Double and triple sequential shocks reduce ventricular defibrillation threshold in dogs with and without myocardial infarction.

The role of optimal placement of electrodes and mode of shock delivery from a defibrillator was examined in dogs with and without myocardial infarction. Single, double and triple truncated exponential shocks separated by 1 ms were delivered through various electrode combinations and cardiac vectors after electrical induction of ventricular fibrillation. A single shock through a pathway not incorporating the interventricular septum (catheter electrodes or epicardial patches between anterior and posterior left ventricle) required the highest total energy (22.6 and greater than 26.4 J, respectively) and peak voltage (1,004 and greater than 1,094 V, respectively) to terminate ventricular fibrillation. A single shock through a pathway including the interventricular septum required lower total energy and peak voltage to defibrillate. Combinations of two sequential shocks between an intracardiac catheter electrode and anterior left ventricular epicardial patch, between the catheter electrode and subcutaneous extrathoracic plate and between three ventricular epicardial patches all significantly reduced total energy (7.7, 8.7 and 7.8 J, respectively) and peak voltage (424, 436 and 424 V, respectively) needed to defibrillate. Three sequential shocks exerted no significant additional reduction in total energy of the defibrillation threshold than did two sequential shocks. Infarcted canine heart required less peak voltage but not total energy to terminate ventricular fibrillation than did noninfarcted heart. Therefore, two sequential shocks over different pathways reduce both total energy and peak voltage required to terminate ventricular fibrillation.

Inhibition of premature ventricular extrastimuli by subthreshold conditioning stimuli.

The purpose of this study was to determine whether trains of subthreshold high frequency conditioning stimuli (333 Hz, 1 ms duration, 2 ms interval) delivered to the canine ventricle inhibited the response to a premature stimulus (S2) more effectively than did a single subthreshold conditioning stimulus. It was found that trains of conditioning stimuli (mean 1.21 mA) inhibited the response to S2 152 ms beyond expiration of the ventricular effective refractory period, whereas a single conditioning stimulus inhibited S2 only 20 ms or less beyond the ventricular effective refractory period. In late diastole, trains of conditioning stimuli failed to inhibit S2 when the train of stimuli caused ventricular depolarization or the latter occurred in response to the next sinus impulse. Trains of conditioning stimuli did not induce ventricular arrhythmias. Lidocaine or autonomic blockade did not alter the response to trains of conditioning stimuli. Trains of conditioning stimuli or a single conditioning stimulus inhibited the response to S2 only when they were delivered at the same electrode site. By lengthening the ventricular effective refractory period, trains of conditioning stimuli could prevent or terminate tachycardias, but this possibility is constrained, at present, by the spatial limitations of the technique.

Improved internal defibrillation with twin pulse sequential energy delivery to different lead orientations in pigs.

Internal cardiac defibrillation with an intravascular catheter was compared with a new method for internal cardiac defibrillation using 2 pulses delivered in sequence directly to the myocardium. For the sequential pulses, the first pulse was passed through an intravascular catheter (Medtronic 6880), between the anode in the superior vena cava-atrial junction region and the cathode in the apex of the right ventricle. The second pulse was delivered between the catheter tip in the right ventricular apex as cathode and an oval plaque electrode (Medtronic TX-7) secured on the epicardium of the left ventricular free wall as anode. Defibrillation pulses were of truncated, trapezoidal waveform (65% tilt), separated by 1, 10 and 100 ms. Using the catheter alone, 36 normal pig hearts could be defibrillated by 44 J. However, 22 pig hearts (60%) could not be defibrillated with energies below 35 J. Defibrillation threshold was improved with sequential twin pulses, the improvement being dependent on pulse separation (42, 34 and 19 J, at 100-, 10- and 1-ms separation, respectively; F = 14.6, df = 2.29, p less than 0.01). In conclusion, sequential twin pulse defibrillation provides a considerable reduction in energy necessary for defibrillation in comparison to single pulses using the catheter alone. In this study, the optimal separation was 1 ms.

Low energy countershock using an intravascular catheter in an acute cardiac care setting.

We examined the feasibility, effectiveness, and safety of using an intravascular catheter positioned in the right ventricular apex for countershock in a coronary care unit setting in 8 patients who had recurrent ventricular tachyarrhythmia. Countershock using 2.5 to 40 J stored energy (damped sinusoidal wave form) was attempted 115 times to terminate 100 episodes of ventricular tachyarrhythmia (ventricular tachycardia, 91; ventricular flutter, 3; ventricular fibrillation, 6). Eighty-six (87%) of 99 countershock attempts for ventricular tachycardia, 3 (60%) of 5 for ventricular flutter, and 4 (36%) of 11 for ventricular fibrillation were successful using this technique. The catheters remained in stable position for 1 to 16 days without dislodgment. A majority of the countershocks were delivered by the regular nursing staff in the coronary unit. We conclude that low energy countershock through an intravascular catheter system is feasible, safe, and effective in a coronary care unit setting. Such a system should be beneficial in the acute management of patients who have recurrent ventricular tachycardia or fibrillation. The catheter lead may also prove useful in managing ventricular tachyarrhythmias that occur during electrophysiologic studies.

Preliminary single center clinical experience of the use of a new implantable cardioverter defibrillator.

