A comparison of biventricular and conventional transvenous defibrillation: a computational study using patient derived models

Conventional transvenous defibrillation is performed with an ICD using a dual current pathway. The defibrillation energy is delivered from the RV electrode to the superior vena cava (SVC) electrode and the metallic case (CAN) of the ICD. Biventricular defibrillation uses an additional electrode placed in the LV free wall with sequential shocks to create an additional current vector. Clinical studies of biventricular defibrillation have reported a 45% reduction in mean defibrillation threshold (DFT) energy. The aim of the study was to use computational methods to examine the biventricular defibrillation fields together with their corresponding DFTs in a variety of patient derived models and to compare them to simulations of conventional defibrillation. A library of thoracic models derived from nine patients was used to solve for electric field distributions. The defibrillation waveform consisted of a LV --> SVC + CAN monophasic shock followed by a biphasic shock delivered via the RV --> SVC + CAN electrodes. When the initial voltage of the two shocks is the same, the simulations show that the biventricular configuration reduces the mean DFT by 46% (3.5 +/- 1.3 vs 5.5 +/- 2.7 J, P = 0.005). When the leading edge of the biphasic shock is equal to the trailing edge of the monophasic shock, there is no statistically significant difference in the mean DFT (4.9 +/- 1.9 vs 5.5 +/- 2.7 J, P > 0.05) with the DFT decreasing in some patients and increasing in others. These results suggest that patient-specific computational models may be able to identify those patients who would most benefit from a biventricular configuration.     

Low Voltage Shocks Have a Significantly Higher Tilt of the Internal Electric Field Than Do High Voltage Shocks

Typically, an implantable cardioverter defibrillator(ICD) uses a cardioversion shock that is a lower voltage pulse of the same morphology and tilt as its defibrillation pulse. We investigated the internal electric field resulting from an ICD low voltage shock to determine whether its field characteristics matched those of the internal electric field of a high voltage shock. We attached epicardial patch electrodes, for shock delivery, to five fresh pig hearts placed in a diluted, heparinized saline bath. We inserted two plunge electrodes into the myocardium to measure an internal voltage proportional to the electric field. Monophasic 20-msec shocks, from a 140 μF capacitor, ranging from 0.1–30 joules, were delivered through the patches. We measured the current, external voltage, and internal voltage every 0.1 msec throughout the duration of a shock. For each shock, we calculated the time point that represented the 65% tilt position as measured across the patch electrodes. At this 65% tilt time position, we measured the pulse widths and calculated the internal tilt from the internal voltage. We found that the initial internal voltage for the 30-joule shock was 173 ± 40 volts compared to 10 ± 2 volts for the 0.1-joule shock. Similarly, we found that the final internal voltage for the 30-joule shock was 56 ± 14 volts compared to 2 ± 1 volts for the 0.1-joule shock. Thus, the internal tilt for the 30-joule shock was 68 ± 1% versus 82 ± 3% for the 0.1-joule shock (P < 0.05). Hence, a defibrillation shock (30 J) has an internal tilt close to its external tilt. A cardioversion shock (0.1 J), on the other hand, has a significantly higher internal tilt. The higher internal tilt of low strength, tilt-based shocks should be investigated as a possible factor in the proarrhythmia of cardioversion therapy.     

Potential benefit of transesophageal defibrillation: an experimental evaluation

INTRODUCTION: Because of the proximity of the esophagus to the heart, transesophageal defibrillation might increase defibrillation success. We assessed the defibrillation threshold (DFT) of transesophageal defibrillation compared with standard transthoracic defibrillation. METHODS: Defibrillation success and DFTs were determined in 22 female pigs with high (68+/-4 kg, n=12) or low body weight (39+/-1 kg, n=10). After induction of ventricular fibrillation, biphasic shocks were delivered between two cutaneous patch electrodes (sternal and apical position) or between an esophageal and two cutaneous patch electrodes in a sternal and apical position. The esophageal electrode was integrated into a latex sheath covering a standard transesophageal echocardiography probe. RESULTS: In 5 of 12 pigs with high body weight, external defibrillation failed despite 3 consecutive 200-J shocks, whereas subsequent transesophageal defibrillation was successful with the first shock. In the remaining 7 pigs, a more than 50% reduction in DFT was obtained with transesophageal defibrillation compared with standard biphasic external defibrillation (67+/-27 vs 164+/-23 J, P<.001). Pigs with lower body weight were successfully defibrillated by both transthoracic and transesophageal shocks. The DFT in pigs with low body weight was significantly lower using transesophageal defibrillation compared with transthoracic shocks (65+/-15 vs 99+/-38 J, P<.05). CONCLUSIONS: In this animal model, nonresponders to standard external defibrillation could successfully be defibrillated via an esophageal-cutaneous electrode configuration. Overall, an almost 50% DFT reduction was achieved by transesophageal defibrillation. Transesophageal defibrillation may provide an additional tool for terminating VF, which is refractory to external defibrillation, eg, in patients with very high body weight.     

