating energy sequence (50/75/86 J) of attenuated adult-dose biphasic shocks (pediatric dosing) is at least as effective as escalating monophasic weight-based dosing for prolonged VF in piglets, but this approach has not been compared to standard adult biphasic dosing.

METHODS

Following 7 min of untreated VF, piglets weighing 13 to 26 kg (19 +/- 1 kg) received either biphasic 50/75/86 J (pediatric dose) or biphasic 200/300/360 J (adult dose) therapies during simulated prehospital life support.

RESULTS

Return of spontaneous circulation was attained in 15 of 16 pediatric-dose piglets and 14 of 16 adult-dose piglets. Four hours postresuscitation, pediatric dosing resulted in fewer elevations of cardiac troponin T (0 of 12 piglets vs. 6 of 11 piglets, p = 0.005) and less depression of left ventricular ejection fraction (p < 0.05). Most importantly, more piglets survived to 24 h with good neurologic scores after pediatric shocks than adult shocks (13 of 16 piglets vs. 4 of 16 piglets, p = 0.004).

CONCLUSIONS

In this model, pediatric shocks resulted in superior outcome compared with adult shocks. These data suggest that adult defibrillation dosing may be harmful to pediatric patients with VF and support the use of attenuating electrodes with adult biphasic AEDs to defibrillate children.

Ventricular fibrillation: basic concepts

This brief overview serves as an introduction to the vast array of basic and clinical concepts that are pertinent to the basic understanding of ventricular fibrillation, its genesis, and its clinical management.

A randomized trial comparing monophasic and biphasic waveform shocks for external cardioversion of atrial fibrillation

BACKGROUND

We compared efficacy of and pain felt after biphasic truncated exponential (BTE) and monophasic damped sine (MDS) shocks in patients undergoing external cardioversion of atrial fibrillation (AF).

METHODS

Patients with AF were randomized to BTE or MDS waveform cardioversion. Successive shocks were delivered at 70, 100, 200, and 360 J until successful cardioversion, with one 360 J attempt of the alternate waveform when all 4 shocks failed. Success was determined by blinded over-read of electrocardiograms. Peak current was calculated from energy and impedance. Patients rated their pain at 1 and 24 hours after cardioversion.

RESULTS

Fourteen of 37 (38%) patients treated with MDS and 34 of 35 (97%) treated with BTE shocks were cardioverted at < or =200 J (P <.0001). Success rates of MDS versus BTE shocks were 5.4% versus 60% for 70 J, 19% versus 80% for < or =100 J, and 86% versus 97% for < or =360 J. BTE shocks cardioverted with less peak current (14.0 +/- 4.3 vs 39.5 +/- 11.2 A, P <.0001), less energy (97 +/- 47 vs 278 +/- 120 J, P <.0001), and less cumulative energy (146 +/- 116 vs 546 +/- 265 J, P <.0001). Patients felt less pain after BTE than MDS shocks at 1 hour (P <.0001) and 24 hours (P <.0001) after cardioversion.

CONCLUSION

This BTE waveform is superior to the MDS waveform for cardioversion of AF, requiring much less energy and current, and causing less postprocedural pain.

[Transthoracic defibrillation. Physiologic and pathophysiologic principles and their role in the outcome of resuscitation]

As one major link in the chain of survival, early transthoracic (external) cardiac defibrillation is aimed at the termination of ventricular flutter and ventricular fibrillation. Most important to the success of defibrillation is the passage of a defined amount of current through a critical mass of heart muscle. Different transthoracic resistances reduce the effective density of the current within the heart. As for other therapeutic intervention procedures, recommendations for the optimal strength of current to be applied to the fibrillating heart need to be evaluated and defined for therapeutical defibrillation too. Unnecessarily high current density causes damage to the heart and should be prevented. By using biphasic waveforms in contrast to monophasic impulses, the amount of current can be reduced but the same or even higher efficacy is attained. Therefore possible myocardial damage might be clearly reduced. Even with individually altered thoracic impedance effective conversion of cardiac rhythm can be achieved by device-controlled compensation and biphasic waveforms. According to their different mechanisms or origin (electrically induced or spontaneously caused by organic heart disease) the probability of successful conversion of the cardiac rhythm by one single electrical impulse varies. The optimum point in time for defibrillation during resuscitation needs to be redefined. In order to improve comparability, further studies should use standardized definitions for successful defibrillation relating to the resulting cardiac rhythm.

