Epicardial Patch Research Articles

Lead-related complications in 340 patients with an implantable cardiverter/defibrillator

From February 1991 to May 1998, 340 patients had a cardioverter/defibrillator (ICD) implanted. Mean age was 64 +/- 9 years, 278 male and 62 female. 60% of patients had coronary artery disease and 31% dilatative cardiomyopathy. Ejection fraction was 38 +/- 14%. The indication for an implantable cardioverter/defibrillator was in 57% of patients ventricular tachycardia, in 43% ventricular fibrillation. 298 patients had a single-chamber cardioverter/defibrillator implanted, 42 patients a dual-chamber cardioverter/defibrillator. In 25 patients additional subcutaneous patch or array electrodes and in 2 patients additional epicardial patch electrodes were implanted. Implantation site was in 92 patients abdominal and in 248 pectoral. Over a period of 7 years 34 lead-related complications occurred in 33 patients (9.7%), after a median of 2 months after implantation. Diagnosis was made by routine chest x-ray in about 55% of lead-related complications, by clinical presentation (inadequate therapy, pain) in 24%, and by electrical parameters in 21%. In patients with an abdominal implantation site, lead-related complications occurred in 20%, in contrast to 6% in patients with a pectoral implantation site. Regarding patients with pectoral implantation site, lead-related complications were observed in 12% of patients with a dual-chamber ICD vs 4% with a single-chamber ICD (p = 0.05), due to dislocation of atrial electrodes with dual-chamber ICD. There were no differences in clinical parameters between patients with pectoral vs abdominal and between single vs dual-chamber ICD. There were no deaths due to lead-related complications.

Implantable defibrillators impedance measurement using pacing pulses versus shock delivery with intact and modified high voltage lead system.

At present the only method for measuring the high voltage system lead impedance in patients with an ICD is to deliver a low energy test shock. This is painful, requires sedation, and carries a risk of ventricular fibrillation induction. We sought to assess the shock lead and electrode function by calculating IMP using low voltage pacing pulses, and compared it to the measured impedance of a shock through the same lead. This was performed in both an intact and a modified lead system in order to mimic common clinical scenarios that alter lead system IMP (e.g., lead fracture). In an anesthesized canine model (n = 12) a standard (S) transvenous defibrillation lead (TDL), a modified (M) TDL (two-thirds of coil covered with heat-shrunk tubing), an active can (AC), and a M epicardial patch (EP) (two of four coils were disconnected) were used. Three configurations (C) were tested: C1:S/TDL-->AC, C2:M/TDL-->AC, and C3:M/TDL-->MEP. A measured IMP was obtained by an ICD using a 5-J shock as control. IMP was calculated using a 5-J shock, pacing pulses of 10-, 5-, 2-, and 1-V amplitude, as well as from a square wave drive train of low amplitude/high frequency signals (1 and 0.2 V, at 10 kHz) in all Cs. Ohm's law (V = IR) was utilized for measuring calculated IMP. As the surface area of the high voltage lead system decreased, the mean measured IMP (control) increased from C 1 to 3 (63 +/- 10, 95 +/- 4, and 127 +/- 20 omega, respectively). The correlation of calculated IMP from all Cs to measured impedance (control) remained high throughout the IMP range (range of correlation coefficient (r): 0.921-0.981). Calculated IMP using delivery of pacing pulses is highly correlated to IMP measured during shock delivery. This correlation remains high over a clinically significant range of high voltage lead system IMP changes. This study suggests that pacing pulses can be used to predict the IMP changes in the high voltage lead system which may occur clinically, reducing the need to deliver a shock for IMP measurement.

Left ventricular pseudoaneurysm after epicardial patch electrode placement

There are many known complications of implantable cardioverter-defibrillator placement. We treated a patient in whom a left ventricular pseudoaneurysm developed secondary to epicardial patch electrode placement and cardioverter-defibrillator implantation. The presenting symptoms, diagnostic evaluation, and surgical repair are described.

