Low-energy endocardial defibrillation using an axillary or a pectoral thoracic electrode location

Cardiac pacing and defibrillation in children and young adults

The population of children and young adults requiring a cardiac pacing device has been consistently increasing. The current generation of devices are small with a longer battery life, programming capabilities that can cater to the demands of the young patients and ability to treat brady and tachyarrhythmias as well as heart failure. This has increased the scope and clinical indications of using these devices. As patients with congenital heart disease (CHD) comprise majority of these patients requiring devices, the knowledge of indications, pacing leads and devices, anatomical variations and the technical skills required are different than that required in the adult population. In this review we attempt to discuss these specific points in detail to improve the understanding of cardiac pacing in children and young adults.

Successful reduction of a high defibrillation threshold by a combined implantation of a subcutaneous array and azygos vein lead

A 72-year-old man with nonischemic cardiomyopathy was referred because his implantable cardioverter defibrillator had failed to terminate spontaneous ventricular fibrillation (VF). Defibrillation threshold (DFT) testing confirmed that 830-V shocks failed to defibrillate VF despite optimization of the biphasic waveform and reversal of shock polarity. The placement of a new right ventricular lead and the addition of a subcutaneous array failed to defibrillate VF at 830 V. The combination of a subcutaneous array and azygos vein coil successfully defibrillated VF. The mechanism for successful DFT reduction was likely greater current supplied to the posterior basal left ventricle by the azygos vein lead.

Azygos Vein Lead Implantation:. A Novel Adjunctive Technique for Implantable Cardioverter Defibrillator Placement

High defibrillation thresholds (DFTs) occasionally are encountered during placement of implantable cardioverter defibrillators (ICDs). There are multiple strategies to lower DFTs in such patients, including reassessment of right ventricular lead position, alteration of the shock waveform, and implantation of subcutaneous arrays. This article describes a novel technique of implanting a high-voltage lead in the azygos vein. This procedure may serve as an adjunctive approach to reduce DFTs. The anatomic location of the azygos vein posterior to the heart provides a suitable shocking vector between the right ventricular electrode, a high-voltage lead placed in the azygos vein, and the ICD can.

Implantable Cardioverter Defibrillator: Current Progress and Management

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

ICD therapy in the new millennium

Remarkable progress has been made in the 15 years since ICD therapy was approved for human use. The early "shock boxes" had almost no diagnostic capabilities and required thoracotomy for epicardial patch implantation with typical duration of hospitalization of about a week. Pulse-generator longevity was less than 2 years. Modern devices provide detailed information about the morphology and rate of electrocardiographic signals before, during, and after arrhythmia therapy. The down-sizing of pulse generators and improvements in lead design and shock waveforms allow the simplicity of defibrillator implantation to approach that of pacemakers, with defibrillation thresholds comparable with those initially observed with epicardial patches. Despite the marked reduction in size and increase in diagnostic capabilities, device longevity is now longer than 6 years. Routine outpatient ICD implantation is presently feasible and will increase in frequency if ongoing primary prevention trials prove beneficial. Further advances in lead technology and arrhythmia discrimination should increase the efficacy and reliability of therapy. Finally, devices have the capabilities to treat multiple problems in addition to life-threatening ventricular arrhythmias including atrial arrhythmias and congestive heart failure.

Increased defibrillation threshold with right-sided active pectoral can

The aim of this study was to identify the optimal position on the chest wall to place an implant able cardioverter defibrillator in a two-electrode system, consisting of a right ventricular electrode and active can.

METHODS AND RESULTS

Defibrillation thresholds (DFT) were measured in 10 anaesthetised pigs (weight 33-45 kg). An Angeflextrade mark lead was introduced transvenously to the right ventricular apex. The test-can (43 cc) was implanted submuscularly in each of four locations: left pectoral (LP), right pectoral (RP), left lateral (LL) and apex (A). The sequence in which the four locations were tested was randomized. Ventricular fibrillation (VF) was induced using 60 Hz alternating current. Rectangular biphasic shocks were delivered 10 seconds after VF induction. The DFT was measured using a modified four-reversal binary search. The results of the four configurations were: LP, 14.6+/- 4.0 J; RP, 18.8+/- 4.2 J; LL, 14.7+/- 4.1 J; A, 14.9+/- 3.1 J. Repeated measures analysis of variance showed that the DFT of RP was significantly higher than LP, LL and A (p < 0.05).

