Adverse effects of permanent cardiac internal defibrillator patches on external defibrillation

Stimulatory current at the edge of an inactive conductor in an electric field: role of nonlinear interfacial current-voltage relationship

Cardiac electric field stimulation is critical for the mechanism of defibrillation. The presence of certain inactive epicardial conductors in the field during defibrillation can decrease the defibrillation threshold. We hypothesized this decrease is due to stimulatory effects of current across the interface between the inactive conductor and the heart during field stimulation. To examine this current and its possible stimulatory effects, we imaged transmittance of indium-tin-oxide (ITO) conductors, tested for indium with X-ray diffraction, created a computer model containing realistic ITO interfacial properties, and optically mapped excitation of rabbit heart during electric field stimulation in the presence of an ITO conductor. Reduction of indium decreased transmittance at the edge facing the anodal shock electrode when trans-interfacial voltage exceeded standard reduction potential. The interfacial current-voltage relationship was nonlinear, producing larger conductances at higher currents. This nonlinearity concentrated the interfacial current near edges in images and in a computer model. The edge current was stimulatory, producing early postshock excitation of rabbit ventricles. Thus, darkening of ITO indicates interfacial current by indium reduction. Interfacial nonlinearity concentrates current near the edge where it can excite the heart. Stimulatory current at edges may account for the reported decrease in defibrillation threshold by inactive conductors.

Influence of epicardial patches on defibrillation threshold with nonthoracotomy lead configurations

BACKGROUND

In previous studies, epicardial patch electrodes decreased transthoracic defibrillation efficacy. We studied the effects of two inactive epicardial 14-cm2 titanium mesh patches on defibrillation energy requirements with nonthoracotomy internal lead configurations.

METHODS AND RESULTS

A 6/6-millisecond biphasic shock wave-form was delivered via several electrode configurations 10 seconds after ventricular fibrillation was initiated with a 60-Hz generator. In two series, a total of 16 dogs (weight, 23.3 +/- 2.4 kg) underwent an up-down defibrillation protocol. In the first series, the defibrillation threshold (DFT) was determined for each electrode configuration in the presence of two inactive epicardial patches. In the second series, DFTs were determined in the presence of an inactive right ventricular (RV) or left ventricular (LV) patch alone. For several nonthoracotomy lead configurations tested in the first 8 dogs, the mean +/- SD DFT energy increased 49% to 97% with two inactive patches on the heart compared with no patches on the heart as follows: RV to superior vena caval (SVC) electrode, from 8.9 +/- 2.6 to 18.0 +/- 14.3 J; RV to SVC plus subcutaneous array electrode, from 7.0 +/- 2.4 to 10.7 +/- 5.3 J; RV to subcutaneous pectoral plate electrode, from 6.2 +/- 1.3 to 11.4 +/- 4.0 J (P < or = .05). The lowest DFT was achieved by defibrillating between the epicardial patches (3.8 +/- 3.3 J). The second series showed that DFT voltage requirements increased significantly for all three nonthoracotomy lead configurations with the inactive LV patch alone (P < or = .05) but not with the inactive RV patch alone.

CONCLUSIONS

Inactive epicardial patches can significantly increase the defibrillation energy requirements for nonthoracotomy lead configurations. This negative impact may be due to an insulating effect of the patches and to a disturbance of the potential gradient field under the patches. If the same holds true in patients, these results have clinical implications. Functioning epicardial patch leads should be incorporated in the defibrillation lead system if already present. If the LV patch is nonfunctioning, such as because of a lead fracture, the marked increase in DFT due to an inactive LV patch calls for thorough DFT testing during surgery and, in selected patients, may necessitate patch removal to produce an effective transvenous-based system.

