Relation between shock-related myocardial injury and defibrillation efficacy of monophasic and biphasic shocks in a canine model

2024 RECOVER Guidelines: Advanced Life Support. Evidence and knowledge gap analysis with treatment recommendations for small animal CPR

OBJECTIVE

To systematically review the evidence and devise clinical recommendations on advanced life support (ALS) in dogs and cats and to identify critical knowledge gaps.

DESIGN

Standardized, systematic evaluation of literature pertinent to ALS following Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) methodology. Prioritized questions were each reviewed by Evidence Evaluators, and findings were reconciled by ALS Domain Chairs and Reassessment Campaign on Veterinary Resuscitation (RECOVER) Co-Chairs to arrive at treatment recommendations commensurate to quality of evidence, risk:benefit relationship, and clinical feasibility. This process was implemented using an Evidence Profile Worksheet for each question that included an introduction, consensus on science, treatment recommendations, justification for these recommendations, and important knowledge gaps. A draft of these worksheets was distributed to veterinary professionals for comment for 4 weeks prior to finalization.

SETTING

Transdisciplinary, international collaboration in university, specialty, and emergency practice.

RESULTS

Seventeen questions pertaining to vascular access, vasopressors in shockable and nonshockable rhythms, anticholinergics, defibrillation, antiarrhythmics, and adjunct drug therapy as well as open-chest CPR were reviewed. Of the 33 treatment recommendations formulated, 6 recommendations addressed the management of patients with nonshockable arrest rhythms, 10 addressed shockable rhythms, and 6 provided guidance on open-chest CPR. We recommend against high-dose epinephrine even after prolonged CPR and suggest that atropine, when indicated, is used only once. In animals with a shockable rhythm in which initial defibrillation was unsuccessful, we recommend doubling the defibrillator dose once and suggest vasopressin (or epinephrine if vasopressin is not available), esmolol, lidocaine in dogs, and/or amiodarone in cats.

CONCLUSIONS

These updated RECOVER ALS guidelines clarify the approach to refractory shockable rhythms and prolonged CPR. Very low quality of evidence due to absence of clinical data in dogs and cats continues to compromise the certainty with which recommendations can be made.

Relationship between Exercise Test Parameters, Device-Delivered Electric Shock and Adverse Clinical Events in Patients with an Implantable Cardioverter Defibrillator for Primary Prevention

(1) Background: Receiving the first internal electric shock is a turning point for patients with an implantable cardioverter defibrillator (ICD) for primary prevention. However, no study has investigated whether patients who receive a first device-delivered electric shock have a poor prognosis even at the time of ICD implantation. (2) Methods: We retrospectively identified 55 patients with ischemic (n = 31) or dilated (n = 24) cardiomyopathy who underwent ICD implantation for primary prevention with exercise test at the time of implantation. We recorded baseline characteristics, exercise test parameters, and clinical events. (3) Results: After a median follow-up of 5 years, we observed an association between an appropriate device-delivered electric shock, the occurrence of death or heart transplant, and the occurrence of the composite endpoint. There was also a significant relation between a VE/VCO2 slope >35 and the occurrence of the composite endpoint. Conversely, there was no significant association between negative outcomes on the exercise test and the occurrence of a device-delivered electric shock. (4) Conclusions: The exercise test performed at the time of ICD implantation do not predict the occurrence of device-delivered electric shock. The exercise test and the first electric shock are two independent markers of poor prognosis.

Ascending Defibrillation Waveform Significantly Reduces Myocardial Morphological Damage and Injury Current

OBJECTIVES

This study tested the hypothesis that a biphasic defibrillation waveform with an ascending first phase (ASC) causes less myocardial damage by pathology and injury current than a standard biphasic truncated exponential (BTE) waveform in a swine model.

BACKGROUND

Although lifesaving, defibrillation shocks have significant iatrogenic effects that reduce their benefit for patient survival.

METHODS

An ASC waveform with an 8-ms linear ramp followed by an additional positive 0.5-ms decaying portion with amplitudes of 20 J (ASC 20J) and 25 J (ASC 25J) was used. The control was a 25-J BTE conventional waveform (BTE 25J) RESULTS: The ASC 20J and ASC 25J shocks were both successful in 6 of 6 pigs, but the BTE 25J was successful in only 6 of 14 pigs (p < 0.05). Post-shock ST-segment elevation (injury current) in the right ventricular electrode was significantly greater with BTE 25J than with ASC 20J and ASC 25J. With a blinded pathology reading, hemorrhage, inflammation, thrombi, and necrosis 24 h post-shock were significantly greater with BTE 25J than with ASC 20J and ASC 25J. Troponin levels were also markedly lower at 3, 4, 5, and 6 h post-shock.

CONCLUSIONS

Defibrillation shocks cause electrophysiological, histological, and biochemical signs of myocardial damage and necrosis. These signs of damage are markedly less for an ASC waveform than for a conventional BTE waveform.

Relation between total shock energy and mortality in patients with implantable cardioverter-defibrillator

BACKGROUND

Implantable Cardioverter-Defibrillator (ICD) shocks have been associated with mortality. However, no study has examined the relation between total shock energy and mortality. The aim of this study is to assess the association of total shock energy with mortality, and to determine the patients who are at risk of this association.

METHODS

Data from 316 consecutive patients who underwent initial ICD implantation in our hospital between 2000 and 2011 were retrospectively studied. We collected shock energy for 3 years from the ICD implantation, and determined the relation of shock energy on mortality after adjusting confounding factors.

RESULTS

Eighty-seven ICD recipients experienced shock(s) within 3 years from ICD implantation and 43 patients had died during the follow-up. The amount of shock energy was significantly associated with all-cause death [adjusted hazard ratio (HR) 1.26 (per 100 joule increase), p < 0.01] and tended to be associated with cardiac death (adjusted HR 1.30, p = 0.08). The survival rate of patients with high shock energy accumulation (≥182 joule) was lower (p < 0.05), as compared to low shock energy accumulation (<182 joule), likewise to no shock. Besides, the relation between high shock energy accumulation and all-cause death was remarkable in the patients with low left ventricular ejection fraction (LVEF ≤40%) or atrial fibrillation (AF).

CONCLUSIONS

Increase of shock energy was related to mortality in ICD recipients. This relation was evident in patients with low LVEF or AF.

The Use of Automated External Defibrillators in Infants: A Report From the American Red Cross Scientific Advisory Council

OBJECTIVE

Automated external defibrillators (AEDs) have been used successfully in many populations to improve survival for out-of-hospital cardiac arrest. While ventricular fibrillation and pulseless ventricular tachycardia are more prevalent in adults, these arrhythmias do occur in infants. The Scientific Advisory Council of the American Red Cross reviewed the literature on the use of AEDs in infants in order to make recommendations on use in the population.

METHODS

The Cochrane library and PubMed were searched for studies that included AEDs in infants, any external defibrillation in infants, and simulation studies of algorithms used by AEDs on pediatric arrhythmias.

RESULTS

There were 4 studies on the accuracy of AEDs in recognizing pediatric arrhythmias. Case reports (n = 2) demonstrated successful use of AED in infants, and a retrospective review (n = 1) of pediatric pads for AEDs included infants. Six studies addressed defibrillation dosages used. The algorithms used by AEDs had high sensitivity and specificity for pediatric arrhythmias and very rarely recommended a shock inappropriately. The energy doses delivered by AEDs were high, although in the range that have been used in out-of-hospital arrest. In addition, there are data to suggest that 2 to 4 J/kg may not be effective defibrillation doses for many children.

