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PubMed1999

Energy levels for defibrillation: what is of real clinical importance?

Today, transthoracic and intracardiac defibrillation offer a well-accepted and widely used form of therapy for patients with life-threatening ventricular arrhythmias. Despite the wide clinical use of defibrillators, the mechanisms by which an electrical shock halts fibrillation are still not completely understood. During a shock, different amounts of current flow through the different parts of the heart and the current distribution is highly uneven. This current distribution is affected by changes in the shock potential gradient through the heart, changes in fiber orientation, and changes in myocardial conductivity caused by connective tissue barriers. It would be ideal if the potential gradient distribution throughout the ventricles could be measured directly for each individual patient during defibrillator implantation and follow-up and the shock strength could be programmed based on this measurement, but so far this is not possible. A more feasible approach is to determine, by trial and error, the magnitude of the shock strength delivered through the defibrillation electrodes for successful defibrillation. There is no distinct threshold value above which all shocks succeed and below which all shocks fail to defibrillate. Rather, increasing shock strength increases the likelihood the shock will succeed. Therefore, instead of a distinct defibrillation threshold, a probability of success curve exists. However, increasing the shock strength above an optimal range can actually decrease the success rate for defibrillation. One possible explanation is that the high voltage gradients caused by such large shocks damage cells and result in postshock arrhythmias that may reinitiate fibrillation. Another problem that can affect the probability of defibrillation success for a particular programmed energy setting is that the shock strength required for defibrillation may increase over time due to (1) the growth of fibrotic tissue around the defibrillation electrode; (2) migration of the lead; (3) acute ischemia; or (4) other changes in the underlying cardiac disease (e.g., worsening of heart failure). Such possible increases in the defibrillation shock strength requirement should be compensated for before they occur by adding a margin of safety to the shock strength needed for effective defibrillation. When programming an implantable defibrillator, it is important to keep in mind that the defibrillation shock should be (1) strong enough to defibrillate at least 98% of the time with the first shock; (2) weak enough not to cause severe post-shock arrhythmias or reinitiation of fibrillation; but (3) strong enough to compensate for changes of defibrillation energy requirements over time. This usually can be accomplished by setting the defibrillator 7-10 J higher than the defibrillation threshold determined by a standard step-down protocol.

Defibrillators, Implantable↗
PubMed1994

Effect of biphasic waveform pulse on endocardial defibrillation efficacy in humans.

Several clinical studies have proved increased defibrillation efficacy for implantable cardioverter defibrillators with biphasic pulse waveforms compared to monophasic pulse waveforms. This difference in defibrillation efficacy depends on the type of defibrillation lead system used. The influence of biphasic defibrillation pulse waveforms on the defibrillation efficacy of purely endocardial defibrillation lead systems has not yet been sufficiently examined, we, therefore studied 30 consecutive patients with drug refractory ventricular tachyarrhythmias during the implantation of a cardioverter defibrillator. After implanting an endocardial "integrated" sensing/defibrillation lead we performed a prospective randomized comparison of the defibrillation efficacy of monophasic and biphasic defibrillation waveform pulses. For endocardial defibrillation with the biphasic waveform the mean defibrillation threshold was 12.5 +/- 4.9 joules and for the monophasic waveform 22.2 +/- 5.6 joules (P < 0.0001). There was a decrease in the required defibrillation energy of biphasic defibrillation in 29/30 patients. Thus considering purely endocardial defibrillation a statistically significant and clinically relevant increase in defibrillation efficacy can be demonstrated for biphasic defibrillation waveform pulses.

Defibrillators, Implantable↗
PubMed1998

Long-term evaluation of the ventricular defibrillation energy requirement.

INTRODUCTION: Defibrillation energy requirements in patients with nonthoracotomy defibrillators may increase within several months after implantation. However, the stability of the defibrillation energy requirement beyond 1 year has not been reported. The purpose of this study was to characterize the defibrillation energy requirement during 2 years of clinical follow-up. METHODS AND RESULTS: Thirty-one consecutive patients with a biphasic nonthoracotomy defibrillation system underwent defibrillation energy requirement testing using a step-down technique (20, 15, 12, 10, 8, 6, 5, 4, 3, 2, and 1 J) during defibrillator implantation, and then 24 hours, 2 months, 1 year, and 2 years after implantation. The mean defibrillation energy requirement during these evaluations was 10.9+/-5.5 J, 12.3+/-7.3 J, 11.7+/-5.6 J, 10.2+/-4.0 J, and 11.7+/-7.4 J, respectively (P = 0.4). The defibrillation energy requirement was noted to have increased by 10 J or more after 2 years of follow-up in five patients. In one of these patients, the defibrillation energy requirement was no longer associated with an adequate safety margin, necessitating revision of the defibrillation system. There were no identifiable clinical characteristics that distinguished patients who did and did not develop a 10-J or more increase in the defibrillation energy requirement. CONCLUSION: The mean defibrillation energy requirement does not change significantly after 2 years of biphasic nonthoracotomy defibrillator system implantation. However, approximately 15% of patients develop a 10-J or greater elevation in the defibrillation energy requirement, and 3% may require a defibrillation system revision. Therefore, a yearly evaluation of the defibrillation energy requirement may be appropriate.

