Effect of a Single Element Subcutaneous Array Electrode Added to a Transvenous Electrode Configuration on the Defibrillation Field and the Defibrillation Threshold

Even with the use of biphasic shocks, up to 5% of patients need an additional subcutaneous lead to obtain a defibrillation safety margin of at least 10 J. The number of patients requiring additional subcutaneous leads may even increase, because recent generation devices have a < 34 J maximum output in order to decrease their size. In 20 consecutive patients, a single element subcutaneous array lead was implanted in addition to a transvenous lead system consisting of a right ventricular (RV) and a vena cava superior lead using a single infraclavicular incision. The RV lead acted as the cathode; the subcutaneous lead and the lead in the subclavian vein acted as the anode. The biphasic defibrillation threshold was determined using a binary search protocol. Patients were randomized as to whether to start them with the transvenous lead configuration or the combination of the transvenous lead and the subcutaneous lead. In addition, a simplified assessment of the defibrillation field was performed by determining the interelectrode area for the transvenous lead only and the transvenous lead in combination with the subcutaneous lead from a biplane chest X ray. The intraoperative defibrillation threshold was reconfirmed after 1 week, after 3 months, and after 12 months. The mean defibrillation threshold with the additional subcutaneous lead was significantly (P = 0.0001) lower (5.7 ± 2.9 J) than for the transvenous lead system (9.5 ± 4.6 J). With the subcutaneous lead, the impedance of the high voltage circuit decreased from 48.9 ± 7.4 Ω to 39.2 ± 5.0 Ω. In the frontal plane, the interelectrode area increased by 11.3%± 5.5% (P < 0.0001) and in the lateral plane by 29.5%± 12.4% (P < 0.0001). The defibrillation threshold did not increase during follow-up. Complications with the subcutaneous electrode were not observed during a follow-up of 15.8 ± 2 months. The single finger array lead is useful in order to lower the defibrillation threshold and can be used in order to lower the defibrillation threshold.     

Defibrillator Implantation in a Patient with a Persistent Left Superior Vena Cava

The implantation of a transvenous cardioverter defibrillator (PCD 7217B) was performed in a patient with a persistent left superior vena cava. The defibrillation electrodes were positioned in the right ventricle and the superior vena cava via the right subclavian vein. A subcutaneous patch had to be implanted at the left lateral chest wall to achieve sufficient defibrillation thresholds. Three weeks later the system had to be removed because of a generator pocket infection. During the second implantation we placed one electrode in the persistent left superior vena cava perpendicular to the electrode in the right ventricle. Using this configuration transvenous defibrillation was possible without an additional subcutaneous patch.     

A Prospective, Randomized Evaluation of a Nonthoracotomy Implantable Cardioverter Defibrillator Lead System

Nonthoracotomy lead systems for ICDs have been developed that obviate the need for a thoracotomy and reduce the morbidity and mortality associated with implantation. However, an adequate DFT cannot be achieved in some patients using transvenous electrodes alone. Thus, a new subcutaneous "array" electrode was designed and tested in a prospective, randomized trial that compared the DFT obtained using monophasic shock waveforms with a single transvenous lead alone that has two defibrillating electrodes, the transvenous lead linked to a subcutaneous/submuscular patch electrode, and the transvenous lead linked to the investigational array electrode. There were 267 patients randomized to one of the three nonthoracotomy ICD lead systems. All had DFTs that met the implantation criterion of ≤ 25 J. The resultant study population was 82% male and 18% female, mean age of 63 ± 11 years. The indication for ICD implantation was monomorphic VT in 70%, VF in 19%, monomorphic VT/VF in 6%, and polymorphic VT in 4% of the patients, respectively. The mean LVEF was 0.33 ± 0.13. The mean DFT obtained with the transvenous lead alone was 17.5 ± 4.9 J as compared to 16.9 ± 5.5 J with the lead linked to a patch electrode (P = NS), and 14.9 ± 5.6 with the lead linked to the array electrode (array versus lead alone, P = 0.0001; array versus lead/patch, P = 0.007). The results of this investigation suggest that the subcutaneous array may be superior to the standard patch as a subcutaneous electrode to lower the DFT and increase the margin of safety for successful nonthoracotomy defibrillation.     

