Inductionless or Limited Shock Testing Is Possible in Most Patients With Implantable Cardioverter- Defibrillators/Cardiac Resynchronization Therapy Defibrillators
Results of the Multicenter ASSURE Study (Arrhythmia Single Shock Defibrillation Threshold Testing Versus Upper Limit of Vulnerability: Risk Reduction Evaluation With Implantable Cardioverter-Defibrillator Implantations)
Background— Implantable cardioverter-defibrillators and cardiac resynchronization therapy defibrillators have relied on multiple ventricular fibrillation (VF) induction/defibrillation tests at implantation to ensure that the device can reliably sense, detect, and convert VF. The ASSURE Study (Arrhythmia Single Shock Defibrillation Threshold Testing Versus Upper Limit of Vulnerability: Risk Reduction Evaluation With Implantable Cardioverter-Defibrillator Implantations) is the first large, multicenter, prospective trial comparing vulnerability safety margin testing versus defibrillation safety margin testing with a single VF induction/defibrillation.
Methods and Results— A total of 426 patients receiving an implantable cardioverter-defibrillator or cardiac resynchronization therapy defibrillator underwent vulnerability safety margin or defibrillation safety margin screening at 14 J in a randomized order. After this, patients underwent confirmatory testing, which required 2 VF conversions without failure at ≤21 J. Patients who passed their first 14-J and confirmatory tests, irrespective of the results of their second 14-J test, had their devices programmed to a 21-J shock for ventricular tachycardia (VT) or VF ≥200 bpm and were followed up for 1 year. Of 420 patients who underwent 14-J vulnerability safety margin screening, 322 (76.7%) passed. Of these, 317 (98.4%) also passed 21-J confirmatory tests. Of 416 patients who underwent 14-J defibrillation safety margin screening, 343 (82.5%) passed, and 338 (98.5%) also passed 21-J confirmatory tests. Most clinical VT/VF episodes (32 of 37, or 86%) were terminated by the first shock, with no difference in first shock success. In all observed cases in which the first shock was unsuccessful, subsequent shocks terminated VT/VF without complication.
Conclusions— Although spontaneous episodes of fast VT/VF were limited, there was no difference in the odds of first shock efficacy between groups. Screening with vulnerability safety margin or defibrillation safety margin may allow for inductionless or limited shock testing in most patients.
Received September 5, 2006; accepted February 20, 2007.
Implantable cardioverter-defibrillator (ICD) and cardiac resynchronization therapy defibrillator (CRTD) devices have relied on multiple ventricular fibrillation (VF) inductions at implantation to ensure that the device can reliably sense, detect, and convert VF.1,2 Although defibrillation threshold (DFT) testing is considered safe, repeated induction of VF at the time of ICD/CRTD implantation, specifically in patients with advanced heart failure, may result in serious complications such as myocardial depression or ischemia, cerebral hypoperfusion, intractable VF, or death.3–10 Testing for the upper limit of vulnerability (ULV) is an alternative method utilized to estimate the DFT without the need for repeated VF inductions required with formal DFT testing at implantation. The ULV is defined as the lowest shock strength delivered during the vulnerable phase of the cardiac cycle that does not induce VF.11–14 This method of testing has been closely correlated with DFT testing in patients undergoing ICD implantation.2,12,13
Editorial p 2370
Clinical Perspective p 2389
The ASSURE Study (Arrhythmia Single Shock Defibrillation Threshold Testing versus Upper Limit of Vulnerability: Risk Reduction Evaluation With Implantable Cardioverter- Defibrillator Implantations) was the first multicenter trial that prospectively evaluated the efficacy of a potentially inductionless ULV screening at 14 J versus limited DFT screening with 1 VF induction/conversion at 14 J during device implantation. Implant testing results were then correlated with clinical first shock efficacy for fast ventricular tachycardia (VT)/VF ≥200 bpm. Because the present study does not evaluate traditional DFT or ULV testing methodology at the time of ICD/CRTD implantation, ULV screening will henceforth be described as the vulnerability safety margin (VSM) and DFT screening as the defibrillation safety margin (DSM).