Transthoracic impedance does not decrease with rapidly repeated countershocks in a swine cardiac arrest model

STUDY PURPOSE: Successful defibrillation is dependent upon the delivery of adequate electrical current to the myocardium. One of the major determinant of current flow is transthoracic impedance. Prior work has suggested that impedance falls with repeated shocks during sinus rhythm. The purpose of this study was to evaluate changes in transthoracic impedance with repeated defibrillation shocks in an animal model of cardiac arrest due to ventricular fibrillation (VF). METHODS: VF was electrically induced in anesthetized swine. After 5 min of untreated VF, monophasic or biphasic waveform defibrillation was attempted using a standard sequence of 'stacked shocks' (200, 300, then 360 J, if necessary) administered via adhesive electrodes. If one of the first three shocks failed to convert VF, conventional CPR was initiated and defibrillation (360 J) attempted 1 min later. Strength-duration curves for delivered voltage and current were measured during each shock and transthoracic impedance calculated. Animals requiring a minimum of four shocks were selected for study inclusion. Impedance data from sequential shocks were analyzed using mixed linear models to account for the repeated-measures design and the variability of the initial impedance of individual animals. RESULTS: Thirteen animals (monophasic waveform, n=7, biphasic waveform, n=6) required at least four shocks to terminate VF (range 4-6). Transthoracic impedance did not change from the first shock in the 13 animals (46+/-8 Omega) to the fourth shock (46+/-9 Omega). In animals receiving more than four shocks, transthoracic impedance likewise did not change significantly from the first to the last shock, which terminated VF. The lack of a significant change in impedance was also observed when animals were analyzed according to defibrillation waveform. CONCLUSION: Transthoracic impedance does not change significantly with repeated shocks in a VF cardiac arrest model. This is likely due to the lack of reactive skin and soft tissue hyperemia and edema observed in non-arrest models.     

Open-chest epicardial "surgical" defibrillation: biphasic versus monophasic waveform shocks

The aim of the study was to compare biphasic versus monophasic shocks for open-chest epicardial defibrillation. Transthoracic biphasic waveform shocks require less energy to terminate ventricular fibrillation compared to monophasic waveform shocks. However, if biphasic shocks are effective for open-chest epicardial ("surgical") defibrillation has not been established. Twenty-eight anesthetized adult swine (15-25 kg) underwent a midline sternotomy. Ventricular fibrillation was electrically induced. After 15 seconds of ventricular fibrillation, each pig in group 1 (n = 16) randomly received damped sinusoidal monophasic epicardial shocks and truncated exponential biphasic epicardial shocks from large (44.2 cm2) paddle electrodes at eight energy levels (2-50 J). Pigs in group 2 (n = 12) received monophasic and truncated exponential biphasic shocks from small (15.9 cm2) paddle electrodes. In group 1 (large paddle electrodes), the overall percent shock success rose from 15 +/- 9% at 2 J to 97 +/- 3% at 50 J. In this group there was no significant difference in percent of shock success between damped sinusoidal monophasic and biphasic waveform shocks. In group 2 (small paddle electrodes), biphasic shocks yielded a significantly higher percent of shock success than monophasic shocks at mid-energy levels from 7 to 20 J (all P < 0.01). With small surgical paddle electrodes, biphasic waveform shocks demonstrated a significantly higher percent of shock success rate compared to monophasic waveform shocks. With large paddle electrodes, the two waveforms were equally effective.     