A Second Defibrillator Chest Patch Electrode Will Increase Implantation Rates for Nonthoracotomy Defibrillators

Nonthoracotomy defibrillator systems can be implanted with a lower morbidity and mortality, compared to epicardial systems. However, implantation may be unsuccessful in up to 15% of patients, using a monophasic waveform. It was the purpose of this study to prospectively examine the efficacy of a second chest patch electrode in a nonthoracotomy defibrillator system. Fourteen patients (mean age 62 ± 11 years, ejection fraction = 0.29 ± 0.12) with elevated defibrillation thresholds, defined as ≥ 24 J, were studied. The initial lead system consisted of a right ventricular electrode (cathode), a left innominate vein, and subscapular chest patch electrode (anodes). If the initial defibrillation threshold was ≥ 24 J, a second chest patch electrode was added. This was placed subcutaneously in the anterior chest (8 cases), or submuscularly in the subscapular space (6 cases). This resulted in a decrease in the system impedance at the defibrillation threshold, from 72.3 ± 13.3 Ω to 52.2 ± 8.6 Ω. Additionally, the defibrillation threshold decreased from ≥ 24 J, with a single patch, to 16.6 ± 2.8 J with two patches. These changes were associated with successful implantation of a nonthoracotomy defibrillator system in all cases. In conclusion, the addition of a second chest patch electrode (using a subscapular approach) will result in lower defibrillation thresholds in patients with high defibrillation thresholds, and will subsequently increase implantation rates for nonthoracotomy defibrillators.     

Minimally invasive cardioverter defibrillator implantation for children: an animal model and pediatric case report

The smaller venous capacitance in infants and small children may hamper transvenous ICD lead implantation, and epicardial approaches require thoracotomy and have associated complications. The study evaluated the feasibility and performance of subcutaneous arrays and active can ICDs without transvenous shocking coils or epicardial patches. An immature and mature pig were anesthetized and ventilated. A pacing lead was inserted in the right ventricle for fibrillation induction and rate sensing. Subcutaneous arrays were positioned in the right and left chest walls. An ICD emulator was placed in abdominal and prepectoral pockets. Fluoroscopic images were acquired for each electrical vector configuration (array --> can, can --> array, array --> array, array + array --> can). Ventricular fibrillation was induced and DFT testing performed. Defibrillation was achieved in all ten trials in the immature piglet, with DFT < or = 9 J, regardless of vector configuration. Using a single subcutaneous array and active can, the shock impedance ranged from 28-36 ohms. With two arrays, shocking impedance fell to 15-22 ohms. In the adult pig, defibrillation was not accomplished with maximum energy of 40 J, using all vector configurations. Using data garnered from these experiments, this technique was then successfully performed in a 2-year-old child with VT and repaired congenital heart disease, needing an ICD. This study demonstrates the feasibility of leadless ICD implantation in an immature animal and successful implementation in a small child. A single subcutaneous array and active can resulted in excellent implant characteristics and DFTs with a minimally invasive approach. Defibrillation was not possible in a larger animal, possibly due to maximal available energy. This may be of value for small children requiring ICD implantation.     

Polarity Reversal Improves Defibrillation Efficacy in Patients Undergoing Transvenous Cardioverter Defibrillator Implantation with Biphasic Sbocks

The purpose of this study was to determine the influence of polarity reversal on DFT in patients undergoing implantation of nonthoracotomy defibrillators with biphasic shocks. Previous studies have shown higher defibrillation efficacy with using the distal electrode as anode in implantation of nonthoracotomy defibrillators and monophasic shocks. However, it is as yet unclear whether biphasic shock defibrillation will also be influenced by polarity reversal. Using a transvenous lead system with a proximal electrode in the superior caval vein and a distal electrode in the RV apex, 27 patients undergoing defibrillator implantation were randomized to DFT testing with "initial" (distal electrode = cathode) or "reversed" polarity (distal electrode = anode). Defibrillation energy was reduced stepwise until defibrillation failure occurred. At this point, polarity was switched and testing continued until the lowest energy requirement was determined for both polarities. With reversed polarity, DFT was 11.1 ± 5.7 J versus 13.3 ± 5.8 J with initial polarity (P = 0.033). This means a 17% reduction of the DFT. In 10 patients, the threshold was lower with reversed, whereas in 3 patients it was lower with initial polarity. In conclusion, changing electrode polarity in transvenous implantable defibrillators with biphasic shocks may significantly influence defibrillation energy requirements. Therefore, polarity reversal should always be attempted before considering patch implantation.     

