Defibrillation threshold: clinical utility and therapeutic implications

Defibrillation Threshold Testing for Right-sided Device Implants: A Review to Inform Shared Decision-making, in Association with the British Heart Rhythm Society

Prevention of sudden death using ICDs requires the reliable delivery of a high-energy shock to successfully terminate VF. Until more recently, the device implant procedure included conducting defibrillation threshold (DFT) testing involving VF induction and shock delivery to ensure efficacy. Large clinical trials, including SIMPLE and NORDIC ICD, have subsequently demonstrated that this is unnecessary, with a practice of omitting DFT testing having no impact on subsequent clinical outcomes. However, these studies specifically excluded patients requiring devices implanted on the right side, in whom the shock vector is significantly different and smaller studies suggest a higher DFT. In this review, the data regarding the use of DFT testing, focusing on right-sided implants, and the results of a survey of current UK practice are presented. In addition, a strategy of shared decision-making when it comes to deciding on the use of DFT testing during right-sided ICD implant procedures is proposed.

Defibrillation threshold testing during implantable cardioverter defibrillator implantation: 5-year follow-up

PURPOSE

Defibrillation threshold (DFT) testing is a routine practice in some Asian countries for patients receiving an implantable cardioverter defibrillator (ICD). However, there are few long-term data about the necessity of intraoperative DFT testing in an Asian population. We investigated the safety of DFT testing and the long-term clinical outcomes in Asian patients undergoing ICD implantation.

METHODS

All patients undergoing de novo transvenous ICD implantation were randomized to undergo periprocedural DFT testing. The study included 67 patients (50 males; 51.5 ± 16.9 years) who underwent ICD implantation with (n = 33) or without (n = 34) intraoperative DFT testing between March 2012 and February 2014. We compared first-shock success, composite safety end points (the sum of complications recorded at 30 days), arrhythmic death, and all-cause mortality.

RESULTS

The baseline clinical characteristics and the procedural-related adverse event rate (3.0% with DFT vs. 0% with non-DFT, p = 0.214) did not differ between groups. The programmed output of the first shock was lower in the DFT testing group (22.9 ± 4.4 J vs. 25.3 ± 5.4 J, p = 0.007). However, there were no significant differences between groups for all-cause mortality (12.1% vs. 17.6%, p = 0.526) or first-shock success rate for ventricular arrhythmia (100% vs. 88.2%, p = 0.471).

CONCLUSIONS

There were no between-group differences in periprocedural safety, complications, and long-term clinical outcomes. Our results suggest that DFT testing in Asian patients allows reduction of the programmed output of the first shock, but does not affect long-term clinical outcomes.

Measurement of the Upper Limit of Vulnerability during Defibrillator Implantation can Substitute Defibrillation Threshold Measurement

We investigated whether defibrillation thresholds (DFTs) could be measured more safely during defibrillator implantation by measuring the upper limit of vulnerability (ULV) without using any special equipment.

Nonthoracotomy ICD implantation with endocardial leads was performed in 13 patients, and through the use of the ICD function itself, ULV and DFT were measured using the delayed four-episode up-down algorithm. Myocardial injures caused by high-energy current were assessed by electrocardiograms and serial CPK-MB.

ULV was confirmed in all cases, and it strongly correlated with DFT. The average ULV was 5.9 ± 3.3J, while the average DFT was 7.9 ± 4.3J (r = 0.89, p < 0.0001, DFT = 1.20+1.14x ULV). The average ULV was thus significantly lower (p < 0.01). Although six patients were on amiodarone therapy, the strong correlation between ULV and DFT was also maintained (r = 0,97), p < 0.01) in these patients.

In all cases, the CPK-MB failed to increase, and no myocardial injuries were detectable on electrocardiograms.

We confirmed that ULV could be easily and safety measured during ICD implantation, and that ULV could be used instead of DFT.

Subcutaneous Implantable Cardioverter Defibrillators Implantation Without Defibrillation Threshold Testing: A Single Center Experience

Background

Subcutaneous implantable cardioverter defibrillator (S-ICD) system has been proven to be an effective therapy for prevention of sudden cardiac death (SCD) in selected patients. Although the Shockless IMPLant Evaluation (SIMPLE) trial has shown that defibrillation threshold (DFT) testing is not necessary for transvenous ICD (TV-ICD) systems, it is still recommended for S-ICD systems. We aimed to study the efficacy and safety of S-ICD implantation without DFT in our Heart Center with the comparison of S-ICD patients’ outcome to those with a single chamber TV-ICD without DFT in the same period.

Methods

A retrospective analysis of patients underwent S-ICD without DFT from December 2014 to May 2016 with the comparison to single chamber TV-ICD patients implanted during the same period.

Results

Thirty consecutive patients (23 males (76.7%); mean age 41 ± 13 years; mean left ventricular ejection fraction 30±12%) received a S-ICD for primary (25 patients, 83.3%) or secondary prevention (five patients, 16.7%) of SCD. During a mean follow-up of 710.6 ± 190 days, three patients received 38 appropriate ICD shocks (90.5%), and two patients received four inappropriate shocks (9.5%). There were two mortalities (6.7%): one cardiac and one non-cardiac. When compared to 30 consecutive who received a single chamber TV-ICD during the same period, there was no significant difference in mortality.

Conclusions

Implantation of S-ICD using intermuscular approach without DFT seems to be safe and effective. Data from large S-ICD registries with long-term follow-up, and preferably randomized controlled studies, are needed to confirm this finding.

Defibrillation testing during implantable cardioverter-defibrillator implantation in Italian current practice: the Assessment of Long-term Induction clinical ValuE (ALIVE) project

BACKGROUND

Clinical practice with regard to defibrillation threshold (DFT) testing during implantable cardioverter-defibrillator (ICD) implantation varies considerably, even among experienced implanting centers. International guidelines do not as yet mandate DFT testing.

OBJECTIVE

The objective of this project is to assess current clinical decision making regarding DFT testing during ICD implantation.

METHODS

The ALIVE project collected data on DFT testing from a multicenter network of Italian clinicians sharing a common system for the collection, management, analysis, and reporting of clinical and diagnostic data from patients with Medtronic (Minneapolis, MN) implantable devices.

RESULTS

Data on 2,082 consecutive patients implanted with a Medtronic ICD in 111 Italian centers, over the period 2007 to 2010, were analyzed. Defibrillation threshold testing was performed in 33% of cases (678/2,082). The main reasons for performing the test were physician's clinical practice ("I always perform DFT") (80%) and secondary prevention implantation (12%). The main reasons for not performing DFT testing were centers' practice (44%), primary prevention (31%), and device replacement (15%). In 22 patients, ventricular fibrillation induction was not achieved; 656 patients completed DFT testing: 633 patients (96%) performed a single test, 19 patients (3%) performed a second induction test, and 4 patients (0.6%) underwent an additional induction test.

CONCLUSIONS

The preliminary results of the ALIVE project show that a great number of implant procedures are performed without DFT testing in the common practice of the participating centers. We also measured an inhomogeneous, center-dependent DFT testing behavior, which suggests the importance of defining a common guideline for ICD implant testing. Follow-up data on our patients will provide more information on the clinical value of the test.