Patients who were 18 years or older and not pregnant who met the standard inclusion criteria for initial ICD/CRTD implantation or replacement were recruited and consented from multiple centers to participate in the study with approval from each local institutional review board. Using a single-blinded study design, a total of 426 patients were prospectively enrolled between June 18, 2003, and October 21, 2004, and followed up for 1 year from the date of implantation, with the last follow-up and termination of this multicenter trial occurring on November 17, 2005.
Only left-sided pectoral implantations with dual-coil shocking leads placed in the right ventricular apex were included in the study. Primary device implantations and replacements with a Guidant ICD generator, including CRTD devices, with either single- or dual-chamber lead systems were included. Patients who met all inclusion criteria as described above were included in the study.
The order of implant testing (VSM or DSM, both at 14 J) was randomly assigned in a crossover fashion for all patients (Figure 1). After both the VSM and DSM testing at 14 J, patients underwent confirmation testing, which required 2 successful VF conversions without a failure at ≤21 J. The 21-J VF induction/conversion tests were completed regardless of the 14-J VSM or DSM outcome. Patients who successfully passed their first randomly assigned 14-J test and confirmatory testing at 21 J had their devices programmed at 21 J to terminate fast VT/VF ≥200 bpm and were followed up in the analysis cohort of the ASSURE Study. Patients who did not pass their first randomly assigned 14-J testing method and/or confirmatory testing had their devices programmed to receive a maximum energy shock of ≥31 J for fast VT/VF ≥200 bpm and were followed in the registry cohort of the study. System changes, including polarity reversal, lead repositioning, and upgrading to a high-energy device, were permitted in this group to achieve a satisfactory DFT safety margin. Device reprogramming was left to the discretion of the investigator only after each patient completed their required 1-year follow-up or withdrew before completing the study.
VSM screening consisted of three 14-J T-wave shocks in standard polarity (distal to proximal) at −20, 0, and 20 ms to the peak of the latest-peaking monophasic T wave after a paced drive train of 8 beats from the lead in the right ventricular apex at a cycle length of 500 ms. The peak of the latest-peaking monophasic T wave was determined by a standard 12-lead ECG machine or electrophysiology laboratory recording system (Figure 2). In the rare instance in which a 12-lead ECG was not available for VSM screening, a 3-lead ECG was recorded with a ZOOM programmer (model 2920 or 3120; Guidant Corp, St. Paul, Minn) and displayed at 100 mm/s. In this case, 4 T-wave shocks were delivered at −20, 0, 20, and 40 ms to allow for the possible underestimation of the peak of the latest peaking T wave. The method of VSM screening, as performed in the present study, differs from formal step-down ULV testing in that VSM screening consisted of 3 to 4 T-wave shocks at a single-shock energy of 14 J, similar to what has been described previously by Swerdlow.2
To achieve proper sensing of VF in VSM-tested patients, primary implants and device replacements required a minimum right ventricular R wave of 7 and 5 mV, respectively. Defibrillation safety margin screening consisted of a single VF induction with conversion at 14 J in standard polarity. Confirmation testing with VF induction/conversion at 21 J was also conducted in standard polarity.
Total test time was defined as the total elapsed time taken to perform each implant test. For VSM screening, total time included attachment of additional leads, pacing to determine peak of the T wave, device programming, a 1-minute time delay between shocks, and postshock recovery. For DSM screening, total time included device programming, VF induction, shock delivery, and postshock recovery. Blood pressure stabilization after adequate sedation was achieved both before and after VSM and DSM confirmation tests were applied. To assess for potential clinically significant hemodynamic changes with shock testing, a uniform sedation protocol was performed with the anesthesia department at a single institution. Any systolic blood pressure drop of >20 mm Hg, as determined by continuous arterial blood pressure monitoring for >1 minute after the delivered shock, was considered significant for hemodynamic compromise.
To ensure VSM screening uniformity among investigators, this method was validated by University of California at Los Angeles Cardiac Arrhythmia Center personnel, who evaluated T-wave measurements from ECGs collected from a subset of ASSURE patients who underwent VSM testing at implantation.