Ventricular fibrillation and defibrillation thresholds in sheep and dogs

Ventricular defibrillation studies normally use dogs rather than other large species. To investigate the suitability of sheep, which are often cheaper and more readily available, we compared ventricular fibrillation and defibrillation thresholds (VFT, DFT) in sheep and dogs. A total of 12 sheep (31±5 kg) and six dogs (19±1 kg) were anesthetised with halothane. Fibrillation was induced via epicardial pacing leads, using a 1 s 50 Hz pulse. Biphasic defibrillation shocks were delivered across epicardial patches. Voltage–response curves for both fibrillation and defibrillation were generated. Logistic regression analysis was used to determine 50 and 90% probability of success for fibrillation induction and defibrillation. VFT was similar in sheep and dogs. DFT at 50% probability of success was significantly higher in sheep (369±14 V) than in dogs (299±31 V, P<0.04) but within each species there was no correlation between heart weight and DFT. After defibrillation sheep took longer to return to sinus rhythm than dogs and electro-mechanical dissociation was observed in sheep, but not in dogs. Thus, sheep may not be an ideal model for ventricular defibrillation research but further studies of the intrinsic differences between sheep and dogs may provide insights into basic mechanisms of defibrillation.

High failure rate for an epicardial implantable cardioverter-defibrillator lead: implications for long-term follow-up of patients with an implantable cardioverter-defibrillator.

The purpose of this study was to determine the risk of epicardial lead failure during long-term follow-up and its mode of presentation. Despite the high prevalence of epicardial lead-based implantable cardioverter-defibrillators, their long-term performance is unknown, and appropriate follow-up has not been established. The study group comprised all patients in whom an epicardial lead system was implanted at the Mayo Clinic between October 31, 1984 and November 3, 1994. The number of lead fractures and leads with fluid within the insulation and the mode of presentation were determined retrospectively by review of patient visits, radiographs of lead systems and data derived from formal lead testing. At 4 years, the survival rate free of lead malfunction, using formal lead testing, for 160 Medtronic epicardial patches (models 6897 and 6921) was 72% compared with 92.5% for the 179 Cardiac Pacemaker, Inc. (CPI) patches (models 0040 and 0041) (p = 0.01). In addition, five Medtronic patches in three patients had fluid within the lead insulation but no obvious fracture. No CPI patches had fluid identified within the leads. Of 330 Medtronic epicardial pace/sense leads (model 6917), the 4-year survival rate free of lead malfunction as assessed by lead testing was 96%. In all, 19 presentations of lead malfunction were found in 17 patients (2 patients had more than one lead fracture at different times). In 11 (58%) of these presentations, the patients were asymptomatic despite the presence of obvious lead fracture. Epicardial lead malfunction is common on long-term follow-up, and some leads have a failure rate of 28% at 4 years. Many patients with fractured leads remain asymptomatic, despite involvement of multiple leads in some cases. Therefore, consideration should be given to regular periodic lead testing in addition to routine X-ray examination, as asymptomatic lead malfunction can present with normal chest X-ray findings.

Implantable cardioverter-defibrillators and pregnancy: a safe combination?

The purpose of this investigation was to evaluate the outcome of pregnancy in women with implantable cardioverter-defibrillators (ICDs). A multicenter retrospective analysis was performed on women with an ICD who became pregnant. Data were collected on 44 patients. The mean age and ejection fraction at the time of the initial ICD implant were 25.6+/-4.9 years and 49.8+/-9.7%, respectively. The mean follow-up after the ICD implant was 4.8+/-2.8 years. Forty-two women had abdominally implanted generators, and 2 had a prepectoral device. Thirty had epicardial lead sensing systems, and 14 had transvenous. Thirty-six (82%) experienced no complications, and 8 (18%) had a medical or device-related complication. The ICD-related problems included tenderness at the ICD pocket scar (2 patients), generator migration (1), and pericarditis secondary to the epicardial patches (1). Medical complications were pulmonary embolism (1), therapeutic abortion (1), worsening hyperthyroidism (1), congestive heart failure (1), and weight loss (1). Thirty-seven women delivered vaginally, and 7 underwent cesarean section. Thirty-nine babies were born healthy, 1 was stillborn, 2 were small for gestational age, 1 had transient hypoglycemia, and 1 woman had a therapeutic abortion unrelated to the ICD. During pregnancy, 33 women received no ICD therapy, 8 had 1 shock, 1 had 5 discharges, 1 had 11 shocks, and 1 had 5 shocks. The total number of shocks during pregnancy ranged from 0 to 11, with an average of 0.66+/-1.9 discharges. Five women had 7 additional pregnancies without an ICD shock. The mere presence of an ICD should not defer a women from becoming pregnant unless she has an underlying structural cardiac disease that is considered a contraindication. Pregnancy does not increase the risk of major ICD-related complications or result in a high number of ICD discharges.