Patient-tailored implantable cardioverter defibrillator testing using the upper limit of vulnerability: the TULIP protocol

AIMS

We evaluated the feasibility of the TULIP (Threshold test using Upper Limit during ImPlantation) protocol, which was designed to provide a confirmed, low defibrillation energy value during implantable cardioverter defibrillator (ICD) implantation with only two induced ventricular fibrillation (VF) episodes.

METHODS AND RESULTS

Ninety-eight patients (62 +/- 12 years, 86 male) from 13 clinical centres underwent an active can ICD implantation. A single coupling interval derived from electrocardiogram lead II during ventricular pacing was used for VF induction shocks at 13, 11, 9, and 6 J in a step-down manner until the upper limit of VF induction (ULVI) was determined. If ULVI >or=9 J, a defibrillation energy of ULVI + 4 J was tested. For ULVI <9 J, the defibrillation test energy was 9 J. In 79/98 patients (80.6%), two induced VF episodes were sufficient to obtain confirmed defibrillation energy of 11.1 +/- 3.3 J. The mean strength of the successful VF induction shock was 6.8 +/- 4.3 J, the coupling interval was 303 +/- 35 ms, and the number of delivered induction shocks until the first VF induction was 3.9 +/- 1.6.

CONCLUSION

TULIP is a safe and simple device testing procedure allowing the determination of confirmed, low defibrillation energy in most patients with two VF episodes induced at a single coupling interval.

Successful low-energy cardioversion using a novel biodegradable gel pad: Feasibility of treating postoperative atrial fibrillation in animals

OBJECTIVE

Postoperative atrial fibrillation is one of the most frequent complications of cardiac surgery. We developed a novel biodegradable gel pad consisting of biopolymers that directly attach to the myocardium by electrostatic interaction. The present study examines the feasibility and effectiveness of low-energy internal cardioversion using these pads.

METHODS

The hearts of 6 pigs were exposed through a median sternotomy under general anesthesia, and 2 monopolar pacing wires were placed on the left pulmonary veins (chest open group). Two biodegradable cardioversion gel pads were placed on the right appendage and the left atria without suturing. All wires were extruded through the skin and secured with a suture. Sustained atrial fibrillation was induced by burst-pacing from the pulmonary veins in continuous 20-ms cycles. Shock intensity started at 0.5 J, and the energy level was increased in 0.5-J increments until cardioversion occurred. This protocol was repeated 5 times per pig. In a second group of 6 pigs (chest closed group), the epicardial cardioversion electrode gel pads and pacing wire electrodes were positioned as described above. Shock intensity was started at 0.5 J. If the shock was unsuccessful, the energy level was increased in 0.5-J increments until 2 consecutive cardioversions were achieved at a single energy level. At postoperative days 1, 3, 5, and 7, the defibrillation threshold was determined with the chest closed. At postoperative day 10, the cardioversion wires were removed. At predetermined time intervals, the heart was reexposed and the extent of degradation in vivo was visually evaluated and histologically assessed after sacrifice.

RESULTS

All pigs with induced atrial fibrillation were cardioverted to sinus rhythm on the determined postoperative day. The mean energy and lead impedance in the chest open group were 0.65 +/- 0.23 J and 97.6 +/- 5.52 Omega, respectively, and the overall values of mean energy and lead impedance in the chest closed group were 1.67 +/- 1.00 J and 75.9 +/- 13.3 Omega, respectively. No complications were observed after wire removal. The gel pads became degraded and decreased in thickness, and signs of mild inflammation were evident on the gel pad. However, the gel pads did not elicit significant severe inflammatory reactions according to both gross and histologic assessments at 1 month after the surgery.

CONCLUSION

Atrial cardioversion using novel biodegradable gel pads that are easily affixed may afford a straightforward and effective treatment for atrial fibrillation after cardiac surgery.

Testing the Implantable Cardioverter-Defibrillator After Implantation?Is It Necessary?

The results of intraoperative and postoperative predischarge implantable cardioverter-defibrillator (ICD) testing of 211 consecutive patients, starting at 15 J and requiring two successful terminations of induced VT/VF with a relative defibrillation safety margin (DSM) of >10 J, were reviewed. The aim was to define the type of intraoperative response that would make postoperative predischarge testing unnecessary. The intraoperative responses were divided into three types: A, a DSM > or =10 J and an absolute energy level of < or =20 J; B, a DSM of > or =10 J and an absolute energy level of >20 J; and C, a DSM <10 J and an absolute energy level of >20 J. At operation, the responses to defibrillation were A, 88.6%; B, 7.1%; and C, 4.3%. Accepting an A response only would leave 11.4% of the patients for postoperative testing. The positive and negative predictive values for diagnosing a postoperative C response were 0.78 and 0.97, respectively. Similarly, the predictive values for diagnosing a postoperative B or C response were 0.71 and 0.97, respectively. The postoperative testing responses were A, 89.1%; B, 4.3%; and C, 6.6%. In summary, an intraoperative A response was sufficient to make a postoperative defibrillation testing unnecessary, while it was found that intraoperative B and C responders should undergo postoperative testing. Applying these criteria, approximately 90% of the patients could be discharged without any postoperative induction test.

The dilemma of ICD implant testing

Ventricular fibrillation (VF) has been induced at implantable cardioverter defibrillator (ICD) implant to ensure reliable sensing, detection, and defibrillation. Despite its risks, the value was self-evident for early ICDs: failure of defibrillation was common, recipients had a high risk of ventricular tachycardia (VT) or VF, and the only therapy for rapid VT or VF was a shock. Today, failure of defibrillation is rare, the risk of VT/VF is lower in some recipients, antitachycardia pacing is applied for fast VT, and vulnerability testing permits assessment of defibrillation efficacy without inducing VF in most patients. This review reappraises ICD implant testing. At implant, defibrillation success is influenced by both predictable and unpredictable factors, including those related to the patient, ICD system, drugs, and complications. For left pectoral implants of high-output ICDs, the probability of passing a 10 J safety margin is approximately 95%, the probability that a maximum output shock will defibrillate is approximately 99%, and the incidence of system revision based on testing is < or = 5%. Bayes' Theorem predicts that implant testing identifies < or = 50% of patients at high risk for unsuccessful defibrillation. Most patients who fail implant criteria have false negative tests and may undergo unnecessary revision of their ICD systems. The first-shock success rate for spontaneous VT/VF ranges from 83% to 93%, lower than that for induced VF. Thus, shocks for spontaneous VT/VF fail for reasons that are not evaluated at implant. Whether system revision based on implant testing improves this success rate is unknown. The risks of implant testing include those related to VF and those related to shocks alone. The former may be due to circulatory arrest alone or the combination of circulatory arrest and shocks. Vulnerability testing reduces risks related to VF, but not those related to shocks. Mortality from implant testing probably is 0.1-0.2%. Overall, VF should be induced to assess sensing in approximately 5% of ICD recipients. Defibrillation or vulnerability testing is indicated in 20-40% of recipients who can be identified as having a higher-than-usual probability of an inadequate defibrillation safety margin based on patient-specific factors. However, implant testing is too risky in approximately 5% of recipients and may not be worth the risks in 10-30%. In 25-50% of ICD recipients, testing cannot be identified as either critical or contraindicated.

Inductionless or Limited Shock Testing Is Possible in Most Patients With Implantable Cardioverter- Defibrillators/Cardiac Resynchronization Therapy Defibrillators

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.