CONCLUSIONS

In the absence of prompt defibrillation for ventricular fibrillation or pulseless ventricular tachycardia, survival is unlikely. Automated external defibrillators should be used in infants with suspected cardiac arrest, if a manual defibrillator with a trained rescuer is not immediately available. Automated external defibrillators that attenuate the energy dose (eg, via application of pediatric pads) are recommended for infants. If an AED with pediatric pads is not available, the AED with adult pads should be used.

Updates in the American Heart Association guidelines for cardiopulmonary resuscitation and potential applications to veterinary patients

OBJECTIVE

To review the updates in the American Heart Association (AHA) guidelines for cardiopulmonary resuscitation (CPR) and identify potential applications to veterinary patients.

ETIOLOGY

Cardiopulmonary arrest is common in veterinary emergency and critical care, and consensus guidelines are lacking. Human resuscitation guidelines are continually evolving as new clinical and experimental studies support updated recommendations. Synthesis of human, experimental animal model, and veterinary literature support the potential for updates and advancement in veterinary CPR practices.

THERAPY

This review serves to highlight updates in the AHA guidelines for CPR and evaluate their application to small animal veterinary patients. Interventions identified will be evaluated for trans-species potential, raise questions regarding best resuscitation recommendations, and offer opportunities for further research to continue to advance veterinary CPR.

PROGNOSIS

The prognosis for any patient undergoing cardiopulmonary arrest remains guarded.

Monophasic versus biphasic defibrillation for pediatric out-of-hospital cardiac arrest patients: a nationwide population-based study in Japan

Introduction

Conventional monophasic defibrillators for out-of-hospital cardiac-arrest patients have been replaced with biphasic defibrillators. However, the advantage of biphasic over monophasic defibrillation for pediatric out-of-hospital cardiac-arrest patients remains unknown. This study aimed to compare the survival outcomes of pediatric out-of-hospital cardiac-arrest patients who underwent monophasic defibrillation with those who underwent biphasic defibrillation.

Methods

This prospective, nationwide, population-based observational study included pediatric out-of-hospital cardiac-arrest patients from January 1, 2005, to December 31, 2009. The primary outcome measure was survival at 1 month with minimal neurologic impairment. The secondary outcome measures were survival at 1 month and the return of spontaneous circulation before hospital arrival. Multivariable logistic regression analysis was performed to identify the independent association between defibrillator type (monophasic or biphasic) and outcomes.

Results

Among 5,628 pediatric out-of-hospital cardiac-arrest patients (1 through 17 years old), 430 who received defibrillation shock with monophasic or biphasic defibrillator were analyzed. The number of patients who received defibrillation shock with monophasic defibrillator was 127 (30%), and 303 (70%) received defibrillation shock with biphasic defibrillator. The survival rates at 1 month with minimal neurologic impairment were 17.5% and 24.4%, the survival rates at 1 month were 32.3% and 35.6%, and the rates of return of spontaneous circulation before hospital arrival were 24.4% and 27.4% in the monophasic and biphasic defibrillator groups, respectively. Hierarchic logistic regression analyses by using generalized estimation equations found no significant difference between the two groups in terms of 1-month survival with minimal neurologic impairment (odds ratio (OR), 1.57; 95% confidence interval (CI), 0.87 to 2.83; P = 0.14) and 1-month survival (OR, 1.38; 95% CI, 0.87 to 2.18; P = 0.17).

Conclusions

The present nationwide population-based observational study could not confirm an advantage of biphasic over monophasic defibrillators for pediatric OHCA patients.

Ascending-ramp biphasic waveform has a lower defibrillation threshold and releases less troponin I than a truncated exponential biphasic waveform

BACKGROUND

We tested the hypothesis that the shape of the shock waveform affects not only the defibrillation threshold but also the amount of cardiac damage.

METHODS AND RESULTS

Defibrillation thresholds were determined for 11 waveforms-3 ascending-ramp waveforms, 3 descending-ramp waveforms, 3 rectilinear first-phase biphasic waveforms, a Gurvich waveform, and a truncated exponential biphasic waveform-in 6 pigs with electrodes in the right ventricular apex and superior vena cava. The ascending, descending, and rectilinear waveforms had 4-, 8-, and 16-millisecond first phases and a 3.5-millisecond rectilinear second phase that was half the voltage of the first phase. The exponential biphasic waveform had a 60% first-phase and a 50% second-phase tilt. In a second study, we attempted to defibrillate after 10 seconds of ventricular fibrillation with a single ≈30-J shock (6 pigs successfully defibrillated with 8-millisecond ascending, 8-millisecond rectilinear, and truncated exponential biphasic waveforms). Troponin I blood levels were determined before and 2 to 10 hours after the shock. The lowest-energy defibrillation threshold was for the 8-milliseconds ascending ramp (14.6±7.3 J [mean±SD]), which was significantly less than for the truncated exponential (19.6±6.3 J). Six hours after shock, troponin I was significantly less for the ascending-ramp waveform (0.80±0.54 ng/mL) than for the truncated exponential (1.92±0.47 ng/mL) or the rectilinear waveform (1.17±0.45 ng/mL).

CONCLUSIONS

The ascending ramp has a significantly lower defibrillation threshold and at ≈30 J causes 58% less troponin I release than the truncated exponential biphasic shock. Therefore, the shock waveform affects both the defibrillation threshold and the amount of cardiac damage.

Electroporation induced by internal defibrillation shock with and without recovery in intact rabbit hearts

Defibrillation shocks from implantable cardioverter defibrillators can be lifesaving but can also damage cardiac tissues via electroporation. This study characterizes the spatial distribution and extent of defibrillation shock-induced electroporation with and without a 45-min postshock period for cell membranes to recover. Langendorff-perfused rabbit hearts (n = 31) with and without a chronic left ventricular (LV) myocardial infarction (MI) were studied. Mean defibrillation threshold (DFT) was determined to be 161.4 ± 17.1 V and 1.65 ± 0.44 J in MI hearts for internally delivered 8-ms monophasic truncated exponential (MTE) shocks during sustained ventricular fibrillation (>20 s, SVF). A single 300-V MTE shock (twice determined DFT voltage) was used to terminate SVF. Shock-induced electroporation was assessed by propidium iodide (PI) uptake. Ventricular PI staining was quantified by fluorescent imaging. Histological analysis was performed using Masson's Trichrome staining. Results showed PI staining concentrated near the shock electrode in all hearts. Without recovery, PI staining was similar between normal and MI groups around the shock electrode and over the whole ventricles. However, MI hearts had greater total PI uptake in anterior (P < 0.01) and posterior (P < 0.01) LV epicardial regions. Postrecovery, PI staining was reduced substantially, but residual staining remained significant with similar spacial distributions. PI staining under SVF was similar to previously studied paced hearts. In conclusion, electroporation was spatially correlated with the active region of the shock electrode. Additional electroporation occurred in the LV epicardium of MI hearts, in the infarct border zone. Recovery of membrane integrity postelectroporation is likely a prolonged process. Short periods of SVF did not affect electroporation injury.

Preventing Implantable Cardioverter Defibrillator Shocks Improves Survival

Implanted cardioverter defibrillators (ICDs) reduce sudden cardiac death and all-cause mortality in patients at an elevated risk of ventricular arrhythmia (VA). Patients with ICDs who receive shocks for VA have an increased mortality primarily due to worsening heart failure. Although VA and ICD shocks are a marker of progression of the underlying cardiomyopathy and disease process, there is evidence suggesting that ICD shocks are directly harmful to the myocardium and may contribute to the increase in mortality. Thus, although ICD shocks are a lifesaving therapy, they are also harmful and should be avoided whenever possible.