CONCLUSIONS

Implanting an active can in the RP position increases the DFT by 29% compared to LP, LL and A sites. The can position on the left thorax does not appear to have a significant influence on DFT.

Defibrillation thresholds are increased by right-sided implantation of totally transvenous implantable cardioverter defibrillators

Whether an ICD is placed via a left- or right-sided approach depends on venous access, the presence of a preexisting pacemaker, and other factors. Since the DFT is affected by lead position, which in turn is determined in part by the side of access, right-sided venous access could adversely affect DFTs. Furthermore, right-sided active can placement directs electric current toward the right hemithorax, which could further increase DFTs. This study sought to determine whether DFTs were increased by right-sided vascular access, and whether active can technology was beneficial or detrimental with right-sided ICD placement. Stepdown to failure DFTs were found in 290 patients receiving transvenous systems at the time of initial ICD implantation. Of these, 271 (93%) received left-sided systems and 19 (7%) received right-sided systems. The mean DFT in systems placed via left-sided vascular access was 11.3 +/- 5.3 J versus 17.0 +/- 4.9 J for right-sided implantation (P < 0.0001); right-sided DFTs were elevated for both active can and cold can systems. Right-sided active can devices had a lower DFT than right-sided cold can systems (15 +/- 4.1 J vs 19 +/- 4.8 J, P = 0.05). The right-sided implantation of implantable defibrillators results in significantly higher DFTs than the left-sided approach. This may be due to the less favorable distribution of the defibrillating field relative to the myocardium with the devices on the right. When right-sided implantation is clinically mandated, active can devices result in lower thresholds and should be used.

Implantable cardioverter defibrillators

Implantable cardioverter defibrillators have proven to be an effective therapy for life-threatening ventricular arrhythmias. Given the ever-increasing number of patients who have these devices, increasing numbers of patients are likely to present to emergency departments with defibrillator-related problems. This article discusses normal device function, indications for implantation, and technique of implantation. It also focuses on the evaluation and management of patients with these devices presenting to the emergency department.

Antiarrhythmic therapy--future trends and forecast for the 21st century

This article discusses recent changes in antiarrhythmic therapy, with a focus on nonpharmacologic therapy (electrode catheter ablation, implantable cardioverter-defibrillators [ICDs]), and puts them into perspective for the coming years. The treatment of supraventricular tachycardias and tachycardia involving accessory pathways is likely to remain the domain of catheter ablation. With promising new techniques under investigation, the spectrum of arrhythmias that can be cured will probably be expanded. Treatment of life-threatening ventricular arrhythmias is likely to remain the domain of the ICD in the foreseeable future. With the safety net of the ICD in place, new antiarrhythmic drugs or other forms of antiarrhythmic therapy can be developed and tested.

Low-energy endocardial defibrillation using dual, triple, and quadruple electrode systems

The feasibility of achieving both universal application of nonthoracotomy leads and low (< or = 15 J) defibrillation energy requirements by optimizing lead system configuration for use with low-output (<30 J) biphasic shock pulse generators was examined. Sixteen patients (mean age 62 +/- 8 years and mean left ventricular ejection fraction of 38 +/- 15%) were included in the study. All patients had either experienced syncope with induced ventricular tachycardia (n = 4) or had documented sustained ventricular tachycardia (n = 7) or ventricular fibrillation (n = 5). Defibrillation threshold testing was performed in 2 stages on different days in these patients. In the first stage, 2 defibrillation catheter electrodes were positioned in the right ventricle and superior vena cava with an axillary cutaneous patch. Fifteen-joule, 10- and 5-J biphasic shocks were delivered across 3 different electrode configurations-right ventricle to superior vena cava, right ventricle to axillary patch, right ventricle to a combination of superior vena cava and axillary patch. In the second stage, an 80-ml can electrode was added subcutaneously in a pectoral location to the previous leads. Configurations compared were the right ventricle to pectoral can, and right ventricle to an "array"-combining superior vena cava, can, and axillary patch leads. The defibrillation threshold was determined using a step-down method. In stage 1, mean defibrillation threshold for the right ventricle to axillary patch (12.7 +/- 5.9 J) and right ventricle to superior vena cava plus axillary patch (9.8 +/- 5.2 J) configurations was lower than the right ventricle to superior vena cava configuration (14.2 +/- 6.4 J, p <0.05). In stage 2, the defibrillation was higher for the right ventricle to pectoral can (9.2 +/- 5.1 J) configuration compared with the right ventricle to the array (5.6 +/- 3.6 J, p < or =0.05). The right ventricle to array had the lowest defibrillation threshold, whereas the right ventricle to pectoral can was the best dual electrode system. Low-energy endocardial defibrillation (< or =10 J) was feasible in 72% of tested patients with > 1 electrode configuration at 10 J, whereas only 53% of successful patients could be reverted at >1 electrode configuration at 5 J (p <0.05). Reduction in maximum pulse generator output to < or =25 J using these electrode configurations with bidirectional shocks is feasible and maintains an adequate safety margin.