Clinical efficacy of a wearable defibrillator in acutely terminating episodes of ventricular fibrillation using biphasic shocks

The Wearable Cardioverter Defibrillator (WCD) automatically detects and treats ventricular tachyarrhythmias without the need for assistance from a bystander, while at the same time allowing the patient to ambulate freely. It represents an alternative to emergency medical services for outpatient populations with a temporary risk of sudden cardiac death. While the original devices used a monophasic truncated exponential waveform for cardioversion/defibrillation shocks, a new, biphasic shock was developed for the next device generation. In 12 patients undergoing electrophysiological testing for ventricular tachyarrhythmias, termination of electrically induced ventricular fibrillation (VF) was attempted via the WCD. In 22 episodes, induced VF was promptly terminated by the first 70 J (n=12) or 100 J (n=10) biphasic shocks. Time between arrhythmia initiation and shock delivery was 22 +/- 6 seconds (70 J) and 21 +/- 6 seconds (100 J) (P=NS). The measured transthoracic impedance was 71 +/- 5 Ohms (64-79 Ohms) for the 70 J shock and 64 +/- 8 Ohms (47-72 Ohms) for the 100 J shock. The present study demonstrates that a single low energy biphasic shock delivered by the WCD, reliably terminates electrically induced VF (100% of episodes). The results of this study suggest that there is an acceptable safety margin to the maximum output of the device (150 J). Despite our promising data, we recommend that programming all shocks for maximum energy output should be done when using the WCD in ambulatory patients.

The Strength-Duration Relationship of Monophasic Waveforms with Varying Capacitance Sizes in External Defibrillation

The shape of the shock waveform influences defibrillation efficacy. However, the optimal combination between capacitance size and truncation/tilt which can determine monophasic waveform's shape, has not been determined for external defibrillation. The purpose of this study was to assess the effects of varying capacitance and tilt on external defibrillation using exponential monophasic waveforms. In a pig model of external defibrillation (n = 10, 30 +/- 6 kg), nine exponential monophasic waveforms combining three capacitance values (30 microF, 60 microF, and 120 microF) and three tilt values (55%, 75%, and 95%) were tested randomly. The energy and leading edge voltage at 50% defibrillation success (E50 and V50) were used to evaluate defibrillation efficacy. E50 and V50 were determined by the Bayesian technique. The lowest stored E50 for the 30microF, 60 microF, and 120 microF waveforms were 90 +/- 12 J (95% tilt), 106 +/- 45 J (55% tilt), and 107 +/- 52 J (75% tilt), respectively. The lowest V50 for the 30 microF, 60 microF, and 120 microF waveforms were 2,439 +/- 166 V (95% tilt), 1,849 +/- 375 V (55% tilt), and 1,301 +/- 322 V (75% tilt), respectively. The average current at external defibrillation threshold demonstrated a strength versus pulse duration relationship similar to that seen with pacing. Reducing capacitance has the same effect as truncating the waveform. The E50 is more sensitive to tilt values changes in larger capacitance waveforms. This study suggests that the optimal combination between capacitance and tilt may be 120 microF and 55%-75% for external defibrillation.

Biphasic versus monophasic cardioversion in shock-resistant atrial fibrillation:

Biphasic versus Monophasic Cardioversion.

INTRODUCTION

Cardioversion of atrial fibrillation using monophasic transthoracic shocks occasionally is ineffective. Biphasic cardioversion requires less energy than monophasic cardioversion, but its efficacy in shock-resistant atrial fibrillation is unknown. Thus, we compared the efficacy of cardioversion using biphasic versus monophasic waveform shocks in patients with atrial fibrillation previously refractory to monophasic cardioversion.

METHODS AND RESULTS

Fifty-six patients with prior failed monophasic cardioversion were randomized to either a 360-J monophasic damped sinusoidal shock or biphasic truncated exponential shocks at 150 J, followed by 200 J and then 360 J, if necessary. If either waveform failed, patients were crossed over to the other waveform. The primary endpoint was defined as the proportion of patients achieving sinus rhythm following initial randomized therapy. Stepwise multivariate logistic regression examined independent predictors of shock success, including patient age, sex, left atrial diameter, body mass index, drug therapy, and waveform. Twenty-eight patients were randomized to the biphasic shocks and 28 to the monophasic shocks. Sinus rhythm was restored in 61% of patients with biphasic versus 18% with monophasic shocks (P = 0.001). Seventy-eight percent success was achieved in patients who crossed over to the biphasic shock after failing monophasic cardioversion, whereas only 33% were successfully cardioverted with a monophasic shock after crossover from biphasic shock (P = 0.02). Overall, 69% of patients who received a biphasic shock at any point in the protocol were cardioverted successfully, compared to 21% with the monophasic shock (P < 0.0001). The type of shock was the strongest predictor of shock success (P = 0.0001) in multivariate logistic regression.

CONCLUSION

An ascending sequence of 150-, 200-, and 360-J transthoracic biphasic cardioversion shocks are successful more often than a single 360-J monophasic shock. Thus, biphasic shocks should be the recommended configuration of choice for all cardioversions.

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.