High-energy defibrillation impairs myocyte contractility and intracellular calcium dynamics

OBJECTIVES

We examined the effects of energy delivered with electrical defibrillation on myocyte contractility and intracellular Ca2+ dynamics. We hypothesized that increasing the defibrillation energy would produce correspondent reduction in myocyte contractility and intracellular Ca2+ dynamics.

DESIGN

Randomized prospective study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Ventricular myocytes from male Sprague-Dawley rat hearts.

MATERIALS AND METHODS

Ventricular cardiomyocytes loaded with Fura-2/AM were placed in a chamber mounted on an inverted microscope and superfused with a buffer solution at 37 degrees C. The cells were field stimulated to contract and mechanical properties were assessed using a video-based edge-detection system. Intracellular Ca2+ dynamics were evaluated with a dual-excitation fluorescence photomultiplier system. Myocytes were then randomized to receive 1) a single 0.5-J biphasic shock; 2) a single 1-J biphasic shock; 3) a single 2-J biphasic shock; and 4) a control group without shock. After the shock, myocytes were paced for an additional 4 mins.

RESULTS

A single 0.5-J shock did not have effects on contractility and intracellular Ca2+ dynamics. Higher energy shocks, i.e., 1- or 2-J shocks, significantly impaired contractility and intracellular Ca2+ dynamics. The adverse effects were greater after a 2-J shock compared with a 1-J shock.

CONCLUSIONS

Higher defibrillation energy significantly impairs ventricular contractility at the myocyte level. Reductions in cardiomyocyte shortening and intracellular Ca2+ dynamics abnormalities were greater when higher energy shock was used.

Transient local injury current in right ventricular electrogram after implantable cardioverter-defibrillator shock predicts heart failure progression

OBJECTIVES

This study aimed to identify an early marker of functional impairment after an implantable cardioverter-defibrillator (ICD) shock as a predictor of heart failure progression.

BACKGROUND

The ICD population has substantial risk of death due to progressive pump failure.

METHODS

Near-field (NF) bipolar right ventricular (RV) electrograms (EGMs) during induced ventricular fibrillation (VF) and 10 s after rescue ICD shock were analyzed in 310 patients (mean age 59 +/- 14.5 years, 219 men [71%]) with structural heart disease, New York Heart Association functional class I to III, and implanted with a single- or dual-chamber Medtronic (Minneapolis, Minnesota) ICD for primary (245 patients, 79%) or secondary prevention of sudden cardiac arrest. A local injury current (LIC) on NF RV EGM was defined as a deviation of EGM potential > or =1 mV or > or =15% of the preceding R-wave peak-to-peak amplitude.

RESULTS

During mean follow-up of 29.3 +/- 15.0 months, the combined end point of death or hospitalization due to congestive heart failure (CHF) exacerbation was documented in 40 patients (12.9%, or 5.3% per person-year of follow-up). LIC was observed in 106 patients. In multivariate risk analysis, after adjustment for baseline prognostic factors (ejection fraction, history of atrial fibrillation, diabetes mellitus) and appropriate ICD shocks during follow-up, patients with observed LIC after induced VF rescue ICD shock at ICD implantation were more likely to die or to be hospitalized (hazard ratio: 2.69; 95% confidence interval: 1.41 to 5.14; p = 0.003).

CONCLUSIONS

Transient LIC on bipolar NF RV EGM after induced VF rescue ICD shock is associated with increased risk of CHF progression, future hospitalizations due to CHF exacerbation, and subsequent heart failure death.

Lethal effect of electric fields on isolated ventricular myocytes

Defibrillator-type shocks may cause electric and contractile dysfunction. In this study, we determined the relationship between probability of lethal injury and electric field intensity (E in isolated rat ventricular myocytes, with emphasis on field orientation and stimulus waveform. This relationship was sigmoidal with irreversible injury for E > 50 V/cm . During both threshold and lethal stimulation, cells were twofold more sensitive to the field when it was applied longitudinally (versus transversally) to the cell major axis. For a given E, the estimated maximum variation of transmembrane potential (Delta V(max)) was greater for longitudinal stimuli, which might account for the greater sensitivity to the field. Cell death, however, occurred at lower maximum Delta V(max) values for transversal shocks. This might be explained by a less steep spatial decay of transmembrane potential predicted for transversal stimulation, which would possibly result in occurrence of electroporation in a larger membrane area. For the same stimulus duration, cells were less sensitive to field-induced injury when shocks were biphasic (versus monophasic). Ours results indicate that, although significant myocyte death may occur in the E range expected during clinical defibrillation, biphasic shocks are less likely to produce irreversible cell injury.

Postresuscitation myocardial stunning and its outcome: new approaches

What is the successful cardiopulmonary resuscitation? It is the few minutes postcardiopulmonary arrest that can answer. Twenty to 40 percent of patients who sustained cardiac arrest are initially resuscitated, but only 10% survive to hospital discharge, and more than 60% of victims succumb within 24 hours. This high fatality rate in the early hours and days after successful resuscitation is mainly related to the acute, intense, and reversible form of postresuscitation myocardial dysfunction (stunning) together with the ventricular tachyarrhythmia. It is a reversible process, provided that we are aware of the pertinent pathophysiology and then intervene accordingly. Herein I reviewed most of the published relevant articles concerning the causes, underlying mechanism, and the updated trials for management of postresuscitation myocardial stunning. I do agree that not only the restoration of the circulation but also long-term outcome should be the aim of resuscitation, and I readdress the role of epinephrine, dobutamine, biphasic defibrillator, with the new promising agent (ie, potassium channel opener), Delta-opioid receptor agonist, unloading intracellular calcium, antioxidants, and therapeutic hypothermia to halt this period of stunning. This will improve the outcome of the resuscitation efforts.

Prediction of successful defibrillation in human victims of out-of-hospital cardiac arrest: a retrospective electrocardiographic analysis

In the present study we sought to examine the efficacy of an electrocardiographic parameter, 'amplitude spectrum area' (AMSA), to predict the likelihood that any one electrical shock would restore a perfusing rhythm during cardiopulmonary resuscitation in human victims of out-of-hospital cardiac arrest. AMSA analysis is not invalidated by artefacts produced by chest compression and thus it can be performed during CPR, avoiding detrimental interruptions of chest compression and ventilation. We hypothesised that a threshold value of AMSA could be identified as an indicator of successful defibrillation in human victims of cardiac arrest. Analysis was performed on a database of electrocardiographic records, representing lead 2 equivalent recordings from automated external defibrillators including 210 defibrillation attempts from 90 victims of out-of-hospital cardiac arrest. A 4.1 second interval of ventricular fibrillation or ventricular tachycardia, recorded immediately preceding the delivery of the shock, was analysed using the AMSA algorithm. AMSA represents a numerical value based on the sum of the magnitude of the weighted frequency spectrum between two and 48 Hz. AMSA values were significantly greater in successful defibrillation (restoration of a perfusing rhythm), compared to unsuccessful defibrillation (P < 0.0001). An AMSA value of 12 mV-Hz was able to predict the success of each defibrillation attempt with a sensitivity of 0.91 and a specificity of 0.97. In conclusion, AMSA analysis represents a clinically applicable method, which provides a real-time prediction of the success of defibrillation attempts. AMSA may minimise the delivery of futile and detrimental electrical shocks, reducing thereby post-resuscitation myocardial injury.