Using the Upper Limit of Vulnerability to Assess Defibrillation Efficacy at Implantation of ICDs

The upper limit of vulnerability (ULV) is the weakest shock strength at or above which ventricular fibrillation (VF) is not induced when the shock is delivered during the vulnerable period. The ULV, a measurement made in regular rhythm, provides an estimate of the minimum shock strength required for reliable defibrillation that is as accurate or more accurate than the defibrillation threshold (DFT). The ULV hypothesis of defibrillation postulates a mechanistic relationship between the ULV-measured during regular rhythm-and the minimum shock strength that defibrillates reliably. Vulnerability testing can be applied at implantable cardioverter defibrillator (ICD) implant to confirm a clinically adequate defibrillation safety margin without inducing VF in 75%-95% of ICD recipients. Alternatively, the ULV provides an accurate patient-specific safety margin with a single fibrillation-defibrillation episode. Programming first ICD shocks based on patient-specific measurements of ULV rather than programming routinely to maximum output shortens charge time and may reduce the probability of syncope as ICDs age and charge times increase. Because the ULV is more reproducible than the DFT, it provides greater statistical power for clinical research with fewer episodes of VF. Limited evidence suggests that vulnerability testing is safer than conventional defibrillation testing.

Effect of defibrillation testing on management during implantable cardioverter-defibrillator implantation

BACKGROUND

Verification of defibrillation efficacy by defibrillation threshold (DFT) testing during implantable cardioverter-defibrillator implantation is the current standard. Generally, defibrillation of ventricular fibrillation at 10 J below the maximum output of a device is felt to establish an adequate safety margin. Nonetheless, DFT testing adds to cost and carries some potential for morbidity, whereas its impact on outcomes in the modern era of defibrillator technology is unclear. We aimed to determine the frequency that DFT testing resulted in a change at device implant and to identify clinical and echocardiographic predictors of the need for DFT testing.

METHODS

We reviewed all implantable cardioverter-defibrillators that were implanted at the London Health Sciences Centre (Ontario, Canada) from June 1999 to August 2003 and used multivariate analysis to determine variables associated with DFT test failures and elevated DFT values. When a defibrillation failure was not observed, a lowest energy to defibrillate (LED) was recorded.

RESULTS

Among 168 implants, DFT testing was successful with a minimum 10-J safety margin in 152 (90%), whereas the remaining 16 required changes at device implant. In a multivariate analysis, use of amiodarone was independently associated with DFT failure (odds ratio, 4.6; 95% confidence interval, 1.2-17.0). Significantly higher mean DFT/LED values were observed among patients on amiodarone (1.36 J; P = .0041). Those with nonischemic cardiomyopathy had a higher mean DFT/LED compared with those with ischemic cardiomyopathy (1.44 J; P = .028).

CONCLUSIONS

Use of amiodarone is associated with a 4-fold increase in risk of DFT failure and subsequent need for changes at implant to achieve a safe threshold. Defibrillation threshold testing appears to be most useful for patients taking amiodarone.

Effect of an active abdominal pulse generator on defibrillation thresholds with a dual-coil, transvenous ICD lead system

INTRODUCTION

Many patients with implantable cardioverter defibrillators (ICDs) have older lead systems, which are usually not replaced at the time of pulse generator replacement unless a malfunction is noted. Therefore, optimization of defibrillation with these lead systems is clinically important. The objective of this prospective study was to determine if an active abdominal pulse generator (Can) affects chronic defibrillation thresholds (DFTs) with a dual-coil, transvenous ICD lead system.

METHODS AND RESULTS

The study population consisted of 39 patients who presented for routine abdominal pulse generator replacement. Each patient underwent two assessments of DFT using a step-down protocol, with the order of testing randomized. The distal right ventricular (RV) coil was the anode for the first phase of the biphasic shocks. The proximal superior vena cava (SVC) coil was the cathode for the Lead Alone configuration (RV --> SVC). For the Active Can configuration, the SVC coil and Can were connected electrically as the cathode (RV --> SVC + Can). The Active Can configuration was associated with a significant decrease in shock impedance (39.5 +/- 5.8 Omega vs. 50.0 +/- 7.6 Omega, P < 0.01) and a significant increase in peak current (8.3 +/- 2.6 A vs. 7.2 +/- 2.4 A, P < 0.01). There was no significant difference in DFT energy (9.0 +/- 4.6 J vs. 9.8 +/- 5.2 J) or leading edge voltage (319 +/- 86 V vs. 315 +/- 83 V). An adequate safety margin for defibrillation (> or =10 J) was present in all patients with both shocking configurations.

CONCLUSION

DFTs are similar with the Active Can and Lead Alone configurations when a dual-coil, transvenous lead is used with a left abdominal pulse generator. Since most commercially available ICDs are only available with an active can, our data support the use of an active can device with this lead system for patients who present for routine pulse generator replacement.

A comparison of the output characteristics of several implantable cardioverter-defibrillators

BACKGROUND

Implantable cardioverter-defibrillators (ICDs) are effective for primary and secondary prevention of sudden cardiac death due to ventricular arrhythmias. However, despite wide clinical use, there are no generally accepted standardized protocols to characterize and report the output capabilities of ICDs.

OBJECTIVE

The objective of this study was to measure and compare the output characteristics of standard-output and high-output ICDs from several manufacturers under a common set of conditions.

METHODS

The output characteristics of ICDs randomly selected from hospital stock were measured. The energy delivered for each shock to a range of fixed loads (25-75 Omega) was computed from the voltage waveform and the corresponding load.

RESULTS

Delivered energy varied by approximately 4 J over the range of loads tested and varied between devices (high-output 33.8-35 J; standard-output 26.7-28.6 J, at 50 Omega). Leading-edge voltage varied by approximately 6% over the range of loads tested and varied between devices (high-output 738-792 V; standard-output 593-797 V, at 50 Omega). Pulse width varied by a factor of approximately 3 over the range of loads tested and varied between devices (high-output 10-14.5 ms; standard-output 9-12.2 ms, at 50 Omega). Observed variations between devices and with load were significant (P <.001).

CONCLUSIONS

Potentially important differences in output characteristics of different ICD systems exist and merit further clinical investigation. The reporting of ICD output characteristics should be standardized. Additionally, it is recommended that manufacturers report output characteristics as a function of load over the typical range of patient loads clinically encountered.

Intraoperative testing of the implantable cardioverter-defibrillator: how much is enough?

INTRODUCTION

Defibrillation testing of the implantable cardioverter-defibrillator (ICD) is considered a standard and required practice at the time of implantation. How much testing, if any in some cases, should be performed, however, remains unknown.

METHODS AND RESULTS

Included in this retrospective analysis were 835 patients (77% men; age 65 +/- 13 years) who received transvenous ICDs between January 1996 and December 2003. One hundred twenty-nine (15.5%) had intraoperative defibrillation threshold (DFT) testing, 503 (60.2%) had limited defibrillation safety margin testing, and 203 (24.3%) had no defibrillation testing. We compared the outcome (success of ICD therapies against spontaneous VT/VF events and survival) of the three groups of patients, who in some respects had important clinical differences. The success of the first delivered shocks against VT/VF was similar for DFT (91%), safety margin testing (91%), and no-testing (92%) groups; and the second shocks terminated the remaining episodes in all three groups. Sudden-death-free survival rates were similar in the three groups, however, the overall long-term survival rate was significantly lower in the no-testing group (58%) than in the DFT (74%) and safety margin testing (69%) groups (P < 0.0005). Multivariate analysis found no strong predictors of sudden death, but there were several independent predictors of overall mortality including lack of ICD testing (HR: 2.031, CI: 1.253-3.290, P = 0.004).

