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(Circulation. 1997;96:1217-1223.)
© 1997 American Heart Association, Inc.

Articles

Probability of Successful Defibrillation at Multiples of the Defibrillation Energy Requirement in Patients With an Implantable Defibrillator

S. Adam Strickberger, MD; Emile G. Daoud, MD; Theresa Davidson, RNr; Raul Weiss, MD; Frank Bogun, MD; Bradley P. Knight, MD; Marwan Bahu, MD; Rajiva Goyal, MD; K. Ching Man, DO; ; Fred Morady, MD

From the Department of Internal Medicine, University of Michigan Medical Ctr, Ann Arbor, Mich.

Correspondence to S. Adam Strickberger, MD, University of Michigan Medical Center, 1500 E Medical Center Dr, Ann Arbor, MI 48109-0022.

	Abstract

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Background The probability of successful defibrillation has been determined in normal animals but not in patients undergoing defibrillator implantation. Therefore, the purpose of this prospective study was to determine the probability of successful defibrillation in humans on the basis of a step-down defibrillation energy requirement.

Methods and Results Fifty-three consecutive patients underwent five separate inductions of ventricular fibrillation after the defibrillation energy requirement was determined with the use of small decrements and a step-down protocol (20, 15, 12, 10, 8, 6, 5, 4, 3, 2, 1, and 0.8 J). The first shock energy for defibrillation was either 1.0, 1.3, 1.5, 1.7, or 2.0 times the defibrillation energy requirement, and the likelihoods of successful defibrillation were 70±27%, 84±12%, 86±25%, 80±29%, and 88±32%, respectively (P=.03). The frequencies of uniformly successful defibrillation (5 of 5 defibrillation attempts) were 30%, 27%, 60%, 64%, and 73%, respectively (P=.01). Seven patients in whom the defibrillation energy requirement was <4 J had an overall rate of successful defibrillation of 54±20% compared with 86±20% in the remaining 47 patients (P=.002). The likelihood of successful defibrillation at twice the defibrillation energy requirement was 98% in the 46 patients with a defibrillation energy requirement of >4 J and 67% in the 7 patients with a defibrillation energy requirement of <4 J (P=.17). An absolute safety margin of 7 J was associated with a 96% probability of successful defibrillation.

Conclusions The probability of successful defibrillation is 70% at the defibrillation energy requirement. The probability plateaus at 88%, at twice the defibrillation energy requirement. A 96% probability of successful defibrillation is achieved at an absolute safety margin of 7 J, and a 98% success rate is achieved at energies that are twice the defibrillation energy requirement if the defibrillation energy requirement is >4 J. If the defibrillation energy requirement is <4 J, larger multiples of the defibrillation energy requirement are needed to achieve a high probability of successful defibrillation.

Key Words: fibrillation • defibrillation • implantable defibrillator • arrhythmias

	Introduction

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Implantable defibrillators commonly are used to treat lethal ventricular arrhythmias. Defibrillation efficacy is assessed by various methods at the time of device implantation to ensure an adequate defibrillation safety margin.^{1 2 3 4} Successful defibrillation is not associated with a clearly demarcated threshold but is a probability function.^{1 2 3 4} Curves of the probability of successful defibrillation have been determined in animals but never in humans undergoing defibrillator implantation.^{5 6 7 8} Therefore, the purpose of this prospective study was to determine the probability of successful defibrillation in humans on the basis of a step-down defibrillation energy requirement.

	Methods

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Study Design Rationale
This study was designed to determine the probability of successful defibrillation in patients with an implantable defibrillator. Construction of a probability of successful defibrillation curve in experimental studies requires as many as 40 inductions of ventricular fibrillation,² which is not feasible in the clinical setting. Experimental data suggest that a step-down defibrillation energy requirement is associated with 70% successful defibrillation.^{1 8 9 10} In animals, multiples of this measurement have been used to construct a probability of successful defibrillation curve.^{1 6 8} Therefore, in the present study, after determination of the step-down defibrillation energy requirement, patients were assigned to shock strengths for defibrillation that were a multiple of the defibrillation energy requirement, and ventricular fibrillation was induced five times. The multiples were 1.0, 1.3, 1.5, 1.7, and 2.0 times the defibrillation energy requirement. The data were then pooled and a probability of successful defibrillation curve was constructed.