Transthoracic impedance study with large self-adhesive electrodes in two conventional positions for defibrillation

External defibrillation requires the application of high voltage electrical impulses via large external electrodes, placed on selected locations on the thorax surface. The position of the electrodes is one of the major determinants of the transthoracic impedance (TTI) which influences the intracardiac current flow during electric shock and defibrillation success. The variety of factors which influence TTI measurements raised our interest to investigate the range of TTI values and the temporal TTI variance during long-term application of defibrillation self-adhesive electrodes in two conventional positions on the patient's chest--position 1 (sub-clavicular/sub-axillar position) and position 2 (antero-posterior position). The prospective study included 86 randomly selected volunteers (39 male and 49 female, 67 patients with normal skin, 13 patients with dry skin and 6 patients with greasy skin, 16 patients with chest pilosity and 70 patients without chest pilosity). The TTI was measured according to the interelectrode voltage drop obtained by passage of a low-amplitude high-frequency current (32 kHz) between the two self-adhesive electrodes (active area about 92 cm2). For each patient, the TTI values were measured within 10 s, 1 min and 5 min after sticking the electrodes to the skin surface, independently for the two tested electrode positions. We found that the expected TTI range is between 58 Omega and 152 Omega for position 1 and between 55 Omega and 149 Omega for position 2. Although the two TTI ranges are comparable, we measured significantly higher TTI mean of about (107.2 +/- 22.3) Omega for position 1 compared to (96.6 +/- 19.2) Omega for position 2 (p = 0.001). This fact suggested that the antero-posterior position of the electrodes is favourable for defibrillation. Within the investigated time interval of 5 min, we observed a significant TTI reduction with about 6.9% (7.4 Omega/107.2 Omega) for position 1 and about 5.3% (5.1 Omega/96.6 Omega) for position 2. We suppose that the long-term application of self-adhesive electrodes would lead to improvement of the physical conditions for conduction of the defibrillation current and to diminution of energy loss in the electrode-skin contact impedance. We found that gender is important when position 1 is used because women have significantly higher TTI (111 +/- 20.3) Omega compared to the TTI of men (102.6 +/- 24) Omega (p = 0.0442). Although we found some specifics of the electrode-skin contact layer, we can conclude that because of the insignificant differences in TTI, the operator of the defibrillator paddles does not need to take into consideration the skin type and pilosity of the patients. Analysis of the correlations between TTI and the individual patient characteristics (chest size, weight, height, age) showed that these patient characteristics are unreliable factors for prediction of the TTI values and optimal defibrillation pulse parameters and energy.     

Performance of temporary epicardial stainless steel wire electrodes used to treat atrial fibrillation: a study in patients following open heart surgery

AF is the most common arrhythmia following open heart surgery. Transthoracic cardioversion is used when pharmacological treatment fails to restore SR, or is ineffective in controlling ventricular response rate. We report on the performance of temporary atrial defibrillation wire electrodes implanted on the epicardium of patients undergoing open heart surgery. Epicardial stainless steel wire electrodes for both pacing/sensing and atrial defibrillation were placed at the left and right atrium during open heart surgery in 100 consecutive patients (age 65 +/- 9 years; male/female 77/23). Electrophysiological studies performed postoperatively revealed a test shock (0.3 J) impedance of 96 +/- 12 omega (monophasic) and 97 +/- 13 omega (biphasic). AF was induced by burst stimulation in 84 patients. All patients were successfully converted to SR. The mean energy of successful shocks was 3.1 +/- 1.9 J. Atrial pacing and sensing were accomplished in all patients. Pacing threshold was 1.9 +/- 1.7 V (0.5 ms) in the left atrium and 2.1 +/- 2 V in the right atrium. P wave sensing was 2.5 +/- 1.6 mV in the left atrium and 2.3 +/- 1.4 mV in the right atrium. No complications were observed with shock application, nor with lead extraction. Atrial defibrillation using temporary epicardial wire electrodes can be performed successfully and safely in patients following cardiac operations. The shock energy required to restore SR is low. Electrical cardioversion in the absence of anesthesia should be feasible.     

Washing Machine Associated 50 Hz Detected As Ventricular Fibrillation by An Implanted Cardioverter Defibrillator

SABATÉ, X., et al. : Washing Machine Associated 50 Hz Detected As Ventricular Fibrillation by An Implanted Cardioverter Defibrillator. This case report describes a patient with an automatic ICD who suffered a defibrillation shock without warning symptoms. An electrical interference can be observed in the stored EGM of the episode. The patient explained that the moment he felt the shock he was touching a washing machine. After correct grounding of this machine the patient did not suffer more inappropriate shocks.     

