© 2005 The European Society of Cardiology. Published by Elsevier Ltd. All rights reserved.
Defibrillation efficacy testing: Long-term follow-up and mortality
Department of Cardiology, Erasmus MC Rotterdam, The Netherlands
Manuscript submitted 24 September 2004. Accepted after revision 18 April 2005.
*Corresponding author. Erasmus MC, Department of Clinical Electrophysiology, Bd416, Dr. Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. Tel.: +31 10 463 2938; fax: +31 10 463 2701. E-mail address: d.theuns{at}erasmusmc.nl
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionTechnical aspects of...Clinical aspects of...Changes in defibrillation...Study limitationsConclusionReferences |
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AIMS: Extensive defibrillation threshold testing is no longer necessary to perform as devices have become more effective. We assessed the lowest effective defibrillation (LED) level at implantation and before hospital discharge and related this to outcome.
METHODS AND RESULTS: One hundred and twenty-seven consecutive patients with biphasic shock and active can devices were studied at intraoperative and predischarge testing. A subgroup of 67 patients had 
3 VF inductions at implant. Improvement was defined when LED decreased by 3 J. The LED was significantly higher at implantation compared with predischarge (P < 0.001). Improvement was seen in 73/127 patients (58%). In the group with 3 VF inductions, an implantation LED > 9 J was related to a lower LVEF (P < 0.01); 34/67 patients (51%) had improvement in LED. During follow-up, 18/127 patients died, four received heart transplantation. No different outcome was observed in patients with and without improvement. However, for those with 3 VF inductions, an independent predictor of mortality was implantation LED > 9 J without improvement on the second test. Safety margin < 10 J was not related to mortality.
CONCLUSION: Repeated defibrillation efficacy testing before hospital discharge may confirm that a relatively high defibrillation energy is required. This is related to a higher mortality in long-term follow-up.
Key Words: arrhythmias, implantable cardioverter defibrillator, sudden death, survival, ventricular fibrillation
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Introduction |
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The implantable cardioverter defibrillator (ICD) has become widely accepted for the treatment of patients with severe life-threatening ventricular tachyarrhythmias [1–
3]. The functions of the ICD and the shock lead integrity are usually tested after implantation or prior to hospital discharge [4]. Device-related problems leading to ICD malfunction have become less common due to advances in ICD technology. Improvements in lead technology have reduced the risk of lead malfunction [5,6]. The introduction of biphasic shock waveforms and active can devices improved defibrillation efficacy [7–10]. These enhancements have led to a general feeling that defibrillation threshold or efficacy testing is no longer important. In order to determine the necessity of predischarge testing, the results of ICD testing at implant and predischarge were studied and related to the patient's outcome. |
Methods |
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Patients
The study population consisted of 127 patients who received an ICD in combination with an endocardial lead system. Baseline clinical characteristics, including age, gender, left ventricular ejection fraction (LVEF), the presence of coronary artery disease (CAD), cardiomyopathy, cardiothoracic (CT) ratio, presenting arrhythmia, and pharmacological treatment were documented. The patient characteristics are shown in Table 1.
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ICD implantation
The implantation procedure was performed in the electrophysiology laboratory under local anaesthesia. The biphasic shock, active can ICD pulse generator and the transvenous lead system were inserted through a single left pectoral incision. A left cephalic vein cutdown and/or a left subclavian puncture were used for lead insertion. The atrial lead was located in the right atrial appendage or lateral free wall by active fixation. The right ventricular lead was placed in the right ventricular apex by active fixation. The right ventricular lead had either one or two defibrillation coils. For biventricular devices, the left ventricular lead was placed in a tributary of the coronary sinus. The ICD pulse generators and defibrillation leads are summarised in Table 2.
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Defibrillation efficacy testing
During the implantation procedure, defibrillation efficacy was tested with the use of a step-down defibrillation protocol. The initial delivered shock energy for testing was 15 J. If successful, the energy was decreased in steps of 3 J in successive tests until defibrillation failed. In case of failure of the initial 15-J shock, the energy was increased in 3-J steps in subsequent tests until defibrillation was successful. Testing was performed under short-duration deep sedation by the administration of diazepam combined with etomidate. The lowest energy, successful in converting ventricular fibrillation to sinus rhythm, was defined as the lowest effective defibrillation (LED). For acceptance of the configuration, the LED had to be equal to or less than the maximum defibrillation energy of the device minus a safety margin of 10 J. Ventricular fibrillation (VF) was induced via the test programme of the ICD by a 50-Hz burst or a T wave shock [11]
. VF was defined as a fast polymorphic ventricular rhythm with a cycle length < 250 ms that resulted in no phasic blood pressure. In case of unsuccessful defibrillation, an internal rescue shock using the maximum energy of the device or an external maximal shock from a precharged defibrillator via cutaneous self-adhesive patches was delivered. In patients with severe LV dysfunction, the procedure was shortened to demonstrate that two consecutive shocks with a safety margin of 10 J were successful.