Effect of electroporation on cardiac electrophysiology

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells, resulting in resynchronization of electrical activity in the heart. If shock-induced transmembrane potentials are large enough, they can cause transient tissue damage due to electroporation. In this review, evidence is presented that electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field and that electroporation can affect the outcome of defibrillation therapy, being both pro- and antiarrhythmic.Here, we present experimental evidence for electroporation in cardiac tissue, which occurs above a threshold of 25 V/cm as evident from propidium iodide uptake, transient diastolic depolarization, and reductions of action potential amplitude and its derivative. These electrophysiological changes can induce tachyarrhythmia, due to conduction block and possibly triggered activity; however, our findings provide the foundation for future design of effective methods to deliver genes and drugs to cardiac tissues, while avoiding possible side effects such as arrhythmia and mechanical stunning.

Ventricular fibrillation and the use of automated external defibrillators on children

The use of automated external defibrillators (AEDs) has been advocated in recent years as a part of the chain of survival to improve outcomes for adult cardiac arrest victims. When AEDs first entered the market, they were not tested for pediatric usage and rhythm interpretation. In addition, the presumption was that children do not experience ventricular fibrillation, so they would not benefit from use of AEDs. Recent literature has shown that children do experience ventricular fibrillation, and this rhythm has a better outcome than do other cardiac arrest rhythms. At the same time, the arrhythmia software on AEDs has become more extensive and validated for children, and attenuation devices have become available to downregulate the energy delivered by AEDs to allow their use in children. Pediatricians are now being asked whether AED programs should be implemented, and where they are being implemented, pediatricians are being asked to provide guidance on the use of AEDs in children. As AED programs expand, pediatricians must advocate on behalf of children so that their needs are accounted for in these programs. For pediatricians to be able to provide guidance and ensure that children are included in AED programs, it is important for pediatricians to know how AEDs work, be up-to-date on the literature regarding pediatric fibrillation and energy delivery, and understand the role of AEDs as life-saving interventions for children.

Ventricular fibrillation and defibrillation

Cardiac arrest in children is not often due to a disturbance in rhythm that is amenable to electrical defibrillation, contrary to the situation in adults. When a shockable rhythm is present, defibrillation using an external electric shock applied at an early stage after pre-oxygenation and chest compressions is of proven efficacy. Success at conversion of ventricular fibrillation is dependent on the delay before delivering the shock and defibrillation efficiency, which is itself a function of thoracic impedance, energy dose and waveform.

Combining stimulus direction and waveform for optimization of threshold stimulation of isolated ventricular myocytes

Electric field stimulation is widely used for heart pacing and arrhythmia reversion. In this study, we analysed the influence of waveform and direction of external stimulating electric field on the excitation threshold of isolated ventricular myocytes. The threshold field (E(T)) was lower when the field was applied longitudinally (E(T,L)) rather than transversally (E(T,T)) to the cell major axis. Rheobase was greater for transversal stimulation, but chronaxie and estimated membrane polarization were similar for both directions. The calculated maximal variation in membrane potential at the threshold (DeltaV(T) approximately 15 mV) was insensitive to field direction. As DeltaV(T) values were similar, we assumed that the E(T,T)/E(T,L) ratio might be described solely as the ratio of the major and minor cell semi-axes. Accordingly, the ratio thus estimated was comparable to that determined experimentally. Stimulus waveform significantly affected both E(T) and DeltaV(T), which were greater for monophasic versus biphasic stimuli. Direction and waveform effects were independent. We conclude that (a) direction affects E(T) by its influence on the ability of a given field intensity to cause threshold membrane polarization and (b) threshold-lowering effects of longitudinal stimulation and biphasic waveforms apparently depend on different mechanisms, are additive and thus may be combined to decrease the energy requirement for myocardial stimulation.

Future shock: automatic external defibrillators

PURPOSE OF REVIEW

This review provides a practical overview of the performance capabilities of automatic external defibrillators (AEDs), and of advances in technology and dissemination programmes for these devices.

RECENT FINDINGS

Arrhythmia analysis by AEDs is extremely reliable in most settings (sensitivity 81-100%, specificity 99.9-97.6%). Accurate detection of arrhythmias has also been demonstrated in children, leading the US Food and Drug Administration to approve the use of several AEDs in children aged 8 years or younger. Factors that potentially may reduce the quality of arrhythmia detection are the presence of wide complex supraventricular tachycardia and location of an arrythmic event near to high-power lines. AED use by professional basic life support providers resulted in increased survival in the prehospital setting. However, provision of AEDs to nonmedical rescue services did not result in universal improvement in patient outcome. Public access defibrillation programmes have led to higher rates of survival from cardiac arrest. The role of AEDs in hospitals has yet to be elucidated, although in-hospital mortality from ventricular arrhythmias has been shown to decrease following AED deployment.

SUMMARY

Given the correct setting, AEDs can ensure that defibrillation is not limited by lack of medical knowledge or difficulties in decision making. However, event-related variables and operator-related factors, that are yet to be determined, can significantly affect the efficacy of automatic external defibrillation.

Mechanisms of unpinning and termination of ventricular tachycardia

High-energy defibrillation shock is the only therapy for ventricular tachyarrhythmias. However, because of adverse side effects, lowering defibrillation energy is desirable. We investigated mechanisms of unpinning, destabilization, and termination of ventricular tachycardia (VT) by low-energy shocks in isolated rabbit right ventricular preparations (n = 22). Stable VT was initiated with burst pacing and was optically mapped. Monophasic "unpinning" shocks (10 ms) of different strengths were applied at various phases throughout the reentry cycle. In 8 of 22 preparations, antitachycardia pacing (ATP: 8-20 pulses, 50-105% of period, 0.8-10 mA) was also applied. Termination of reentry by ATP was achieved in only 5 of 8 preparations. Termination by unpinning occurred in all 22 preparations. Rayleigh's test showed a statistically significant unpinning phase window, during which reentry could be unpinned and subsequently terminated with E80 (magnitude at which 80% of reentries were unpinned) = 1.2 V/cm. All reentries were unpinned with field strengths < or = 2.4 V/cm. Unpinning was achieved by inducing virtual electrode polarization and secondary sources of excitation at the core of reentry. Optical mapping revealed the mechanisms of phase-dependent unpinning of reentry. These results suggest that a 20-fold reduction in energy could be achieved compared with conventional high-energy defibrillation and that the unpinning method may be more effective than ATP for terminating stable, pinned reentry in this experimental model.

Electroporation of the heart

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells resulting in resynchronization of electrical activity in the heart. If shock-induced changes in transmembrane potential are large enough, they can cause transient tissue damage due to electroporation. In this review evidence is presented that (a) electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field, and (b) electroporation can affect the outcome of defibrillation therapy; being both pro- and anti-arrhythmic.

Effectiveness and safety of internal rectilinear biphasic versus monophasic defibrillation in patients undergoing cardiac surgery

BACKGROUND

Recently it has been shown that biphasic external shocks are more effective in the treatment of ventricular fibrillation (VF) compared with monophasic external shocks in terms of number of defibrillation attempts and maximal energy used for termination of VF. Biphasic defibrillators apply different biphasic impulse forms, depending on technology. To the authors' knowledge, there are no existing data concerning the effects of rectilinear biphasic internal shocks in patients undergoing cardiac surgery. The purpose of this study was to compare monophasic with rectilinear biphasic internal shock waveforms for termination of VF in patients undergoing cardiac surgery.