Open-chest epicardial "surgical" defibrillation: biphasic versus monophasic waveform shocks

The aim of the study was to compare biphasic versus monophasic shocks for open-chest epicardial defibrillation. Transthoracic biphasic waveform shocks require less energy to terminate ventricular fibrillation compared to monophasic waveform shocks. However, if biphasic shocks are effective for open-chest epicardial ("surgical") defibrillation has not been established. Twenty-eight anesthetized adult swine (15-25 kg) underwent a midline sternotomy. Ventricular fibrillation was electrically induced. After 15 seconds of ventricular fibrillation, each pig in group 1 (n = 16) randomly received damped sinusoidal monophasic epicardial shocks and truncated exponential biphasic epicardial shocks from large (44.2 cm2) paddle electrodes at eight energy levels (2-50 J). Pigs in group 2 (n = 12) received monophasic and truncated exponential biphasic shocks from small (15.9 cm2) paddle electrodes. In group 1 (large paddle electrodes), the overall percent shock success rose from 15 +/- 9% at 2 J to 97 +/- 3% at 50 J. In this group there was no significant difference in percent of shock success between damped sinusoidal monophasic and biphasic waveform shocks. In group 2 (small paddle electrodes), biphasic shocks yielded a significantly higher percent of shock success than monophasic shocks at mid-energy levels from 7 to 20 J (all P < 0.01). With small surgical paddle electrodes, biphasic waveform shocks demonstrated a significantly higher percent of shock success rate compared to monophasic waveform shocks. With large paddle electrodes, the two waveforms were equally effective.

Use of biphasic electrical cardioversion for treatment of idiopathic atrial fibrillation in two horses

Rectilinear biphasic cardioversion was used in 2 horses with idiopathic sustained atrial fibrillation; 1 horse converted to sustained sinus rhythm. Variables that potentially affected outcome of the electrical cardioversion procedures in these horses included duration of arrhythmia, placement of cardioverter pads and paddles, serum electrolyte concentrations, and treatment with quinidine. Serum cardiac troponin I concentration, measured to determine whether the myocardium was damaged from the electrical shocks, was within the reference range in both horses after the procedure. Biphasic electrical cardioversion may provide an alternative to pharmacologic cardioversion with quinidine in horses. The rectilinear biphasic defibrillator-cardioverter uses a unique biphasic waveform to deliver constant current to the myocardium during cardioversion, regardless of transthoracic impedance. Biphasic cardioversion is safer and more effective than traditional monophasic cardioversion in humans and animals.

Biphasic waveform external defibrillation thresholds for spontaneous ventricular fibrillation secondary to acute ischemia

OBJECTIVES

The goal of this study was to determine if the defibrillation threshold (DFT) after spontaneous ventricular fibrillation (VF) secondary to acute ischemia differs from the DFT for electrically induced VF in the absence of ischemia in anesthetized, closed-chest dogs and pigs.

BACKGROUND

The efficacy of external defibrillators has been tested mainly in animals and humans using E-VF, yet external defibrillators are often used in patients to halt S-VF.

METHODS

Protocol 1: biphasic truncated exponential (BTE) waveform shocks were delivered through electrodes placed in an anterior-anterior (A-A) position (left and right lateral thorax) in nine dogs. After measuring the E-VF DFT, acute ischemia was induced with an angioplasty balloon in either the left anterior descending or left circumflex coronary artery, and the S-VF DFT was determined. Protocol 2: in a group of 12 pigs, the E-VF DFT and S-VF DFT were determined for electrodes in the A-A position and in the anterior-posterior position (A-P). Protocol 3: the E-VF DFT was determined in seven pigs. Then up to three shocks 1.5x the E-VF DFT were delivered to S-VF. If defibrillation did not occur, a step-up protocol was used until defibrillation occurred.

RESULTS

Protocol 1: the DFT for E-VF was 65 +/- 28 J (mean +/- SD) compared with 226 +/- 97 J for S-VF, p < 0.05. Protocol 2: the DFT was 152 +/- 58 J for E-VF and 315 +/- 123 J for S-VF for A-A electrodes. The DFT was 100 +/- 43 J for E-VF and 206 +/- 114 J for S-VF for A-P electrodes. Protocol 3: 11/37 shocks of strength 1.5x E-VF DFT (182 +/- 40 J) stopped the arrhythmia. The episodes of S-VF not halted by these shocks required energy levels of up to 400 J for defibrillation.

CONCLUSIONS

External defibrillation of S-VF induced by acute ischemia requires significantly more energy than VF induced by 60-Hz current in the absence of ischemia. A safety margin >1.5x the DFT for electrically induced VF may be necessary in BTE external defibrillators to defibrillate S-VF.

Bench to bedside: resuscitation from prolonged ventricular fibrillation

Ventricular fibrillation (VF) remains the most common cardiac arrest heart rhythm. Defibrillation is the primary treatment and is very effective if delivered early within a few minutes of onset of VF. However, successful treatment of VF becomes increasingly more difficult when the duration of VF exceeds 4 minutes. Classically, successful cardiac arrest resuscitation has been thought of as simply achieving restoration of spontaneous circulation (ROSC). However, this traditional approach fails to consider the high early post-cardiac arrest mortality and morbidity and ignores the reperfusion injuries, which are manifest in the heart and brain. More recently, resuscitation from cardiac arrest has been divided into two phases; phase I, achieving ROSC, and phase II, treatment of reperfusion injury. The focus in both phases of resuscitation remains the heart and brain, as prolonged VF remains primarily a two-organ disease. These two organs are most sensitive to oxygen and substrate deprivation and account for the vast majority of early post-resuscitation mortality and morbidity. This review focuses first on the initial resuscitation (achieving ROSC) and then on the reperfusion issues affecting the heart and brain.