CONCLUSION

In this select patient cohort, success of ICD therapies and sudden-death-free survival were similar in patients who had DFT, safety margin testing, and no testing, but overall survival was significantly lower in the no-testing group. Thus in the absence of prospective mortality data, a minimum of safety margin ICD testing should remain standard practice.

Coronary sinus electrode does not reduce atrial defibrillation thresholds

BACKGROUND

Atrial defibrillation can be achieved with a conventional dual-coil, active pectoral implantable cardioverter-defibrillator (ICD) lead system. Shocking vectors that incorporate an additional electrode in the CS have been used, but it is unclear if they improve atrial DFTs.

OBJECTIVE

The objective of this prospective, randomized study was to determine if a coronary sinus (CS) electrode reduces atrial defibrillation thresholds (DFTs).

METHODS

This was a prospective study of 36 patients undergoing initial ICD implant for standard indications. A defibrillation lead with superior vena cava (SVC) and right ventricular (RV) shocking coils was implanted in the RV. An active can emulator (Can) was placed in a pre-pectoral pocket. A lead with a 4 cm long shocking coil was placed in the CS. Atrial DFTs were determined in the following 3 shocking configurations in each patient, with the order of testing randomized: RV --> SVC + Can (Ventricular Triad), distal CS --> SVC + Can (Distal Atrial Triad), and proximal CS --> SVC + Can (Proximal Atrial Triad).

RESULTS

The Proximal and Distal Atrial Triad configurations were both associated with significant reductions in peak current (p < 0.01), but this effect was offset by significant increases in shock impedance (p < 0.01), resulting in no net change in the peak voltage or DFT energy in comparison to the Ventricular Triad configuration (Ventricular Triad: 4.9 +/- 6.6 J, Proximal Atrial Triad: 3.3 +/- 4.1J, Distal Atrial Triad: 4.4 +/- 6.7 J, p > 0.2).

CONCLUSION

Shocking vectors that incorporate a CS coil do not significantly improve atrial defibrillation efficacy. Since the Ventricular Triad shocking pathway provides reliable atrial and ventricular defibrillation, this configuration should be preferred for combined atrial and ventricular ICDs.

Cardioverter-defibrillator implantation in high-risk patients with hypertrophic cardiomyopathy

BACKGROUND

Implantable cardioverter-defibrillators (ICDs) are used with increasing frequency in hypertrophic cardiomyopathy (HCM) patients of all ages for primary and secondary sudden death prevention. Concerns may arise regarding the safety of device implantation because of unique clinical and phenotypic expressions of HCM.

OBJECTIVES

The purpose of this study was to assess the efficacy and safety of ICD placement in high-risk patients with HCM.

METHODS

We analyzed the experience with ICDs and transvenous lead systems in 75 consecutive HCM patients at the Minneapolis Heart Institute from 1993 to 2004.

RESULTS

The age of the study group patients was 12 to 79 years (mean 36 +/- 16). Patients received ICDs for secondary (n = 4, after cardiac arrest) or primary prevention (n = 71, with > or = 1 risk factor). Thirty-one patients demonstrated disease features that potentially impacted methodology and safety of the implant procedure, most commonly massive left ventricular (LV) hypertrophy and outflow obstruction > or = 50 mmHg. There were no procedure-related deaths; defibrillator implants were successful and uneventful in 71 of 75 patients (95%). In 3 of the 75 patients (4%), defibrillation was unsuccessful because of high thresholds, associated with extreme hypertrophy (wall thickness > 45 mm) and/or ongoing amiodarone therapy. In two of these patients, thoracotomy with epicardial lead placement achieved successful defibrillation; ICD therapy was abandoned in the other patient.

CONCLUSION

ICD placement in children and adults with HCM is generally safe and effective. However, in some patients with massive LV hypertrophy and/or prior administration of amiodarone, transvenous defibrillation proved difficult, and epicardial lead placement was required. High-energy ICD devices and defibrillation threshold testing are recommended for most high-risk HCM patients.

Detection of the Defibrillation Threshold Using the Upper Limit of Vulnerability Following Defibrillator Implantation

OBJECTIVE

This study was designed to test defibrillation threshold (DFT) with the least number of fibrillation inductions using upper limit of vulnerability (ULV) and to describe the most practical set of ICD during DFT following implantation.

BACKGROUND

Although the correlation between ULV and DFT has been well described, there has been no uniform DFT testing protocol taking the advantage of ULV after defibrillator (ICD) implantation.

METHODS

A total of 26 patients undergoing a new ICD implantation had a DFT induced with scanned T wave shock. The hypothesis that ventricular fibrillation (VF) could be defibrillated with 5 J higher than the highest T wave shock needed to induce VF or with 10 J if the T wave shock needed to induce VF was less than 5 J, was tested and 20 patients fulfilled these criteria. The methodology is improved by detecting peak T wave with 12-lead ECG, applying biphasic T wave shock and scanning the T wave shock in a wider window.

RESULTS

Five patients in the first group (n = 15) and one patient in the second group (n = 11) did not fulfill the above hypothesis. The common features of six patients who did not fulfill the hypothesis were that T wave shock needed to induce VF was either under 5 J (5 patients) or high (1 patient).

CONCLUSION

This study revealed the importance of methodology in studies regarding ULV and DFT. Following ICD implantation, we propose the first biphasic T wave detected by 12-lead ECG and rescue shock set at 10 and 15 J, respectively. If any of the scanned T wave (40 ms before and 40 ms after the peak T wave with decrements and increments of 20 ms) shocks could not induce VF, then the T wave and the first rescue shock should be set at 5 and 10 J, respectively. If the induction of VF has been unsuccessful with T wave shock at 5 J, then a safe defibrillation with 10 J should be expected in majority.

One conversion of ventricular fibrillation is adequate for implantable cardioverter-defibrillator implant: An analysis from the Low Energy Safety Study (LESS)

OBJECTIVES

The purpose of this study was to analyze defibrillation conversion data from the Low Energy Safety Study (LESS) to determine how implant criteria that use fewer inductions of ventricular fibrillation (VF) correlate with outcome and, in particular, to assess the reliability of using a single VF induction and test shock at 14 J.

BACKGROUND

A safety margin of 10 J has become standard for implantation of an implantable cardioverter-defibrillator (ICD), but the specifics and rigor of the implant test sequence are not standardized.

METHODS

In LESS, 611 ICD recipients completed a rigorous VF induction test scheme that began at 14 J and continued until the energy that succeeded three times without a failure was determined (DFT++). The data were analyzed to determine how well the outcome of the first 14-J shock and various other combinations of first and/or second shocks predicted a rigorous gold standard of DFT++ < or =21 J (i.e., three successes at < or =21 J).