Comparison of early and late complications in patients undergoing coronary artery bypass graft surgery with and without concomitant placement of an implantable cardioverter defibrillator

Previous studies have reported a significant morbidity and mortality associated with coronary artery bypass graft (CABG) surgery in conjunction with the placement of an implantable cardioverter defibrillator (ICD) with an epicardial lead system. In the absence of a control group, how significantly the component of concomitant placement of the ICD system contributes to these untoward outcomes remains unknown. The purpose of this study was to assess the short- and long-term complications in patients undergoing CABG surgery in conjunction with the placement of an ICD with epicardial leads and to compare these complications with those of patients who had only CABG surgery (control group). The study group (group A) consisted of 56 patients who underwent CABG surgery and placement of an ICD pulse generator with epicardial leads. A control group (group B) consisted of 56 patients who underwent CABG surgery only during the same time period. The two groups were matched for age, sex distribution, left ventricular function, surgical approach, number of bypass grafts per patient, bypass pump time, and length of follow-up period. The early mortality for group A was 7.1% versus 1.8% for group B (p > 0.05). The incidence of early morbidity (congestive heart failure, infection, supraventricular and ventricular arrhythmias) for groups A and B was similar (26.8% vs 25.0%, p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of defibrillation probability of success curves for an endocardial lead configuration with and without an inactive epicardial patch

OBJECTIVES

This study sought to assess the effect of passive "bystander" epicardial electrodes on defibrillation efficacy.

BACKGROUND

We hypothesized that an inactive epicardial patch placed in an area of low potential gradient from an endocardial electrode shock might affect defibrillation efficacy through its effects on the shock field and the underlying potential gradient.

METHODS

We studied the effects of an inactive 18-cm2 titanium mesh patch placed on the anterolateral left ventricular epicardium on the 50% probability of successful defibrillation. A biphasic shock with both phases 6 ms in duration was delivered between superior vena cava and right ventricular catheter electrodes 10 s after the electrical induction of ventricular fibrillation. Six dogs underwent an up/down defibrillation protocol randomized with or without the patch on the heart.

RESULTS

Mean 50% (+/-) probability point for energy doubled with the conductive patch on the heart, from 8.0 +/- 3.2 to 16.8 +/- 7.0 J (p < 0.01), and leading-edge voltage increased from 334 +/- 64 to 477 +/- 98 V (p < 0.01). Mean 50% probability points for energy and leading-edge voltage were not significantly changed when the procedure was repeated using a nonconductive patch in another six dogs as a control group. In a saline-saturated foam model, measurements from electrodes placed around and under the patch revealed a 72% mean decrease in the potential gradient in the foam under the conductive patch.

CONCLUSIONS

A passive defibrillator patch can markedly increase the energy requirements for defibrillation, probably by decreasing the potential gradient under the patch. These results suggest the use of caution when passive electrodes are present, for example, when a patient receives a nonthoracotomy defibrillator system while epicardial electrodes from a previously implanted system are left in place.

Atrial fibrillation after implantable cardioverter defibrillator implantation

Atrial fibrillation is a reported complication of automatic defibrillator implantation. Its incidence, risk factors, time-course, and complications have not been well-defined. Accordingly, data from 117 patients who underwent defibrillator implantation via a thoracotomy (n = 71) or nonthoracotomy (n = 46) approach were reviewed. To identify risk factors, 15 variables of potential predictive value were chosen and analyzed. Atrial fibrillation developed in 26/117 patients (22%) during the early postoperative period and all but one of these 26 patients had undergone thoracotomy (P < 0.001). Patients who developed atrial fibrillation tended to be older than those who did not (63 +/- 2 vs 58 +/- 2 years, P = 0.04) and more frequently had a prior history of paroxysmal atrial fibrillation (31% vs 10%, P = 0.02). They were also less likely to be taking Class I or III antiarrhythmic drugs (1/26 vs 24/91, P = 0.01). By multivariate analysis, operative approach (P < 0.001), the absence of antiarrhythmic drug therapy (P = 0.006), and a prior history of atrial fibrillation (P = 0.003) were significant independent variables. Digoxin neither prevented the occurrence of atrial fibrillation nor slowed the maximal ventricular response. The mortality and complication rates did not differ between the two groups. The major adverse effect of postimplant atrial fibrillation was automatic defibrillator discharge; six patients received between 1 and 11 discharges for atrial fibrillation with rapid ventricular rates.