Defibrillators, Implantable↗
PubMed2006

Potential benefit of transesophageal defibrillation: an experimental evaluation.

INTRODUCTION: Because of the proximity of the esophagus to the heart, transesophageal defibrillation might increase defibrillation success. We assessed the defibrillation threshold (DFT) of transesophageal defibrillation compared with standard transthoracic defibrillation. METHODS: Defibrillation success and DFTs were determined in 22 female pigs with high (68+/-4 kg, n=12) or low body weight (39+/-1 kg, n=10). After induction of ventricular fibrillation, biphasic shocks were delivered between two cutaneous patch electrodes (sternal and apical position) or between an esophageal and two cutaneous patch electrodes in a sternal and apical position. The esophageal electrode was integrated into a latex sheath covering a standard transesophageal echocardiography probe. RESULTS: In 5 of 12 pigs with high body weight, external defibrillation failed despite 3 consecutive 200-J shocks, whereas subsequent transesophageal defibrillation was successful with the first shock. In the remaining 7 pigs, a more than 50% reduction in DFT was obtained with transesophageal defibrillation compared with standard biphasic external defibrillation (67+/-27 vs 164+/-23 J, P<.001). Pigs with lower body weight were successfully defibrillated by both transthoracic and transesophageal shocks. The DFT in pigs with low body weight was significantly lower using transesophageal defibrillation compared with transthoracic shocks (65+/-15 vs 99+/-38 J, P<.05). CONCLUSIONS: In this animal model, nonresponders to standard external defibrillation could successfully be defibrillated via an esophageal-cutaneous electrode configuration. Overall, an almost 50% DFT reduction was achieved by transesophageal defibrillation. Transesophageal defibrillation may provide an additional tool for terminating VF, which is refractory to external defibrillation, eg, in patients with very high body weight.

Animals↗
PubMed1993

Use of multiple patches during implantation of epicardial defibrillator systems.

During implantation of epicardial automatic defibrillator systems, occasional patients have difficulty in obtaining adequate defibrillation thresholds. Of 236 consecutive patients undergoing implantation of epicardial defibrillator systems, 18 patients received a 3-patch (n = 15) or 4-patch (n = 3) defibrillator system. Twelve patients who received a multiple-patch defibrillator system had a best 2-patch defibrillation energy requirement of > or = 30 J; in the remaining 6 patients less stringent clinical criteria were used in the decision to add a third defibrillator patch (defibrillation energy requirement > 18 J in 4 patients, and > 20 J in 2 patients). Technically, multiple-patch systems were made possible with either the use of Y-connectors or defibrillators allowing output to 3 patches. In 3 patients, addition of a third epicardial patch still resulted in a defibrillation energy requirement of > or = 30 J; in these 3 patients, addition of a fourth patch resulted in a defibrillation energy requirement of < or = 20 J. All patients receiving a multiple-patch defibrillator system had a reduction in defibrillation energy requirement, and 12 patients had a reduction in defibrillation energy requirement of > or = 10 J over the best 2-patch defibrillation energy requirement. In the patients who eventually had placement of a multiple-patch system, the best 2-patch defibrillation energy requirement was > 18 J in 4 patients, > 20 J in 2 patients, > or = 30 J in 9 patients, and > 40 J in 3 patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-Arrhythmia Agents↗
PubMed1994

[Initial defibrillation by emergency physicians or by first aid assistants? A prospective, comparative multicenter study in outpatients with ventricular fibrillation].