Effects of biphasic vs monophasic defibrillation on the scaling exponent in a swine model of prolonged ventricular fibrillation

OBJECTIVE

Mathematical analyses of ventricular fibrillation (VF) have resulted in the derivation of a measure termed the scaling exponent (ScE) that characterizes the duration of VF and probability of defibrillation success. The purpose of this study was to compare the effects of biphasic defibrillation waveform (BDW) and monophasic defibrillation waveform (MDW) rescue shocks on ScE in a swine model of prolonged VF.

METHODS

Utstein guidelines for the laboratory study of cardiopulmonary resuscitation were followed. Twenty mixed-breed domestic swine (mass range 20.5-26.8 kg) were instrumented and randomized to receive either MDW or BDW rescue shocks. Ventricular fibrillation was induced and untreated for a nonintervention interval of 8 minutes. Rescue shocks were delivered at 8, 10, and 12 minutes of elapsed VF time. The energy sequence for the three MDW shocks was 70, 100, and 150 J (approximately 3, 4, and 6 J/kg). All BDW shocks were delivered at 50 J (approximately 2.5 J/kg). Only VF was shocked. Chest compressions and drugs were not provided. Rhythm analysis and ScE calculation were performed offline. Continuous and discontinuous linear regression models were fit to plots of ScE vs time. Defibrillation success and progression of ScE slope were analyzed using Fisher's exact test, paired t-tests, and repeated-measures analysis of variance (ANOVA).

RESULTS

Baseline characteristics were similar for both groups. Successful termination of VF occurred on the first rescue shock in 1 of 10 (10%) in the MDW group and 3 of 10 (30%) in the BDW group; this difference was not statistically significant (p = 0.58). No other defibrillation successes were observed. No animals achieved return of spontaneous circulation. The ScE values during the protocol progressed from 1.330 (95% CI = 1.287 to 1.373) to 1.724 (95% CI = 1.603 to 1.845) for MDW and 1.338 (95% CI = 1.261 to 1.415) to 1.639 (95% CI = 1.530 to 1.745) for BDW. Both groups showed a trend toward increasing ScE values with successive rescue shocks. Repeated-measures ANOVA using both continuous and discontinuous models demonstrated no difference in overall ScE slope progression between study groups.

CONCLUSIONS

Mode of defibrillation waveform (BDW vs MDW) does not appear to impact ScE trends. Additional studies must be performed to better evaluate the clinical implications of this finding.

Biphasic and monophasic transthoracic defibrillation in pigs with acute left ventricular dysfunction

OBJECTIVE

Our purpose was to compare biphasic versus monophasic shock success for VF termination in a porcine model of acute left ventricular (LV) dysfunction.

BACKGROUND

For the termination of ventricular fibrillation (VF), transthoracic biphasic waveform shocks achieve higher success rates than monophasic shocks. However, the effectiveness of biphasic versus monophasic defibrillation in a setting of left ventricular dysfunction has not been reported.

METHODS

In 23 open-chest adult swine (15-25 kg), LV dysfunction [> or =25% decline in cardiac output (CO)] was induced by continuous inhalation of halothane (1-1.75%). Each pig randomly received transthoracic biphasic and monophasic shocks at three energy levels (30, 50 and 100 J) in two conditions: baseline and LV dysfunction. Halothane effect on left ventricular size and contraction was measured by echocardiography in three additional swine.

RESULTS

With halothane, pigs demonstrated a decline in CO (baseline 4.16+/-0.19, halothane 2.72+/-0.19 l/min, P<0.01), mean arterial pressure (baseline 107.2+/-3.5, halothane 80.1+/-3.4 mmHg, P<0.01) and increased left ventricular end-diastolic pressure (baseline 6.4+/-0.9, halothane 12.7+/-0.8 mmHg, P<0.01). LV diameters increased and fractional shortening fell. During baseline, biphasic shocks achieved significantly greater success (termination of VF) compared to monophasic waveforms (100 J: biphasic 83.3+/-9.5 versus monophasic 38.9+/-9.5%, P<0.01; 50 J: biphasic 67.1+/-8.8 versus monophasic 11.8+/-5.7%, P<0.01; 30 J: biphasic: 31.9+/-6.4 versus monophasic 0+/-0%, P<0.01). The superiority of the biphasic waveform to terminate VF was retained during LV dysfunction at all energy levels (100 J: biphasic 78.3+/-7.3 versus monophasic 37.5+/-8.1%, P<0.01; 50 J: biphasic 65.5+/-11.5 versus monophasic 11.7+/-5.9%, P<0.01; 30 J: biphasic: 40.6+/-8.0 versus monophasic 3.1+/-3.1%, P<0.01). Within both waveforms, there were no significant differences in percent shock success at any energy level comparing baseline with LV dysfunction.

CONCLUSION

In this porcine model of acute LV dysfunction, biphasic waveform shocks were not only superior to monophasic waveform shocks for termination of VF during baseline, but retained superiority to monophasic waveform shocks when LV dysfunction was present.