METHODS

One hundred thirty-four patients scheduled for elective cardiac surgery were prospectively randomized either to monophasic (group A) or biphasic (group B) internal defibrillation. Defibrillation was started with 7 J and increased stepwise to 30 J in each group until successful termination of VF after aortic declamping. The number of defibrillations, as well as the cumulative and maximal energy for termination of VF, were determined. Preoperatively, intraoperatively, and postoperatively troponin T, total creatine phosphokinase (CPK), and CPK- MB isoenzymes were measured.

RESULTS

In 64 patients (47%) VF occurred. The groups consisted of 32 patients each. The number of defibrillations (1.3 +/- 0.6 v 1.9+/- 1.2; p = 0.013), maximal energy per patient (7.9 +/- 2.5 v 11.6 +/- 7.32; p = 0.006), and cumulative energy (10.1 +/-6.1 v 21.3 +/- 24.1; p = 0.016) for successful termination of VF were significantly reduced in group B. Troponin T, CPK, and CPK-MB did not differ between groups.

CONCLUSIONS

Results of this study indicate that rectilinear biphasic internal defibrillation is more effective in the treatment of VF during cardiac surgery than is monophasic defibrillation. However, no significant difference in myocardial damage could be detected between groups.

The resuscitation outcome: revisit the story of the stony heart

Postresuscitation syndrome is a state of myocardial dysfunction after the restoration of circulation by successful resuscitation. Despite several advances in the field of resuscitation, the management of out-of-hospital cardiac arrest is still suboptimal. The high fatality rate shortly after successful resuscitation is mainly related to postresuscitation myocardial dysfunction. Postresuscitation myocardial stunning is reversible, while stony heart is irreversible due to prolonged unsuccessful resuscitation. This article reviews most of the published articles concerning the causes, mechanism, pathophysiology, and the updated trials for management of postresuscitation myocardial dysfunction. Further studies are warranted to highlight postresuscitation disease and its hemodynamic sequences and then to intervene according to the different phases of cardiac arrest. By modifying the conventional modalities of resuscitation together with new promising agents, the rescuers will be able to salvage the jeopardized postresuscitation myocardium and prevent its progression to the dismal stony heart. Community awareness and staff education are crucial to shorten resuscitation time and improve short-term and long-term outcomes. There is an urgent need to revise the guidelines for cardiopulmonary resuscitation in community setting, but how? It is a matter of where and when it is of enough value to be efficacious and cost-effective.

Effect of transthoracic shocks on left ventricular function

Although defibrillating shocks are thought to depress ventricular function transiently, the independent effects of high strength shocks (without the metabolic sequelae of pre-shock fibrillation) have not been assessed systematically in humans. Therefore, we delivered three consecutive synchronized monophasic transthoracic shocks (200, 200 and 360 J) at 60s intervals during sinus rhythm and evaluated the effect on left ventricular chamber size and function as determined by transesophageal echocardiography in 11 patients (mean age 67+/-8 years, 9M/2F) with depressed left ventricular function (left ventricular ejection fraction: 14-37%). The shocks did not alter hemodynamics consistently. On average, the shocks did not alter stroke volume, cardiac output, left ventricular ejection fraction or regional wall thickening (all p>0.05 versus baseline). This effect was highly variable and 36% of patients experienced a >25% reduction in cardiac output by the final shock. There was a tendency for regional wall thickening to worsen in the best baseline sextant with an offsetting significant increase in thickening in the worst baseline sextant (p=0.05). Thus, repetitive defibrillation strength transthoracic shocks do not impair left ventricular function consistently in patients with cardiomyopathy. However, the effect is widely variable and potentially important depression of left ventricular function does occur in some patients.

Defibrillation waveform and post-shock rhythm in out-of-hospital ventricular fibrillation cardiac arrest

BACKGROUND

The importance of the defibrillation waveform on the evolving post-shock cardiac rhythm is uncertain. The primary objective of this study was to evaluate cardiac rhythms following the first defibrillation shock, comparing biphasic truncated exponential (BTE), monophasic damped sinusoidal (MDS), and monophasic truncated exponential (MTE) waveforms in patients experiencing out-of-hospital ventricular fibrillation cardiac arrest (OHCA).

METHODS

We reviewed the automated external defibrillator (AED) and emergency medical services (EMS) records of 366 patients who suffered OHCA and were treated with defibrillation shocks by first-tier emergency responders between 1 January 1999 and 31 August 2002 in King County, Washington. The post first shock rhythms were determined at 5, 10, 20, 30, and 60 s and compared according to defibrillation waveform.

RESULTS

The MDS and BTE waveforms were associated with significantly higher frequency of defibrillation than the MTE waveform, though only the BTE association persisted to 30 and 60 s. No difference in defibrillation rates was detected between MDS and BTE waveforms. By 60 s, an organized rhythm was present in a greater proportion for BTE (40.0%) compared with MDS (25.4%, P=0.01) or MTE (26.5%, P=0.07).

CONCLUSION

In this retrospective cohort investigation, MDS and BTE waveforms had higher first shock defibrillation rates than the MTE waveform, while patients treated with the BTE waveform were more likely to develop an organized rhythm within 60 s of the initial shock. The results of this investigation, however, do not provide evidence that these surrogate advantages are important for improving survival. Additional investigation is needed to improve the understanding of the role of waveform and its potential interaction with other clinical factors in order to optimize survival in OHCA.

Fixed-energy biphasic waveform defibrillation in a pediatric model of cardiac arrest and resuscitation

OBJECTIVE

For adults, 150-J fixed-energy, impedance-compensating biphasic truncated exponential (ICBTE) shocks are now effectively used in automated defibrillators. However, the high energy levels delivered by adult automated defibrillators preclude their use for pediatric patients. Accordingly, we investigated a method by which adult automated defibrillators may be adapted to deliver a 50-J ICBTE shock for pediatric defibrillation.

DESIGN

Prospective, randomized study.

SETTING

A university-affiliated research institution.

SUBJECT

Domestic piglets.

INTERVENTIONS

We initially investigated four groups of anesthetized mechanically ventilated piglets weighing 3.8, 7.5, 15, and 25 kg. Ventricular fibrillation was induced with an AC current delivered to the right ventricular endocardium. After 7 mins of untreated ventricular fibrillation, a conventional manual defibrillator was used to deliver up to three 50-J ICBTE shocks. If ventricular fibrillation was not reversed, a 1-min interval of precordial compression preceded a second sequence of up to three shocks. The protocol was repeated until spontaneous circulation was restored, or for a total of 15 mins. In a second set of experiments, we evaluated a 150-J biphasic adult automated defibrillator that was operated in conjunction with energy-reducing electrodes such as to deliver 50-J shocks. The same resuscitation protocol was then exercised on piglets weighing 3.7, 13.5, and 24.2 kg.

MEASUREMENTS AND MAIN RESULTS

All animals were successfully resuscitated. Postresuscitation hemodynamic and myocardial function quickly returned to baseline values in both experimental groups, and all animals survived.

CONCLUSION

An adaptation of a 150-J biphasic adult automated defibrillator in which energy-reducing electrodes delivered 50-J shocks successfully resuscitated animals ranging from 3.7 to 25 kg without compromise of postresuscitation myocardial function or survival.