A Subcutaneous Lead Array for Implantable Cardioverter Defibrillators

Two patients received an implantable cardioverter defibrillator with the combination of a transvenous lead and a subcutaneous lead array with three branches. This approach allowed us to find low defibrillation threshoids in both patients (≤10 and ≤ 15 joules [J], respectively], which was impossible with a transvenous catheter. In a third patient, a crinkled subcutaneous patch was replaced by an array. The defibrillation threshoid with the array was ≤ 20 J, as opposed to ≤24 J with the patch. No surgical problems occurred. The subcutaneous array is a technical improvement for tiie therapy with impiantable defibrillators, when a single catheter system is not sufficient to ensure a safety margin for defibrillation, or when surgicai or postsurgical problems occur with a subcutaneous patch.     

Intracardiac Emergency Defibrillation for Refractory Ventricular Fibrillation During Implantation of Cardioverter Defibrillators with Nonthoracotomy Lead Systems

Implantable cardioverter defibrillators (ICDs) are being implanted in increasing numbers. At inlraoperative defibrillation threshold tests refractory ventricular fibrillation (VF) requiring emergency open chest resuscitation is a major concern during impiantation of nonthoracotomy ICD lead systems. A new method of high energy endocardial/extrathoracic defibrillotion via the implanted ICD transvenous defibrillation electrode (TDE) was used to terminate refractory VF. During implantation of ICD with TDE in 20 patients refractory VF occurred in two patients. The arrhythmia was terminated with endocardial/extrathoracic defibrillation in both cases, and no complications were observed.     

Nonthoracotomy Implantation of Cardioverter Defibrillators: Preliminary Experience with a Defibrillation Lead Placed at the Right Ventricular Outflow Tract

Although morbidity and mortality associated with defibrillator implantation using a nonthoracotomy approach have decreased as compared with a thoracotomy approach, dfifihrillation thresholds have been higher and fewer patients satisfied implan t criteria. It may be possible to improve on the success of nonthoracotomy defibrillator implantation by the placement of a right ventricular (HV) outflow defibrillation lead. Implnntable car-dioverter defibrillator implantation data of 30 consecutive patients with clinical VT or VF were reviewed. Three defibrillation leads were routinely used. When either pacing threshold at the RV apex ivas inadequate (n - 2) or 18-J shocks were not successful in terminating VF in 3 of 4 trials (n = 8). the RV apex lead was positioned to the HV outflow tract attaching to the septum. Defibrillation testing was first performed with the RV apex lead in combination with CS, SVC. and/or subcutaneous leads. Twenty patients satisfied implant criteria with a defibrillation threshold of 13.5 ± 3.6 J. In 7 of the 10 patients, whose RV lead was repositioned to the RV outflow tract, this lead in combination with SVC, CS, or subcutaneous leads produced successful defibrillation at < 18 J or in 3 of 4 trials. This approach improved the overall success of nonthoracotomy implantation of defibrillators from 69% to 90%, After a follow-up of 27 ± 6 months, there was no dislodgment of the HV outflow tract defibrillation leads. Conclusions: This article reports the preliminary observation that placement of defibrillation leads to the RV outflow tract in humans was possible and without dislodgment. RV outflow tract offers an alternative for placement of defibrillation leads, which may improve on the success of nonthoracotomy defibrillator implantation.     