Permanent Device Programming
Devices in both cohorts were programmed to deliver shock therapy for spontaneously occurring ventricular tachyarrhythmias at rates ≥200 bpm. Patients in the analysis cohort had their devices programmed to deliver a lower-energy first shock at 21 J. At the discretion of the investigator, if the first shock, programmed at 21 J, was unsuccessful at terminating spontaneous episodes, subsequent shocks could be reprogrammed to a maximal energy of 31 J for most patients and 41 J for a few registry cohort patients with a higher DFT requirement during implant testing. A 1-second detection window was programmed to ensure uniformity of episodic detection and therapy delivery. Additional rate zones and therapies, such as ventricular antitachycardia pacing, were programmed only for ventricular arrhythmias ≤200 bpm.
Descriptive statistics were used to characterize the demographic and clinical characteristics of the cohort, whereas χ2 and t tests were used to compare groups of patients. Two-way tables were used to describe the outcomes of implant testing procedures. Positive predictive accuracy, defined as the conditional probability of successful conversion with the 21-J confirmation test given a successful VSM (or 14-J DSM) test, was calculated along with corresponding 2-sided 95% CIs to assess the accuracy of each testing procedure. The odds of successful 21-J confirmation testing conditional on successful 14-J testing were compared between the 2 different 14-J tests with logistic regression models fit with generalized estimating equations. This method accounts for the binary nature of the outcome (pass/fail) and accounts for the repeated measurements on patients due to the crossover design.15 Terms for 14-J test, period, and carryover were included in this model. Logistic regression models fit with generalized estimating equations were also used to compare the odds of successful 21-J confirmation testing given the success of the 14-J VSM and DSM tests and the per-episode first shock termination rate, again to account for repeated measurements on patients. The mean time required to perform each test was compared with a paired t test, which compared the average of the patient-specific differences in test times between the VSM and DSM tests. All analyses were performed with SAS version 9.1. Probability values less than 0.05 were deemed statistically significant.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
Table 1 describes the patient demographics for the present study. Of 426 implanted patients, 79.8% were male, with a mean age of 66.3±13.0 years and mean QRS width of 131.2±39.3 ms. Coronary artery disease was present in 70.9% of patients. The majority of patients (50.7%) had a left ventricular ejection fraction between 21% and 30%, and 20.4% had a left ventricular ejection fraction <20%. A total of 8.9% were classified as New York Heart Association (NYHA) class I, 32.4% as class II, 49.3% as class III, and 4.9% as class IV. Nearly half of the patients (48.6%) met the indications for ICD implantation (a prior myocardial infarction and left ventricular ejection fraction of 30% or less) as defined by the Multicenter Automatic Defibrillator Implantation Trial II (MADIT II). Most patients were taking β-blockers (69.3%) and angiotensin-converting enzyme inhibitors (60.3%) at the time of implantation.
Implant testing results were compared between patients who passed or failed 14-J VSM screening, 14-J DSM screening, and 21-J VF induction/conversion testing. Of the 420 patients who underwent VSM screening at 14 J and 21-J confirmation testing, 322 (76.7%) passed VSM screening (VF not induced), and 317 (98.4%) of these 322 passed both 14-J VSM screening and 21-J confirmation tests (Table 2). Of the 98 patients (23.3%) who failed VSM screening at 14 J (VF induced), 21 had a DFT >21 J.
Similarly, of the 416 patients who underwent 14-J DSM screening and 21-J confirmation testing, 343 (82.5%) passed (VF terminated with a 14-J shock), and 338 (98.5%) of these 343 passed both 14-J DSM screening and 21-J confirmation tests (Table 2). Of 73 patients (17.6%) who failed 14-J DSM screening (VF not terminated with a 14-J shock), 20 had a DFT >21 J. The number of patients who failed VSM or DSM screening and 21-J confirmation testing was similar (21 versus 20, or 5.0% versus 4.8%, respectively). Overall, the positive predictive accuracy of passing 14-J VSM or 14-J DSM screening and having a DFT ≤21 J (10-J safety margin) was 98.4% (95% CI 96.6% to 99.5%) and 98.5% (95% CI 96.4% to 99.5%), respectively. The odds of successful outcome of 21-J testing conditional on successful 14-J testing were not significantly different between the two 14-J tests, and there were no significant period or carryover effects (all P>0.10 from a logistic regression generalized estimating equations model).