New concepts in transthoracic defibrillation

The transition of biphasic waveforms from ICDs to external defibrillators constitutes a significant technological advances for transthoracic defibrillation. Impedance compensation has enabled the delivery of defibrillating current adapted to each patient and each shock in the same patient. Optimally designed biphasic waveforms have been shown clinically to have greater efficacy in the termination of VF when compared with monophasic waveforms, and because peak current delivery is less, these waveforms are likely to be less injurious to myocardial function. Advances in the understanding of the mechanisms of fibrillation and defibrillation have identified the electrophysiologic events that initiate and sustain VF and the effects of defibrillation shocks on those events. Definition of the role of VEP and postshock excitation has clarified the mechanisms by which shocks can either fail or succeed. The ability of the second phase of optimal biphasic waveform shocks to exploit recruited sodium channels in negatively polarized areas and thus induce rapid propagation of postshock excitation assures uniform depolarization and prevention of re-entry. This appears to be the major mechanism of greater efficacy of biphasic waveforms. It seems certain that continuing investigation of virtual electrodes will enhance our understanding of defibrillation and optimal waveforms. At the same time, much more needs to be known regarding translation of these experimental observations to mechanisms of defibrillation in human hearts with long-standing underlying structural heart disease, which often arises of multiple factors. This represents a major challenge in defibrillation research.

Defibrillation threshold and cardiac responses using an external biphasic defibrillator with pediatric and adult adhesive patches in pediatric-sized piglets

Before recommendations for using an automatic external defibrillator on pediatric patients can be made, a protocol for the energy of a biphasic waveform energy dosing needs to be determined that will allow ventricular defibrillation of 8 year olds while causing only a minimal amount of cardiac damage to infants. Pediatric- and adult-sized electrode patches were alternately applied to 10 isoflurane-anesthetized piglets weighing 3.8-20.1 kg to approximate the body weights of newborns to children < 8 years old. The defibrillation threshold (DFT) was determined for biphasic truncated exponential waveform shocks. Additional shocks, varying from the DFT to 360 Joules (J), were delivered during sinus rhythm or following 30 s of ventricular fibrillation (VF). The DFT was 2.4+/-0.81 and 2.1+/-0.65 J/kg for pediatric and adult patches, respectively (P = N.S.). The change in left ventricular (LV) dP/dt from baseline as a function of shock strength was significantly different at 1 and 10 s after shocks of increasing energy that were delivered in sinus rhythm, and 1, 10, 20, and 30 s after defibrillation shocks. There was no significant difference in LV dP/dt with increasing shock energy at 60 s with either patch size. The time to return of sinus rhythm, ST-segment deviation, and cardiac output were also not significantly different from baseline 60 s following shocks of up to 360 J delivered during sinus rhythm or VF with either patch. The same amount of energy delivered with a biphasic external defibrillator successfully defibrillated VF whether adult or pediatric patches were used. Cardiac rhythm and hemodynamic variables were unaltered at 60 s after shocks delivered at energies of up to 360 J. These data suggest that there is a substantial safety margin above a DFT strength shock for this biphasic waveform in piglets.

Cariporide for pharmacologic defibrillation after prolonged cardiac arrest

BACKGROUND

We hypothesized that cariporide, a sodium-hydrogen exchange inhibitor, would be as cardioprotective during the global myocardial ischemia of prolonged cardiac arrest as it is in settings of coronary occlusion.

METHODS AND RESULTS

Fifteen Sprague-Dawley rats were randomized to receive bolus injections of cariporide or placebo in a dose of 3 mgxkg(-1) into the right atrium either 5 minutes before, or at 8 minutes after, onset of ventricular fibrillation. Ventricular fibrillation was electrically induced and untreated for 8 minutes. Precordial compression, together with mechanical ventilation, was then started and continued for an interval of 8 minutes prior to attempted resuscitation. All but one placebo-treated animal were successfully resuscitated. Spontaneous defibrillation with restoration of circulation was observed in both cariporide-pretreatment and post-treatment groups but in none of the placebo-treated animals. Postresuscitation cardiac index, end-tidal CO(2), mean aortic pressure, left ventricular systolic pressure, left ventricular end-diastolic pressure, and left ventricular contractile and lusitropic functions (dP/dt(40), and -dP/dt) were significantly less impaired after cariporide, especially in the pretreated group, compared to electrically defibrillated controls. Postresuscitation ventricular premature beats were significantly reduced after cariporide. The duration of post-resuscitation survival was significantly increased in animals pretreated with cariporide.

CONCLUSIONS

Cariporide, when administered prior to and during cardiac arrest, improved both the success of resuscitation and postresuscitation myocardial function.

Predicting the success of defibrillation by electrocardiographic analysis

BACKGROUND

We investigated an electrocardiographic signal analysis technique for predicting whether an electrical shock would reverse ventricular fibrillation (VF) in an effort to minimize the damaging effects of repetitive shocks during CPR.

METHODS AND RESULTS

An established model of CPR was utilized. VF was electrically induced in anesthetized 40 kg domestic pigs. Defibrillation was attempted after either 4 or 7 min of untreated VF. Failing to reverse VF, a 1 min interval of precordial compression and mechanical ventilation preceded each subsequent defibrillation attempt. The amplitude frequency spectrum of digitally filtered VF wavelets was computed with Fourier analysis during uninterrupted precordial compression from conventional right infraclavicular and left apical electrodes. Of a total of 34 electrical defibrillation attempts, 24 animals were restored to spontaneous circulation (ROSC). An amplitude spectrum analysis (AMSA) value of 21 mV Hz had a negative predictive value of 0.96 and a positive predictive value of 0.78.

CONCLUSIONS

AMSA predicted when an electrical shock failed to restore spontaneous circulation during CPR with a high negative predictive value. This method potentially fulfills the need for minimizing ineffective defibrillation attempts and their attendant adverse effects on the myocardium.

Outcome of out-of-hospital postcountershock asystole and pulseless electrical activity versus primary asystole and pulseless electrical activity

OBJECTIVE

In the prehospital setting, countershock terminates ventricular fibrillation (VF) in about 80% of cases. However, countershock is most commonly followed by asystole or pulseless electrical activity (PEA). The consequences of such a countershock outcome have not been well studied. The purpose of this investigation was to compare the outcome of prehospital VF victims shocked into asystole or PEA with that of patients whose first documented rhythm was asystole or PEA (primary asystole or PEA).

DESIGN

Observational, retrospective study conducted over 5 yrs (1995-1999).

SETTING

A municipal hospital with a catchment area of >200,000.

PATIENTS

Consecutive adult patients with out-of-hospital nontraumatic cardiopulmonary arrest of cardiac origin. Patients found in VF who developed asystole or PEA after countershocks (group 1) and patients found in asystole or PEA (primary asystole or PEA) (group 2) were included if the reported downtime was <10 min.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Study end points included restoration of circulation (defined as a pulse for any duration), survival to hospital admission, and survival to hospital discharge. Ratios were determined, 95% confidence intervals were calculated, and observed differences were compared. For group 1 patients (n = 101), 61% of patients had a bystander-witnessed collapse and 34% received bystander cardiopulmonary resuscitation. For group 2 patients (n = 140), collapse was bystander witnessed in 71% and 45% received bystander cardiopulmonary resuscitation. These differences were not statistically significant. Restoration of circulation was significantly more frequent in group 2 than group 1 (42% vs. 16%, p <.001) as was survival to hospital admission (36% vs. 11%, p =.001). Survival to hospital discharge was greater in group 2 patients, but the difference failed to achieve statistical significance (10% vs. 3%, p =.062).

CONCLUSIONS

Countershock of prolonged VF followed by a nonperfusing rhythm has a worse prognosis than primary asystole or PEA and may be related to myocardial electrical injury.