Surgical repair of subacute left ventricular free wall rupture.

The natural course of subacute ventricular free wall rupture (FWR) as a complication of acute myocardial infarction (MI) is usually lethal. The aim of this study was to investigate the curability of this entity and to report on five patients successfully treated by rapid diagnosis, hemodynamic stabilization, and emergency surgical repair. Five patients with subacute FWR of the left ventricle after previous MI were operated on. Infarctectomy with subsequent closure of the ruptured area was carried out in two patients with anterolateral infarction. Three other patients (two with posterior and one with lateral infarction) were treated by direct closure and the application of a patch. Furthermore, in two patients, concomitant myocardial revascularization was performed. All patients survived the procedure and were alive and well at long-term follow-up (mean 36.4 months). None of the patients suffered recurrent MI. Our experience and a review of the literature shows that prompt diagnosis and emergency surgical intervention may save the patient. Anterior rupture (with a moderate sized infarcted area) is best treated by infarctectomy and subsequent closure of the ventriculotomy with sutures buttressed with felt, whereas posterior rupture may be treated by direct closure and the application of an epicardial patch. Considering our results, we cannot conclude whether additional coronary artery bypass grafting is beneficial or not. Our suggestion is to perform additional myocardial revascularization only if indicated.

Simulated internal defibrillation in humans using an anatomically realistic three-dimensional finite element model of the thorax.

Determination of the optimal electrode configuration during implantable cardioverter defibrillator (ICD) implantation remains largely an empirical process. This study investigated the feasibility of using a finite element model of the thorax to predict clinical defibrillation metrics for internal defibrillation in humans. Computed defibrillation metrics from simulations of three common electrode configurations with a monophasic waveform were compared to pooled metrics for similar electrode and waveform configurations reported in humans. A three-dimensional finite element model was constructed from CT cross-sections of a human thorax. Myocardial current density distributions for three electrode configurations (epicardial patches, right ventricular [RV] coil/superior vena cava [SVC] coil, RV coil/SVC coil/subcutaneous patch) and a truncated monophasic pulse with a 65% tilt were simulated. Assuming an inexcitability threshold of 25 mA/cm2 (10 V/cm) and a 75% critical mass criterion for successful defibrillation, defibrillation metrics (interelectrode impedance, defibrillation threshold current, voltage, and energy) were calculated for each electrode simulation. Values of these metrics were within 1 SD of sample-size weighted means for the corresponding metrics determined for similar electrode configurations and waveforms reported in human clinical studies. Simulated myocardial current density distributions suggest that variations in current distribution and uniformity partially explain differences in defibrillation energy requirements between electrode configurations. Anatomically realistic three-dimensional finite element modeling can closely simulate internal defibrillation in humans. This may prove useful for characterizing patient-specific factors that influence clinically relevant properties of current density distributions and defibrillation energy requirements of various ICD electrode configurations.

Effect of preexisting epicardial patch electrodes on defibrillation thresholds of unipolar defibrillators

We conclude that patients with failing epicardial lead systems can be safely and effectively treated with the implantation of a fourth generation unipolar implantable cardioverter defibrillator.

Relationship between "extension of refractoriness" and probability of successful defibrillation.

The "extension of refractoriness" hypothesis, which suggests that the shock halts fibrillation by extending the refractory period, has not been directly tested. Defibrillation (5 isolated rabbit hearts; 111 episodes) was attempted by 8-ms pulses (65% tilt) delivered through epicardial patches. Monophasic action potentials were recorded in a low current density region (6.3 V/cm at 90% success). Fifty shocks failed to convert; 61 shocks successfully defibrillated. Postshock response duration (from shock to repolarization) was significantly longer for successful type A (with no postshock activations) defibrillation (102.3 +/- 7.5 ms) than for unsuccessful defibrillation (47.6 +/- 4.3 ms; P < 0.0001) for shocks occurring during the last 40% of the fibrillation action potential. Probability of success and postshock response duration both increased with current intensity. However, at each intensity, response durations for successful defibrillation were significantly longer than those for unsuccessful defibrillation. A minimum prolongation of 75 ms was associated with type A defibrillation. These results suggest that shock-induced response duration correlates with successful defibrillation and that a response of 75 ms is required to completely block fibrillation wavefronts.