Of 65 patients who had a discrepancy between VSM and DSM crossover screening (those who passed 1 screening methodology but failed the other), 60 (94%) of 64 patients with 21-J testing data available passed their 21-J confirmation testing (Table 3). Results of testing were similar to those randomized to VSM screening first compared with those who were randomized to DSM screening first (Table 4).
Of the 426 patients enrolled in the ASSURE Study, 26 (6.2%) of 422 with 21-J conversion testing results available had an initial DFT >21 J. Patients with more advanced heart failure were more likely to have an initial DFT >21 J (3.5% for those with NYHA I/II versus 8.7% for those with NYHA III/IV, P=0.03).
The average time required to perform the 3 T-wave shocks was 4.5±3.6 minutes for 14-J VSM screening and 2.4±4.6 minutes for 14-J DSM screening (single VF induction/conversion). Although this difference was statistically significant (P<0.01 from a paired t test), three fourths of all VSM tests were performed within 5 minutes, and the interquartile range for the difference in test times (VSM−DSM) was 1 to 4 minutes.
The incidence of hemodynamic compromise was assessed at 1 designated research site (LDS Hospital) after implant testing procedures. Of 87 study patients who received implants at this site, 2 (2.3%, 95% CI 0.3% to 8.1%) had systolic blood pressure drops >20 mm Hg that persisted longer than 1 minute after 14-J DSM screening as determined by continuous arterial blood pressure monitoring. Both events resolved of their own accord without treatment. Prolonged hypotension with sinus rhythm T-wave shocks from VSM screening was not observed during the present study.
The total follow-up time was 339 patient-years, with an overall average of 9.5±4.5 months of follow-up per patient. A total of 45 treated spontaneous tachyarrhythmia episodes ≥200 bpm occurred among 25 patients, for an episode rate of 13.3% per year (45 episodes/339 patient-years). Of the 45 episodes, 25 were classified as VF, 17 as VT, and 3 as polymorphic VT. The majority of episodes (38 of 45, or 84%; 95% CI 71% to 92%) were terminated by the first shock. In all observed cases in which the first shock was unsuccessful, subsequent shocks terminated fast VT/VF without complication. Among the 328 patients in the analysis cohort, a total of 25 episodes of fast VT/VF occurred among the 170 patients randomized to VSM screening. Of these 25 episodes, 23 (92%) were terminated with the first shock. Similarly, 12 episodes of fast VT/VF occurred among 158 patients in the analysis cohort randomized to DSM screening. Of these 12 episodes, 9 were terminated with the first shock. There were a total of 8 episodes of fast VT/VF among the 98 patients in the registry cohort, of which 6 (75%) were terminated with the first shock. There was no statistical difference in the first clinical shock success rate between the analysis or registry cohorts (P>0.05 from a logistic regression generalized estimating equations model). Of the 24 reported patient deaths that occurred during the study, none were reported as sudden cardiac death, and the number of deaths was not significantly different between the cohorts.
The assessment of DFT has long been the “gold standard” of ICD testing at implantation; however, it is not without additional procedural time and risk to the patient. Some have even suggested that DFT testing at the time of implantation may not be warranted given the advances in device and lead technology.3 Although device DFT has become more predictable over time, there is still a small subgroup of patients, especially those with more advanced heart failure, for whom adequate defibrillation energy requirements cannot be achieved by standard methods. Furthermore, it is important to test the integrity of the device shocking system at the time of implantation.