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

INTRODUCTION

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.

METHODS AND RESULTS

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.

CONCLUSION

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.

Transvenous defibrillation leads: is there an ideal position of the defibrillation anode?

A potential benefit of two-lead transvenous defibrillation systems is the ability to independently position the defibrillation electrodes, changing the vector field and possibly decreasing the DFT. Using the new two-lead transvenous TVL lead system, we studied whether DFT is influenced by SVC lead position and whether there is an optimal position. TVL leads and Cadence pulse generators were implanted in 24 patients. No intraoperative or perioperative complications were observed. In each patient, the DFTs were determined for three SVC electrode positions, which were tested in random order: the brachiocephalic vein, the mid-RA, and the RA-SVC junction. The mean DFTs in the three positions were not statistically different, nor was any single lead position consistently associated with lower DFTs. However, an optimal electrode position was identified in 83% of patients, and the DFT from the best lead position for each patient was significantly lower than for any one of the electrode positions (P < 0.01). The mean safety margin for the best SVC lead position was approximately 27 J. These results demonstrate the advantage of a two-lead system, as well as the importance of testing multiple SVC lead positions when the patient's condition permits. Both of these factors can decrease the DFT and maximize the defibrillation safety margin. This will become increasingly important as pulse generator capacitors become smaller (as part of the effort to decrease generator size) and the energy output of the generators consequently decreases.

[Influence of waveform and configuration of electrodes on the defibrillation threshold of implantable cardioverter-defibrillators]

The defibrillation threshold (DFT) is no threshold in the true sense. Between energy levels which defibrillate in all cases and energy levels which never defibrillate, a broad range of energies exists which might or might not defibrillate. Thus, the value of the DFT is dependant on the protocol used for its determination. Usually the DFT presents an energy at which the implantable cardioverter-defibrillator (ICD) will defibrillate successfully at a rate of approximately 75%. To achieve a 100% success rate the energy has to be programmed 15 J above the DFT or twice the DFT.Using DFT measurements the energy needed for internal defibrillation could be gradually reduced in the last years. Major break throughs have been the introduction of the biphasic defibrillation waveform and the use of pectorally implanted ICD shells as defibrillation electrodes. The shortening of the defibrillation impulse by the use of lower capacitances could not improve DFTs but allowed to construct ICDs of smaller volume. Addition of a superior vena cava electrode or a subcutaneous array electrode at the left lateral chest to the standard bipolar electrode system (right ventricle, pectoral ICD can) allowed for tri- and quadripolar lead configurations which reduced DFTs on average only slightly but reduced the standard deviation of DFTs significantly and thus helped to avoid high DFTs. Besides building smaller ICDs, reduction of DFTs and thus programming of lower defibrillation ICD energies allows for improved battery longevities and reduced capacitor charging times and thus a lower incidence of syncopes.

Ultrastructural alterations in the right and left ventricular myocardium following multiple low energy endocardial countershocks in anesthetized dogs

Both high energy transthoracic and direct epicardial defibrillation can result in RV and LV myocardial damage, but little is known about the damage due to defibrillation using an endocardial RV electrode. Furthermore, disturbances in postdefibrillation oxidative metabolism have been reported and may be caused by primary injury of mitochondrial integrity and function, but information about ultrastructural mitochondrial alterations is rare. We therefore studied, in 13 fox hounds, RV and LV ultrastructural alterations following multiple low energy endocardial countershocks. Using an ICD and an endocardial defibrillation system a median of 54 (43-74) countershocks with a cumulative energy of 1,558 J (844-2,141 J) was delivered. After termination of countershocks, RV and LV myocardium was examined by electron microscopy. In both ventricles, severe myocardial alterations were found, including swollen mitochondria, disruption of mitochondrial crests, and loss of integrity of the mitochondrial inner and outer membranes. At the first time a semiquantitative score, originally developed for postischemic injury, was successfully used to grade the postcountershock mitochondrial alteration, which showed a more pronounced damage in the RV (2.69 +/- 0.22 points) compared to the LV (2.18 +/- 0.22 P = 0.021). We conclude that even the use of endocardial lead systems with low energy countershocks may lead to severe mitochondrial damage, especially in the RV.

