Clinical predictors of defibrillation energy requirements in patients treated with a nonthoracotomy defibrillator system. The ResQ Investigators

The relationship between defibrillation threshold and total mortality

BACKGROUND

The relationship between the defibrillation threshold (DFT) and total mortality is unclear.

METHODS

A university hospital prospectively maintained implantable defibrillator (ICD) database identified 508 patients who underwent ICD implantation and had DFT testing performed at implant. Patients were placed in one of three groups based on the implant DFT (high (≥20 J), moderate (19-11 J), or low DFT (≤10 J)).

RESULTS

Sixty-seven patients had a high DFT, 216 had a moderate DFT, and 225 had a low DFT. The mean left ventricular ejection fraction (LVEF) was 0.25, 0.28, and 0.30 in the high, moderate, and low DFT groups, respectively, (p = 0.04). There were significantly more patients with a subcutaneous electrode in the high DFT group (p < 0.001), more patients taking amiodarone (p < 0.001), and more patients on oral anticoagulation (p = 0.04). There were a total of 140 deaths during the follow-up period (mean 3.2 ± 1.7 years). There were 24 deaths in the high DFT group (35.8%), 62 in the moderate DFT group (28.7%), and 54 in the low DFT group (24.0%) (p = 0.05). Implant DFT was a significant predictor of mortality (p = 0.01), as was age, LVEF (p < 0.001), CAD (p = 0.01), amiodarone use (p = 0.02), and hematoma at implant (p = 0.01). An elevated DFT was an independent predictor of mortality after controlling for all significant univariate variables (p = 0.004).

CONCLUSIONS

A high-implant DFT predicts an adverse prognosis, even when an adequate ICD safety margin is present.

Is defibrillation testing required for defibrillator implantation?

The assessment of defibrillation (DFT) efficacy has long been the standard of care during defibrillator implantation. To ensure an acceptable DFT safety margin, early defibrillator systems frequently required that the shock polarity and the location, type, or number of electrodes had to be altered. Advances in defibrillator and lead technology have resulted in lower and more consistent DFT thresholds in the range of 10 J, with an infrequent requirement to modify the DFT system. Yet, one can make an argument for and against continuation of DFT testing at the time of defibrillator implantation. The goal of this paper is to address both the data that do support and the data that do not support continuation of DFT testing at the time of device implantation. Scientifically, DFT testing should be abandoned only when prospective evidence demonstrates that defibrillator implantation without testing is as safe and has the same mortality benefits as implantation with testing. The most attractive aspect of eliminating DFT efficacy testing is that more patients may have the opportunity to be treated with this life-saving therapy. Perhaps there are alternative strategies to improve accessibility to defibrillator therapy without possibly eroding its effectiveness. In the end, will lives be saved or lost if we discontinue DFT efficacy testing and lower the barriers to implantable defibrillator therapy?

Long-term evaluation of the ventricular defibrillation energy requirement

INTRODUCTION

Defibrillation energy requirements in patients with nonthoracotomy defibrillators may increase within several months after implantation. However, the stability of the defibrillation energy requirement beyond 1 year has not been reported. The purpose of this study was to characterize the defibrillation energy requirement during 2 years of clinical follow-up.

METHODS AND RESULTS

Thirty-one consecutive patients with a biphasic nonthoracotomy defibrillation system underwent defibrillation energy requirement testing using a step-down technique (20, 15, 12, 10, 8, 6, 5, 4, 3, 2, and 1 J) during defibrillator implantation, and then 24 hours, 2 months, 1 year, and 2 years after implantation. The mean defibrillation energy requirement during these evaluations was 10.9+/-5.5 J, 12.3+/-7.3 J, 11.7+/-5.6 J, 10.2+/-4.0 J, and 11.7+/-7.4 J, respectively (P = 0.4). The defibrillation energy requirement was noted to have increased by 10 J or more after 2 years of follow-up in five patients. In one of these patients, the defibrillation energy requirement was no longer associated with an adequate safety margin, necessitating revision of the defibrillation system. There were no identifiable clinical characteristics that distinguished patients who did and did not develop a 10-J or more increase in the defibrillation energy requirement.

CONCLUSION

The mean defibrillation energy requirement does not change significantly after 2 years of biphasic nonthoracotomy defibrillator system implantation. However, approximately 15% of patients develop a 10-J or greater elevation in the defibrillation energy requirement, and 3% may require a defibrillation system revision. Therefore, a yearly evaluation of the defibrillation energy requirement may be appropriate.

Simplified implantation of single-lead pectoral cardioverter defibrillators using device-based testing

Device-based testing of single-lead pectoral defibrillators (defibrillation efficacy testing without an external defibrillation system after complete implantation of the device) resulted in an adequate defibrillation threshold (< or = 20 J) in 45 of 50 study patients (90%). Mean surgical implantation time (skin to skin) was 62 +/- 29 minutes without perioperative mortality and without implantable cardioverter defibrillator infection during follow-up. Thus, device-based testing appears to be a simple and safe method to test defibrillation efficacy of single-lead pectoral defibrillators.