RESULTS

The positive predictive accuracy for the 91% of patients in whom the first 14-J shock succeeded was virtually identical to the positive predictive accuracy for the commonly used criteria of two successes at < or =17 J (99.1% vs 99.0%, P = .69), and slightly higher than the positive predictive accuracy for two successes at < or =21 J (98.8%, P = .51). A single success at 17 J or 21 J had a somewhat lower positive predictive accuracy of 98.2% (P = .17). Eliminating VF induction testing would have resulted in a significantly lower positive predictive accuracy of 97.1% (P = .01).

CONCLUSIONS

A single conversion success at 14 J on the first VF induction provides similar positive predictive accuracy as two successes at 17 J or 21 J. Using this criterion, 91% of patients meet implant criteria with a single induction of ventricular fibrillation.

Clinical predictors of atrial defibrillation thresholds with a dual-coil, active pectoral lead system

OBJECTIVES

The purpose of this study was to identify clinical predictors of atrial defibrillation thresholds (DFTs) with standard implantable cardioverter-defibrillator (ICD) leads.

BACKGROUND

Atrial defibrillation can be achieved with active pectoral, dual-coil transvenous ICD lead systems. If clinical predictors of atrial defibrillation efficacy with these lead systems were identified, they could be used to predict which patients may require more complex lead systems for atrial defibrillation, such as a coronary sinus electrode.

METHODS

This was a prospective study of 135 consecutive patients undergoing initial ICD implant for standard indications. The lead system evaluated was a transvenous defibrillation lead with coils in the superior vena cava (SVC) and right ventricular apex (RV), and a left pectoral pulse generator emulator (CAN). The shocking pathway was RV-->SVC+CAN. Atrial DFT was measured using a step-up protocol. Clinical and echocardiographic parameters were evaluated as predictors of atrial DFT and multiple linear regression was performed.

RESULTS

Mean atrial DFT was 4.6 +/- 3.8 J. Atrial DFT was < or =3 J in 70 patients (52%) and < or = 10 J in 97% of patients. The highest atrial DFT was 20 J (one patient). Left atrial size (r = 0.21, P = .01) and left ventricular end-diastolic diameter (r = 0.19, P = .02) were independent predictors of atrial DFT. However, these two predictors accounted for only 6% of the variability in atrial DFT.

CONCLUSIONS

Clinical parameters are of limited use in predicting atrial DFT with a dual-coil, active pectoral ICD lead system. Because the RV--> SVC + CAN shocking pathway provides reliable atrial and ventricular defibrillation, this configuration should be preferred for combined atrial and ventricular ICDs.

Optimization of Atrial Defibrillation with a Dual-Coil, Active Pectoral Lead System

INTRODUCTION

Atrial defibrillation can be achieved with standard implantable cardioverter defibrillator (ICD) leads, but the optimal shocking configuration is unknown. The objective of this prospective study was to compare atrial defibrillation thresholds (DFTs) with three shocking configurations that are available with standard ICD leads.

METHODS AND RESULTS

This study was a prospective, randomized, paired comparison of shocking configurations on atrial DFTs in 58 patients. The lead system evaluated was a transvenous defibrillation lead with coils in the superior vena cava (SVC) and right ventricular apex (RV) and a left pectoral pulse generator emulator (Can). In the first 33 patients, atrial DFT was measured with the ventricular triad (RV --> SVC + Can) and unipolar (RV --> Can) shocking pathways. In the next 25 patients, atrial DFT was measured with the ventricular triad and the proximal triad (SVC --> RV + Can) configurations. Delivered energy at DFT was significantly lower with the ventricular triad compared to the unipolar configuration (4.7 +/- 3.7 J vs 10.1 +/- 9.5 J, P < 0.001). Peak voltage and shock impedance also were significantly reduced (P < 0.001). There was no significant difference in DFT energy when the ventricular triad and proximal triad shocking configurations were compared (3.6 +/- 3.0 J vs 3.4 +/- 2.9 J for ventricular and proximal triad, respectively, P = NS). Although shock impedance was reduced by 13% with the proximal triad (P < 0.001), this effect was offset by an increased current requirement (10%).

CONCLUSION

The ventricular triad is equivalent or superior to other possible shocking pathways for atrial defibrillation afforded by a dual-coil, active pectoral lead system. Because the ventricular triad is also the most efficacious shocking pathway for ventricular defibrillation, this pathway should be preferred for combined atrial and ventricular defibrillators.

Comparison of Step-Down and Binary Search Algorithms for Determination of Defibrillation Threshold in Humans

Determination of DFT is an integral part of ICD implantation. Two commonly used methods of DFT determination, the step-down method and the binary search method, were compared in 44 patients undergoing ICD testing for standard clinical indications. The step-down protocol used an initial shock of 18 J. The binary search method began with a shock energy of 9 J and successive shock energies were increased or decreased depending on the success of the previous shock. The DFT was defined as the lowest energy that successfully terminated ventricular fibrillation. The binary search method has the advantage of requiring a predetermined number of shocks, but some have questioned its accuracy. The study found that (mean) DFT obtained by the step-down method was 8.2 +/- 5.0, whereas by the binary search method DFT was 8.1 +/- 0.7 J, P = NS. DFT differed by no more than one step between methods in 32 (71%) of patients. The number of shocks required to determine DFT by the step-down method was 4.6 +/- 1.4, whereas by definition, the binary search method always required three shocks. In conclusion, the binary search method is preferable because it is of comparable efficacy and requires fewer shocks.

Distal Right Ventricular Coil Position Reduces Defibrillation Thresholds

Distal RV Coil Position Reduces DFTs.

INTRODUCTION

Understanding the factors that affect defibrillation thresholds (DFTs) has important implications both for optimization of defibrillation efficacy and for the design of new transvenous leads. The aim of this prospective study was to test the hypothesis that defibrillation efficacy is improved with the right ventricular (RV) coil in a distal position compared with a more proximal RV coil position.

METHODS AND RESULTS

A novel defibrillation lead with three adjacent RV defibrillation coils (distal 0.8 cm, middle 3.7 cm, proximal 0.8 cm) was used for this study to permit comparison of DFTs with the proximal and distal RV coil positions without lead repositioning. In the distal RV configuration, the distal and middle RV coils were connected electrically as the anode for defibrillation. In the proximal RV configuration, the middle and proximal coils were the anode. A superior vena cava (SVC) coil and active can were connected electrically as the cathode (reversed polarity, RV-->Can+SVC). In each patient, the DFT was measured twice using a binary search protocol with the distal RV and proximal RV configurations, with the order of testing randomized. The study cohort consisted of 31 subjects (mean age 65 +/- 12 years, mean left ventricular ejection fraction 30% +/- 16%, 81% male predominance). The mean delivered energy (8.2 +/- 5.3 J vs 11.2 +/- 6.1 J), leading-edge voltage (335 +/- 109 V vs 393 +/- 118 V), and peak current (11.6 +/- 5.2 A vs 14.9 +/- 7.3 A) at DFT all were significantly lower with the distal RV configuration compared to the proximal RV configuration (P < 0.01 for all comparisons).

CONCLUSION

DFTs are significantly reduced with the distal RV configuration compared to the proximal RV configuration. Defibrillation leads should be designed with the shortest tip to coil distance that can be achieved without compromising ventricular fibrillation sensing.