A prospective, randomized evaluation of a nonthoracotomy implantable cardioverter defibrillator lead system. Endotak/PRX Investigator Group

Nonthoractomy 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 < or = 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.

Complications associated with pectoral cardioverter-defibrillator implantation: comparison of subcutaneous and submuscular approaches. Worldwide Jewel Investigators

OBJECTIVES

The aim of this study was to compare complications in a large cohort of patients undergoing pectoral cardioverter-defibrillator implantation with a subcutaneous or submuscular approach.

BACKGROUND

Pectoral placement of implantable cardioverter-defibrillator (ICD) pulse generators is now routine because of downsizing of these devices. subcutaneous implantation has been advocated by some because it is a simple surgical procedure comparable to pacemaker insertion. Others have favored submuscular insertion to avoid wound complications. These surgical approaches have not been compared previously.

METHODS

The subjects for this study were 1,000 consecutive patients receiving a Medtronic Jewel ICD at 93 centers worldwide. Cumulative follow-up for all patients was 633.7 patient-years, with 64.9% of patients followed up for > or = 6 months. The complications evaluated were erosion, pocket hematoma, seroma, wound infection, dehiscence, device migration, lead fracture and dislodgment.

RESULTS

Subcutaneous implantation was performed in 604 patients and submuscular implantation in the remaining 396. The median procedural times were shorter for subcutaneous implantation (p = 0.014). In addition, the cumulative percentage of patients free from erosion was greater for subcutaneous implantations (p = 0.03, 100% vs. 99.1% at 6 months). However, lead dislodgment was more common with subcutaneous implantations (p = 0.019, 2.3% vs. 0.5% at 6 months) and occurred primarily during the first month postoperatively. Overall, there were no significant differences in cumulative freedom from complications between groups (4.1% vs. 2.5%, p = 0.1836).

CONCLUSIONS

Subcutaneous pectoral implantation of this ICD can be performed safely and has a low complication rate. This approach requires a simple surgical procedure and, compared with the submuscular approach, is associated with shorter procedure times and comparable overall complication rates. However, early follow-up is important in view of the increased lead dislodgment rate.

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 > or = 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 > or = 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 omega to 52.2 +/- 8.6 omega. Additionally, the defibrillation threshold decreased from > or = 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.

Transvenous defibrillation lead systems

The use of the implantable cardioverter defibrillator has grown dramatically over the past 10 years. One of the major advances in defibrillation technology is the development of transvenous lead systems. Compared with traditional epicardial lead systems, transvenous defibrillation leads reduce perioperative mortality, hospitalization, and costs. Transvenous lead systems provide reliable sensing of ventricular tachyarrhythmias, although redetection of ventricular fibrillation can be prolonged, especially with integrated lead systems. Both ramp and burst adaptive pacing are equally effective for the termination of ventricular tachycardia and are successful in up to 90% of spontaneous events. Defibrillation thresholds are higher with transvenous leads than with epicardial patches. These thresholds are reduced with the use of multiple transvenous leads, subcutaneous patches, or with reversing shock polarity. However, the development of biphasic waveforms has made the largest impact on the efficacy of these lead systems, allowing dual coil transvenous systems to be effective in about 90% of patients. Defibrillation efficacy is further enhanced and implantation simplified by the incorporation of an active pulse generator located in the left pectoral region. Active pectoral pulse generators with biphasic waveforms will be the primary lead system for new implants.