Patient Population
The study population consisted of 53 patients undergoing evaluation of the defibrillation energy requirement before hospital discharge after the implantation of a transvenous implantable defibrillator. Their mean age was 59±13 years, and 36 were men. Thirty-seven patients had coronary artery disease, 9 patients had idiopathic cardiomyopathy, 5 patients had other forms of cardiomyopathy, and 2 patients had no structural heart disease. The mean left ventricular ejection fraction was 0.30±0.12. The presenting symptom was cardiac arrest in 19 patients, and the remaining patients presented either with syncope or ventricular tachycardia. All patients underwent baseline electrophysiological testing.

Defibrillator System and Implantation
The patients provided written informed consent under a protocol approved by the Human Research Committee at the University of Michigan. All patients came to the operating room in a postabsorptive state. Therapy with antiarrhythmic medications was discontinued at least 5 half-lives before device implantation except in 14 patients in whom amiodarone therapy had been ineffective.

A transvenous defibrillation lead with a distal electrode of 295 mm² and a proximal electrode of 617 mm², separated by a distance of 11.5 cm, was used in this study (Cardiac Pacemakers, Inc, St Paul, Minn, Endotak model 0074 or 125). The defibrillation lead was positioned in the right ventricular apex through the subclavian vein under fluoroscopic guidance. The distal shocking coil was placed in the right ventricular apex, and the proximal shocking coil was positioned in the right atrium or at the junction of the right atrium and the superior vena cava. After an adequate defibrillation energy requirement was determined (see below), the defibrillator was implanted.

Each patient in this study received a defibrillator with a truncated, fixed-tilt biphasic waveform with a first-phase tilt of 60% and a second-phase tilt of 50% (Cardiac Pacemakers, Inc, defibrillator model 1625, 1635, 1715, 1720, or 1740).

During the implantation procedure, a step-down defibrillation protocol was used to determine the defibrillation energy requirement. The defibrillation energy requirement was defined as the lowest energy successful at converting ventricular fibrillation to sinus rhythm. A defibrillation energy requirement adequate for device implantation was 10 J less than the maximum output of the defibrillator. Shock energies of 20, 15, 10, 5, 3, and 1.0 J were delivered until ventricular fibrillation failed to convert to sinus rhythm. The shocks were either delivered from an external defibrillator (CPI, ECD II) or directly from the implantable defibrillator. Ventricular fibrillation was induced with 1 to 2 seconds of alternating current or with ventricular pacing with a 15-V pulse delivered every 30 ms with a duration of 1.1 ms for 1 to 3 seconds. Shocks were delivered after ventricular fibrillation was sensed and the device (either the external defibrillator or the implantable defibrillator) charged. At least 5 minutes were allowed to elapse between each induction of ventricular fibrillation.

Study Protocol
Within 48 hours of implantation, the patients were brought to the electrophysiology laboratory in a postabsorptive state. All inductions of ventricular fibrillation, arrhythmia sensing, and shock delivery were performed by the implanted defibrillator. Ventricular fibrillation was induced with 1 to 3 seconds of ventricular pacing at an amplitude of 15 V and delivered every 30 ms with a duration of 1.1 ms. At least 5 minutes was allowed to elapse between each induction of ventricular fibrillation. The defibrillation energy requirement was determined in small decrements using a step-down protocol. The delivered energies were 25, 20, 15, 12, 10, 8, 6, 5, 4, 3, 2, 1, and 0.5 J. The energy level selected for the first ventricular fibrillation induction was the energy equal to the defibrillation energy requirement at the time of device implantation. The step-down defibrillation energy requirement was then determined using the decrements described above. In the unusual event that the first shock energy selected was not effective in terminating ventricular fibrillation, the shock energy selected for the next induction of ventricular fibrillation was two steps above the defibrillation energy requirement determined during device implantation. The step-down defibrillation energy requirement was then determined from that point. The defibrillation energy requirement was again defined as the lowest energy successful at converting ventricular fibrillation to sinus rhythm. After determination of the defibrillation energy requirement in the study protocol, patients underwent the investigational protocol, which required five inductions of ventricular fibrillation. At least 5 minutes were allowed to elapse between each induction of ventricular fibrillation. Patients were assigned to a first shock energy for each of these ventricular fibrillation inductions of either 1.0, 1.3, 1.5, 1.7, or 2.0 times the defibrillation energy requirement. The success of each of the five shocks at the assigned multiple of the defibrillation energy requirement was noted. The programmed energy after a failed shock was selected at the discretion of the investigator.