Surgical open-chest ventricular defibrillation: triphasic waveforms are superior to biphasic waveforms

Triphasic shocks have been evaluated for endocardial defibrillation but not for open-chest epicardial defibrillation. The purpose of this study was to compare the efficacy and safety of biphasic versus triphasic shocks for epicardial defibrillation in a porcine model. Twenty-two adult swine (18-28 kg) were deeply anesthetized and intubated. After 30 seconds electrically induced VF, each pig received truncated exponential biphasic (7.2-ms positive pulse duration and 7.2-ms negative pulse duration, total waveform duration 14.4 ms) and triphasic (4.8/4.8/4.8 ms, total waveform duration 14.4 ms) epicardial shocks. Pigs in group 1 (n = 11) received epicardial biphasic and triphasic shocks from large hand held paddle electrodes (44.2 cm2); pigs in group 2 (n = 11) received shocks from small paddle electrodes (15.9 cm2). Shocks were given at five selected energy levels (3-30 J) in random sequence. Four shocks were delivered at each energy level to construct an energy versus percentage of success curve. In group 1 (large paddle electrodes), percentage of shock success was significantly higher for triphasic shocks at the energy levels of 3, 5, 10, and 20 J compared to biphasic shocks. In group 2 (small paddle electrodes), triphasic shocks yielded a significantly higher percentage of shock success than biphasic shocks at the energy levels of 5, 10, and 20 J). Shock induced ventricular tachycardia was similar for both waveforms; asystole was rare. For open-chest defibrillation, triphasic waveform shocks were superior to biphasic waveform shocks for VF termination at energy levels of 3-20 J and were as safe as biphasic shocks.     

The Left Subclavian Vein as an Alternative Site for Implantation of the Second Defibrillation Lead

The optimal placement for the second defibrillation lead in a twolead system has never been addressed. We retrospectively reviewed the data of 33 patients with an average age of 59.2 years (range 41–78 years), predominantly mala (n = 29), who underwent implantation of a cardioverter defibrillator (ICD) for treatment of ventricular tachycardia (n = 19) or ventricular fibrillation (n = 14). In all patients an attempt was made to implant an endovenous ICD device (leads only, no subcutaneous patch). In group I (n = 18) the defibrillation anode, a separate unipolar lead, was placed in the common position, the superior vena cava. In group II (n =15) the lead was placed in the left subclavian vein. At least two consecutive shocks reverting ventricular fibrillation at energies ±24J were required for implantation of the ICD device. All shocks were monophasic. The success rate of endovenous defibrillation was significantly higher in group II than in group I (67% vs 28%, P < 0.05). Thus, it could be demonstrated that the position of the defibrillation anode can influence the defibrillation efficacy in transvenous ICD systems. Prospective randomized trials are needed to investigate the optimal position for the second defibrillation electrode, which may gain increasing importance as soon as dual chamber ICDs become available.     

A percutaneous catheter-based system for the measurement of potential gradients applicable to the study of transthoracic defibrillation

BACKGROUND: The local electric (E) field or potential gradient produced by a shock reliably predicts VF termination. In this study we evaluated a multiple electrode, catheter-based device for closed-chest 3D measurements of E field from transthoracic defibrillation shocks. METHODS: Catheters with multiple electrodes on the tip were placed in intracardiac locations in anesthetized swine. An empirically derived calibration matrix and custom microprocessor was used to transform simultaneously measured voltages into orthogonal E field vector components. E fields produced in six intracardiac locations by 30 and 300 J shocks were compared in eight animals. Correlations were determined for measured current and E field at various shock strengths at two different transthoracic impedances in five additional animals. VF was induced in 12 animals and E field measured during defibrillation attempts. RESULTS: The E field measurements resulting for 30 J transthoracic shocks were not significantly different among different intracardiac sites. At 300 J, however, significant differences were observed between sites with the greatest intensities recorded in the coronary sinus and right ventricle. Within animals, the variability of the measurement at each site was small, ranging from 2.8 +/- 1.6% to 5.7 +/- 4.5%. Significant correlations (P < 0.001) between measured E field and peak current were observed at native impedance (34 +/- 4 Omega, r = 0.81) and at adjusted impedance (76 +/- 4 Omega, r = 0.78) with transthoracic shocks of 200, 300, and 360 J. In VF studies, the probability of defibrillation was closely fit by a sigmoidal dose response curve in the coronary sinus E field with an approximate threshold of 4.7 V/cm with 50% defibrillation success at 9.3 V/cm. CONCLUSIONS: The measured intracardiac E field variability within animals and at a specific site was small, exhibiting a median value of 5.1%, contrasted to median variabilities across animals of 5-11% suggesting the capacity of this measurement system to provide subject specific information on the distribution of E fields. The measured E field magnitudes across animals in the coronary sinus were linearly correlated with applied shock current with a very strong linear relation to effective shock voltage observed in vitro in a saline tank. When evaluated as a predictor of shock success, the observed values were consistent with previously reported critical fields. This technique may be of value in evaluating waveforms for transthoracic defibrillation as well as electrode size, placement, and composition.     