Prospective randomized comparison of anodal monophasic shocks versus biphasic cathodal shocks on defibrillation energy requirements

Biphasic shocks are believed to be superior to monophasic shocks. Monophasic anodal shocks, as opposed to cathodal shocks, are associated with improved defibrillation energy requirements (DERs). However, it is unclear how the DER of anodal monophasic shocks compare with conventional biphasic shocks. Therefore the purpose of this study was to prospectively compare the DER of an anodal monophasic shock with that of a cathodal biphasic shock. A transvenous defibrillation lead with distal and proximal shocking electrodes was used. The subjects of this study were 20 consecutive patients with a mean age of 64.2 +/- 10.5 years ( +/- SD) and a mean left ventricular ejection fraction of 0.36 +/- 0.18. Six had had cardiac arrest. The DER, defined as the lowest energy that converted ventricular fibrillation to sinus rhythm, was determined twice with a step-down protocol (25 J, 20 J, 15 J, 10 J, 5 J, 3 J, 1 J). If the DER was > or = 25 J, then a subcutaneous patch was deemed necessary for system implantation. In random order the DER was determined with a monophasic anodal shock (distal electrode positive) and then with a cathodal (first phase, distal electrode negative) biphasic shock. The mean DER with anodal monophasic shocks was 15.1 +/- 8.5 J compared with 13.6 +/- 8.1 J with cathodal biphasic shocks (p = 0.4). A DER > or = 25 J was present in three patients with the monophasic waveform and in three patients with the biphasic waveform (p = NS). In conclusion, the DER and frequency of subcutaneous patch use with an anodal monophasic waveform is comparable to that obtained with cathodal biphasic waveform.

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

Many factors can influence defibrillation energy requirements (DER) in patients with a nonthoracotomy defibrillator. No large studies, however, have correlated clinical characteristics with the DER. In this study, 124 patients underwent the same DER protocol with the identical biphasic waveform, nonthoracotomy lead system, and lead configuration. These patients were 63 +/- 12 years old (mean +/- SD); 99 were men; the ejection fraction was 0.32 +/- 0.13, and 36 were taking an antiarrhythmic medication. New York Heart Association congestive heart failure class I was present in 28, class II in 70, and class III in 26 patients. Male sex (454 +/- 94 V vs 406 +/- 91 V for female sex) was associated with a significantly higher DER (p = 0.02) and an increased risk of a DER > 550 V (p = 0.047). No other clinical variable was associated with the DER or a DER > 550 V. In conclusion, women tend to have lower DERs than men.

Clinical efficacy and safety of atrial defibrillation using biphasic shocks and current nonthoracotomy endocardial lead configurations

We undertook a prospective randomized clinical trial evaluating efficacy and safety of internal atrial defibrillation in patients with drug-refractory atrial fibrillation (AF). Consecutive patients with paroxysmal or chronic AF were randomly tested with 3 internal atrial defibrillation lead configurations and biphasic shocks. Patients with implanted cardiac pacemakers were tested with the right atrium (RA) and left pulmonary artery or coronary sinus (CS) configuration. Shocks were initially delivered without anesthesia to assess patient tolerance. The need for backup ventricular defibrillation and pacing support was evaluated. Eighteen patients with (n = 15) or without (n = 3) structural heart disease, mean left ventricular ejection fraction 36 +/- 14%, and mean left atrial diameter 4.5 +/- 0.6 cm were studied. The mean defibrillation threshold in the best randomized lead configuration was 9.9 +/- 7.7 J. Mean defibrillation threshold for the right ventricle (RV) and superior vena cava configuration was 13.3 +/- 5 J, which was significantly lower than the RA and axilla configuration (20.1 +/- 7.4 J, p < 0.04) but not the RV to RA configuration (16.5 +/- 11 J, p > 0.2). The mean defibrillation threshold using the RA-left pulmonary artery/CS configuration was 8.9 +/- 9 J (p > 0.2 vs RV-superior vena cava). There was a bimodal distribution of defibrillation thresholds. Low atrial defibrillation thresholds correlated with absence of heart disease, higher ejection fraction, and smaller left ventricular end-diastolic diameter. Shocks were hemodynamically well tolerated, but 2 of 18 patients (11%) had nonsustained ventricular tachycardia after shock delivery. Six of 18 patients (33%) had postshock bradyarrhythmias. Fourteen of 16 patients perceived shocks > or = 3 J as intolerable.(ABSTRACT TRUNCATED AT 250 WORDS) [corrected]

Elevated defibrillation threshold when right-sided venous access is used for nonthoracotomy implantable defibrillator lead implantation. The Endotak Investigators

INTRODUCTION

Although myriad factors influence the defibrillation threshold, the relation between the site of transvenous lead entry into the vascular system and the defibrillation threshold has not been reported. This study examines the influence that venous entry site has on defibrillation success for a transvenous implantable cardioverter defibrillator lead with two defibrillating coils.