Waveform analysis of biphasic external defibrillators

BACKGROUND AND OBJECTIVE

All internal defibrillators and some external defibrillators use biphasic waveforms. The study analysed the discharged waveform pulses of two manual and two semi-automated biphasic external defibrillators.

METHODS AND RESULTS

The defibrillators were discharged into resistive loads of 25, 50 and 100 Omega simulating the patient's transthoracic impedance. The tested biphasic defibrillators differed in initial current as well as initial voltage, varying from 10.9 to 73.3 A and from 482.8 to 2140.0 V, respectively. The energies of the manual defibrillators set at 100, 150 and 200 J deviated by up to +19.1 or -28.9% from the selected energy. Impedance-normalised delivered energy varied from 1.0 to 12.5 J/Omega. Delivered energy, shock duration and charge flow were examined with respect to the total pulse, its splitting into positive and negative phases and their impedance dependence. For three defibrillators pulse duration increased with the resistive load, whereas one defibrillator always required 9.9 ms. All tested defibrillators showed a higher charge flow in the positive phase. Defibrillator capacitance varied between approximately 200 and 100 mu F and internal resistance varied from 2.0 to 7.6 Omega. Defibrillator waveform tilt ranged from -13.1 to 61.4%.

CONCLUSIONS

The tested defibrillators showed remarkable differences in their waveform design and their varying dependence on transthoracic impedance.

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.

Transthoracic monophasic and biphasic defibrillation in a swine model: a comparison of efficacy, ST segment changes, and postshock hemodynamics

OBJECTIVE

Biphasic waveforms for transthoracic defibrillation (DF) have been tested extensively after brief (15 s) episodes of VF in animal models and in patients undergoing electrophysiologic testing. The purpose of this study was to compare the effects mono- and biphasic waveforms for DF on postdefibrillation ST segments and left ventricular pressure, markers of myocardial injury, after more extended periods of VF (30 and 90 s).

METHODS

21 anesthetized and instrumented swine were randomized to truncated exponential monophasic or biphasic waveform DF. VF was induced electrically and 30 s later, DF with the designated waveform was attempted with a shock dose of 200 J. If unsuccessful, 300 J and then 360 J were administered if necessary. Following return to control hemodynamic values and normalization of the surface ECG, VF was again induced and, after 90 s, DF was attempted as in the 30 s VF period. CPR was not performed during VF and each animal was countershocked with only one waveform for both VF episodes. Waveforms were compared for frequency of first shock defibrillation success, surface ECG indicators of myocardial injury (ST segment changes at 10, 20, and 30 s after countershock) and time to return to pre-VF hemodynamics after successful DF, an indicator of postshock ventricular function.

RESULTS

Successful first shock conversion rates at 30 and 90 s were 60 and 63% for monophasic and 64 and 82% for biphasic (NS). Biphasic DF after 30 s produced ST segment changes (measured 10 s after DF) in 1/10 animals while six of eight animals in the monophasic group showed ST segment changes (P=0.013). After 90 s of VF, ST segment changes were observed in 6/8 in the monophasic group and 2/10 in the biphasic group (P=0.054). Differences in the time to hemodynamic recovery (return to control peak left ventricular pressure) were not observed between biphasic and monophasic waveforms after 30 or 90 s of VF.

CONCLUSIONS

Monophasic and biphasic transthoracic defibrillation are equally effective in terminating VF of 30 and 90 s duration and restoring a perfusing rhythm. The biphasic waveform produced less ECG evidence of transient myocardial injury. However, there was no difference in the rate of return to control hemodynamics. ST segment changes following countershock of VF of brief duration are transient and of questionable significance.

Monophasic versus biphasic transthoracic countershock after prolonged ventricular fibrillation in a swine model

OBJECTIVE

We sought to compare the defibrillation efficacy of a low-energy biphasic truncated exponential (BTE) waveform and a conventional higher-energy monophasic truncated exponential (MTE) waveform after prolonged ventricular fibrillation (VF).

BACKGROUND

Low energy biphasic countershocks have been shown to be effective after brief episodes of VF (15 to 30 s) and to produce few postshock electrocardiogram abnormalities.

METHODS

Swine were randomized to MTE (n = 18) or BTE (n = 20) after 5 min of VF. The first MTE shock dose was 200 J, and first BTE dose 150 J. If required, up to two additional shocks were administered (300, 360 J MTE; 150, 150 J BTE). If VF persisted manual cardiopulmonary resuscitation (CPR) was begun, and shocks were administered until VF was terminated. Successful defibrillation was defined as termination of VF regardless of postshock rhythm. If countershock terminated VF but was followed by a nonperfusing rhythm, CPR was performed until a perfusing rhythm developed. Arterial pressure, left ventricular (LV) pressure, first derivative of LV pressure and cardiac output were measured at intervals for 60 min postresuscitation.

RESULTS

The odds ratio of first-shock success with BTE versus MTE was 0.67 (p = 0.55). The rate of termination of VF with the second or third shocks was similar between groups, as was the incidence of postshock pulseless electrical activity (15/18 MTE, 18/20 BTE) and CPR time for those animals that were resuscitated. Hemodynamic variables were not significantly different between groups at 15, 30 and 60 min after resuscitation.