Quadriphasic waveforms are superior to triphasic waveforms for transthoracic defibrillation in a cardiac arrest swine model with high impedance

BACKGROUND: We have demonstrated previously that triphasic waveform shocks were superior to biphasic waveform shocks for transthoracic defibrillation. Our purpose was to compare the efficacy and safety of quadriphasic versus triphasic shocks for transthoracic defibrillation in a porcine model. METHODS: Sixteen adult swine (19-25 kg, mean: 21.5 kg) were deeply anesthetized and intubated. To simulate impedance of the human chest, fixed electrical resistors (25 or 50 ohms) was placed in series with the defibrillator and the chest of each pig. After 30 s of electrically induced VF, each pig received transthoracic shocks, using either a truncated exponential triphasic waveform (5 ms positive pulse duration, 5 ms negative pulse duration and 5 ms positive pulse duration, total waveform duration 15 ms) or a quadriphasic waveform (5/5/5/5 ms, total waveform duration 20 ms). Each pig received transthoracic triphasic and quadriphasic shocks at three selected energy levels (50, 100 and 150 J) in random sequence. Four shocks were delivered at each energy level to construct an energy versus % success curve. Success was defined as VF termination at 5 s after shock. The total shocks were divided into three groups based on the delivered energy actually delivered to the animal: <40, 40-65 and >65 J. Delivered energy = (animal impedance/total impedance) times selected energy of the shock. RESULTS: For high-impedance animals (86-102 ohms), quadriphasic waveform shocks achieved significantly higher percent shock success than triphasic shocks for the termination of VF at the energy levels of >65 J actually delivered (quadriphasic 72.7+/-12.2%, triphasic 38.9+/-7.7%, p<0.02). No differences in the shock success between quadriphasic and triphasic waveforms were found for other two energy levels. There were no differences in ventricular tachycardia or asystole after shocks between quadriphasic and triphasic waveforms. CONCLUSION: In this porcine model, 20 ms (5/5/5/5) quadriphasic shocks were superior to 15 ms (5/5/5) triphasic shocks for transthoracic defibrillation in animals with impedances that simulated high impedance in humans.     

A Prospective Evaluation of Single and Dual Current Pathways for Transvenous Cardioversion in Rapid Ventricular Tachycardia

By using a prospective randomized study design, we compared the clinical efficacy and safety of single unidirectional and bidirectional transvenous cardioversion shocks for termination of rapid ventricular tachycardia (VT) having cycle lengths less than 300 ms. A Medtronic 6880 catheter was placed in the right ventricular apex and an R2 skin patch electrode was placed over the left scapula. Patients were randomized into two groups. Group A patients received unidirectional transvenous shocks using the two catheter electrodes (right ventricular apical cathode and superior vena caval anode) which resulted in a single current pathway. Group B patients received bidirectional transvenous shocks using a common cathode (right ventricular apex) and two separate anodes (superior vena caval and R2 patch) resulting in two current pathways. Identical shocks with total energies of 2.7, 5.0 and 10.0 J and waveform tilt of 27% were delivered to Groups A and B. In selected Group B patients, delivered shock currents through the right ventricular apex/superior vena caval and right ventricular apex/R2 patch electrode pairs were measured. We analyzed the initial episode of VT with a cycle length less than 300 ms in 33 patients with organic heart disease (mean age, 64 +/- 9 years; mean VT cycle length, 248 +/- 37 ms) who underwent programmed electrical stimulation. Transvenous cardioversion shocks terminated 31% of 16 VT episodes in Group A and 41% of 17 VT episodes in Group B (p greater than .2). The mean successful shock energy was 6.1 +/- 3.7 J in Group A and 3.0 +/- 0.9 J in Group B (p less than .05). Forty percent of all successfully cardioverted episodes in Group A and 86% of all successfully cardioverted VT episodes in Group B were terminated at an energy of 2.7 J (p = .09). Analysis of shock waveforms in Group B revealed 47 to 74% of the total current was transmitted through the right ventricular apex/superior vena caval electrodes and 26 to 53% through the right ventricular apex/R2 electrodes. We conclude that single bidirectional transvenous shocks are effective for rapid VT termination in selected patients. Dual current pathways decrease energies needed for successful transvenous cardioversion in this patient population.     