Prospective randomized comparison of two defibrillation safety margins in unipolar, active pectoral defibrillator therapy

Various techniques are used to establish defibrillation efficacy and to evaluate defibrillation safety margins in patients with an ICD. In daily practice a safety margin of 10 J is generally accepted. However, this is based on old clinical data and there are no data on safety margins using current ICD technology with unipolar, active pectoral defibrillators. Therefore, a randomized study was performed to test if the likelihood of successful defibrillation at defibrillation energy requirement (DER) + 5 J and + 10 J is equivalent. Ninety-six patients (86 men; age 61.0 +/- 10.3 years; ejection fraction 0.341 +/- 0.132; coronary artery disease [n = 65], dilated cardiomyopathy [n = 18], other [n = 13]) underwent implantation of an active pectoral ICD system with unidirectional current pathway and a truncated, fixed tilt biphasic shock waveform. The defibrillation energy requirement (DER) was determined with the use of a step-down protocol (delivered energy 15, 10, 8, 6, 4, 3, 2 J). The patients were then randomized to three inductions of ventricular fibrillation at implantation and three at predischarge testing with shock strengths programmed to DER + 5 J at implantation and + 10 J at predischarge testing or vice versa. The mean DER in the total study population was 7.88 +/- 2.96 J. The number of defibrillation attempts was 288 for + 5 J and 288 for + 10 J. The rate of successful defibrillation was 94.1% (DER + 5 J) and 98.9% (DER + 10 J; P < 0.01 for equivalence). Charge times for DER + 5 J were significantly shorter than for DER + 10 J (3.65 +/- 1.14 vs 5.45 +/- 1.47 s; P < 0.001). A defibrillation safety margin of DER + 5 J is associated with a defibrillation probability equal to the standard DER + 10 J. In patients in whom short charge times are critical for avoidance of syncope, a safety margin of DER + 5 J seems clinically safe for programming of the first shock energy.

Safety of a single successful conversion of ventricular fibrillation before the implantation of cardioverter defibrillators

Multiple successful conversions of ventricular fibrillation (VF) at 10 J below the maximum output of implantable cardioverter defibrillator (ICD) have been recommended as a minimum device implantation criterion. This recommendation is based on the probabilistic properties of defibrillation that necessitates multiple shocks to establish an adequate safety margin for the conversion of subsequent spontaneous arrhythmias. We hypothesized that a single successful shock at a 14 J may suffice.

METHODS AND RESULTS

The Low Energy Safety Study (LESS) enrolled 720 patients undergoing initial ICD implantation with a dual-coil transvenous lead and active pulse generator. At implant, an enhanced defibrillation threshold (DFT++) was determined by a rigorous protocol beginning at 14 J, and requiring at least 4 shocks. Fifty percent of all patients were then randomized to full output shock energy and the conversion rates for spontaneous ventricular tachyarrhythmias at rates > 200 beats/min were measured. There were 318 patients randomized to 31 J, of whom 254 were successfully defibrillated by an initial 14 J shock. During a mean follow-up of 24 +/- 12 months, 112 spontaneous VF episodes occurred in 31 patients. The combined conversion success of the first and second shock (when needed) did not differ between the subgroup of patients who were successfully defibrillated by an initial 14 J shock, regardless of the results of additional testing, and the whole cohort who underwent more systematic testing (97% vs 97%). All spontaneous episodes of VF were successfully treated during long-term follow-up.

CONCLUSIONS

A first successful shock of 14 J may be a sufficient endpoint to allow the implantation of ICDs with the Triad lead configuration, when programming all shocks to 31 J.

Efficacy and temporal stability of reduced safety margins for ventricular defibrillation: primary results from the Low Energy Safety Study (LESS)

BACKGROUND

Traditionally, a safety margin of at least 10 J between the maximum output of the pulse generator and the energy needed for ventricular defibrillation has been used because lower safety margins were associated with unacceptably high rates of failed defibrillation and sudden cardiac death. The Low Energy Safety Study (LESS) was a prospective, randomized assessment of the safety margin requirements for modern implantable cardioverter-defibrillator (ICD) systems.

METHODS AND RESULTS

A total of 636 patients undergoing initial ICD implantation with a dual-coil lead and active pulse generator were evaluated. The defibrillation threshold (DFT) and enhanced DFT (DFT+ and DFT++) were measured using a modified step-down protocol. Conversion testing of induced ventricular fibrillation before discharge, at 3 months, and at 12 months was performed, as was randomization to chronic programming at either 2 steps above DFT++ or maximal output. The induced ventricular fibrillation data had conversion success rates of 91.4%, 97.9%, 99.1%, 99.6%, and 99.8% for safety margins of 0, 1, 2, 3, and 4 steps above the DFT++, respectively. A margin of 4 to 6 J was adequate to maintain high conversion success over time (98.9% before discharge versus 99.2% at 12 months; P=NS). Over a mean follow-up of 24+/-13 months, conversion of spontaneously occurring ventricular tachyarrhythmias >200 bpm was identical (97.3%), despite a safety margin difference of 5.2+/-1.1 J for the 2-step group versus 20.8+/-4.2 J for maximal output.

CONCLUSIONS

With a rigorous implantation algorithm, a safety margin of about 5 J is adequate for safe implantation of modern ICD systems.

Effect of shock polarity on the efficacy of transthoracic atrial defibrillation

BACKGROUND

The energy requirement for internal ventricular defibrillation is reduced by reversal of shock polarity. The influence of shock polarity on the efficacy of transthoracic atrial defibrillation is unknown.

METHODS

This prospective, randomized study enrolled 110 consecutive patients who were referred for elective cardioversion of persistent atrial fibrillation (AF). The electrodes were placed in the anteroposterior position. The patients were randomized to receive either standard (anterior pad = cathode) or reversed polarity (anterior pad = anode) shocks with a damped sinusoidal monophasic waveform. A step-up protocol was used to estimate the cardioversion threshold. The initial shock energy was 50 J, with subsequent increments to 100, 200, 300, and 360 J in the event of cardioversion failure.

RESULTS

Sixty-four percent of the patient population were men, with a mean age of 66 +/- 13 years and a mean duration of AF of 242 +/- 556 days. The overall success rates of cardioversion were 84% for standard polarity and 78% for reversed polarity (P not significant). Among the patients who were successfully cardioverted, the mean atrial defibrillation threshold was 198 +/- 103 J for standard polarity and 212 +/- 107 J for reversed polarity (P not significant).

CONCLUSIONS

Reversal of shock polarity does not improve transthoracic cardioversion efficacy with a standard damped sinusoidal monophasic waveform. Alternate strategies should be considered for patients who fail external cardioversion, such as adjunctive pharmacologic treatment, use of a biphasic shock waveform, or internal cardioversion.

Transvenous biventricular defibrillation halves energy requirements in patients

BACKGROUND

Defibrillation thresholds (DFT) with standard implantable cardioverter-defibrillator leads in the right ventricle (RV) may be determined by weak shock field intensity in the myocardium of the left ventricle (LV). Adding a shocking electrode in a coronary vein on the middle of the LV free wall, thereby establishing biventricular defibrillation, substantially reduced defibrillation requirements in animals. We investigated the feasibility of this approach in 24 patients receiving an implantable cardioverter-defibrillator using a prototype over-the-wire temporary LV defibrillation lead.