Statistical Analysis
Continuous variables are expressed as mean±1 SD and were compared with the use of the Student's t test. Multiple continuous variables were compared by ANOVA and nominal variables were compared by ² analysis. A Kendall's Tau test was used to examine the association of the multiple of the defibrillation energy requirement with the rate of successful defibrillation. For the purposes of this study, a low defibrillation energy requirement was defined as a defibrillation energy requirement that was 1 SD less than the mean defibrillation energy requirement, and a high defibrillation energy requirement was defined as a defibrillation energy requirement 1 SD greater than the mean defibrillation energy requirement. An attempt was also made to identify results that were outliers relative to the population means. Outliers in each multiplier group were identified by subtracting 2 SD of the mean probability of successful defibrillation for a particular multiple from the mean probability of successful defibrillation for that multiple group.

	Results

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Major Findings
The mean defibrillation energy requirement was 8.7±4.9 J in the entire population of 53 patients. Ten patients each were assigned to receive shocks that were 1.0 and 1.5 times the defibrillation energy requirement, and 11 patients each were assigned to receive shocks that were 1.3, 1.7, and 2.0 times the defibrillation energy requirement. There were no identifiable differences in the clinical characteristics between patients assigned to receive shocks at the various multiples of the defibrillation energy requirement (Table).

View this table: [in this window] [in a new window]   Table 1. Patient Characteristics and Defibrillation Energy Requirements

The results are shown in Fig 1. The overall likelihood of successful defibrillation was 70±27% in the patients assigned to defibrillation testing at 1.0 times the defibrillation energy requirement, 84±12% in patients assigned to defibrillation testing at 1.3 times the defibrillation energy requirement, 86±25% in patients assigned to defibrillation testing at 1.5 times the defibrillation energy requirement, 80±29% in patients assigned to defibrillation testing at 1.7 times the defibrillation energy requirement, and 88±32% in patients assigned to defibrillation testing at 2.0 times the defibrillation energy requirement (P=.03).

View larger version (14K): [in this window] [in a new window]   Figure 1. Results in individual patients. The multiple of the defibrillation energy requirement is represented on the x-axis; the mean likelihood of successful defibrillation is represented on the y-axis. Each open circle represents individual patient data for a patient with a defibrillation energy requirement of 4 J. Each closed circle represents individual patient data for a patient with a defibrillation energy requirement <4 J. Outliers are indicated by a square around the circle. Mean likelihoods of successful defibrillation at each multiple of the defibrillation energy requirement are connected by a solid line; error bars represent 1 SD.

Among the 53 patients tested, successful defibrillation was achieved with each of five shocks in 27 patients (Fig 1). Specifically, the frequencies of uniformly successful defibrillation for patients assigned to 1.0, 1.3, 1.5, 1.7, and 2.0 times the defibrillation energy requirement were 30%, 27%, 60%, 64%, and 73%, respectively. The likelihood of five successful defibrillation attempts was statistically different between groups (P=.01). Additional analysis demonstrated that compared with the patients tested at 1.0 times the defibrillation energy requirement, the likelihood of 100% successful defibrillation was significantly greater only in the patients tested at twice the defibrillation energy requirement (P=.02).

Successful Defibrillation With a Low Defibrillation Energy Requirement
There were 7 patients who had a low defibrillation energy requirement, that is, <3.8 J (Fig 1). Among these 7 patients, no patients were assigned to 1.0 times the defibrillation energy requirement, 1 each was assigned to 1.3 and 1.5 times the defibrillation energy requirement, 2 patients were assigned to 1.7 times the defibrillation energy requirement, and 3 patients were assigned to 2.0 times the defibrillation energy requirement (P=.3). The overall rate of successful defibrillation was 54±20% in the 7 patients with a low defibrillation energy requirement compared with 86±20% in the other 46 patients (P=.002). The likelihoods of successful defibrillation in patients with and without a low defibrillation energy requirement were 80% and 82±12%, respectively, at 1.3 times the defibrillation energy requirement, 20% and 93±10%, respectively, at 1.5 times the defibrillation energy requirement, 40±0% and 87±28%, respectively, at 1.7 times the defibrillation energy requirement (P=.052), and 67±58% and 98±8%, respectively, at 2.0 times the defibrillation energy requirement (P=.17).