METHODS AND RESULTS

The study population comprised 345 patients. Their mean age was 61 +/- 13 years and, left ventricular ejection fraction was 0.33 +/- 0.13. A left-sided approach was used in 324 (93.9%) of the patients, and a right-sided approach was used in the remaining 21 (6.1%) patients. There was no difference in the gender, age, left ventricular ejection fraction, or underlying cardiac disease in the two groups. For all patients, with a transvenous lead used either alone or with a submuscular or subcutaneous patch, the biphasic defibrillation threshold was 9.9 +/- 4.8 J when a left-sided approach was used, and 14.0 +/- 7.3 J when a right-sided approach was used (P = 0.02). When a transvenous lead was used with a submuscular or subcutaneous patch (115 patients), the biphasic defibrillation threshold was 9.5 +/- 4.3 J when a left-sided approach was used, and 12.0 +/- 10.0 J when a right-sided approach was used (P = 0.98). When a transvenous lead was used without a submuscular or subcutaneous patch (230 patients), the biphasic defibrillation threshold was 10.1 +/- 5.0 J when a left-sided approach was used, and 14.6 +/- 6.6 J when a right-sided approach was used (P < 0.01). For the entire group of patients and for each specific lead arrangement, there was no significant difference in the defibrillating lead system impedance when right-sided versus left-sided approaches were compared.

CONCLUSION

Left-sided approaches to implant transvenous leads with two coils for defibrillation result in lower biphasic defibrillation thresholds than when right-sided approaches are used.

The subcutaneous array: a new lead adjunct for the transvenous ICD to lower defibrillation thresholds

Despite the benefits of transvenous implantable cardioverter defibrillators (ICDs), concern exists that patients with high defibrillation thresholds (DFTs) have an inadequate safety margin between the DFT and the maximum defibrillator energy. A new transvenous ICD lead adjunct, a subcutaneous lead array (SQ Array), was developed to increase safety margins by lowering DFTs. Composed of three lead elements joined in a common yoke, the SQ Array is tunneled subcutaneously in the left lateral chest. Serving as their own controls, 20 patients were studied intraoperatively comparing transvenous lead-alone DFTs with lead-SQ Array DFTs. Seventeen males and three females were randomized to receive the SQ Array through the CPI Ventak PRx/Endotak 70 series protocol. Mean patient age was 63.7 +/- 2.5 years and mean ejection fraction 0.34 +/- 0.04. DFTs were determine using a precise protocol of step-down/step-up testing commencing at 20 joules. Lead-alone DFTs were tested using the proximal coil as the anode (+). For the lead-SQ Array, the proximal coil and the array were linked as a common anode. The lead-SQ Array resulted in a statistically significant reduction in mean monophasic DFT from 23.3 +/- 2.3 joules (lead-alone) to 13.5 +/- 1.9 joules (lead-SQ Array) (P < 0.001). Six patients had lead-alone DFTs > 25 joules but did not require thoracotomy because of adequate DFT reduction with the SQ Array. We conclude that the SQ Array adjunct to the transvenous ICD lead lowers monophasic DFTs an average of 9.8 joules (40.6%) obviating the need for a thoracotomy in selected patients.

Effect of first-phase polarity of biphasic shocks on defibrillation threshold with a single transvenous lead system

OBJECTIVES

The purpose of this study was to determine whether the polarity of the first phase of a biphasic shock affects the defibrillation threshold.

BACKGROUND

The polarity of a monophasic shock has been shown to affect the defibrillation threshold.

METHODS

A transvenous defibrillation lead with distal and proximal shocking electrodes was used in this study. In 15 consecutive patients, the defibrillation threshold was determined twice using a step-down protocol, in random order: with the distal coil as the anode for the initial phase (anodal biphasic shock) and with the polarity reversed (cathodal biphasic shock). The power to detect a 5.0-J difference in this study is 0.96. These patients were 61 +/- 11 years old (mean +/- SD), and the mean left ventricular ejection fraction was 0.32 +/- 0.10.

RESULTS

Mean defibrillation threshold using anodal biphasic shocks was 9.9 +/- 4.8 J, compared with 9.5 +/- 4.2 J using cathodal biphasic shocks (p = 0.8). In three patients the defibrillation threshold was lower by a mean of 6.3 +/- 2.9 J with the former configuration; in three patients the defibrillation threshold was lower by a mean of 6.7 +/- 2.5 J with the latter configuration; and in nine patients it was the same. Using the standard cathodal configuration, a defibrillation threshold < or = 10 J was obtained in approximately 70% of patients, and a subcutaneous patch was not required in any patient.