CONCLUSIONS

Monophasic and biphasic waveforms were equally effective in terminating prolonged VF with the first shock, and there was no apparent clinical disadvantage of subsequent low-energy biphasic shocks compared with progressive energy monophasic shocks. Lower-energy shocks were not associated with less postresuscitation myocardial dysfunction.

Recent progress in advanced cardiac life support

The revised guidelines for advanced cardiac life support (ACLS) from the American Heart Association are anticipated in the fall of 2000. Although dramatic changes in the approach to adult basic and ACLS are not anticipated, several controversies and new drugs on the horizon may radically change our approach to emergent cardiac resuscitation. This article features some of the evolving thinking on the emergent treatment of the adult with ventricular fibrillation or ventricular tachycardia, the critical rhythms seen in most cases of acute cardiac distress. Approaches to airway therapy drug administration and new agents also are described.

Higher energy synchronized external direct current cardioversion for refractory atrial fibrillation

OBJECTIVES

We sought to evaluate the safety and efficacy of higher energy synchronized cardioversion in patients with atrial fibrillation refractory to standard energy direct current (DC) cardioversion.

BACKGROUND

Standard external electrical cardioversion fails to restore sinus rhythm in 5% to 30% of patients with atrial fibrillation.

METHODS

Patients with atrial fibrillation who failed to achieve sinus rhythm after at least two attempts at standard external cardioversion with 360 J were included in the study. Two external defibrillators, each connected to its own pair of R-2 patches in the anteroposterior position, were used to deliver a synchronized total of 720 J.

RESULTS

Fifty-five patients underwent cardioversion with 720 J. Mean weight was 117 +/- 23 kg (body mass index 48.3 +/- 4.1 kg/m2). Structural heart disease was present in 76% of patients. Mean left ventricular ejection fraction was 45 +/- 12%. Atrial fibrillation was present for over three months in 55% of the patients. Sinus rhythm was achieved in 46 (84%) of the 55 patients. No major complications were observed. No patient developed hemodynamic compromise and no documented cerebrovascular accident occurred within one month after cardioversion. Of the 46 successful cardioversions, 18 patients (39%) remained in sinus rhythm over a mean follow-up of 2.1 months.

CONCLUSIONS

External higher energy cardioversion is effective in restoring sinus rhythm in patients with atrial fibrillation refractory to standard energy DC cardioversion. This method is safe and does not result in clinical evidence of myocardial impairment. It may be a useful alternative to internal cardioversion because it could be done within the same setting of the failed standard cardioversion and obviates the need to withhold protective anticoagulation for internal cardioversion.

Biphasic truncated exponential waveform defibrillation

This paper presents data from studies that have compared the efficacies of biphasic truncated exponential (BTE) and monophasic damped sine (MDS) waveform defibrillation in patients with out-of-hospital cardiac arrest and in in-hospital defibrillation. When a shock is delivered, rhythms evolve rapidly in a variety of directions and take different courses, even over a short time. When defibrillation is defined as termination of ventricular fibrillation at 5 seconds postshock, whether to an organized rhythm or asystole, low-energy BTE shocks appear to be more effective than high-energy MDS shocks in out-of-hospital arrest. For future research, the terms associated with defibrillation should be standardized and used uniformly by all investi-gators. In particular, there should be an agreed-upon definition of shock efficacy.

External exponential biphasic versus monophasic shock waveform: efficacy in ventricular fibrillation of longer duration

Ventricular fibrillation (VF) duration may be a factor in determining the defibrillation energy for successful defibrillation. Exponential biphasic waveforms have been shown to defibrillate with less energy than do monophasic waveforms when used for external defibrillation. However, it is unknown whether this advantage persists with longer VF duration. We tested the hypothesis that exponential biphasic waveforms have lower defibrillation energy as compared to exponential monophasic waveforms even with longer VF duration up to 1 minute. In a swine model of external defibrillation (n = 12, 35 +/- 6 kg), we determined the stored energy at 50% defibrillation success (E50) after both 10 seconds and 1 minute of VF duration. A single exponential monophasic (M) and two exponential biphasic (B1 and B2) waveforms were tested with the following characteristics: M (60 microF, 70% tilt), B1 (60/60 microF, 70% tilt/3 ms pulse width), and B2 (60/20 microF, 70% tilt/3 ms pulse width) where the ratio of the phase 2 leading edge voltage to that of phase 1 was 0.5 for B1 and 1.0 for B2. E50 was measured by a Bayesian technique with a total often defibrillation shocks in each waveform and VF duration randomly. The E50 (J) for M, B1, and B2 were 131 +/- 41, 57 +/- 18,* and 60 +/- 26* with 10 seconds of VF duration, respectively, and 114 +/- 62, 77 +/- 45,* and 72 +/- 53* with 1 minute of VF duration, respectively (*P < 0.05 vs M). There was no significant difference in the E50 between 10 seconds and 1 minute of VF durations for each waveform. We conclude that (1) the E50 does not significantly increase with lengthening VF durations up to 1 minute regardless of the shock waveform, and (2) external exponential biphasic shocks are more effective than monophasic waveforms even with longer VF durations.