For 1.0, 1.3, 1.5, 1.7, and 2.0 times the defibrillation energy requirement, outliers were defined to be patients with probabilities of successful defibrillation of less than 16%, 60%, 36%, 22%, and 24%, respectively. Only three patients, who had a mean defibrillation energy requirement of 3.0±2.6 J, were outliers (Fig 1). The mean defibrillation energy requirement in the outliers was significantly less than in the remainder of the patients (9.0±4.8 J; P=.04). Furthermore, patients with a low defibrillation energy requirement, that is, <3.8 J, were statistically more likely to be outliers (P=.04).

Probability of Successful Defibrillation With a 15-J or 20-J Defibrillation Energy Requirement
There were four patients who had a defibrillation energy requirement of 15 J and four patients who had a defibrillation energy requirement of 20 J. These eight patients were tested at energies of 20 to 34 J. Overall, successful defibrillation was achieved in 90% of the induced episodes. Two of the four patients with a defibrillation energy requirement of 15 J were tested at a multiple of 1.3 times the defibrillation energy requirement, or 20 J, and two of the patients were tested at 25 J, a multiple of 1.7 times the defibrillation energy requirement. Successful defibrillation was observed in 9 of 10 defibrillation attempts in the two patients tested at 20 J. The other two patients who had a defibrillation energy requirement of 15 J were tested at 25 J and were always successfully defibrillated. One of the four patients with a 20-J defibrillation energy requirement was tested at 20 J (1.0 times the defibrillation energy requirement), one patient was tested at 30 J (1.5 times the defibrillation energy requirement), and two patients were tested at 34 J (1.7 times the defibrillation energy requirement). The 20-J shocks were always effective. A single failed defibrillation attempt was noted in the patient tested at 30 J. In the two patients tested at 34 J, one patient had three successful defibrillation attempts and one patient had five successful defibrillation attempts.

Absolute Safety Margins
Fig 2 illustrates the relationship between the absolute safety margin, defined as the tested energy minus the defibrillation energy requirement, and successful defibrillation. There were 10 patients tested at an energy of 7 J or more above the defibrillation energy requirement, and a 96% probability of successful defibrillation was achieved. Two failed defibrillation attempts were observed in a patient with a 20-J defibrillation energy requirement who was tested at 34 J.

View larger version (10K): [in this window] [in a new window]   Figure 2. Individual results with the absolute energy margin (J) on the x-axis and the probability of successful defibrillation on the y-axis. Absolute energy margin is defined as the difference between the tested energy and the defibrillation energy requirement.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Major Findings
The results of this study demonstrate that the defibrillation energy requirement identifies the energy level associated with a 70% probability of successful defibrillation. As energy levels increase from the defibrillation energy requirement to twice the defibrillation energy requirement, the probability of successful defibrillation plateaus but does not reach 100%. Energies of twice the defibrillation energy requirement are associated with a 98% probability of successful defibrillation if the defibrillation energy requirement is >4 J. Larger safety margins relative to the defibrillation energy requirement are needed to achieve a high probability of successful defibrillation if the defibrillation energy requirement is <4 J. An absolute safety margin of 7 J is associated with a 96% probability of successful defibrillation. In clinical scenarios in which a 10-J safety margin is used, failed defibrillation attempts should be expected occasionally in patients with a 20-J defibrillation energy requirement who receive shocks at 1.7 times the defibrillation energy requirement, that is, 34 J, and in patients with a 20-J defibrillation energy requirement who are tested at 30 J, or 1.5 times the defibrillation energy requirement.