Improved defibrillation threshold with a new epicardial carbon electrode compared with a standard epicardial titanium patch

BACKGROUND

Recent studies show that depending on the type of shock morphology used, 5% to 15% of patients requiring implantable defibrillators cannot be treated with a nonthoracotomy system. In these cases, an epicardial patch-based system becomes necessary. In this study, we investigated a newly developed epicardial carbon electrode as an alternative to a standard epicardial titanium patch.

METHODS AND RESULTS

A tubular epicardial braided carbon electrode of 7F diameter and 14-cm length applied in a U-shape to the epicardium was compared with a standard left ventricular epicardial 15-cm2 titanium mesh patch (CPI Inc). As cathode, a CPI endocardial lead, a Medtronic lead, or a carbon-platinum-iridium prototype electrode was used. Ventricular fibrillation was induced with a 60-Hz generator and allowed to continue for 10 seconds before a shock was given. Two different biphasic shock waveforms (3.2/2- and 6/6-millisecond) were delivered by the six electrode configurations. Eight dogs (weight, 24.5 +/- 1.3 kg) underwent an up-down defibrillation protocol. The order of testing the epicardial electrodes, the endocardial cathodes, and the waveform was randomized. With the epicardial carbon electrode, the mean defibrillation threshold (DFT) energy decreased 39% to 56% and the voltage decreased 24% to 35% compared with the titanium patch: from 8.3 +/- 2.5 to 4.9 +/- 3.6 J with the CPI lead and the 3.2/2-millisecond waveform, from 6.2 +/- 2.5 to 2.9 +/- 2.1 J with the carbon-platinum-iridium prototype, and from 6.4 +/- 3.4 J to 3.5 +/- 2.6 J with the Medtronic lead (P < or = .05). The DFT determinations with the 6/6-millisecond biphasic waveform showed a similar trend with slightly higher values.

CONCLUSIONS

Compared with a titanium patch, the new braided epicardial electrode significantly decreases the defibrillation energy requirements. This effect can be maximized by using an endocardial carbon-platinum-iridium prototype as cathode and a short duration biphasic waveform.

Use of multiple patches during implantation of epicardial defibrillator systems

During implantation of epicardial automatic defibrillator systems, occasional patients have difficulty in obtaining adequate defibrillation thresholds. Of 236 consecutive patients undergoing implantation of epicardial defibrillator systems, 18 patients received a 3-patch (n = 15) or 4-patch (n = 3) defibrillator system. Twelve patients who received a multiple-patch defibrillator system had a best 2-patch defibrillation energy requirement of > or = 30 J; in the remaining 6 patients less stringent clinical criteria were used in the decision to add a third defibrillator patch (defibrillation energy requirement > 18 J in 4 patients, and > 20 J in 2 patients). Technically, multiple-patch systems were made possible with either the use of Y-connectors or defibrillators allowing output to 3 patches. In 3 patients, addition of a third epicardial patch still resulted in a defibrillation energy requirement of > or = 30 J; in these 3 patients, addition of a fourth patch resulted in a defibrillation energy requirement of < or = 20 J. All patients receiving a multiple-patch defibrillator system had a reduction in defibrillation energy requirement, and 12 patients had a reduction in defibrillation energy requirement of > or = 10 J over the best 2-patch defibrillation energy requirement. In the patients who eventually had placement of a multiple-patch system, the best 2-patch defibrillation energy requirement was > 18 J in 4 patients, > 20 J in 2 patients, > or = 30 J in 9 patients, and > 40 J in 3 patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of an engineering oriented medical research group in developing improved methods and devices for achieving ventricular defibrillation: the University of Missouri experience

Physical scientists and engineers have played important roles in helping to expand our understanding of the factors that influence the defibrillation process and in developing improved methods and devices for achieving cardiac ventricular defibrillation. The long-term experience of one engineering oriented group, based in a clinical department of a medical school, is summarized. Emphasized are the features of a series of research defibrillators that facilitated the generation of an extensive experimental database from studies in dogs and calves, the development of the first automatic implantable defibrillator to be successfully used in dogs, and studies that furnished the rationale for the widespread use of the uniphasic truncated exponential waveform and for the increasing interest in a variety of biphasic and multiphasic waveforms. Also considered are studies concerning the scaling of the defibrillatory shock with subject size and the role of compound units, defibrillation threshold, and contour graphs in the presentation and interpretation of data.