Evaluation of biphasic transthoracic defibrillation in an animal model of prolonged ventricular fibrillation

OBJECTIVES

To determine whether a biphasic defibrillation waveform (BDW) would produce a superior rate of converting prolonged ventricular fibrillation (VF) into a perfusing rhythm and delay the occurrence of asystole and/or pulseless electrical activity (PEA) during the resuscitation attempt, when compared with a monophasic defibrillation waveform (MDW).

METHODS

The authors performed a prospective, randomized, blinded experiment using an established swine model of prolonged VF. Thirty-four mixed-breed domestic swine (mean mass 22.9 kg) were sedated (ketamine/xylazine), anesthetized (isoflurane), and intubated. Aortic and femoral venous catheters were placed. ECG was monitored continuously. The animals were shocked into VF (3-s, 100-mA, 60-Hz shock), and were untreated for 8 minutes. Advanced Cardiac Life Support (ACLS) began with 1 minute of standardized (Thumper) chest compressions and ventilation. The animals were randomized to treatment with either BDW or MDW. Standard ACLS protocols were followed. The energy sequence was 2.5 J/kg first, 3.5 J/kg second, and 4.5 J/kg for all subsequent shocks. Outcome variables were time to event of asystole/PEA, return of spontaneous circulation (ROSC), and one-hour survival. Data were analyzed with two-tailed Fisher's exact test and Kaplan-Meier survival plots (alpha = 0.05).

RESULTS

ROSC occurred more frequently in the BDW group (7/17) compared with the MDW group (1/17); p = 0.04. Survival analysis showed that the BDW significantly delayed the occurrence of asystole/PEA during the resuscitation attempt when compared with the MDW; log-ranked p = 0.02. One-hour survival rates (5/17 BDW and 1/17 MDW, p = 0.17) did not differ.

CONCLUSIONS

BDW resulted in a superior rate of ROSC and delay in the occurrence of asystole/ PEA during the resuscitation attempt when compared with MDW.

Fully discharging phases. A new approach to biphasic waveforms for external defibrillation

BACKGROUND

Phase-2 voltage and maximum pulse width are dependent on phase-1 pulse characteristics in a single-capacitor biphasic waveform. The use of 2 separate output capacitors avoids these limitations and may allow waveforms with lower defibrillation thresholds. A previous report also suggested that the optimal tilt may be >70%. This study was designed to determine an optimal biphasic waveform by use of a combination of 2 separate and fully (95% tilt) discharging capacitors.

METHODS AND RESULTS

We performed 2 external defibrillation studies in a pig ventricular fibrillation model. In group 1, 9 waveforms from a combination of 3 phase-1 capacitor values (30, 60, and 120 microF) and 3 phase-2 capacitor values (0=monophasic, 1/3, and 1.0 times the phase-1 capacitor) were tested. Biphasic waveforms with phase-2 capacitors of 1/3 times that of phase 1 provided the highest defibrillation efficacy (stored energy and voltage) compared with corresponding monophasic and biphasic waveforms with the same capacitors in both phases except for waveforms with a 30-microF phase-1 capacitor. In group 2, 10 biphasic waveforms from a combination of 2 phase-1 capacitor values (30 and 60 microF) and 5 phase-2 capacitor values (10, 20, 30, 40, and 50 microF) were tested. In this range, phase-2 capacitor size was more critical for the 30-microF phase-1 than for the 60-microF phase-1 capacitor. The optimal combinations of fully discharging capacitors for defibrillation were 60/20 and 60/30 microF. Conclusions-Phase-2 capacitor size plays an important role in reducing defibrillation energy in biphasic waveforms when 2 separate and fully discharging capacitors are used.

Waveforms of external defibrillators: analysis and energy contribution

BACKGROUND AND OBJECTIVE

Defibrillation is the most important therapy for terminating ventricular fibrillation in cardiac arrest patients. In addition to performing defibrillation at the earliest possible time, appropriate pulse energy and optimal waveform seem to be crucial for success. Emergency medical service personnel use different defibrillators and rely on their similarity of energy content. This study examined the true pulse energy content and waveform of 17 commonly used defibrillators.

METHODS AND RESULTS

Defibrillation energies were selected to be 30, 200 or 360 J and defibrillators were discharged into test resistors, simulating transthoracic impedances of 25, 50 or 100 Ohms. Pulse energy deviated by up to +23% or -29% from the selected energy. Pulse energy within the initial 8 ms ranged from 90 to 30% of total pulse energy. Fourteen defibrillators utilising damped sinusoidal waveforms produced a monophasic pulse when discharged into resistances of 50 Ohms and 100 Ohms.

CONCLUSIONS

Defibrillators used at the same energy settings do not necessarily produce the same defibrillation pulse energy. All but one defibrillator actually use monophasic waveforms, leaving the potential advantage of biphasic waveforms unused. Energy accuracy of defibrillators needs to be improved, and biphasic waveforms should be used more.