CONCLUSIONS

The polarity of the first phase of a biphasic shock used with a single transvenous lead does not affect the defibrillation threshold.

Transvenous-subcutaneous defibrillation leads: effect of transvenous electrode polarity on defibrillation threshold

INTRODUCTION

The defibrillation threshold (DFT) of a transvenous-subcutaneous electrode configuration is sometimes unacceptably high. To obtain a DFT with a sufficient safety margin, the defibrillation field can be modified by repositioning the electrodes or more easily by a change of electrode polarity. In a prospective randomized cross-over study, the effect of transvenous electrode polarity on DFT was evaluated.

METHODS AND RESULTS

In 21 patients receiving transvenous-subcutaneous defibrillation leads, the DFT was determined intraoperatively for two electrode configurations. Two monophasic defibrillation pulses were delivered in sequential mode between either the right ventricular (RV) electrode as common cathode and the superior vena cava (SVC) and subcutaneous electrodes as anodes (configuration I) or the SVC electrode as common cathode and the RV and subcutaneous electrodes as anodes (configuration II). In each patient, both electrode configurations were used alternately with declining energies (25, 15, 10, 5, 2 J) until failure of defibrillation occurred. The DFT did not differ between both configurations (18.3 +/- 8.2 J vs 18.9 +/- 8.9 J; P = 0.72). Eleven patients had the same DFT with both electrode configurations, 5 patients a lower DFT with the RV electrode as cathode, and 5 patients a lower DFT with the SVC as cathode. Four patients had a sufficiently low DFT (< or = 25 J) with only 1 of the 2 configurations.

CONCLUSION

A change of electrode polarity of transvenous-subcutaneous defibrillation electrodes may result in effective defibrillation if the first electrode polarity tested fails to defibrillate. In general, neither the RV electrode nor the SVC electrode is superior if used as a common cathode in combination with a subcutaneous anodal chest patch.

Third- and fourth-generation implantable cardioverter defibrillators: current status and future development

Implantable cardioverter defibrillator (ICD) therapy has become the mainstay of therapy for patients with a history of sudden cardiac death or life-threatening ventricular arrhythmias. The current generation of ICDs used for secondary prevention combines features for tachycardia reversion with demand ventricular pacing, antitachycardia pacing, programmable shock therapy, and tachycardia events memory. Although demand pacing and defibrillation is indicated for primary prevention usage of ICDs, the application of antitachycardia pacing modes is more controversial. High energy cardioversion and defibrillation shocks remaining the mainstay of sudden death prevention will be redefined as more effective defibrillation shock modes and lead systems are developed. Fourth-generation ICD systems accomplished a significant reduction of device size and almost universal success using an endocardial lead configuration and pectoral implant. A variety of new directions of ICD therapy in clinical practice such as primary prevention applications and the adjunctive role of antiarrhythmic drug therapy are currently being examined in clinical trials. The concepts underlying initiation of tachyarrhythmias are being studied to develop new approaches to tachycardia prevention. These include rate support, subthreshold stimulation, and multiple site pacing. The current developments of ICD therapy promise continued growth of this technology.

Troubleshooting new ICD systems

Advances in ICD technology have improved arrhythmia detection and termination, and the development of nonthoracotomy lead systems has reduced operative mortality and morbidity. Despite these important developments, patients with ICDs continue to experience untoward events that are usually attributable to lead failures, the effects of antiarrhythmic drugs, problems related to signal processing, or the need to modify the ICD program. It is incumbent on physicians who implant ICDs and monitor long-term therapy to appreciate the mechanisms by which these events occur, approaches needed to establish a diagnosis, and therapeutic interventions that can resolve problems associated with ICDs.

Clinical outcome of patients with malignant ventricular tachyarrhythmias and a multiprogrammable implantable cardioverter-defibrillator implanted with or without thoracotomy: an international multicenter study. PCD Investigator Group

OBJECTIVES

The long-term efficacy and safety of a third-generation implantable cardioverter-defibrillator implanted with thoracotomy and nonthoracotomy lead systems was evaluated in a multicenter international study.

BACKGROUND

The clinical impact of transvenous leads for nonthoracotomy implantation and pacing for bradyarrhythmias and tachyarrhythmias in implantable cardioverter-defibrillator systems is not well defined.