Factors Associated With Elevated Impedance With a Nonthoracotomy Defibrillation Lead System

Peak current flow across the heart determines the success of defibrillation and is inversely dependent on impedance between defibrillation electrodes. Factors associated with elevated impedance in patients with implantable defibrillators using nonthoracotomy lead systems have not been well described. Clinical and echocardiographically derived variables were analyzed in 41 patients in whom implantation of a nonthoracotomy lead system was attempted. Lead impedance was measured at end-expiration with 5-J monophasic shocks. Successful defibrillation with or without addition of a subcutaneous patch with ≤20 J with a monophasic waveform was required for nonthoracotomy lead placement. Patients were divided into 2 groups based on impedance: low (≤47 ohms, n = 30) and high (>47 ohms, n = 11). Twenty-four patients had successful defibrillator implantation using a transvenous lead alone, 13 required placement of a subcutaneous patch, and 4 required epicardial patch placement. The mean left ventricular end-diastolic and end-systolic volumes were significantly smaller (p = 0.01 for both) in patients in the low- versus high-impedance groups and were significantly correlated with impedance (r = 0.44, p < 0.005 for both). Impedance was not significantly different between patients with successful defibrillation using a transvenous lead alone compared with those who required either subcutaneous or epicardial patches. Thus, impedance using a nonthoracotomy lead system with monophasic shocks is significantly correlated with both end-systolic and end-diastolic volumes, but elevated impedance does not predict increased defibrillation energy requirements.This study examined the influence of clinical, radiographic, and echocardiographic variables on impedance measured with 5-J monophasic shocks at end-expiration with a nonthoracotomy lead system in 41 patients. Significant correlations (r = 0.44, p < 0.005 for both) were found between impedance and both end-systolic and end-diastolic volumes, but elevated impedance did not predict increased defibrillation energy requirements with a nonthoracotomy lead or the need for either subcutaneous or epicardial patches.

Pharmacologic enhancement of atrial electrical defibrillation efficacy: role of ibutilide.

The study was designed to identify the influence of ibutilide on activation during atrial fibrillation (AF) and determine its role in enhancing the efficacy of electrical conversion of AF. Vagally-mediated acute AF was induced in 12 anesthetized dogs. In 5 dogs, a cross-shaped epicardial patch containing 16 recording electrodes was placed on the right atrium. In 7 dogs, defibrillation patch electrodes overlying both atria were used to deliver biphasic shocks (50% tilt; 90 microF). Measurements were made at baseline and following ibutilide (0.075 mg/kg bolus followed by 0.075 mg/kg infusion over 10 minutes). Right atrial multisite electrogram recordings revealed significant prolongation in AF cycle length at all sites following ibutilide. In all dogs, AF cycle length increased with ibutilide from 90 +/- 23 to 130 +/- 49 ms (p < 0.005). Whereas, atrial defibrillation threshold decreased from 0.83 +/- 0.5 to 0.53 +/- 0.29 J (p = 0.020). In conclusion, ibutilide prolongs cycle length of canine vagally-mediated acute AF. Furthermore, ibutilide facilitates electrical conversion of AF by lowering energy requirement. Thus, controlled drug infusion in conjunction with electrical defibrillation may be useful for managing AF.

Effect of changing capacitors between phases of a biphasic defibrillation shock.