Optimized Pulse Durations Minimize the Effect of Polarity Reversal on Defibrillation Efficacy with Biphasic Shocks

There are conflicting results on the effect of polarity change on the defibrillation efficacy of biphasic shocks possibly caused by different shock durations. The goal of the present study was to investigate the influence of polarity reversal on defibrillation efficacy for different biphasic shock durations in a porcine animal model. In eight anesthesized pigs using a transvenous/submuscular lead system DFTs for 4 phase 1 durations were determined: 8.1 ms, 6 ms, 3.8 ms and 1.7 ms. The phase 1/phase 2 ratio was constant at 60%/40%. For cathodal shocks, the defibrillation coil in the right ventricular apex was the cathode during phase 1 and for anodal shocks it was the anode. For both polarities, the strength-duration curve revealed a DFT minimum at 3.8 ms (cathodal shocks: 21.3 +/- 6.4 J, P < 0.001; anodal shocks: 21.9 +/- 8 J, P = 0.05). For anodal shocks and phase 1 durations of 1.7, 3.8, and 6 ms there was no significant difference of the stored energy at the DFT compared to cathodal shocks. In contrast, significantly lower DFTs were observed for anodal shocks with a phase 1 duration of 8.1 ms (28.8 +/- 6.4 J compared to 33.1 +/- 5.9 J for cathodal shocks, P = 0.006). The effect of lower defibrillation energy requirements with polarity reversal depends on the total biphasic shock duration; for the pulse duration with the lowest DFT, polarity reversal does not increase defibrillation efficacy of biphasic shocks.     

Safety of External Cardioversion/Defibrillation in Patients with Internal Defibrillation Patches and No Device

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

Effect of ventricular fibrillation duration on the defibrillation threshold in humans

Early during ventricular fibrillation, the defibrillation threshold may be low, as ventricular fibrillation most probably arises from a localized area with only a few wavefronts and the effects of global ischemia, ventricular dilatation, and sympathetic discharge have not yet fully developed. The purpose of this study was to explore the effect of the timing of shock delivery in humans. During implantation of an ICD in 26 patients (24 men, 60 +/- 11 years, 19 coronary artery disease, NYHA 2.2 +/- 0.4, left ventricular ejection fraction 0.42 +/- 0.16), the defibrillation threshold was determined after approximately 10 and 2 seconds of ventricular fibrillation. Ventricular fibrillation was induced by T wave shocks. Mean defibrillation threshold was 9.9 +/- 3.6 J after 10.3 +/- 1.0 seconds. Within 2 seconds, 20 of 26 patients could be successfully defibrillated with < or = 8 J. In these patients, the mean defibrillation threshold was 4.0 +/- 2.1 J after 1.4 +/- 0.3 seconds compared to 9.5 +/- 3.1 J after 10.2 +/- 1.1 seconds (P < 0.001). There were no clinical differences between patients who could be successfully defibrillated within 2 seconds and those patients without successful defibrillation within 2 seconds. In the majority of patients, the defibrillation threshold was significantly lower within the first few cycles of ventricular fibrillation than after 10 seconds of ventricular fibrillation. These results should lead to exploration of earlier shock delivery in implantable devices. This could possibly reduce the incidence of syncope in patients with rapid ventricular tachyarrhythmias and ICDs.     

The Characterization of Human Transmyocardial Impedance during Implantation of the Automatic Internal Cardioverter Defibrillator

We set out to determine in a prospective fashion the characteristics of energy delivery related to defibrillation in a population of patients receiving the AICD. Specifically, we examined the characteristics of the delivered current, transmyocardial voltage, and transmyocardial impedance. Secondly, we determined the relationship between the energy delivered, the impedance encountered and the defibrillation threshold. Since the AICD will deliver a succession of shocks if the initial shock does not cardiovert, the effects of consecutive shocks at different energy levels on the transmyocardial impedance were also assessed.     