Although repeated VF induction, particularly in patients with advanced heart failure, may result in serious complications such as myocardial depression or ischemia, cerebral hypoperfusion, immediate cardiac dilation, acute pulmonary edema, elevation of cardiac enzymes, intractable VF, or even death,4–7 the present study did not observe any complications from shock testing at the time of implantation. Among the 87 patients monitored at a single center that used a standardized anesthesia protocol for prolonged hypotension with hemodynamic compromise, these conditions were observed in only 2 patients who underwent DSM screening at 14 J (single VF induction), and both resolved of their own accord. Conversely, no significant hypotension was observed for patients who underwent VSM screening at 14 J. These findings are consistent with our previously reported findings of a study of 194 patients who underwent device testing, in which prolonged hypotension was observed in 2% and 8% of patients after VSM and DSM screening, respectively (P=0.006).16
It has been shown that patients with advanced heart failure are more likely to have an elevated DSM.17 In fact, this patient type trended toward a higher DSM in the present study, 3.5% for those classified as NYHA I/II versus 8.7% for those deemed NYHA III/IV (P=0.0345). Similarly, in the Inhibition of Unnecessary RV Pacing With AV Search Hysteresis in ICDs (INTRINSIC RV Study) involving 1530 patients, 59 patients had an initial DFT >21 J (3.9%), and this was more commonly observed among patients with more advanced heart failure.18
Of interest, 5 patients (1.2%) in both the VSM and DSM groups, respectively, who passed their randomized screening tests at 14 J went on to fail higher-energy confirmation testing at 21 J (Table 2). It is possible that by the time 21-J confirmatory testing was performed, the patients’ underlying heart failure status may have deteriorated from the randomized crossover testing per protocol. Alternatively, the more likely reason is that the probabilistic nature of DFT testing may explain why a lower-energy shock may terminate VF from 1 induction, whereas a higher-energy shock delivered on subsequent induction may fail to terminate VF.
For any given patient, shocks delivered at the DFT energy level may or may not terminate VF. In fact, the “probability of defibrillation” was proposed to explain the lack of a preset, fixed DFT (eg, shocks delivered at an energy level below the DFT may occasionally terminate VF, and shocks delivered at an energy level above the DFT may occasionally fail to terminate VF). Although the mechanism of DFT testing is not well understood, it may be related to the electrical state of the heart at the instant of a shock,19 VF regularity,20 VF voltage amplitude,21 or the amount of myocardial tissue affected in its vulnerable period at the time of the shock.22
From the present study, VSM or DSM screening at 14 J provided a similar positive predictive accuracy (98.4% and 98.5%, respectively) of having a DFT of 21 J or less (10-J safety margin), and the odds of 21-J success given 14-J success did not vary significantly by testing method. These findings are consistent with the results of the Low Energy Safety Study (LESS), which reported that 1 successful VF conversion at 14 J was associated with a 99.1% positive predictive accuracy of the clinically accepted testing criteria of 2 VF shock conversions at 21 J.1,23 Although both screening methods appear to provide a similar degree of safety with regard to first clinical shock success by programming a device based on these screening methods, VSM screening at 14 J could have avoided intentional VF induction in 76.7% of the patients.
Two criticisms of VSM screening suggest that this methodology is difficult to learn and adds time to device implantation. On the basis of the present findings, VSM screening added a mere 2.1 minutes to the total case time. Moreover, even though most investigators in the present study had not previously performed VSM testing, evaluation of VSM measurements by the ECG Core Laboratory demonstrated uniformity among investigators.
Based on either VSM or DSM screening, the first clinical shock success rate for fast VT/VF was 92% and 75%, respectively. Although there was no statistical difference in first shock efficacy between the randomized implant testing methods, it is possible that the first clinical shock success rate for fast VT/VF in the present study could have been higher had devices been programmed to deliver the first clinical shock at maximal energy of 31 J. A lower-energy first clinical shock of 21 J was selected to minimize device charge time and potentially avoid the postshock mechanical dysfunction and electromechanical dissociation that may occur with higher-energy shocks.6,24 With ICD/CRTD devices currently available, charge times are generally short for any shock energy strength. Thus, the potential risks of a higher first energy shock may be offset by an increased first clinical shock success rate for spontaneously occurring fast VT/VF. Among all observed cases in which the first shock was unsuccessful, subsequent shocks terminated fast VT/VF without complication. Moreover, of the reported deaths that occurred during follow-up, on postmortem device evaluation, none were due to failure of programmed shocks to terminate a ventricular arrhythmia.