METHODS AND RESULTS

The LV lead was inserted through the coronary sinus, using a guide catheter and guidewire, into a posterior or lateral coronary vein whose location was determined by retrograde venography. Paired DFT testing compared a standard system (RV to superior vena cava plus can emulator [SVC+Can], 60% tilt biphasic shock) to a system including the LV lead. The biventricular system was tested with a dual-shock waveform (20% tilt monophasic shock from LV-->SVC+Can, then 60% tilt biphasic shock from RV-->SVC+Can). Twenty patients completed DFT testing. Venography and LV lead insertion time was 46+/-40 minutes. The biventricular system reduced mean DFT by 45% (8.9+/-1.1 J versus 4.9+/-0.5 J, P<0.001). Twelve patients (60%) had a standard system DFT >/=8 J, and the biventricular system gave a lower DFT in all patients. There were no adverse events related to the use of the LV lead, which was removed after testing.

CONCLUSIONS

Internal defibrillation using a transvenously inserted LV lead is feasible, produces significantly lower DFTs, and seems safe under the conditions tested. Biventricular defibrillation may be a useful option for reducing DFTs or could be added to an LV pacing lead for heart failure.

Temporal decline in defibrillation thresholds with an active pectoral lead system

OBJECTIVES

The objective of this study was to characterize temporal changes in defibrillation thresholds (DFTs) after implantation with an active pectoral, dual-coil transvenous lead system.

BACKGROUND

Ventricular DFTs rise over time when monophasic waveforms are used with non-thoracotomy lead systems. This effect is attenuated when biphasic waveforms are used with transvenous lead systems; however, significant increases in DFT still occur in a minority of patients. The long-term stability of DFTs with contemporary active pectoral lead systems is unknown.

METHODS

This study was a prospective assessment of temporal changes in DFT using a uniform testing algorithm, shock polarity and dual-coil active pectoral lead system. Thresholds were measured at implantation, before discharge and at long-term follow-up (70 +/- 40 weeks) in 50 patients.

RESULTS

The DFTs were 9.2 +/- 5.4 J at implantation, 8.3 +/- 5.8 J before discharge and 6.9 +/- 3.6 J at long-term follow-up (p < 0.01 by analysis of variance; p < 0.05 for long-term follow-up vs. at implantation or before discharge). The effect was most marked in a prespecified subgroup with high implant DFTs (> or =15 J). No patient developed an inadequate safety margin (< 9 J) during follow-up.

CONCLUSIONS

The DFTs declined significantly after implantation with an active pectoral, dual-coil transvenous lead system, and no clinically significant increases in DFT were observed. Therefore, routine defibrillation testing may not be required during the first two years after implantation with this lead system, in the absence of a change in the cardiac substrate or treatment with antiarrhythmic drugs.

Implantation of cardioverter defibrillators without induction of ventricular fibrillation

BACKGROUND

The upper limit of vulnerability (ULV) is the weakest shock at which ventricular fibrillation (VF) is not induced by a T-wave shock. This study tested the hypothesis that a vulnerability safety margin based on the ULV can be used as an implantable cardioverter-defibrillator implantation criterion.

METHODS AND RESULTS

Implantable cardioverter-defibrillators were implanted in 80 patients if T-wave shocks did not induce VF and the baseline-rhythm R wave was >/=7 mV. The T-wave shock was 10 J in the first 45 patients (group A) and 15 J in the last 35 patients (group B). After inductionless implantations, the first VF shock was programmed to the T-wave shock plus 5 J. If T-wave shocks induced VF, the ULV was measured and the first shock was programmed to the ULV+5 J. Inductionless implantations were performed in 58 patients (72%), 28 in group A (62%) and 30 in group B (86%; P=0.04). If T-wave scanning had been done at 15 J in group A patients, inductionless implantations could have been performed in 84% of them. At 3 months, VF was induced twice during electrophysiological study in 75 patients (94%). All VFs were detected in </=4.7 s and were terminated by the first shock. During follow-up, 197 of 198 appropriate first shocks for rapid ventricular tachycardia or VF (99%) were successful in patients who had inductionless implantations (95% confidence intervals, 97% to 100%).

CONCLUSION

Inductionless implantations can be performed in >80% of implantable cardioverter-defibrillator recipients using a vulnerability safety margin based on a T-wave scan at 15.

Implantable cardiac defibrillators

The implantable cardioverter defibrillator (ICD) represents an important development in the effort to reduce the incidence of sudden cardiac death (almost 400,000 yearly in the United States). Early generation ICDs, which required epicardial lead systems and abdominal placement of the pulse generator, have been replaced by transvenous leads and pectoral implants. Other important refinements, which include biphasic waveforms, extensive memory capability, antitachycardia pacing, and enhanced sensing algorithms, have greatly improved patient tolerance. Ongoing trials and those in the planning stages will continue to expand the indications for ICDs and will focus on cost-effectiveness.

The value of predischarge ICD tests in patients with a successful peroperative test

An internal cardioverter defibrillator (ICD) is normally extensively tested during implantation. The necessity of retesting prior to discharge of the patient is a matter of debate. In our material of 30 patients undergoing first-time implantation of a transvenous internal defibrillator system, we retrospectively compare the predischarge defibrillation test with the peroperative test. A successful peroperative defibrillation test with no failed shocks at 10 J below maximal energy level was followed by a successful predischarge test with the same safety margin in 18/19 patients, while one patient required a maximal energy ICD shock for conversion at the predischarge test. We conclude that the predischarge defibrillation test can be omitted if the peroperative test was successful, with no failed shocks at 10 J below maximal energy level and if the shock therapy is set to maximal energy level.

High defibrillation threshold at cardioverter defibrillator implantation under amiodarone treatment: favorable effects of D, L-sotalol

A 57-year-old man with primary dilated cardiomyopathy and obesity received an implantable cardioverter defibrillator because of recurrent, poorly tolerated ventricular tachycardia despite continuous treatment with amiodarone. When the device was implanted, assessment of the ability to defibrillate induced ventricular fibrillation showed high energy requirements, with a lack of conventional safety margin between energies effective at defibrillation testing and maximal device output. Treatment with oral amiodarone was withdrawn and substituted with oral sotalol. A repeat defibrillation test, performed 54 days after amiodarone withdrawal and during D,L -sotalol treatment, showed a reduction in defibrillation energy requirements. In view of this experience, replacement of amiodarone treatment with an alternate class III agent (D,L -sotalol or other agents, if available) can be considered as a possible option in case of high defibrillation threshold at the time of the implantation in a patient receiving continuous amiodarone treatment.

Implantable Cardioverter Defibrillator: Current Progress and Management

With greater technologic advances during the past decade, use of the implantable cardioverter defibrillator (ICD) has increased to more than 200,000 implants worldwide to date. Indications for ICD implant have expanded to include both patients who have survived sudden cardiac death (secondary prevention of cardiac arrest) and those who are at high risk for experiencing lethal arrhythmias (primary prevention of cardiac ar rest). Thus, it is likely that physicians will encounter defibrillators in their clinical practice and must be familiar with their indications for implant, basic opera tion, and long-term management of devices. Several prospective clinical trials have recently shown the long- term efficacy of ICD therapy at aborting sudden death in the high-risk patient population. Although still evolving, general guidelines and indications for ICD implant have been put forth and are discussed in this review. From the first defibrillation in humans during surgery in 1947 to the sophisticated dual-chamber pacing and memory functions of the modern device, ICD development has led to ever smaller devices with more complex technol ogy. The implant procedure of current ICDs parallels that used to place pacemakers. However, the anesthe sia team plays a vital role in initial ICD implantation by monitoring cardiopulmonary status during defibrilla tion threshold (DFT) testing. Additionally, long-term management of ICDs often requires repeat DFT testing with anesthesia involvement. Finally, possible electro magnetic (environmental) interactions with the ICD of which physicians should be aware are described in this article.