The Economic Impact of Transvenous Defibrillation Lead Systems

The purpose of this study was to compare implant charges and convalescence for transvenous and epicardial defibrillation systems. Hospital stay, intensive care utilization, professional fees, and hospital bills were compared in 44 patients who underwent implantation of a cardiac defibrillator between September 1991 and May 1993. Twenty-five consecutive patients received an epicardial lead system, while 19 consecutive patients underwent implantation of the entire transvenous defibrillation system in the electrophysiology laboratory. There were no significant differences between the two groups in mean age or left ventricular ejection fraction. There was a significant reduction in postoperative hospital convalescence from 7.2 ± 2.0 days with epicardial systems to 3.1 ± 1.5 days with transvenous systems (P < 0.001). Postoperative intensive care unit stay was significantly reduced with transvenous systems compared with epicardial systems (0.1 ± 0.2 vs 1.5 ± 0.9 days; P < 0.001). Hospital charges were also significantly reduced with the transvenous lead system implants. Mean implant charges were lower with transvenous systems; $32,090 ±$2,620 vs $38,307 ±$2,701 (P < 0.001); convalescence charges were lower: $5,861 ±$5,010 vs $12,447 ±$4,969 (P < 0.001); the total hospital bill was also significantly lower with transvenous systems: $53,459 ±$12,588 vs $71,981 ±$16,172 (P < 0.001). Professional fees for implantation ($4,131 ±$1,724 vs $6,100 ± 0, P < 0.001), convalescence care ($1,258 ±$960 vs $2,846 ±$1,770; P < 0.001), and total professional fees ($12,925 ±$4,772 vs $15,731 ±$4,055, P < 0.05) were lower in the transvenous defibrillation group. In conclusion, transvenous defibrillation lead systems are associated with significantly shorter postoperative recovery and significantly lower hospital and professional charges.     

Subpectoral Implantation of ICD Generators: Long-Term Follow-Up

A nonthoracotomy surgical approach using an endocardial electrode and combined implantation of a subcutaneous patch and the implantable cardioverter defibrillator (ICD) generator in a Subpectoral pocket has been described. We report the long-term follow-up results in patients undergoing implantation using this approach. The patient population consisted of 28 patients (22 men and 6 women) with a mean age of 59 ± 12 years. The underlying heart disease consisted of coronary artery disease in 20 patients and dilated cardiomyopathy in 8 patients. Sustained ventricular tachycardia was the mode of presentation in 16 patients and sudden cardiac death in 12 patients. The mean left ventricular ejection fraction was 31%± 6%. The lead system consisted of an 8 French bipolar passive fixation rate sensing lead positioned at the right ventricular apex, an 11 French spring coil electrode positioned at the superior vena cava-right atrial junction (surface area 700 mm²), and submuscular placement of a large patch (surface area 28 cm²) on the anterolateral chest wall near the cardiac apex via a submammary incision. A defibrillation threshold of ≤ 15 joules (J) was required for implantation. This criterion was not satisfied in five patients; thus, a limited thoracotomy was performed via the submammary incision, and the large patch was placed epicardially. The mean R wave amplitude was 12 ± 3 mV, the mean pacing threshold was 1.0 ± 0.5 V at 0.5 msec, and the mean defibrillation threshold was 12.6 ± 3 J. ICD generators implanted were the Ventak-P in 17, PCD-7217 in 5, and the Cadence V-l00 in 6 patients. These patients have been followed for a mean of 14.6 ± 6 months. There was no perioperative mortality, and none of the patients developed an infection during follow-up. Generator migration or significant discomfort requiring ICD repositioning was not observed, although one patient developed an erosion requiring surgical repair.Conclusions: Subpectoral implantation of the ICD generator is feasible and was well tolerated by all patients with an acceptable complication rate (3.5%). As the size of future generation ICDs is reduced, subpectoral implantation may become the preferred approach.     