One limitation of “inductionless implants” is that they do not assess sensing of VF by the ICD. In the present study, a minimum right ventricular R wave of 7 and 5 mV was required for primary implants and device replacements, respectively. On the basis of these criteria, VF undersensing during DSM and confirmatory testing was not observed. Certainly, in cases in which one could not obtain a satisfactory right ventricular R wave at the time of implantation, VF induction would be helpful to ensure that the device could appropriately detect VF.
Notably, the main limitation of the ASSURE Study was that there were fewer than expected spontaneously occurring clinical episodes of fast VT/VF during follow-up. Thus, given the limited number of clinical shocks for fast VT/VF, our calculated sample size and follow-up duration were unable to detect any statistically significant differences in first clinical shock success between VSM or DSM screening at the time of device implantation.
There was no difference in the odds of successful implant DFT testing at 21 J or lower conditional on the success of VSM or DSM screening at 14 J during device implantation. Although clinical episodes of fast VT/VF were limited, first shock efficacy was similar. Screening patients with VSM or DSM testing may allow for inductionless or limited shock testing in most patients. Low-energy VSM or DSM screening provides a reasonable balance between identifying those patients who do not require significant implant testing and those who may have an increased DSM and may require more formal DSM testing. Furthermore, this may prevent unnecessary additional VF inductions during implantation in most patients.
We thank Christopher M. Mullin and Christopher Pulling of Integra Clinical Trial Solutions for their statistical support. We also thank Stephen Hahn for providing scientific expertise on the study design and methodologies of implant testing.
Sources of Funding
The ASSURE Study was funded and supported by Boston Scientific CRM (formerly Guidant Corporation).
Dr Doshi has received modest honoraria from the speaker’s bureau at Guidant and St. Jude Medical and serves as a consultant to Cameron Health. Dr Belott serves as a consultant to Cameron Health, Cardiodynamics Inc, and Guidant and serves on the medical advisory boards of both Guidant and Cardiodynamics Inc; he also serves on the editorial board of PACE and has received modest honoraria from the speaker’s bureau for Guidant. Dr Birgersdotter-Green has worked on behalf of the speaker’s bureau and has received research grants from both Guidant and Medtronic. Dr Lee has received modest honoraria from both Medtronic and St. Jude Medical. Dr Wiener has received research grants and has served on behalf of the speaker’s bureau for Guidant, Medtronic, and St. Jude Medical. Dr Grogin has received modest consulting honoraria from Guidant. Dr Chun has received research grants from both Medtronic and St. Jude Medical. Dr Crandall has worked both on behalf of the speaker’s bureau and as an investigator for both Guidant and Medtronic. Dr Neuman is employed by Boston Scientific CRM. The remaining authors report no conflicts.
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The method of upper-limit-of-vulnerability testing has been closely correlated with defibrillation threshold testing among patients undergoing implantable cardioverter-defibrillator implantation. Although traditional defibrillation threshold testing is considered safe, multiple inductions of ventricular fibrillation may result in serious complications. The present study demonstrated that most patients could safely undergo low-energy inductionless or limited shock testing during implantation. Moreover, both screening methods used in the present study appeared to provide a similar degree of safety with regard to first clinical shock success. Although the majority of spontaneously occurring arrhythmias ≥200 bpm were terminated by the first shock, subsequent shocks terminated fast arrhythmias without consequence. The method of vulnerability safety margin screening added to the total case time; however, total time to conduct both tests was short. Three fourths of all vulnerability safety margin tests were performed within 5 minutes. Low-energy vulnerability safety margin or defibrillation safety margin screening may be beneficial for selected patients who have good sensing parameters from the implantable cardioverter-defibrillator lead and who do not have factors, such as heart failure, that warrant more extensive testing. The utilization of low-energy inductionless or limited shock testing could prevent unnecessary additional ventricular fibrillation inductions during implantation in many patients.
Clinical trial registration information—URL: http://www.clinicaltrials.gov. Unique identifier: NCT00231426.