Optimizing timing of ventricular defibrillation

OBJECTIVE

Our intent was to evolve a prognosticator that would predict the likelihood that an electrical shock would restore a perfusing rhythm. Such a prognosticator was to be based on conventional electrocardiographic signals but without constraints caused by artifacts resulting from precordial compression. The adverse effects of "hands off" intervals for rhythm analyses would therefore be minimized. Such a prognosticator was further intended to reduce the number of electrical shocks and the total energy delivered and thereby minimize postresuscitation myocardial dysfunction.

DESIGN

Observational study.

SUBJECTS

Medical research laboratory of a university-affiliated research and educational institute.

SUBJECTS

Domestic pigs.

INTERVENTIONS

Ventricular fibrillation was induced in an established porcine model of cardiac arrest. Recordings of scalar lead 2 over the frequency range of 4-48 Hz were utilized. The area under the curve representing the amplitude and frequency was defined as the amplitude spectrum area (AMSA).

MEASUREMENTS AND MAIN RESULTS

A derivation group of 55 animals yielded a threshold value of AMSA that uniformly predicted successful resuscitation. A separate group of 10 animals, a validation group, confirmed that an AMSA value of 21 mV.Hz predicted restoration of perfusing rhythm after 7 of 8 electrical shocks and failure of electrical conversion in 21 of 23 electrical shocks, yielding sensitivity and specificity of about 90%. The negative predictive value of AMSA was 95% and statistically equivalent to that of coronary perfusion pressure, mean amplitude, and median frequency. The positive predictive value that would prompt continuation of cardiopulmonary resuscitation without interruption for an unsuccessful defibrillation attempt was greatly improved with AMSA (78%) as compared with coronary perfusion pressure (42%), mean amplitude (32%), and median frequency (29%).

CONCLUSION

AMSA has the potential for guiding more optimal timing of defibrillation without adverse interruption of cardiopulmonary resuscitation or the delivery of unsuccessful high energy electrical shocks that contribute to postresuscitation myocardial injury.

A comparison of biphasic and monophasic waveform defibrillation after prolonged ventricular fibrillation

STUDY OBJECTIVE

To compare the effects of biphasic defibrillation waveforms and conventional monophasic defibrillation waveforms on the success of initial defibrillation, postresuscitation myocardial function, and duration of survival after prolonged duration of untreated ventricular fibrillation (VF), including the effects of epinephrine.

DESIGN

Prospective, randomized, animal study.

SETTING

Animal laboratory and university-affiliated research and educational institute.

PARTICIPANTS

Domestic pigs.

INTERVENTIONS

VF was induced in 20 anesthetized domestic pigs receiving mechanical ventilation. After 10 min of untreated VF, the animals were randomized. Defibrillation was attempted with up to three 150-J biphasic waveform shocks or a conventional sequence of 200-J, 300-J, and 360-J monophasic waveform shocks. When reversal of VF was unsuccessful, precordial compression was performed for 1 min, with or without administration of epinephrine. The protocol was repeated until spontaneous circulation was restored or for a maximum of 15 min.

MEASUREMENTS AND RESULTS

No significant differences in the success of initial resuscitation or in the duration of survival were observed. However, significantly less impairment of myocardial function followed biphasic shocks. Administration of epinephrine reduced the total electrical energy required for successful resuscitation with both biphasic and monophasic waveform shocks.

CONCLUSIONS

Lower-energy biphasic waveform shocks were as effective as conventional higher-energy monophasic waveform shocks for restoration of spontaneous circulation after 10 min of untreated VF. Significantly better postresuscitation myocardial function was observed after biphasic waveform defibrillation. Administration of epinephrine after prolonged cardiac arrest decreased the total energy required for successful resuscitation.

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.

Patient outcomes following defibrillation with a low energy biphasic truncated exponential waveform in out-of-hospital cardiac arrest

PRIMARY OBJECTIVE

To determine the outcome of patients with out-of-hospital cardiac arrest and ventricular fibrillation as the presenting rhythm while using automated external defibrillators (AEDs) that delivered non-escalating, impedance-compensated low-energy (150 J) shocks.

MATERIALS AND METHODS

AEDs delivering low-energy biphasic truncated exponential (BTE) shocks were employed in an emergency medical services (EMS) system in which first-arriving personnel - police, firefighters or paramedics - delivered the initial shocks. Patients were classified according to their response to shocks: restoration of sustained spontaneous circulation (ROSC) without need for epinephrine and other advanced life support (ALS) interventions; and ALS, those requiring epinephrine in all instances. The primary end-point was neurologically-intact discharge survival. Secondary end-points were ROSC with shocks only and the call-to-shock time interval.

RESULTS

Of 42 patients with VF arrest treated with BTE shocks, 35 were bystander-witnessed. Of these 35, 14 (38%) regained a sustained ROSC on-scene with shocks only, needing no epinephrine for ROSC. All 14 survived to discharge home. Of the remaining 21 patients needing ALS intervention, only two (9.5%) survived to discharge. Overall, 16/35 patients (46%) survived to discharge home, an outcome comparable to our experience with patients treated with escalating high-energy monophasic waveform shocks.

CONCLUSIONS

Low-energy (150 J) non-escalating biphasic truncated exponential waveform shocks terminate VF in out-of-hospital cardiac arrest with high efficacy; patient outcome is comparable with that observed with escalating high-energy monophasic shocks. Low-energy shocks, in addition to high efficacy, may confer the advantage of less shock-induced myocardial dysfunction, though this will be difficult to define in the clinical circumstance of long-duration VF provoked by a pre-existing diseased myocardial substrate.

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.

Transvenous biventricular defibrillation

The recent success of biventricular pacing with transvenously implantable left ventricular leads suggests that left ventricular leads may be useful for other modes of therapy. Animal studies showed small leads inserted into a left ventricular vein dramatically reduced defibrillation strength requirements. This article describes a human investigation of the feasibility of biventricular defibrillation. Fifty-one patients undergoing implantable cardioverter defibrillator (ICD) implantation were enrolled. After insertion of a standard ICD lead, a prototype over-the-wire left ventricular defibrillation lead was inserted through the coronary sinus and into a vein on the left ventricle. Lead insertion was guided by retrograde venography. The left ventricular lead's location was randomized to the anterior or posterior vein. Randomized, paired defibrillation threshold (DFT) testing was performed to compare a standard ICD shock configuration (Control: right ventricle- --> superior vena cava+ + CAN+) to 1 of 3 biventricular shock configurations. In the anterior vein, the left ventricular lead was tested with either a single biphasic shock from right ventricle + left ventricle- --> superior vena cava+ + CAN+ or a dual biphasic shock. In the posterior vein, the left ventricular lead was tested with a dual biphasic shock. Dual shocks consisted of a 40% tilt biphasic shock from right ventricle- --> superior vena cava+ + CAN+ followed by another 40% tilt biphasic shock from left ventricle- --> superior vena cava+ + CAN+, delivered from a single 225 microF capacitance. Left ventricular lead positioning was successful in 41 of 46 patients (89%). Mean left ventricular lead insertion time was 17 +/- 17 minutes and 13 +/- 15 minutes for anterior and posterior locations, respectively. Mean DFTs were not statistically lower for the left ventricular shock configurations, but retrospective analysis showed a well-defined region of the posterolateral left ventricle where consistent DFT reduction was achieved with dual shocks (14.0 +/- 2.7 J vs 7.8 +/- 0.9 J; n = 5; p = 0.04). There were no adverse events requiring intervention due to the use of the left ventricular lead. Biventricular defibrillation is feasible and safe under the conditions used in this study. Additional studies are needed to verify whether dual shocks with posterolateral left ventricular lead positions consistently reduce DFTs.