Safety Margins and Defibrillation Energy Requirement Curves
In clinical practice, establishment of a 20-J defibrillation energy requirement ordinarily is considered adequate for implantation of a device that has a maximum output of 30 J. Data from the present study demonstrate that in this situation, failed defibrillation shocks will occasionally be observed. For example, in the present study, a total of 15 shocks with energies of 30 or 34 J were administered, and 80% were effective. Furthermore, these data suggest that the probability of successful defibrillation will most often be achieved when using energies that are twice the defibrillation energy requirement. These data do not address the issue of using energies that are greater than twice the defibrillation energy requirement.

Defibrillation at Twice the Defibrillation Energy Requirement
The likelihood of successful defibrillation was significantly greater at energies that were twice the defibrillation energy requirement, and the likelihood of uniformly successful defibrillation was more likely at twice the defibrillation energy requirement. These data are consistent with the concept that doubling the defibrillation energy requirement should result in an energy on the plateau portion of the defibrillation energy requirement curve.^{9 10} In fact, only one patient who was tested at twice the defibrillation energy requirement did not achieve uniformly successful defibrillation. This patient had a defibrillation energy requirement of only 2 J. However, as discussed below, patients with low defibrillation energy requirements may not reach the plateau of the probability curve when the defibrillation energy requirement is doubled. If patients with low defibrillation energy requirements are excluded, the likelihood of successful defibrillation at twice the defibrillation energy requirement would be 98% and hence well onto the plateau portion of the curve. This observation has important implications as defibrillation energy requirements continue to decrease and low defibrillation energy requirements become more common.

Programming the first shock energy of defibrillators to twice the defibrillation energy requirement may offer advantages, including a more rapid charge time, which may prevent syncope and prolong battery life. In addition, while the results regarding the effect of defibrillation on cardiac function are mixed,^{11 12 13 14 15} lower energy shocks may be less deleterious hemodynamically.

Absolute Safety Margin Energy
This study was not designed to determine if an absolute safety margin is associated with a high degree of successful defibrillation. However, the data suggest that a 7-J safety margin is associated with a high probability of successful defibrillation. In clinical practice, a 10-J safety margin has been associated with very low incidence of sudden death.^{16 17 18 19} First-, second-, and third-generation defibrillators had maximum outputs of approximately 30 to 34 J. These data suggest a 10-J safety margin, or 1.4 or 1.5 times the defibrillation energy requirement, would be expected to be associated with approximately an 85% probability of successful defibrillation. Similarly, with defibrillation energy requirements in the 10-J range, as is often the case today, a 7-J safety margin also might be expected to be associated with a probability of successful defibrillation in the range of 85%.

Low Defibrillation Energy Requirements
There were 7 patients with a low defibrillation energy requirement. In this study, that corresponded to <4 J. The overall probability of successful defibrillation was less in these patients than for the population as a whole. When trying to understand why these patients are different, it is important to realize that the physiological and anatomic variables that determine how the defibrillation energy requirement curve will be shaped in any given patient are poorly understood. However, there are at least three possible explanations for this finding. The first is that there may be autonomic differences in patients with a low defibrillation energy requirement. For instance, some clinical data suggest that epinephrine can decrease the likelihood of a successful shock, although some animal studies have found that high adrenergic tone may not alter the defibrillation energy requirement.^{20 21}

An alternative explanation could be that the probability of a successful defibrillation curve may have a steeper slope in patients with a low defibrillation energy requirement. In fact, a steeper curve would be expected.^{9 10} As noted above, animal experiments and the data contained herein suggest that twice the step-down defibrillation energy requirement will identify the plateau portion of the probability of successful defibrillation curve.^{9 10} For example, if the actual defibrillation energy requirement, or the 70% probability of successful defibrillation is 10 J, then the plateau of the curve is 20 J. If by chance 8 J and not 10 J was identified as the defibrillation energy requirement, then 16 J would be tested and would be 1.6 times the actual 70% probability of successful defibrillation energy (10 J). In this instance, an energy of 1.6 times the defibrillation energy requirement should still be highly efficacious. However, in a patient in whom 2 J corresponds to the actual 70% likelihood of successful defibrillation, then the plateau would be expected at 4 J. If by chance one additional successful defibrillation occurred, then 1 J would have been identified as the defibrillation energy requirement. Doubling it would be 2 J, which is actually the defibrillation energy requirement associated with 70% successful defibrillation. In this example, failed defibrillation attempts would be expected and likely.