Early Changes in Defibrillation Threshold Following Implantation of a Nonthoracotomy System in Dogs

Background: Nonthoracotomy systems are rapidly becoming the preferred surgical method for implantation of cardioverter defibrillators. Testing is performed at the time of implantation to insure an adequate margin of safety for defibrillation. However, this safety margin may change with lead maturation. This study evaluated changes in defibrillation threshold following implantation of a nonthoracotomy system. Methods and Results: Ten dogs underwent implantation of a nonthoracotomy system consisting of a single catheter with a distal coil electrode in the right ventricular apex and a proximal coil electrode in the superior vena cava forming a common anode with a subcutaneous patch over tbe left tborax. Defibrillation threshold testing, using a biphasic waveform, was performed on each animal under general anesthesia at implantation (day 1) and subsequently on postoperative days 3, 7, 10, 17, 24, 31, 38, and 45. E50, the energy associated with a 50% likelihood of successful defibrillation, was determined at each setting. The mean E50 was 12.2 ± 1.1 J at the time of implantation, increasing 36% to 16.8 ± 2.0 J by day 38 (P < 0.01). Individual increases in E50 of 10–12 J were observed in four animals. Conclusions: Energy requirements for defibrillation with a nonthoracotomy system increase during the early postoperative period, with the highest defibrillation threshold observed at 38 days. This increase may be applicable to humans and should be considered when selecting an adequate energy safety margin for defibrillation at time of implantation.     

Comparison of a Unipolar Defibrillation System with a Dual Lead System Using an Enlarged Defibrillation Anode

The unipolar system for transvenous defibrillation, consisting of a single right ventricular lead as the cathode and the device shell as anode, has been shown to combine low de- fibrillation thresholds (DFTs) and simple implantation techniques. We compared the defibrillation efficacy of this system with the defibrillation efficacy of a dual lead system with a 12-cm long defibrillation anode placed in the left subclavian vein. The data of 38 consecutive patients were retrospectively analyzed. The implantation of an active can system was attempted in 20 patients (group 1), and of the dual lead system in 18 patients (group 2). Both groups had comparable demographic data, cardiac disease, ventricular function, or clinical arrhythmia. The criterion for successful implantation was a DFT of > 24 J. This criterion was met in all 18 patients of group 2, The active can system could not be inserted in 3 of the 20 group 1 patients because of a DFT > 24 J. In these patients, the implantation of one (n = 2) or two (n = 1) additional transvenous leads was necessary to achieve a DFT ≤ 24). The DFTs of the 17 successfully implanted group 1 patients were not significantly different from the 18 patients in group 2 (12.3 ± 5.7 f vs 10.8 ± 4.8 J). The defibrillation impedance was similar in both groups (50.1 ± 6.1 ± 48.9 ± 5.2 Ω). In group 1, both operation duration (66.8 ± 17 min vs 80.8 ± 11 min; P < 0.05) and fluoroscopy time (3.3 ± 2.1 min vs 5.7 ± 2.9 min; P < 0,05) were significantly shorter. Thus, the active can system allows reliable transvenous defibrillation and a marked reduction of operation duration and fluoroscopy time. The dual lead system, with an increased surface area defibrillation anode, seems to he a promising alternative for active can failures.     

Impact of Transvenous Lead Position on Active-Can ICD Defibrillation: A Computer Simulation Study

Optimizing lead placement in transvenous defibrillation remains central to the clinical aspects of the defibrillation procedure. Studies involving superior vena cava (SVC) return electrodes have found that left ventricular (LV) leads or septal positioning of the right ventricular (RV) lead minimizes the voltage defibrillation threshold (VDFT) in endocardial lead→SVC defibrillation systems. However, similar studies have not been conducted for active-can configurations. The goal of this study was to determine the optimal lead position to minimize the VDFT for systems incorporating an active can. This study used a high resolution finite element model of a human torso that includes the fiber architecture of the ventricular myocardium to find the role of lead positioning in a transvenous LEAD→can defibrillation electrode system. It was found that, among single lead systems, posterior positioning of leads in the right ventricle lowers VDFTs appreciably. Furthermore, a septal location of leads resulted in lower VDFTs than free-wall positioning. Increasing the number of leads, and thus the effective lead surface area in the right ventricle also resulted in lower VDFTs. However, the lead configuration that resulted in the lowest VDFTs is a combination of a mid-cavity right ventricle lead and a mid-cavity left ventricle lead. The addition of a left ventricular lead resulted in a reduction in the size of the low gradient regions and a change of its location from the left ventricular free wall to the septal wall.     