In this study, we examined the effect of changing capacitor values between phases of a biphasic waveform with the goal of lowering leading edge voltage (LEV), total delivered energy (TDE), and total stored energy (TSE). Defibrillation thresholds were determined in 18 open-chest swine using epicardial patch electrodes. In part I, three combinations of capacitors were tested: 150:150 microF; 150:300 microF; and 300:150 microF. Waveform durations were 6/0, 6/2, 6/4, 6/6, and 6/8 ms. In part II, phase 1 capacitance was 150 microF. Three phase 2 capacitance values were used: 150 microF; 75 microF; and 37.5 microF. Phase 2 LEV was a multiple of phase 1 trailing edge voltage: x 0.5; x 0.75; x 1; x 2; x 3; and x 4. A 3.5/2.0 ms biphasic waveform was used. In part III, thresholds were determined for two sets of capacitor values, which can be created by switching a pair of capacitors from in parallel to in series, 150:37.5 microF and 300:75 microF, and nine waveform durations, 4/0, 4/2, 4/4, 6/0, 6/3, 6/6, 8/0, 8/4 and 8/8 ms. In part I, the 300:150 microF system defibrillated with the lowest LEV, TDE and TSE were not different for any of the biphasic waveforms tested except for the 6/8 ms, which was higher. In part II, there was no difference in LEV among any of the three phase 2 capacitor values. LEV was lowest for the x 2, x 3, x 4 multipliers. Peak voltage was lowest for the x 1 and x 2 multipliers. TDE was lowest for the x 0.5, x 0.75, x 1, and x 2 multipliers. In part III, the 300:75 microF system defibrillated at a lower LEV than did the 150:37.5 microF system. The 150:37.5 microF system defibrillated at a lower total delivered energy than did the 300:75 microF. These results suggest that defibrillation can be accomplished with lower LEV, TDE, and TSE if two capacitors are switched from a parallel configuration to a series configuration between phases of the biphasic waveform.

Defibrillation Shocks Over Epicardial Patches Produce Sympathetic Neural Dysfunction in Man

The purpose of this study was to determine the effect of direct current (DC) shocks on cardiac sympathetic innervation in humans using I-123-metaiodobenzylguanidine (MIBG) scintigraphy. Decreased efferent sympathetic neural function has been demonstrated following > 10-J DC shocks delivered through epicardial patch electrodes in dogs. To evaluate the effect of DC shocks on cardiac sympathetic innervation in humans, we performed MIBG scintigraphy in 11 patients (ages 46 to 75 years) prior to and after receiving shocks from an implantable cardioverter defibrillator (ICD). This study was performed during an ICD generator change in 7 patients with epicardial patch electrodes and at the time of initial ICD implantation in 4 patients: 2 with epicardial patch electrodes, and 2 with a transvenous ICD system. All patients had spontaneous and inducible ventricular tachycardia. Prior to ICD implantation and remote from any cardioversions or shocks, baseline MIBG and thallium-201 scintigraphy were performed. Repeat MIBG scintigraphy was performed after delivery of ICD shocks and compared with the baseline scans to determine the effect of the shock on sympathetic neural function. The baseline scans revealed focal areas of reduced MIBG uptake in areas of thallium perfusion defects in all patients except the patient without structural heart disease whose scans were normal. Postshock, patients with epicardial patch electrodes who received at least one 24-J shock and had the postshock MIBG scan performed within 4 hours demonstrated no cardiac uptake of MIBG. Two patients with epicardial patch electrodes had no change in the postshock MIBG scans: 1 had a maximal shock of 20 J, and the other had the postshock scan delayed for 11 hours. The 2 patients with a transvenous lead system demonstrated no change in the postshock MIBG scan when compared with baseline. This study demonstrates that following DC shocks delivered over epicardial patch electrodes, there is diffuse reduction in MIBG uptake that probably represents cardiac sympathetic neural dysfunction that appears to be transient. Sympathetic function does not appear to be affected by shocks delivered over a transvenous lead system.

Effects of initial polarity on defibrillation threshold with biphasic pulses.

Previous studies have shown that the polarity of epicardial patches significantly affects the defibrillation efficacy of monophasic shocks. However, whether this improvement can be extended to different pulsing methods and lead systems, such as biphasic shocks using endocardial defibrillating electrodes, is unknown. Twenty consecutive patients undergoing testing and permanent implant using an Endotak lead system with a biphasic device were included in the study. In each patient the defibrillation threshold was determined delivering biphasic pulses with the distal coil as the cathode and the proximal coil as the anode during the positive phase and with the polarity reversed. The initial electrode polarity tested was chosen randomly. The defibrillation threshold was defined as the lowest pulse amplitude that effectively terminated ventricular fibrillation induced with 60-Hz alternating current. For each biphasic pulse peak voltage, pulse duration, resistance, and stored energy were recorded. Of the 20 patients, 12 (60%) had lower defibrillation threshold when the proximal coil was negative, whereas only 2 patients had a lower defibrillation threshold when the distal coil was negative. In four patients a subcutaneous patch would have been required if only the biphasic pulse with the distal coil as negative had been tested. The mean stored defibrillation threshold energy was lower with the configuration using the proximal coil as cathode (16.3 +/- 8.8 J vs 21.5 +/- 11 J; P < 0.01). Change in the initial polarity of biphasic shocks may influence defibrillation efficacy and should, therefore, be assessed in each patient to achieve a more satisfactory safety margin and minimize the use of more invasive lead configurations.