Comparison of subxiphoid and traditional approaches for ICD implantation

We compared clinical and electrophysiological data in 18 patients undergoing ICD implantation via a traditional (median sternotomy or left lateral thoracotomy) with 29 patients with a subxiphoid approach. Both groups were similar in terms of age, sex, left ventricular ejection fraction, presence of coronary artery disease, and clinical indication for the device. Fifteen patients (83%) with the traditional approach had previous cardiac surgery compared with 6 patients (21%) who had a subxiphoid approach (P < 0.001). Both groups had similar patch R wave and sensing R wave measurements. Patients with the traditional approach had a lower energy for defibrillation than patients with a subxiphoid approach (13.6 +/- 6.8 J vs 17.9 +/- 4.1 J, P < 0.05). Postoperative hospital days were fewer in the subxiphoid group compared with the traditional approaches (9.8 +/- 5.3 vs 13.7 +/- 7.5 days) but the differences did not reach statistical significance, possibly due to small numbers. The subxiphoid approach appears to be a reasonable alternative approach to the traditional approach in selected patients undergoing ICD implantation.

Retrospective analysis of patients undergoing one- or two-stage strategies for myocardial revascularization and implantable cardioverter defibrillator implantation

Internal defibrillation leads were placed at time of coronary revascularization in 79 patients. In 34, an implantable cardioverter defibrillator (ICD) was placed simultaneously (group I). A two-stage strategy (selective implantation of the ICD in patients with postoperative spontaneous or inducible ventricular tachycardia [VT]) was followed in 45 patients (group II). Group I patients had failed more antiarrhythmic drug trials (2.9 +/- 1.6 vs 1.5 +/- 1.6; P = 0.02), including amiodarone (62% vs 20%; P less than 0.001). There were four operative deaths in each group. Postoperatively, VT was present in 27 group II patients (60%), 25 of whom received an ICD (two refused device implantation). Patients with postoperative VT had a lower left ventricular ejection fraction than those without VT (33 +/- 9 vs 47 +/- 16; P = 0.01). Actuarial survival at 1, 2, and 3 years was 88 +/- 6, 88 +/- 7, and 88 +/- 10 in group I; and 83 +/- 6, 76 +/- 7, and 76 +/- 11 in group II (NS). No patient without an ICD (based on the postoperative electrophysiological study [EPS]) died suddenly. Five patients (6%) had ICD system infection. Sudden death was largely prevented by either strategy, but relatively high rates of operative mortality and ICD system infection were observed. Prospective studies should identify patients more likely to benefit from one or another strategy.

Safety of external cardioversion/defibrillation in patients with internal defibrillation patches and no device

Placement of prophylactic epicardial defibrillation patches at time of open-heart surgery in patients at risk for postoperative arrhythmias has been strongly questioned. Concern has centered on the ability to safely perform subsequent external defibrillation if needed. From 61 patients who were treated with a two-stage strategy we identified 17 who, while wearing epicardial patches and no generator, received external cardioversion/defibrillation for 20 episodes of hemodynamically unstable ventricular arrhythmias. All the patients had one small and one large patch. Eighteen of the episodes were induced during electrophysiological testing (with transthoracic shocks delivered via pad electrodes oriented in an apex-posterior configuration) and two were spontaneous. The episodes occurred at 21 +/- 27 days from patch implant. Thirteen episodes (65%) were converted with one shock at an energy level of 185 +/- 65 J. Seven (35%) required a second shock at 351 +/- 22 J. The accumulated energy requirement was 286 +/- 205 J. No adverse outcomes were noted. The number of episodes requiring more than one shock and the energy requirements were not different from those in a control group of 20 similar arrhythmias treated with the same equipment. Under these conditions, external cardioversion/defibrillation in patients with one large and one small epicardial defibrillation patch was uniformly successful. Further data is needed in the out-of-hospital setting and on the results of external defibrillation in patients with two large patches.