Optimization of transvenous coil position for active can defibrillation thresholds

INTRODUCTION

Lead systems that include an active pectoral pulse generator are now standard for initial defibrillator implantations. However, the optimal transvenous lead system and coil location for such active can configurations are unknown. The purpose of this study was to evaluate the benefit and optimal position of a superior vena cava (SVC) coil on defibrillation thresholds with an active left pectoral pulse generator and right ventricular coil.

METHODS AND RESULTS

This prospective, randomized study was performed on 27 patients. Each subject was evaluated with three lead configurations, with the order of testing randomized. Biphasic shocks were delivered between the right ventricular coil and an active can alone (unipolar), or an active can in common with the proximal coil positioned either at the right atrial/SVC junction (low SVC) or in the left subclavian vein (high SVC). Stored energies at defibrillation threshold were higher for the single-coil, unipolar configuration (11.2 +/- 6.6 J) than for the high (8.9 +/- 4.2 J) or low (8.5 +/- 4.2 J) SVC configurations (P < 0.01). Moreover, 96% of subjects had low (< or = 15 J) thresholds with the SVC coil in either position compared with 81% for the single-coil configuration. Shock impedance (P < 0.001) was increased with the unipolar configuration, whereas peak current was reduced (P < 0.001).

CONCLUSION

The addition of a proximal transvenous coil to an active can unipolar lead configuration reduces defibrillation energy requirements. The position of this coil has no significant effect on defibrillation thresholds.

Analysis of defibrillation efficacy from myocardial voltage gradients with finite element modeling

Increasing defibrillation efficacy by lowering the defibrillation threshold (DFT) is an important goal in positioning implantable cardioverter-defibrillator electrodes. Clinically, the DFT is difficult to estimate noninvasively. It has been suggested that the DFT relates to the myocardial voltage gradient distribution, but this relation has not been quantitatively demonstrated. We analyzed the relation between the experimentally measured DFT's and the simulated myocardial voltage gradients provided by finite element modeling. We performed a series of experiments in 11 pigs to measure the DFT's, and created and solved three-dimensional subject-specific finite element models to assess the correlation between the computed myocardial voltage gradient histograms and the DFT's. Our data show a statistically significant correlation between the DFT and the left ventricular voltage gradient distribution, with the septal region being more significant (correlation coefficient of 0.74) than other myocardial regions. The correlation between the DFT and the right ventricular and the atrial voltage gradient, on the other hand, is not significant.

A four-shock Bayesian up-down estimator of the 80% effective defibrillation dose

INTRODUCTION

New defibrillation techniques are often compared to standard approaches using the defibrillation threshold. However, inference from thresholding data necessitates extrapolation from reactions to relatively ineffective shocks, an error prone procedure requiring large sample sizes for hypothesis testing and large safety margins for defibrillator implantation. In contrast, this article presents a clinically validated statistical model of a minimum error, four-shock defibrillation testing protocol for estimating the 80% effective defibrillation strength for a given patient (ED80).

METHODS AND RESULTS

A Bayesian statistical model was constructed assuming that the defibrillation dose-response curve is sigmoidal, and the ED80 is between 150 and 750 V. The model was used to design a minimum predicted error testing protocol and estimates. To prospectively validate the testing protocol and estimates, 170 patients received voltage-programmed biphasic testing. Four fibrillation episodes were induced and terminated in each patient according to the Bayesian up-down protocol. In addition, a validation attempt was made at the estimated ED80 rounded up to the nearest 50 V. In order to estimate the safety margin, in 136 patients, a defibrillation attempt was made at the rounded ED80 + 100 V. Of the 170 attempts at the rounded ED80, 143 (84%) attempts terminated fibrillation. Of the 136 attempts at the rounded ED80 + 100 V, 133 (98%) were effective.

CONCLUSIONS

The four-shock Bayesian up-down protocol is the first clinical protocol to accurately predict an ED80 voltage. A 100 V increment above the ED80 provides an adequate safety margin. This simple and accurate method for estimating a highly effective defibrillation dose may be a valuable tool for population-based clinical hypothesis testing, as well as defibrillator implantation.

Effect of amiodarone and sotalol on the defibrillation threshold in comparison to patients without antiarrhythmic drug treatment

AIM OF THE STUDY

It is generally accepted that chronic therapy with antiarrhythmic drugs might increase the defibrillation threshold at implantation of an implantable cardioverter defibrillator. A recently published animal study showed a minor effect of the class 1 antiarrhythmic drug lidocaine on the defibrillation threshold if biphasic shocks were used.

METHODS AND RESULTS

We therefore performed a retrospective analysis in 89 patients who received an ICD capable of monophasic (n=18) or biphasic (n=71) shocks with a transvenous lead system. In all patients the defibrillation threshold was determined according to the same step down protocol. In the 18 patients with a monophasic device the effects of chronic therapy with amiodarone (n=7) on the defibrillation threshold were evaluated in comparison to a group without antiarrhythmic treatment (n=11). In those patients receiving a biphasic device the effects of chronic therapy with amiodarone (n=29), sotalol (n=20) or no antiarrhythmic medication (n=22) on the defibrillation threshold were evaluated. The groups receiving a monophasic device did not differ in respect to age, sex, underlying cardiac disease, clinical arrhythmia (VT/VF), clinical functional status, left ventricular ejection fraction and the number of patients with additional subcutaneous electrodes. These parameters as well as the type of implanted device were not different between patient groups receiving a biphasic device. Patients on chronic amiodarone therapy receiving a monophasic device had a significantly higher defibrillation threshold (29.1 +/- 8.8 J) than patients without antiarrhythmic treatment (19.1 +/- 5.1 J, P = 0.021). The groups did not differ significantly in respect to the impedance measured at the shocking lead (P = 0.13). In three patients on chronic amiodarone an epicardiac lead system had to be implanted due to an inadequate monophasic defibrillation threshold compared to no patient without antiarrhythmic drug treatment (P = 0.043). In the patients with a biphasic device the intraoperative defibrillation threshold was not significantly different between the three study groups (P = 0.44). No patient received an epicardiac lead system. The defibrillation threshold in the amiodarone group was 15.3 +/- 7.3 J, in the sotalol group 14.4 +/- 7.2 J and in the patients without antiarrhythmic drug treatment 17 +/- 6.1 J. As well, no significant difference was seen between the groups in respect of the impedance of the high voltage electrode (P = 0.2).

CONCLUSION

With the use of a biphasic device in combination with a transvenous lead system the intraoperative defibrillation threshold is not significantly different between patients on chronic amiodarone in comparison to patients without antiarrhythmic drug treatment or patients on chronic oral sotalol. This is in contrast to our findings with a monophasic device.