Arrhythmias and sudden death in heart failure

Survival of patients with heart failure has improved over the past decade due to advances in medical therapy. Sudden death continues to cause 20 to 50% of deaths. Ventricular arrhythmias are common in patients with heart failure. Ventricular hypertrophy, scars from prior myocardial infarction, sympathetic activation, and electrolyte abnormalities contribute. Some sudden deaths are due to bradyarrhythmias and electromechanical dissociation rather than ventricular arrhythmias. The risks and benefits of antiarrhythmic therapies continue to be defined. Class I antiarrhythmic drugs should be avoided due to proarrhythmic and negative inotropic effects that may increase mortality. For patients resuscitated from sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) amiodarone or an implantable cardioverter defibrillator (ICD) should be considered. ICDs markedly reduce sudden death in VT/VF survivors, but in advanced heart failure, this may not markedly extend survival. Catheter or surgical ablation can be considered for selected patients with bundle branch reentry VT or difficult to control monomorphic VT. For patients who have not had sustained VT/VF antiarrhythmic therapy should generally be avoided, but may benefit some high risk patients. Amiodarone may be beneficial in patients with advanced heart failure and rapid resting heart rates. ICDs may improve survival in selected survivors of myocardial infarction who have inducible VT.

Defibrillation efficacy with endocardial electrodes is influenced by reductions in cardiac preload

Little is known about the effects of cardiac preload and cardiac geometry on defibrillation efficacy with endocardial electrodes. We studied nine pigs implanted with an endocardial lead system in the normal and reduced preload state. In the reduced preload state, a balloon catheter was inflated in the inferior vena cava (IVC) for 20 seconds prior to the induction of ventricular fibrillation (VF). Complete occlusion of the IVC and reductions in preload were confirmed by observing deformation of the contrast-filled balloon, a reduction in cardiac size by fluoroscopy, and reductions in ventricular pressures. Biphasic shocks were delivered after 10 seconds of VF using a recursive up-down protocol. VF was induced 20 times for each preload state, and the 50% effective doses (ED50) for energy, current, and voltage were estimated by averaging all shocks for that state. At reduced preloads, energy decreased from 12.1 +/- 3.0 J (+/- SD) to 10.5 +/- 2.9 J (p < 0.01), voltage decreased from 415 +/- 51 V to 390 +/- 51 V (p < 0.05), and current decreased from 8.6 +/- 1.5 A to 7.6 +/- 1.5 A (p < 0.01), while impedance rose from 49.2 +/- 3.8 omega to 52.8 +/- 4.4 omega (p < 0.001). We conclude that reducing cardiac preload and cardiac size significantly lowers ED50 defibrillation energy, current, and voltage. This outcome may be caused directly by the decrease in blood volume as evidenced by increased impedance and/or may be due to changes in heart geometry and stretch.

A population-based method for the estimation of defibrillation energy requirements in humans. Assessment of time-dependent effects with a transvenous defibrillation system

BACKGROUND

A weighted logistic regression analysis was developed to allow pooling of patient data for the study of the stability of defibrillation energy requirements with a new nonthoracotomy lead defibrillation system.

METHODS AND RESULTS

One hundred twenty patients were prospectively studied with a single-model nonthoracotomy implantable cardioverter defibrillator (ICD) system at the time of implant and at 3 months. The pooled data of all shocks delivered to all patients were fitted to a logistic function to construct a defibrillation voltage/energy dose-response relationship. The crude logit curve was weighted in quartiles according to the average shock energy delivered per patient. Shocks at implant (n = 802; 6.6 +/- 2.5 shocks/patient) and follow-up (n = 292; 2.4 +/- 1.2 shocks/patient) were analyzed. The modeled voltage/energy required for 50% successful defibrillation (95% CI) in the pooled data was 367 V (273, 461) and 9.8 J (6.7, 12.9) at implant and 338 V (264, 412) and 10.5 J (8, 13.0) at follow-up. The conventional measure of lowest successful voltage/energy (95% CI) was 430 V (411, 449) and 12.1 J (11, 13.2) at implant and 415 V (391, 439) and 11.3 J (10, 12.6) at follow-up. There were no statistically significant differences between implant and follow-up energy requirements with either method.

CONCLUSIONS

The nonthoracotomy lead system used in this study demonstrated stability of defibrillation energy requirements at implant and 3-month follow-up. A new technique for the estimation of the defibrillation energy dose-response relationship was derived by using a weighted logistic regression analysis.

[Influence of amiodarone on defibrillation threshold and perioperative complications in patients with implantable cardioverter-defibrillator with transvenous electrodes and biphasic shocks]

In order to evaluate the safety of implantation of a third generation automatic cardioverter-defibrillator (ICD) in patients with amiodarone therapy, we retrospectively analyzed implantation results and perioperative complications in 17 consecutive patients with continued amiodarone therapy and 38 patients without antiarrhythmics at first ICD implantation.ICD implantation could be performed successfully in all 55 patients using a transvenous electrode system. In one of 17 patients with amiodarone (6%) versus three of 38 patients without antiarrhythmics (8%), additional implantation of a subcutaneous patch electrode was necessary to achieve a defibrillation threshold </=24 Joules (p=n.s.). Intraoperatively tested defibrillation threshold was 13+/-5 Joules in the amiodarone group versus 12+/-5 Joules in patients without antiarrhythmics (p=n.s.). The incidence of perioperative complications was not different between patients with amiodarone and patients without antiarrhythmics.In summary, patients with amiodarone did not have higher defibrillation thresholds or a higher incidence of perioperative complications than patients without antiarrhythmics. The occasionally postulated discontinuation of amiodarone with subsequent waiting for the elimination of this antiarrhythmic drug is unnecessary. For final clarification of the influence of amiodarone-therapy on the defibrillation threshold in patients with third generation ICDs, a prospective randomized study in a large patiente cohort with standardized study conditions would be required.