Transthoracic electrical impedance during external defibrillation: comparison of measured and modelled waveforms

The transthoracic electrical impedance is an important defibrillation parameter, affecting the defibrillating current amplitude and energy, and therefore the defibrillation efficiency. A close relationship between transthoracic impedance and defibrillation success rate was observed. Pre-shock measurements (using low amplitude high frequency current) of the impedance were considered a solution for selection of adequate shock voltages or for current-based defibrillation dosage. A recent approach, called 'impedance-compensating defibrillation' was implemented, where the pulse duration was controlled with respect to the impedance measured during the initial phase of the shock. These considerations raised our interest in reassessment of the transthoracic impedance characteristics and the corresponding measurement methods. The purpose of this work is to study the variations of the transthoracic impedance by a continuous measurement technique during the defibrillation shock and comparing the data with results obtained by modelling. Voltage and current impulse waveforms were acquired during cardioversion of patients with atrial fibrillation or flutter. The same type of defibrillation pulse was taken from dogs after induction of fibrillation. The electrodes were located in the anterior position, for both the patients and animals.     

Internal Cardiac Defibrillation: Single and Sequential Pulses and a Variety of Lead Orientations

A sequential pulse system for internal cardiac defibrillation incorporating catheter and patch electrodes with two current pathways has been shown to reduce defibrillation threshold in comparison to the single pulse technique. The relative advantage of the sequential pulse over the single pulse technique with other lead systems is not known. We compared defibrillation thresholds using sequential and single pulses delivered to a variety of lead orientations with the same electrode surface areas, when possible. Defibrillation threshold totals determined in halothane-anesthetized open-chest pigs averaged: For the single pulse shock passed between (1) superior vena cava (SVC) and left ventricular apical patch (LVA), 27.2 +/- 9.1 joules (J) and (2) LV epicardial patch (LVE) to right ventricular epicardial (RVE) patch leads, 16.5 +/- 2.1 J; and for the sequential pulse shock with two pulses passed between: (1) the SVC to RV intracavitary apex (RVA) and a quadripolar catheter in the coronary sinus to the RVA, 11.6 +/- 1.0 J; (2) the SVC to LVA and the LVE to RVE, 9.6 +/- 1.3 J and (3) the SVC to RVA and the LVE to RVA, 8.9 +/- 0.4 J. Defibrillation thresholds for sequential pulse shocks were all significantly lower than either of the defibrillation thresholds for single pulse shocks (p less than 0.001). We conclude that the sequential pulse system provides a substantial reduction in defibrillation threshold over the single pulse regardless of the lead system when the surface area and pulse characteristics are controlled. Sequential pulse technique may be valuable in the design of an implantable automatic defibrillator.(ABSTRACT TRUNCATED AT 250 WORDS)     

Improved Sensing Signals After Endocardial Defibrillation with a Redesigned Integrated Sense Pace Defibrillation Lead

Adequate sensing is a basic requirement for appropriate therapy with ICDs. Integrated sense pace defibrillation leads, which facilitate ICD implantation, show a close proximity of sensing and defibrillation electrodes that might affect the sensing signal amplitude by the high currents of internal defibrillation. In 99 patients, we retrospectively examined two integrated sense pace defibrillation leads, eitherboth with a distance of 6 mm between the tip of the lead (sensing cathode) and the right ventricular defibrillation electrode (sensing anode) or one with a distance of 12 mm. Three seconds after a shock of 20 J, mean sensing signal amplitude during sinus rhythm (SR) decreased from 10.5 ± 4.3 mVto 5.1 ± 3.7 mV (P < 0.001) for the 6-mm lead, but showed no significant decrease for the 12-mm lead. The degree of signal reduction was inversely related to the time passed since defibrillation. Significant differences in reduction of sensing signal amplitude concerning monophasic and biphasic shocks could not be observed. Mean sensing signal amplitude of VF after shocks that failed to terminate it decreased in the same order as during SR (from 8.3 ± 4.1 mV to 4.1 ± 3.2 mV), but resulted in no failure of redetection during ongoing VF. DFTs did not differ for the 6-mm and the 12-mm lead. In conclusion, close proximity of the right ventricular defibrillation coil to the sensing tip of an integrated sense pace defibrillation lead causes energy and time related reductions in sensing signal amplitude after defibrillation, and might cause undersensing in the postshock period. A new lead design with a more proximal position of the right ventricular defibrillation coil avoids these problems without impairing DFTs.