A Long Thin Electrode Is Equivalent to a Short Thick Electrode for Defibrillation in the Right Ventricle

We hypothesized that a long thin right ventricular (RV) electrode would have equivalent defibrillation threshold (DFT) performance to a short thick electrode with approximately the same surface area. This could lead to thinner transvenous lead systems, which would be easier to implant. A thin (5.1 French) lead was compared to a standard control (10.7 French). The thin lead had an 8-cm RV electrode length with a surface area of 4.26 cm². The standard lead had a RV electrode length of 3.7 cm and a surface area of 4.12 cm². A 140-μFrench capacitor 65%/65% tilt biphasic defibrillation shock was delivered between the RV electrode and a 14-cm² subcutaneous patch. DFTs were determined following 10 seconds of fibrillation in 11 dogs by a triple determination averaging technique. The thin lead had a lower resistance (77.1 ± 27.4 Ω vs 88.9 ± 30.3 Ω, P < 0.001) than did the thick lead. There was no significant difference in stored energy DFTs (9.9 ± 2.5 vs 10.3 ± 2.7, P = 0.098 2-sided, P = 0.049 1-sided). This was in spite of the fact that the long thin lead had a portion of its RV coil extending above the tricuspid valve and, thus, not contributing efficiently to the ventricular gradients in the small dog heart. We conclude that a long thin right ventricular electrode and a standard short thick electrode had equivalent defibrillation performance. This preliminary result should be confirmed in clinical studies as it could lead to significantly thinner transvenous lead systems.     

Thoracoscopic Approach to Implantable Cardioverter Defibrillator Patch Electrode Implantation

Even if transvenous lead system for automatic implantable Cardioverter defibrillators (ICDs) has been one of the main surgical advances in the recent past, its major limitation is the high defibrillation thresholds in some cases. Thus, an additional patch may be required and implanted either in a subcutaneous position or in an epicardial position. We describe another possibility: the implantation of extrapericardial patch under video-thoracoscopic control. This new technique allows a deep implantation of the whole material without thoracotomy. Seven patients were included in our preliminary experience. During defibrillation threshold evaluation, two patients required 34 J with the single transvenous lead system, and five patients were not defibrillated with the single lead system; therefore, they required a 300-J external rescue shock. We decided to implant an additional patch in those seven patients with high defibrillation thresholds. This patch was inserted into the pleural cavity through a left subcostal incision. Under video thoracoscopy, it was positioned and stitched onto the pericardium. The defibrillation generator was then implanted through the left subcostal incision in a subdiaphragmatic space. As a result, pre-operative defibrillation thresholds were significantly reduced (14.29 ± 3.45 J, mean ± SD) and remained stable during follow-up controls (eighth day and second month). Long-term follow-up (14 ± 4.5 months) was uneventful, with an excellent tolerance for the patients. In conclusion, extrapericardial implantation of defibrillation patches under video thoracoscopy is an easy technique that allows low defibrillation thresholds.     

Influence of Malpositioned Transvenous Leads on Defibrillation Efficacy With and Without a Subcutaneous Array Electrode

Some patients cannot receive a transvenous lead system because of high defibrillation thresholds (DFTs). We hypothesized that a right ventricular (RV) catheter electrode not extending as far as possible into the RV apex could cause high DFTs, Recently, a subcutaneous array (SQA) electrode has been shown to lower DFTs substantially. We compared the influence of a malpositioned RV catheter electrode on defibrillation efficacy for endocardial lead systems with and without a SQA. In eight anesthetized pigs, defibrillation catheters were placed in the RV apex and near the junction of the superior vena cava (SVC) and right atrium. SQA, formed by three elements, each 20 cm in length, was placed in the left thorax. DFTs were determined for a biphasic waveform using an up/ down protocol with the RV catheter at the apex and with it repositioned 1-cm and 2-cm proximal to the apex. The mean DFT energies for the configurations with a SQA were less than those without a SQA for every catheter position. The placement of the RV catheter away from the apex caused an increase in defibrillation energy for the configurations without a SQA (apex: 17.1 ± 3.8 J [mean ± SD]; 1 cm: 20.1 ± 4.6 J; 2 cm: 27.6 ± 9.5 J; P ± 0.05), but not for the configurations with a SQA (apex: 12.2 ± 2.2 J; 1 cm: 12.3 ± 2.9 J; 2 cm: 12.1 ± 0.9 J: P= NS). These results suggest that a malpositioned RV catheter electrode, at the time of implantation or by late dislodgment, significantly elevates DFTs for a total endocardial system but not for a system that includes a SQA.     