Low-energy biphasic waveform defibrillation reduces the severity of postresuscitation myocardial dysfunction

Both clinical and experimental studies have demonstrated substantial impairment of ventricular function after resuscitation from cardiac arrest. Indeed, postresuscitation myocardial dysfunction has been implicated as a potentially important mechanism, accounting for fatal outcomes after successful resuscitation in 70% of victims within the first 72 hrs. Recent experimental studies implicated the total electrical energy delivered during defibrillation as an important correlate with the severity of postresuscitation myocardial dysfunction and postresuscitation survival. This prompted us to investigate the option of using lower electrical energy biphasic waveform defibrillation. We compared the effects of low-energy biphasic waveform defibrillation with conventional monophasic waveform defibrillation after a short (4 mins), intermediate (7 mins), or prolonged (10 mins) interval of untreated ventricular fibrillation. Biphasic waveform defibrillation with a fixed energy of 150 joules proved to be as effective as conventional monophasic damped sine waveform defibrillation for restoration of spontaneous circulation, with significantly lower delivered energy. This was associated with significantly less severity of postresuscitation myocardial dysfunction. The low-energy biphasic waveform defibrillation is, therefore, likely to be the future direction of transthoracic defibrillation in settings of cardiopulmonary resuscitation.

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.

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.

The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function

OBJECTIVES

The purpose of this study was to compare the effects of biphasic defibrillation waveforms and conventional monophasic defibrillation waveforms on the success of initial defibrillation, postresuscitation myocardial function and duration of survival after prolonged ventricular fibrillation (VF).

BACKGROUND

We have recently demonstrated that the severity of postresuscitation myocardial dysfunction was closely related to the magnitude of the electrical energy of the delivered defibrillation shock. In the present study, the effects of fixed 150-J low-energy biphasic waveform shocks were compared with conventional monophasic waveform shocks after prolonged VF.

METHODS

Twenty anesthetized, mechanically ventilated domestic pigs were investigated. VF was induced with an AC current delivered to the right ventricular endocardium. After either 4 or 7 min of untreated ventricular fibrillation (VF), the animals were randomized for attempted defibrillation with up to three 150-J biphasic waveform shocks or conventional sequence of 200-, 300- or 360-J monophasic waveform shocks. If VF was not reversed, a 1-min interval of precordial compression preceded a second sequence of up to three shocks. The protocol was repeated until spontaneous circulation was restored or for a total of 15 min.

RESULTS

Monophasic waveform defibrillation after 4 or 7 min of untreated VF resuscitated eight of 10 pigs. All 10 pigs treated with biphasic waveform defibrillation were successfully resuscitated. Transesophageal echo-Doppler, arterial pressure and heart rate measurements demonstrated significantly less impairment of cardiovascular function after biphasic defibrillation.

CONCLUSIONS

Lower-energy biphasic waveform shocks were as effective as conventional higher energy monophasic waveform shocks for restoration of spontaneous circulation after 4 and 7 min of untreated VF. Significantly better postresuscitation myocardial function was observed after biphasic waveform defibrillation.

Electrocardiographic evaluation of defibrillation shocks delivered to out-of-hospital sudden cardiac arrest patients

OBJECTIVE

Following out-of-hospital defibrillation attempts, electrocardiographic instability challenges accurate assessment of defibrillation efficacy and post-shock rhythm. Presently, there is no precise definition of defibrillation efficacy in the out-of-hospital setting that is consistently used. The objective of this study was to characterize out-of-hospital cardiac arrest rhythms following low-energy biphasic and high-energy monophasic shocks in order to precisely define defibrillation efficacy and establish uniform criteria for the evaluation of shock performance.

METHODS

Automatic external defibrillators (AEDs) delivering 150 J impedance-compensating biphasic or 200-360 J monophasic damped sine waveform shocks were observed in a combined police and paramedic program. ECGs from 29 biphasic patients and 87 monophasic patients were classified as organized, asystole or VF at post-shock times of 3, 5, 10, 20 and 60 s.

RESULTS

Post-shock time (P<0.0001) and shock waveform type (P = 0.02) affected the classification of post-shock rhythm. At each analysis time, there were more patients in VF following high-energy monophasic shocks than following 150 J biphasic shocks (P<0.0001). The percentage of patients in VF increased with post-shock time. The rate of VF recurrence was not a function of shock type, indicating that refibrillation is largely a function of the patient's underlying cardiac disease.

CONCLUSION

Defibrillation should uniformly be defined as termination of VF for a minimum of 5-s after shock delivery. Rhythms should be reported at 5-s after shock delivery to assess early effects of the defibrillation shock and at 60-s after shock delivery to assess the interaction of the defibrillation therapy and factors such as post-shock myocardial dysfunction and the patient's underlying cardiac disease.

Sudden death in implantable cardioverter-defibrillator recipients: clinical context, arrhythmic events and device responses

OBJECTIVES

We sought to investigate the nature of terminal events and potential contributory clinical and nonclinical (e.g., device-related) factors associated with sudden death (SD) in recipients of an implantable cardioverter-defibrillator (ICD).

BACKGROUND

The ICD is very effective in terminating ventricular tachycardia (VT) or ventricular fibrillation (VF), but protection against SD is not absolute. Little is known about the nature and potential causes of SD in patients with ICDs.

METHODS

We analyzed 25 cases of out-of-hospital SD among patients enrolled in the clinical investigation of the Cadence Tiered-Therapy Defibrillator System.

RESULTS

All patients (24 men and 1 woman, mean age 62+/-10 years) received epicardial lead systems. The majority (92%) had coronary artery disease and a previous myocardial infarction (MI), with a mean left ventricular ejection fraction 0.25+/-0.07. At device implantation, the mean defibrillation threshold was 13+/-5 J. Sudden death occurred 13+/-11 months later. Twenty patients (80%) had received appropriate ICD therapies before death, and 18 (72%) were receiving > or = 1 antiarrhythmic drugs at the time of death. Sudden death was tachyarrhythmia-associated in 16 patients (64%), non-tachyarrhythmia-associated in 7 (28%) and indeterminate in 2 (8%). In the 16 patients with tachyarrhythmia-associated SD, the overall first therapy success rate in tachycardia and fibrillation zones was 60% and 67%, respectively. However, despite protracted therapies (> or = 2 shocks) in 7 (66%) of 12 patients who received fibrillation therapies, the final tachyarrhythmic episode was ultimately terminated by the ICD in 15 (94%) of the 16 patients, whereas 1 patient died after multiple (initially successful) internal and external shocks for intractable VT/VF during exercise. In 10 patients (40%) one or more, primarily clinical, factors potentially contributory to SD were identified: heart failure (n=8), angina (n=2), hypokalemia (n=1), adverse antiarrhythmic drug treatment (n=1) and acute MI (n=1). An additional 10 patients (40%) had experienced an increase in frequency of ICD shocks within 3 months of SD. Appropriate battery voltages and normal circuitry function were found in all devices interrogated and analyzed after death.

CONCLUSIONS

In this select group of patients receiving a third-generation ICD, SD was associated with VT or VF events in nearly two-thirds of patients, and death occurred despite ultimately successful, although often protracted, device therapies. These observations, along with evidence of recent worsening clinical status, suggest acute cardiac mechanical dysfunction as a frequent terminal factor. In recipients with ICDs, SD directly attributable to device failure seems to be rare.