Crinkling of epicardial defibrillator patches: A common and serious problem

The durability and reliability of the implantable cardioverter-defibrillator epicardial patch systems have not been reported. In 128 consecutive patients such systems manufactured by Cardiac Pacemakers, Inc. (St. Paul, Minn.) or Medtronic, Inc. (Minneapolis, Minn.) were implanted with 100% follow-up to investigate the rate of patch crinkling and its consequences. A total of 122 patients survived the operation (operative mortality, 6 patients: 4.7%). Ninety-four patients received Cardiac Pacemakers, Inc. AICD patches and 28 received Medtronic PCD patches. Patients had chest x-ray studies every 3 to 6 months and function of the defibrillator was checked every 3 months. Late mortality occurred in 17 patients (13%) leaving a total of 105 long-term survivors (82%) to the present. Among 122 survivors, severe crinkling of the patches occurred in 48 patients--33 in the Cardiac Pacemakers, Inc. AICD group (36%) and 15 in the PCD group (54%)--within 2 years of the implant. Crinkling of patches caused not only malfunction of the system, but also cardiac pain in three patients. Crinkling occurred as early as 2 months after implant and progressed throughout the period of observation. Fourteen patients later required implant of an additional transvenous defibrillator because of failure of the epicardial system. The percentage of transvenous implantable cardioverter-defibrillator systems needed was higher for the Medtronic group (28%) than for the Cardiac Pacemakers, Inc. AICD group (6.3%). Both systems have shown an unacceptably high rate of patch crinkling that occurs in a relatively short time. There is no difference whether a thoracotomy or midline sternotomy is used or whether the patches are implanted intrapericardially or extrapericardially. The quest should continue for a better patch system design.

Implantation and Follow‐Up of a Third‐Generation Cardioverter Defibrillator: Comparison of Epicardial and Nonthoracotomy Defibrillation Lead System

The intraoperative and follow-up results were compared in 67 patients with ventricular tachyarrhythmias who underwent implantation of the Ventritex Cadence defibrillator with either epicardial patch (EPI, 25 patients) or nonthoracotomy CPI Endotak (ENDO, 42 patients) defibrillation lead systems. There was no significant difference between groups in age, sex, structural heart disease, ejection fraction, arrhythmia history, or drug therapy. Successful implantation was accomplished in all patients using either lead system. In the ENDO group, 35 patients (83%) had a defibrillation threshold < or = 550 V and did not require a subcutaneous patch. Intraoperatively, the defibrillation threshold was 453 +/- 139 V (13 +/- 9 J) for EPI and 490 +/- 113 V (15 +/- 8 J) for ENDO (P = NS). There were no perioperative deaths in either group. At predischarge testing, the defibrillation threshold was 445 +/- 183 V (14 +/- 12 J) for EPI and 439 +/- 133 V (13 +/- 7 J) for ENDO (P = NS). During a mean follow-up of 16 +/- 8 months, there were no sudden deaths, and four patients died from congestive heart failure (3 EPI, 1 ENDO). During follow-up, 916 spontaneous arrhythmia episodes occurred in 16 of 25 EPI patients (64%) and 967 episodes occurred in 31 of 42 ENDO patients (74%) (P = NS). The number of episodes detected as ventricular fibrillation were 192 for EPI (21%) and 232 for ENDO (24%), with first shock success in 76% and 75%, respectively; all episodes were successfully terminated by the device. In the remaining episodes detected as ventricular tachycardia, antitachycardia pacing was attempted and was successful in 672 of 724 episodes (93%) with EPI and 666 of 735 episodes (91%) with ENDO lead systems (P = NS). Acceleration of ventricular tachycardia with antitachycardia pacing occurred in 21 episodes (3%) with EPI and in 37 episodes (5%) with ENDO leads (P = NS). A nonthoracotomy approach using the third generation cardioverter defibrillator Cadence V-100 is safe and effective and has clinical results that are not significantly different from epicardial defibrillation lead systems.