The automated external cardiac defibrillator: lifesaving device for medical emergencies

BACKGROUND

More than 350,000 adult Americans die each year of sudden cardiac arrest, or SCA. The event is unpredictable and can occur in patients with no history of cardiac disease or cardiac symptoms. Drugs and cardiopulmonary resuscitation, or CPR, save only a small percentage of victims. The necessary response is rapid application of electrical shock, and the chances of success are reduced 10 percent for every minute of delay.

TYPES OF STUDIES REVIEWED

The author reviewed the literature on resuscitation of people who have undergone SCA, and examined the emerging technology of automated external defibrillators, or AEDs, for correcting cardiac ventricular fibrillation. Included is a review of the controversies surrounding AED waveforms and energy levels.

RESULTS

Automated cardiac defibrillators are becoming readily available because of improved technology and decreasing prices. AEDs are now commonly found in commercial aircraft, gambling casinos, sports arenas and public buildings, and will soon become as readily available as fire extinguishers. The use of AEDs is being taught in standard CPR courses.

CLINICAL IMPLICATIONS

AEDs are being installed in more public locations, including some dental offices. As costs decrease and availability increases, there is significant potential use for AEDs in managing SCAs in dental offices.

Encircling overlapping multipulse shock waveforms for transthoracic defibrillation

OBJECTIVES

This study was performed to determine the efficacy of new encircling overlapping multipulse, multipathway waveforms for transthoracic defibrillation.

BACKGROUND

Alternative waveforms for transthoracic defibrillation may improve shock success.

METHODS

First, we determined the shock success achieved by three different waveforms at varying energies (18-150 J) in 21 mongrel dogs after short-duration ventricular fibrillation. The waveforms tested included the traditional damped sinusoidal waveform, a single pathway biphasic waveform, and a new encircling overlapping multipulse waveform delivered from six electrode pads oriented circumferentially. Second, in 11 swine we compared the efficacy of encircling overlapping multipulse shocks given from six electrode pads and three capacitors versus encircling overlapping shocks given from a device utilizing three electrodes and one capacitor.

RESULTS

In the first experiment, the encircling overlapping waveform performed significantly better than biphasic and damped sinusoidal waveforms at lower energies. The shock success rate of the overlapping waveform (six pads) ranged from 67+/-4% (at 18-49 J energy) to 99+/-3% at > or = 150 J; at comparable energies biphasic waveform shock success ranged from 26+/-5% (p < 0.01 vs. encircling overlapping waveforms) to 99+/-5% (p = NS). Damped sinusoidal waveform shock success ranged from 4+/-1% (p < 0.01 vs. encircling overlapping waveform) to 73+/-9% (p = NS). In the second experiment the three electrode pads, one capacitor encircling waveform achieved shock success rates comparable with the six-pad, three-capacitor waveform; at 18-49 J, success rates were 45+/-15% versus 57+/-12%, respectively (p = NS). At 100 J, success rates for both were 100%.

CONCLUSIONS

We conclude that encircling overlapping multipulse multipathway waveforms facilitate transthoracic defibrillation at low energies. These waveforms can be generated from a device that requires only three electrodes and one capacitor.

Relative efficacy of monophasic and biphasic waveforms for transthoracic defibrillation after short and long durations of ventricular fibrillation

BACKGROUND

Recently, interest has arisen in using biphasic waveforms for external defibrillation. Little work has been done, however, in measuring transthoracic defibrillation efficacy after long periods of ventricular fibrillation. In protocol 1, we compared the efficacy of a quasi-sinusoidal biphasic waveform (QSBW), a truncated exponential biphasic waveform (TEBW), and a critically damped sinusoidal monophasic waveform (CDSMW) after 15 seconds of fibrillation. In protocol 2, we compared the efficacy of the more efficacious biphasic waveform from protocol 1, QSBW, with CDSMW after 15 seconds and 5 minutes of fibrillation.

METHODS AND RESULTS

In protocol 1, 50% success levels, ED50, were measured after 15 seconds of fibrillation for the 3 waveforms in 6 dogs. In protocol 2, defibrillation thresholds were measured for QSBW and CDSMW after 15 seconds of fibrillation and after 3 minutes of unsupported fibrillation followed by 2 minutes of fibrillation with femoral-femoral cross-circulation. In protocol 1, QSBW had a lower ED50, 16.0+/-4.9 J, than TEBW, 20.3+/-4.4 J, or CDSMW, 27.4+/-6.0 J. In protocol 2, QSBW had a lower defibrillation threshold after 15 seconds, 38+/-10 J, and after 5 minutes, 41.5+/-5 J, than CDSMW after 15 seconds, 54+/-19 J, and 5 minutes, 80+/-30 J, of fibrillation. The defibrillation threshold remained statistically the same for QSBW for the 2 fibrillation durations but rose significantly for CDSMW.

CONCLUSIONS

In this animal model of sudden death and resuscitation, these 2 biphasic waveforms are more efficacious than the CDSMW at short durations of fibrillation. Furthermore, the QSBW is even more efficacious than the CDSMW at longer durations of fibrillation.