METHODS

The safety of the implantation procedure and clinical outcome of 1,221 patients with symptomatic and life-threatening ventricular tachyarrhythmias who underwent implantation of a third-generation cardioverter-defibrillator using either a thoracotomy approach with epicardial leads (616 patients) or a nonthoracotomy approach with endocardial leads (605 patients) in a nonrandomized manner was analyzed. The implantable cardioverter-defibrillator system permitted pacing, cardioversion, defibrillation, arrhythmia event memory and noninvasive tachycardia induction.

RESULTS

Successful implantation of an endocardial lead system was achieved in 605 (88.2%) of 686 patients and an epicardial system in 614 (99.7%) of 616 (p < 0.05). Perioperative 30-day mortality rate was 0.8% (1.8% including crossovers) in endocardial implant recipients compared with 4.2% (p < 0.001) in epicardial implant recipients (3.6% without crossovers, p < 0.05, respectively). Implantation mortality risk was significantly lower for nonthoracotomy systems irrespective of left ventricular ejection fraction or New York Heart Association functional class. Pacing therapies prevented need for cardioversion or defibrillation shocks in 89% of all ventricular tachycardia episodes and were comparably effective for both lead systems. Total survival rate at 2 years was significantly higher in endocardial (87.6%) than epicardial (81.9%) lead recipients (p < 0.001). Elimination of perioperative mortality from the analysis demonstrated comparable survival in both groups (p > 0.2).

CONCLUSIONS

Third-generation cardioverter-defibrillators with monophasic waveforms can be successfully implanted with epicardial (99.7%) and endocardial (88.2%) lead systems. We conclude that endocardial leads should be the implant technique of first choice. Improved patient management and tolerance for device therapy is achieved with the addition of antitachycardia pacemaker capability in these systems.

Innovations in pulse generators and lead systems: balancing complexity with clinical benefit and long-term results

Technologic development of implantable cardioverter defibrillators (ICDs) is now in an exponential growth phase, and many new concepts are being examined. Major innovations have occurred in pulse generators and in lead systems. This may increase ICD use for primary and secondary prevention of sudden cardiac death. Pulse generators now include hybrid pacemaker-defibrillators. Clinical data suggest a need for demand pacing in primary and secondary prevention applications, with antitachycardia pacing being most valuable in the latter group. Atrial leads will allow dual chamber sensing, pacing, and defibrillation. Low energy cardioversion with biphasic shocks can enhance shock efficacy in rapid monomorphic ventricular tachycardia and flutter. Modifications of lead design, biphasic shock waveforms, and optimal thoracic electrode location in axillary or pectoral regions will permit lower energy defibrillation and smaller pulse generators with lower maximum energy outputs of < or = 25 joules. Dual chamber sensing will improve detection of atrial flutter or fibrillation. Minimum data storage requirements for tachycardia events in ICDs still need to be defined. Intracardiac electrogram analysis is still in evolution, and better analytic methods are awaited. Lead system development is likely to support generic pacing and defibrillation catheter electrodes for atrial and ventricular application. Advances in thoracic electrode design are in progress, and a variety of intercostal electrodes are being tested. Improvements in lead design with further impact on defibrillation energy requirements have the potential to permit generator miniaturization. Significant technologic improvements in ICD devices are imminent and should improve clinical results, patient safety, and quality of life.

The impact of implantable cardioverter defibrillator therapy on health care systems

Implantable cardioverter defibrillators (ICDs) are now widely used for the secondary prevention of sudden cardiac death and are being offered as a primary preventive therapy. This technology has potential for significant fiscal impact on health care budgets. Technologic innovation will result in more complex devices that are more effective and better accepted by patients and physicians. The clinical impact of these devices will be predicated, in part, by absolute survival benefits but also by their relative advantages over alternative therapies in terms of survival, safety, morbidity, quality of life, and cost. The impact on public health will depend on the effectiveness of screening methods for identification of populations likely to benefit from primary prevention. Risk stratification algorithms are now being tested in several ongoing clinical trials. Dilution of benefit by competing illnesses may occur to different extents in individual patient populations. The economic impact is predicated on the future cost of ICD systems, limitation of hospitalization costs associated with this therapy, and accurate prospective stratification in primary prevention populations. Cost efficacy analyses and quality of life assessment in ongoing and future clinical trials are essential to the development of this therapy and its diffusion into different health care systems. Achievement of clinical benefits, functional independence, and a return to gainful employment by patients will be important determinants of the support lent by health care systems to the dissemination of this therapy.