ICDS
High defibrillation energy requirements are encountered rarely with modern dual-chamber implantable cardioverter-defibrillator systems
1 Utah Heart Clinic Arrhythmia Services, LDS Hospital, 324 10th Avenue, Suite 206, Salt Lake City, UT 84103, USA; 2 University of Iowa Hospitals, Iowa City, IA, USA; 3 North Ohio Research, Ltd, Elyria, OH, USA; 4 The Integra Group, Brooklyn Park, MN, USA; 5 Boston Scientific CRM, St Paul, MN, USA
Manuscript submitted 4 November 2007. Accepted after revision 14 January 2008.
* Corresponding author: Intermountain Medical Center, Eccles Outpatient Care Center, 5169 Cottonwood Street, Suite 510, Murray, UT 84157, USA. Tel: +1 801 408 3900; fax: +1 801 408 3909; E-mail address: john.day{at}cv-research.org
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Abstract |
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Aims: Defibrillation conversion testing to assure a 10 J safety margin is a standard practice during implantable cardioverter-defibrillator (ICD) implantation. Little data are available on the number of patients who do not have a 10 J margin initially and therefore require system revisions, further testing, or a higher energy output device.
Methods and results: The INTRINSIC RV study enrolled 1530 new ICD recipients who were not in permanent atrial fibrillation who received a VITALITY AVT (Guidant, St Paul, MN, USA) standard energy (31 J maximum) ICD and underwent defibrillation conversion testing at the time of implantation from 108 centres. Among enrolled patients, 59 (3.9%) did not initially meet the 10 J safety margin criterion. In these 59 patients, a 10 J safety margin was achieved by making at least one system revision: reversing shocking polarity (n = 33, 56%), right ventricular lead repositioning (n = 19, 32%), repeat testing at a later date (n = 1, 2%), adding a subcutaneous array (n = 1, 2%), or other means (n = 10, 17%). Only New York Heart Association class (P = 0.001) and no previous myocardial infarction (P = 0.044) predicted a failed initial conversion test. There were no reported complications from ICD shock testing.
Conclusion: Successful defibrillation conversion criteria with the first configuration tested with a standard energy device is almost always met with modern dual-chamber ICD systems. The need for revising the initial ICD shock configuration to achieve a 10 J safety margin appears extremely low and of low risk.
Key Words: Implantable cardioverter-defibrillator, Defibrillation energy requirements, Ventricular fibrillation
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Introduction |
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Implantable cardioverter-defibrillators (ICDs) are now indicated for an increasing number of patients who are at risk for sudden cardiac death.1
Testing defibrillation energy requirements (DERs) with ventricular fibrillation (VF) induction is performed typically during the implantation procedure to ensure that the device will be able to adequately sense and terminate VF.
Currently, the most widely accepted method of defibrillation conversion testing (device testing) at the time of ICD implantation requires at least two VF inductions with subsequent conversion successes using shock energies of at least 10 J below the maximum shock energy deliverable by the ICD (10 J safety margin).2,3 Although repeated VF conversion testing is a relatively safe procedure, repeated induction of VF, particularly in patients with advanced heart failure, may result in serious complications such as myocardial depression or ischaemia, cerebral hypoperfusion, immediate cardiac dilation,4 acute pulmonary oedema,5 elevation of cardiac enzymes,6 intractable VF, or death.7–9
In clinical practice, a high DER is encountered rarely during routine implantation testing with modern biphasic shock ICD systems. However, limited data support this finding.10,11
The purpose of this report is to assess, in a large prospective patient population, defibrillation testing results and frequency of failure to meet a 10 J safety margin in modern ICD systems.
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Methods |
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All patients in the current analysis were enrolled in the Inhibition of Unnecessary RV Pacing with AV Search Hysteresis in ICDs (INTRINSIC RV) study. The INTRINSIC RV study is a 108-site, randomized, controlled clinical trial that investigated programming modalities of a dual-chamber ICD to assess the relative efficacy of an ICD with backup ventricular pacing compared with dual-chamber ICD programming using AV Search Hysteresis to allow intrinsic ventricular conduction. Complete study design12
and results13 have been published previously. To be enrolled in this study, patients of any age or gender had to have an ICD indication. Patients with permanent atrial fibrillation, a previously placed pacemaker or ICD, an indication for cardiac resynchronization therapy, or planned other cardiac procedures were excluded. Patients were enrolled between June 2003 and September 2004. All patients completed the informed consent process before implantation at approved centres.
In this study, all patients underwent implantation of a transvenous right ventricular shocking lead with a dual-chamber VITALITY® AVT (Guidant Corporation, St Paul, MN, USA) ICD. The majority (99.4%) of the right ventricular shocking leads used were dual-coil leads. At the investigator discretion, 93.4% were programmed with initial shock polarity (distal shocking coil to proximal shocking coil and ICD generator) and 6.6% were programmed with reversed shock polarity (proximal shocking coil and ICD generator to distal shocking coil) during implantation testing. The shock waveform was biphasic for all patients.
An acceptable safety margin for defibrillation conversion consisted of two consecutive VF conversions, without a failure, at an energy level at least 10 J below the maximum shock output of the ICD. Specific interventions to correct a high DER and achieve a 10 J safety margin were determined by the implanting physician.
Blood pressure, history and location of myocardial infarction (MI), body mass index, and New York Heart Association (NYHA) functional class were analysed to determine whether they differed between patients who passed or failed initial implantation testing.
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Results |
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Of the 1530 patients undergoing defibrillation testing, 1471 (96.1%) had an adequate DER test (two consecutive VF terminations with at least a 10 J safety margin) and 59 (3.9%) had DER > 21 J. Among the 59 who failed a VF conversion test at 21 J or lower in the initial shock configuration, 14 also failed a 31 J maximum energy rescue shock and required external defibrillation.