Effects of the Pacing Site, Procainamide, and Lead Configuration on the Relationship Between the Upper Limit of Vulnerability and the Defibrillation Threshold

In six open chest dogs, we determined the upper limit of vulnerability (ULV) and defibrillation threshold (DFT) by an up-down algorithm when the pacing site was at the right atrium, at the left ventricular apex, and at the left ventricular base. Monophasic shocks (6 ms) were given to epicardial patches at 20 and 40 ms before the peak of the T wave to bracket the mid-upslope. In an additional six closed-chest dogs, we determined the ULV and the DFT with transvenous leads with an 8-ms biphasic waveform. The S1 pacing site was at the right ventricular apex and the right atrium, and the shocks were given at 20 ms and 40 ms before the peak of the T wave, and on the peak of T wave. The same test was repeated after intravenous procainamide infusion (20 mg/Kg loading, then 2 mg/min maintenance). In the first six dogs, the ULV determined when pacing was given to the left ventricular apex, the left ventricular base, and the right atrium was 4.2 +/- 1.7 J, 4.4 +/- 2.1 J, and 3.9 +/- 1.5 J, respectively; values that were not significantly different from the DFT of 4.8 +/- 1.9 J, 4.5 +/- 1.9 J, and 4.2 +/- 1.3 J, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transmyocardial impedance: Implications for implantable cardioverter-defibrillator implantation

The influence of normothermic and hypothermic cardiopulmonary bypass on defibrillation energy requirements and transcardiac impedance is not well characterized. However, this relationship is of clinical importance during automatic defibrillator implantation done with concomitant cardiac surgery, and there is anecdotal information that criteria for successful implantation are harder to achieve after such operations. We studied the effect of controlled hypothermia on defibrillation energy requirements and transcardiac impedance in a canine model of cardiopulmonary bypass in which 26 animals underwent right atrial and femoral arterial cannulation, as well as continuous hemodynamic and intramyocardial temperature monitoring. The defibrillation energy requirements were evaluated at 60-minute intervals with an epicardial patch system, and transcardiac impedance was measured before and after the multiple inductions and terminations of ventricular fibrillation. In group 1 ( n = 10) defibrillation energy requirements were evaluated immediately after initiation of cardiopulmonary bypass at 37° C (T 0 ), after gradual cooling to 28° C (T 1 ), and after rewarming to 37° C (T 2 ). Group 2 ( n = 16) comprised time controls that were identically instrumented and studied, but maintained at 37° C throughout. Percent successful defibrillation was plotted against delivered energy, and the raw data fit by logistic regression. The energy at which 50% of shocks were successful (E 50 ) was 3.23 ± 0.89 joules at T 0 , 5.12 ± 1.85 joules at T 1 , and 4.42 ± 1.22 joules at T 2 in group 1; this was not significantly different from the corresponding group 2 E 50 values, which were 3.11 ± 1.39 joules, 4.95 ± 2.47 joules, and 5.59 ± 3.18 joules, respectively. Both groups demonstrated a significant increase in E 50 during the first hour of cardiopulmonary bypass (mean increase from T 0 to T 1 was 1.89 joules in group 1 and 1.84 joules in group 2, p < 0.05). Transmyocardial impedance fell progressively during the group 2 experiments from 73.6 ± 12.9 Ω at the beginning of the T 0 shock series to 61.4 ± 8.9 Ω at the end of the T 2 shock series. A similar reduction in transmyocardial impedance was observed during the course of all the group 1 experiments; however, at the beginning of the T 1 shock series impedance was significantly elevated to 77.4 ± 12.3 Ω ( p < 0.05 compared with group 2 and with end T 0 in group 1). There was no relationship between defibrillation energy requirements and transcardiac impedance; there was also no correlation between either of these parameters and intramyocardial extracellular pH or left ventricular end-diastolic pressure. We conclude that defibrillation energy requirements significantly and persistently increase during the first hour of cardiopulmonary bypass and the effect is temperature independent Systemic hypothermia significantly and reversibly elevates transmyocardial impedance but multiple shocks reduce this effect at 28° C as they do at 37° C. (J T HORAC C ARDIOVASC S URG 1995;109:981-8)

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

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