In a controlled prospective randomized study, defibrillation by emergency medical technicians (EMTs) was compared with the current standard of care in Germany (basic life support by EMTs and defibrillation by emergency physicians only) in order to answer the following questions: 1. Does EMT defibrillation improve the survival rate and long-term prognosis of patients in ventricular fibrillation as compared to the current German standards in resuscitation (basic life support by EMTs and defibrillation by emergency physicians)? 2. Are the prerequisites for the use of semiautomatic defibrillators fulfilled in the emergency medical systems (EMS) of the participating centers? METHODS. The study phase includes randomization of 121 adult patients with witnessed cardiac arrest and ventricular fibrillation (VF) as first ECG rhythm. Prior to the onset of the study, all EMTs of the participating EMS systems were retrained in basic life support (BLS) measures. In each center, randomly assessed EMT-Ds (EMTs trained in Defibrillation) were trained to use semiautomatic defibrillators. With the help of one-line tape recording, the time intervals during resuscitation and treatment steps were evaluated. Successfully resuscitated patients were followed up with the help of the Glasgow Coma Scale and the Pittsburgh Cerebral and Overall Performance Categories. RESULTS. From 1 February 1991 until 28 June 1992, 159 patients with VF were randomized. In 121 cases, collapse was witnessed. 25% (14/57) of the patients receiving defibrillation by EMT-Ds (study group = S) were discharged from the hospital alive. In the control group, 52 patients were defibrillated by emergency physicians, following BLS by EMTs [control group 1 = C1; discharged: 29% (15/52)]. Fifty patients received BLS and advanced cardiac life support (ACLS) by the emergency physicians crews [control group 2 = C2; discharged: 18% (9/20)]. In the study group, the median time interval from collapse of the patient until initiation of BLS measures was 7.7 min, 7 min in C1 and 8 min in C2. ACLS measures were initiated significantly earlier (P < 0.05) in the control groups, as compared to the study group [S: 13 min, C1: 11 min; C2: 10.3 min]. Sixty-seven percent (30/45) of the study patients and 46% (36/76) of the control patients were defibrillated within 12 min. Study patients were defibrillated earlier (P < 0.05) (S: 9.9 min; C1: 12.2 min; C2: 12.75 min); return of spontaneous circulation (ROSC) was achieved earlier (P < 0.05) in the study group [S: 14 min; C1: 19 min; C2: 18.2 min] and the number of patients in the study group requiring no epinephrine during resuscitation was higher (P < 0.01) than in the control groups [S: 35.3% (12/34); C1: 10% (4/40); C2: 10.5% (4/38)]. Furthermore, the total amount of epinephrine [mean (+/- standard error)] administered in the study group [S: 2.35 (+/- 0.49) mg; C1: 6.71 (+/- 0.98) mg; C2: 7.71 (+/- 1.31) mg] was significantly lower (P < 0.05). No significant differences in neurological long-term prognosis were found for the groups investigated. CONCLUSION. Neither the initial survival rate the number of patients discharged alive, nor the neurological long-term prognosis was significantly different for any of the groups investigated. Because of apparent differences in indirect prognostic parameters (time interval until ROSC, number of patients requiring no epinephrine) and because of the fact that the time interval to the first defibrillation was reduced by EMT defibrillation, EMT-Ds may perform defibrillation if: (a) they reach the patient before the emergency physician and (b) if they are trained intensively and supervised continuously. In order to increase the efficiency of defibrillation by EMT-Ds, far-reaching changes in our EMS are mandatory: (a) a reduction in the time interval from collapse until initiation of BCLS measures by intensifying layperson CPR training; (b) an increase in the number of emergency units equipped with semiautomatic defibril

Electric Countershock↗
PubMed1994

Early defibrillation by emergency physicians or emergency medical technicians? A controlled, prospective multi-centre study.

UNLABELLED: In a controlled, prospective multi-centre study, defibrillation by emergency medical technicians (EMTs) was compared with the current standard of care in Germany--defibrillation by emergency physicians (EPs)-in order to answer the following questions: can EMTs in a two-tiered emergency medical services (EMS) system with physicians in the field defibrillate earlier than, and as safely as EPs? Does defibrillation by EMTs (study group) affect survival rate and long-term prognosis of patients in ventricular fibrillation (VF), as compared with the current national standards in resuscitation (basic cardiopulmonary resuscitation (CPR) by EMTs, and defibrillation by physicians: control group? METHODS: Prior to the onset of the study, all EMTs completed retraining in basic life support (BLS). Randomly assessed EMTs were then trained to use semi-automatic defibrillators. With the help of on-line tape recordings, the complete resuscitation sequence was evaluated. Follow-up of the patients was carried out with the help of the Glasgow Coma Scale as well as Pittsburgh Cerebral and Overall Performance Categories. RESULTS: A total of 159 patients with VF were included in the study. In 121 cases, collapse was witnessed. Of the patients receiving defibrillation by EMTs 25% were discharged from hospital alive, compared to 24% of the patients defibrillated by EPs. Of the study patients 67% were defibrillated within 12 min, while the percentage of control patients was 46%. Study patients were defibrillated earlier (P < 0.01), the return of spontaneous circulation (ROSC) was achieved earlier (P < 0.05), and the rate of patients requiring no adrenalin during resuscitation was higher in the study group (P < 0.05). The total amount of adrenalin administered in the study group was lower (P < 0.05). No statistically significant differences were found concerning the neurologic long-term prognosis. CONCLUSIONS: In our study, EMT defibrillation was equally effective as defibrillation by EPs, but failed to improve survival rates or long-term outcome of patients in VF significantly, compared to EP defibrillation. Due to a reduction in the time intervals from collapse to defibrillation and to ROSC, as well as in adrenalin doses, by EMT-defibrillation, EMTs in Germany should defibrillate if they reach a patient prior to an EP, provided they have received continuous medical training and supervision.

Cardiopulmonary Resuscitation↗