A third reason may be a methodological consideration. The energies were decremented in steps of 17% to 25% between 25 and 3 J. From 3 J downward, the steps were proportionately larger, ranging from 33% to 50%, and this represents only 1 J at the lower energy levels. Hence, the larger percentage steps may have decreased the accuracy of this measurement at low energies. Alternatively, the smaller 1-J steps used in patients with low defibrillation energy requirements may have biased the defibrillation energy requirement toward lower values of successful defibrillation. In either case, reconfirmation of the step-down defibrillation energy requirement with an additional shock or shocks would probably increase the defibrillation energy requirement toward the actual energy associated with 70% successful defibrillation in some cases and verify the previously established defibrillation energy requirement in other cases. Hence, reconfirmation of the defibrillation energy requirement may be reasonable for patients with low defibrillation energy requirements.

Successful Defibrillation With a 20-J Defibrillation Energy Requirement
A "high" defibrillation energy requirement is not an absolute number but clinically is defined as one that precludes implantation of a defibrillation system. Conventionally, the safety margin for defibrillator implantation is thought of as being 10 J. Therefore, an adequate defibrillation energy requirement for implantation of a 30-J defibrillator would be 20 J and would be 25 J for a 35-J defibrillator. In the present study there were three patients with a defibrillation energy requirement of 20 J who were tested at 30 J or 34 J. Two of these three patients had at least one failed defibrillation attempt. This finding demonstrates that in patients treated with present-day defibrillator technology and a defibrillation energy requirement of 20 J, an occasional failed defibrillation attempt will occur at the maximum output. Clinically, this does not seem to be a frequent occurrence. This statement is supported by the fact that multiple shocks for ventricular fibrillation and sudden cardiac death are uncommon in patients with an implantable defibrillator.^{16 17 18 19} This may be true for several reasons. Most patients do not have a "high" defibrillation energy requirement, and even when they do, there is usually at least a reasonable probability that the defibrillation attempt will work. Next, while the results of studies evaluating the effects of prolonged ventricular fibrillation on the defibrillation energy requirement are variable,^{22 23 24 25} the second and subsequent shocks should have the same reasonably high probability of successful defibrillation as the first shock, especially if the additional shocks are delivered relatively quickly. Finally, ventricular tachycardia often may be the successfully treated arrhythmia, and the energy required for defibrillation of ventricular tachycardia is usually less than for ventricular fibrillation.

Previous Studies
In clinical practice, an acceptable defibrillation safety margin for implantable defibrillators has been arbitrarily defined as 10 J. Neither clinical research or animal experimentation has directly addressed the issue of absolute energy margins. However, two previous retrospective clinical studies evaluated the clinical efficacy of defibrillator therapy in patients with elevated defibrillation energy requirements. Less than a 10-J safety margin in patients treated with defibrillators with maximum outputs of 25 to 35 J were effective, but sudden death was still an important clinical problem.^{26 27} The devices in these studies did not allow determination of the ventricular rhythm responsible for the delivered therapy.^{26 27}

A significant amount of animal experimentation and a minimal number of human studies have addressed the issue of defibrillation energy requirement curves and safety margins. The results of animal studies suggest that a step-down defibrillation energy requirement identifies the energy associated with a 70% probability of successful defibrillation but with a range from 25% to 88%.^{1 8 9 10} If the energy is doubled, then a 100% successful defibrillation, or close to it, should be expected.^{1 8 9 10} The data from these reports suggest that the defibrillation energy requirement curve for an animal with a low defibrillation energy requirement will be narrower and steeper than for an animal with a high defibrillation energy requirement.^{9 10}