Transvenous and Subcutaneous Electrode System for an Implantable Defibrillator, Improved on Large Pigs

The currently required surgical procedure for implantable defibrillator implantation is a limiting factor and several groups are therefore investigating transvenous approaches. Our electrode system consists of a nondistal right ventricular catheter electrode and two or three subcutaneously (SC) placed electrodes. An optimal location for these SC electrodes is important to obtain the lowest possible defibrillation threshold (DFT) by allowing a more homogeneous current distribution within the thorax. An empirical approach consists of placing randomly the SC electrodes to find out the lowest possible DFT. A mathematical approach is to calculate the SC electrode locations for an optimal electric field distribution by using magnetic resonance images of thorax cross-sections and a specially designed computer program. Our recent experimental results are based on a series of 15 pigs weighing between 60 and 102 Kg. DFT ranged between 10 and 26 joules. We conclude that an electrode system with a right ventricular electrode and two or three subcutaneous electrodes can be optimized to reach a DFT for pigs with human-near body weights which is compatible with the energy capabilities of our implantable device.     

Internal Cardiac Defibrillation Threshold: Effects of Acute Ischemia

The influence of myocardial ischemia on defibrillation success was studied using two different lead orientations in halothane-anesthetized pigs. Ischemia was induced by ligating the left anterior descending artery in its distal third. Controls had loosely tied ligatures placed around the artery at the same site. Ventricular fibrillation was induced by electrical stimulation 30 minutes after coronary artery ligation. Defibrillation used a single truncated pulse of approximately 6 ms duration passed to either: a transvenous electrode catheter (Medtronic, 6880) with the cathode in the apex of the right ventricle and the anode in the superior vena cava-atrial junction region, or the cathode in the apex of the right ventricle and a mesh plaque on the epicardium of the basal lateral left ventricle as anode. Ten seconds after the onset of ventricular fibrillation, defibrillation was attempted with increasing incremental energies until defibrillation was achieved. Fibrillation episodes were repeated at 15-minute intervals until the minimum first shock was successful in defibrillating the animal (i.e., defibrillation threshold). The number of animals successfully defibrillated with a minimum energy above or below 30 J was not different between normal and ischemic animals for either electrode configuration (i.e., 3 out of 20 vs 1 out of 13 for the catheter and 5 out of 6 vs 6 out of 7 for the epicardial plaque, respectively). Also, the cumulative percent success as a function of defibrillation energy was similar in both the normal and ischemic groups. There was a significant reduction in the minimum energy necessary for defibrillation when passing current between the right ventricular apex and the left ventricular epicardial plaque. The present results indicate that, despite differences in lead orientations, acute ischemia in the anesthetized pig does not appear to influence defibrillation success.     

An epicardial subxiphoid implantable defibrillator lead: superior effectiveness after failure of standard implants

A single epicardial implantable lead using the subxiphoid approach is described in this article. It consists of a single halo-shaped coil that is implanted under the inferior surface of the heart, including the right and left inferior ventricular surfaces. It has been implanted in four patients who could not be defibrillated with a transvenous system, even with the adjunct use of subcutaneous leads or left chest wall patch. Three of the patients had progressive heart failure due to ischemic myocardiopathy; the fourth patient had a dilated idiopathic myocardiopathy. The approach is simple and appears to be effective due to its ability to encompass the left and right ventricles. This vector seems to significantly lower the threshold for defibrillation, and may offer substantial benefit in the setting of high defibrillation thresholds with conventional leads, or when conventional systems are inadequate to achieve consistent defibrillation.