An improvement in LED was defined as a decrease in defibrillation energy of 3 J on the predischarge test.
Statistical analysis
Continuous variables were expressed as mean ± standard deviation. Chi-square testing was used for analysis of categorical variables, and Student's t-test was used for analysis of continuous variables. Kaplan–Meier actuarial method was used to calculate the survival rate over time. Survival analysis was initiated at the time of ICD implantation. Differences between pairs of survival curves were tested by the log-rank test. Cox proportional hazards model was used to identify independent predictors of mortality. Patients who received cardiac transplantation were censored at the time of the operation. P < 0.05 was considered statistically significant.
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Results |
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Clinical characteristics
Of 146 consecutive patients, 127 underwent testing at implantation and prior to hospital discharge. The clinical characteristics of these patients are summarised in Table 1. The mean age of the patients was 59 ± 14 years (range: 20–82 years). The mean LVEF was 0.35 ± 0.15 (range: 10–76%). Cardiomyopathy was present in 25 (20%) patients, of whom 21 (17%) had dilated cardiomyopathy and four (3%) had hypertrophic cardiomyopathy. Indications for ICD therapy were as follows: non-sustained ventricular tachycardia (NSVT) with subsequent inducible sustained VT in 11 (9%) patients, spontaneous sustained VT in 70 (55%), and VF in 46 (36%).
Survival
During a mean follow-up of 38 ± 14 months, 18 patients died (17 men, mean age 60 ± 16 years). The mortality rates were 4.7%, 9.6%, and 25.1%, at 1, 2, and 5 years, respectively. Deaths were considered to be sudden cardiac in four (22%) and non-sudden cardiac in 10 (55%). In one case, death was attributed to a non-cardiac cause. Three cases (17%) were unwitnessed deaths. There were no deaths related to ICD implantation. Four (3%) patients underwent cardiac transplantation. The mean interval of cardiac transplantation after ICD implantation was 21 months.
Defibrillation data at implantation
A total number of 93 patients (73%) had an LED 
15 J on this test. The proportion of patients with LED 12 J, 9 J, and 6 J was 53%, 38%, and 14%, respectively. A safety margin > 10 J was achieved in 86% of patients.
Defibrillation data prior to hospital discharge
A total number of 108 patients (85%) had an LED 15 J on this test which was performed in a median of 1 day after implantation. The proportion of patients with LED 12 J, 9 J, and 6 J was 75%, 62%, and 34%, respectively. Overall, the LED at implantation was significantly higher compared with the LED at predischarge testing (12.9 ± 4.9 J versus 10.4 ± 5.0 J; P < 0.001). A total number of 73 patients (57%) had an improvement of 3 J.
Variables in relation to LED
There was no significant difference in LED between patients with a single and a dual coil defibrillation lead (n = 88 and 39, respectively) at implantation (12.7 ± 4.8 J versus 13.3 ± 4.9 J) and at predischarge testing (10.2 ± 5.0 J versus 10.7 ± 5.0 J). The shock impedance for all patients significantly decreased from 56 ± 12 
at implantation to 51 ± 10 at predischarge (P < 0.001). At predischarge, the shock impedance significantly decreased in patients with both single and dual coil defibrillation leads. Data related to the safety margin are shown in Table 3.
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Subanalysis of patients with at least three VF inductions at implantation
For this group (n = 67), the average LED was 11.8 ± 5.7 J. In Table 4, the clinical characteristics are summarised for two subgroups, dichotomized at an LED value of 9 J. There were no significant differences between the two patient groups with regard to clinical data such as age, amiodarone use, coronary artery disease, and CT ratio. Only the LVEF was significantly different (P < 0.01). An improvement of
3 J was observed in 34 patients (50.7%). In the 30 patients with implantation LED > 9 J, the LED improved from 16.9 ± 4.4 J to 13.9 ± 5.1 J at predischarge (P < 0.001). In the 37 patients with implantation LED 9 J, it remained unchanged.
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Factors related to mortality
In the group tested on two occasions, mortality was similar in patients with and without improvement in LED. In the group with three VF inductions at baseline, the difference for those without improvement was borderline significant. Subgroup analysis for those with an implantation LED > 9 J revealed a significantly higher mortality when no improvement was observed on the second test (P = 0.02) (Fig. 1). Cox proportional hazard analysis in the total group of 67 patients revealed a baseline LED > 9 J and no improvement on the second test as independent predictors of mortality (P < 0.03). The presence of coronary artery disease, cardiomyopathy, and LVEF were not identified as predictors of mortality. Mortality was not related to the presence of a safety margin
10 J.
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Discussion |
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This study has both technical and clinical implications. Safety margin testing did not prove to be very useful in predicting patient's prognosis. The major clinical finding is that patients without improvement in defibrillation efficacy on the second test tended to have a higher mortality. However, repeated defibrillation testing is equally not very practical at a moment when testing of the upper limit of vulnerability exists. A simple test shock of 1 J might prove that the system senses and shocks as intended. Thus, from a technical point of view, repeated testing of defibrillation efficacy is not necessary.