An intraoperative open chest study in humans that used a monophasic waveform and an epicardial patch electrode defibrillation system found that 100% successful defibrillation could be achieved with an energy that was 2.6 times the step-up defibrillation energy requirement.²⁸ This previous report is different than the present study in several important ways. First, the present study used a completely different defibrillation system, including a biphasic waveform and a transvenous lead system. Next, 98% of the patients in the study by Jones et al²⁸ did not have structural heart disease and were not undergoing implantation of an implantable defibrillator. In the present study, all the patients were undergoing implantation of a defibrillator. Third, the previous study used a step-up defibrillation energy requirement protocol and not a step-down defibrillation energy requirement protocol.²⁸ On the basis of statistical considerations alone, one would expect the step-up defibrillation energy requirement to identify a lower point on the probability of successful defibrillation curve than would a step-down defibrillation energy requirement.^{6 9 10} Therefore, as previously reported, a larger multiple of the defibrillation energy requirement would be expected to reach the plateau of the curve.²⁸ From a clinical standpoint, the results of the present study may be more relevant than those of the previous study because when a threshold protocol is performed, step-down protocols and not step-up protocols are usually employed.^{10 28}

The data contained herein add significantly to the previous literature. First, these data demonstrate that the step-down defibrillation energy requirement is associated with 70% successful defibrillation but with a wide interpatient range of 20% to 100%. This range is wider than observed with defibrillation studies in normal animals.¹ Next, if the entire study population is considered, doubling the defibrillation energy requirement does not result in uniformly successful defibrillation. Specifically, the patients with low defibrillation energy requirements tend to fall off the probability of successful defibrillation curve. While low defibrillation energy requirements are believed to be associated with steeper defibrillation energy requirement curves,^{9 10} it was not previously appreciated that larger multiples of the defibrillation energy requirement are required to reach the plateau of the curve.

Limitations
A limitation of this study is that the data were obtained using only one defibrillation system and hence these data may not be applicable to other defibrillation systems. Second, the patients were not randomly assigned to be tested at a multiple of the defibrillation energy requirement. This was not feasible because of the limited energy levels available with defibrillators. For example, a patient with a defibrillation energy requirement of 20 J could not be tested at twice the defibrillation energy requirement. The third limitation is that energy levels greater than twice the defibrillation energy requirement were not tested. Because of previous animal studies suggesting that the plateau of the probability of successful defibrillation curve should be achieved with twice the defibrillation energy requirement,^{1 6 8 9 10} and because current clinical practice allows a safety margin of 10 J with a defibrillation energy requirement of 25 J, larger multiples of the defibrillation energy requirement were not studied. Fourth, the step-down defibrillation energy requirement protocol used in this study used the smallest possible decrements. Clinically, larger steps are often used. If larger steps were used during defibrillation energy requirement testing, then the defibrillation energy requirement would likely be higher. Therefore, relative to the results in the present study, higher success rates at the various multiples of the defibrillation energy requirement would be expected. Last, clinical follow-up to assess the efficacy of twice the defibrillation energy requirement to successfully convert spontaneous ventricular fibrillation was not performed. Review of the patients at the University of Michigan who received a defibrillator that allows retrieval of ventricular electrograms suggests that spontaneous ventricular fibrillation occurs in <3% of patients, and hence assessment of clinical efficacy would be difficult.

Clinical Implications
The results of this study support the concept that successful defibrillation is a statistical phenomenon and that failed defibrillation attempts will occasionally be observed with the maximum energy that the defibrillator can deliver. Determination of the step-down defibrillation energy requirement and programming of the first shock in the ventricular fibrillation zone at twice the step-down defibrillation energy requirement and subsequent shocks at the maximum energy may be a rational approach to programming the shock energy for treatment of ventricular fibrillation. However, a patient with a defibrillation energy requirement <4 J may require a safety margin greater than twice the defibrillation energy requirement. When an energy of twice the defibrillation energy requirement exceeds the output of the defibrillator, then the maximum energy should be programmed. This approach may offer an alternative to the clinical practice of always programming the defibrillator to the maximum output, or at least to a 10-J safety margin. However, the clinical efficacy of programming the energy to twice the defibrillation energy requirement has not been documented, and prospective studies to evaluate the efficacy of this approach are needed.

	Acknowledgments

 
The authors would like to thank Allyson Navyac for her secretarial support and Seema Sonnad, PhD, from the Consortium for Health Outcomes, Innovations and Cost Effectiveness Studies, at the University of Michigan Medical Center.

	Footnotes

 
Dr Strickberger is a paid consultant to Cardiac Pacemakers, Inc.

Received October 14, 1996; revision received March 7, 1997; accepted March 18, 1997.

	References

Top Abstract Introduction Methods Results Discussion References

 

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