An acceptable implantation (10 J safety margin) was achieved later by making at least one of the following system revisions: reversing polarity (n = 33, 56%), right ventricular lead repositioning (n = 19, 32%), repeat testing on a later date (n = 1, 2%), adding a subcutaneous array (n = 1, 2%), or other means such as restoring proper pin position of the lead within the ICD or capping proximal shocking coils (n = 10, 17%, Figure 1). The final right ventricular lead location to achieve a satisfactory DER was reported at the apex of right ventricle in 90% of the patients.
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The relationship between several baseline characteristics and DER was analysed. Blood pressure and body mass index were not significantly associated with DER (Table 1). Of the baseline clinical characteristics [a history of coronary artery disease, congestive heart failure, diabetes, dilated cardiomyopathy, hypertension, MI location (anterior, inferior, other), and amiodarone or beta blocker usage] analysed, only NYHA functional class (P = 0.001) and no history of MI (P = 0.044) were predictive of a high initial DER (Table 2). Regarding functional class, this difference was driven primarily by a small number of patients with NYHA functional class IV who failed initial DER testing (5/18; 28%). Furthermore, among patients with a history of MI, location of the infarct did not predict DER success independently.
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Amiodarone was prescribed in 144 patients (9.4%) at the time of implantation. There was no significant difference in DER success between those patients prescribed amiodarone and those not taking this medication. Other medications including beta-blockers did not appear to result in any significant difference in DER success.
There was also no significant difference in DER success on the basis of initial shock polarity, right ventricular R wave, impedance, or pacing threshold. In addition, of the 1464 patients in whom a shock impedance value was recorded at the time of implantation (mean 39.0 ± 5.4 
; range 25–74 ), no relationship was found between DERs and shock impedance (Table 3; P = 0.70). There were no reported complications from ICD shock testing.
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Discussion |
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This study indicates that a high DER is encountered rarely with a modern dual-chamber ICD system. These data from the INTRINSIC RV trial represent a wide variety of contemprary patients in one of the largest randomized, controlled, clinical ICD trials completed to date. These data represent a wide variety of contemporary patients in one of the largest randomized, controlled, clinical ICD trials completed to date. Even though patients with permanent atrial fibrillation were excluded from the study, some may have had paroxysmal atrial fibrillation at implantation and at some point during follow-up.
The finding of a high DER in only 3.9% of our patients is consistent with findings of Russo et al.11 Their retrospective evaluation of 1139 patients undergoing generator replacement or revision indicated that 6.2% had high DER.11 Day et al.10 recently demonstrated the same 6.2% incidence of high DER in a prospective study of initial implantations involving a cohort of 422 patients which included both ICDs and biventricular pacemaker-defibrillators. The fact that high DERs are encountered rarely supports the thesis that DER testing may not be required, as in most cases, adequate DERs are achieved.14,15
Defibrillation conversion testing at the time of ICD implantation is certainly not without risk. Indeed, in rare cases, the repeated induction of VF, particularly in patients with advanced heart failure, may result in myocardial depression or ischaemia, cerebral hypoperfusion, immediate cardiac dilation, acute pulmonary oedema, elevation of cardiac enzymes, intractable VF, or even death.4–9 However, in this study, complications from ICD shock testing at the time of implantation were not observed. These data do suggest that patients potentially at highest risk of a complication from intentional VF induction (NYHA class IV) may be the ones more likely to have an elevated DER.
Although it is relatively uncommon to encounter a high DER patient with current ICD technology, there is still a small subgroup of patients, especially those with more advanced heart failure, for whom an adequate DER cannot be achieved by standard methods. Indeed, from our study, 59 patients required a revision of their ICD system in order to obtain a satisfactory defibrillation safety margin at the time of implantation. The long-term outcomes of this small group of patients with high DER, without revision of their ICD system at the time of implantation, are uncertain and not without risk. An alternate strategy would be to employ just a single VF induction test with termination at 
15 J below the maximum energy of the device or screening patients for increased DER with upper limit of vulnerability testing which does not require intentional VF induction.10,14,16–21
Limitations
Although this study did not require a specific protocol to performing defibrillation conversion testing at the time of implantation, an acceptable safety margin for defibrillation conversion consisted of two consecutive VF conversions, without a failure, at an energy level at least 10 J below the maximum shock output of the ICD. This analysis used a single ICD model, and results may not apply to other ICDs, patients with permanent atrial fibrillation, or to cardiac resynchronization therapy with defibrillator systems.
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Conclusion |
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Most patients undergoing ICD implantation for primary and secondary causes, as demonstrated in the INTRINSIC RV study, have acceptable DER at the first test position. A small percentage of patients, especially those with advanced heart failure symptoms, did not meet these criteria initially but had successful ICD implantation and defibrillation conversion testing after changes were made in their ICD shocking system. Although no complications were observed with ICD shock testing, strategies to identify patients with higher DERs prior to ICD implantation may allow for limited defibrillation conversion testing in most patients.
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Funding |
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This trial was supported by Boston Scientific CRM, St Paul, MN, USA.
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Acknowledgements |
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The authors wish to thank the patients who participated in the INTRINSIC RV trial and the participating investigators and institutions.
Conflict of interest: B.O. is a consultant and member of the speakers' bureau for Boston Scientific/Guidant and Medtronic Inc. S.B. is a paid consultant to Boston Scientific. K.Q.S. and D.R.L. own stock in and are employees of Boston Scientific.
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Footnotes |
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This study is registered with www.clinicaltrials.gov (identifier nct00148967
[ClinicalTrials.gov]
). This manuscript was presented in part at the Heart Rhythm Society Annual Scientific Sessions, May 2005, New Orleans, LA, USA. 
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