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Technical aspects of defibrillation efficacy testing |
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The primary function of defibrillation threshold testing is to confirm that the safety margin for defibrillation is adequate. Finding a low effective energy level for conversion of VF to sinus rhythm means that the probability that a patient will be safely converted during future events is high [12,
13]. A difference of 10 J between the maximum output of a device and the lowest effective energy level has been accepted as an adequate safety margin [14]. However, these early studies were conducted in devices with an epicardial lead system and monophasic waveforms. The idea that such a finding is predictive of successful therapy was confirmed with transvenous devices [13]. On the other hand, an absolute safety margin of 10 J does not provide 100% probability of successful defibrillation [15]. The development of active pectoral pulse generators, transvenous lead systems, and biphasic waveforms resulted in lower and more stable defibrillation thresholds. The Low-Energy-Endotak-Trial (LEET) demonstrated that a relative safety margin is just as safe and effective as an absolute safety margin [16]. The rate of successful defibrillation at twice the energy level of the DFT was 99.5%. During follow-up, this study demonstrated no significant difference in conversion rate between twice the DFT and maximum output as first-shock energy. These results were confirmed in the Low Energy Safety Study [17]. A safety margin of 5 J was found to be adequate and safe with a dual coil lead and active can device. |
Clinical aspects of defibrillation efficacy |
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Clinical long-term follow-up data have indicated potential adverse consequences of an elevated defibrillation threshold [18,
19]. A higher sudden cardiac death rate was reported in the presence of a low safety margin [18,19]. Arrhythmic death, accounting for 42% of total mortality in the ICD group, could be attributed to failure of conversion in patients with a high DFT [20]. Several studies were designed to identify characteristics that may predict the finding of an elevated defibrillation threshold. Amiodarone therapy, body surface area, and left ventricular dilatation were the predictors of high thresholds for nonthoracotomy defibrillation with monophasic as well as biphasic waveforms [21–23]. In a recent study, clinical parameters were of limited use in predicting DFTs in a dual coil active can system [24]. In our study, we also failed to find an association between a safety margin 10 J and subsequent mortality. |
Changes in defibrillation efficacy |
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Long-term stability of defibrillation efficacy is important, especially among patients with a high DFT and a low safety margin. Changes in DFT are influenced by several factors, such as the lead system and the defibrillation waveform. A long-term increase in DFT was observed with the use of monophasic defibrillation waveforms in combination with a transvenous lead system [25]
. This was also found in a biphasic series with lead-only and subcutaneous patch configurations [26]. In contrast to these data, biphasic active can devices combined with a transvenous lead system prevented such a rise [27]. Recently, a significant decrease in DFTs over time with a dual coil, active pectoral lead system was reported[28]. In our study, the LED was significantly lower on the second test. The LED at implantation cannot have been influenced by the anaesthetic, as was usual in the era of thoracotomy. However, the implantation values could have been influenced by surgical variables, such as stress and the presence of an oversize pocket. The finding that the impedance changed between the two tests is another argument in favour of the predischarge test being a more "reliable" measurement. With the advances in technology, defibrillation thresholds are lower and remain stable. The risk of finding an increased defibrillation threshold has become lower than in the past, and good safety margins are usually obtained in almost all patients. Even if the safety margin is low, consecutive shocks usually convert the arrhythmia to normal rhythm, and patients are saved from instantaneous arrhythmic death. The fact that our patients with marginal findings had a similar mortality to the others confirms this idea, implying that we are faced with the problem of heart failure rather than lethal arrhythmias. The role of a second defibrillation threshold test after implantation can then be questioned. Our study shows that by using the current generation of biphasic shock, active can pectoral ICDs and a second defibrillation test, it becomes possible to identify patients with a worse prognosis.
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Study limitations |
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This study was not designed as a prospective trial. However, the data used were based on a prospective, continuously updated complete database. The data must be interpreted with caution, as the number of patients with at least three VF inductions is small.
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Conclusion |
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From a technical point of view, extensive defibrillation efficacy testing in conventional situations is no longer necessary for experienced hands [29,
30]. With the advances in technology, defibrillation thresholds are low and stable. A predischarge test probably can be considered as ideal management and used to identify patients with poor outcome. The confirmation that relatively high defibrillation energy is required during repeated testing suggests that mortality is higher during follow-up. Conventional safety margin testing is inadequate to predict a patient's prognosis. |
References |
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