We report a single center's preliminary clinical experience of the Sentinel (Angeion, Minneapolis, MN) implantable cardioverter defibrillator (ICD), which employs novel technologies that offer the potential for significant reduction in ICD size. Thirty-three patients have received Sentinel ICDs with a mean follow-up of 450 (range 150-1023) days. Device shock therapy has been used to defibrillate/cardiovert 43 spontaneous episodes of malignant ventricular arrhythmia and 510 episodes of hemodynamically well tolerated ventricular arrhythmia have been pace-terminated (pace-termination failed in 6 episodes with subsequent delivery of appropriate shock therapy). There has been no arrhythmic death in this patient population. There have been 9 inappropriate shocks in 6 patients (in 2 patients for atrial fibrillation which had satisfied the algorithm detection criteria for high zone ventricular arrhythmia, in 3 for sinus tachycardia [rate greater than 180 beats per min] and in 1 due to device capacitor malfunction). Device replacement has been required for component malfunction in 3 patients. There have been no other major complications. Follow-up time to date is short and longterm device efficacy and performance remain unproven. However, our early clinical experience suggests that the innovations used to manufacture the Sentinel ICD have facilitated reduction in ICD size without compromising therapeutic efficacy.

The middle cardiac vein--a novel pathway to reduce the defibrillation threshold.

UNLABELLED: Defibrillation energy requirements of epicardial implantable cardioverter defibrillator systems are generally lower than endovascular systems currently used. The former has the disadvantage of requiring a thoracotomy and so has a greater morbidity and mortality than an endovascular procedure. The middle cardiac vein (MCV) is an epicardial structure that is accessible by a non-thoracotomy approach. This study investigated the merits of ventricular defibrillation from the middle cardiac vein. METHODS AND RESULTS. Defibrillation thresholds (DFT) were measured in 10 anesthetized pigs, weighing 34.5 +/- 44.1 kg (mean 39 kg). An Angeflex electrode (1.7 mm x 50 mm) was introduced via the left external jugular vein to the right ventricular apex. The MCV was identified with standard angiography techniques and a 4080 (Angeion Corp.) defibrillation electrode (1.6 mm x 65 mm) introduced into the vein. An active can was implanted in the left subpectoral region. The defibrillation thresholds (DFT) of the following defibrillation configurations were assessed using a modified four-reversal binary search: RV-->Can, RV + MCV-->Can and MCV-->Can. The DFT's for the three configurations were 15.5 +/- 2.8 J, 10.8 +/- 3.4 J and 13.7 +/- 2.4 J. Analysis of variance showed that the DFT with the RV + MCV combination was significantly less than the RV alone (p < 0.05) CONCLUSIONS: Defibrillation is possible through the MCV and that incorporating an electrode in the MCV with RV-Can configuration can reduce the DFT by 30%.

Increased defibrillation threshold with right-sided active pectoral can.

UNLABELLED: 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.

A systematic evaluation of conventional and novel transvenous pathways for defibrillation.

INTRODUCTION: Conventional implantable cardioverter defibrillators employ endocardial (shock) electrodes with a lead located in the right ventricular apex (RV) and a "hot-can" electrode located subcutaneously in the left pectoral region. In the event of a high defibrillation threshold (DFT) a third electrode is frequently employed in the superior vena cava (SVC). We report the comparison of conventional and novel locations of additional electrodes with the RV/Can configuration, in a porcine model. METHOD: In 12 anesthetized pigs (30-45 kg), endocardial defibrillation electrodes were randomized to the following locations: RV/Can, RV/Can + SVC, RV/Can + main pulmonary artery (MPA) and RV/Can + left pulmonary artery wedge position (PAW), RV/Can + high inferior vena cava (HIVC), RV/Can + Low inferior vena cava (LIVC). Ventricular fibrillation (VF) was induced using 60 Hz alternating current. After 10 seconds VF a rectangular biphasic shock was delivered by the ARD9000 (Angeion Corp). The DFT was determined for each configuration using a modified four-reversal binary search. All configurations were compared using a repeated measures analysis of variance (ANOVA) statistical test and the five 3-electrode configurations were compared to the RV/Can position using a Dunnett test. RESULTS: Mean DFTs: RV = 21.5 +/- 4.8 J, SVC = 16.8 +/- 4.7 J (p < 0.05 vs. RV), HIVC = 21.1 +/- 4.7 J (p <. 0.05), LIVC = 19.1 +/- 5.7 J (p <. 0.05 vs. RV), MPA = 16.0 +/- 5.8 J (p < 0.01), PAW = 17.5 +/- 4.6 J (p < 0.05 vs. RV). CONCLUSIONS: Relative to the RV/can configuration the addition of a third electrode in the PA, PAW or SVC significantly reduces the DFT in the pig. The addition of an electrode to the IVC did not significantly reduce the DFT in our model.

The influence of opening the thorax on defibrillation threshold in canines.

To determine if intraoperative testing is predictive of implantable defibrillator performance postoperatively, we measured sequential pulse defibrillation thresholds (DFTs) in 16 adult canines (28.0 +/- 3.5 kg, mean +/- SD body weight) at the time of epicardial defibrillation electrode implantation. Three epicardial defibrillation electrodes were sutured directly to the anterior, posterior, and left lateral epicardial surfaces of the heart through a left fifth intercostal thoracotomy. The pericardium was sutured closed over the electrodes and DFT was measured first with the thorax open and again after closing all surgical wounds, evacuating the thorax, and reinflating the lungs. Mean +/- SD DFT voltage, current, and impedance (pulse 1), and total delivered energy (both pulses) for the open chest measurements were 321 +/- 87 volts, 4.2 +/- 1.9 amps, 80 +/- 14 ohms and 5.3 +/- 3.7 joules, respectively. The corresponding DFT values for the closed chest measurements were 321 +/- 92 volts, 5.1 +/- 1.9 amps, 64 +/- 10 ohms and 6.1 +/- 3.9 joules, respectively. Paired Student's t-test comparison of open versus closed chest DFT values indicated that there were no significant differences in voltage (P greater than 0.80) or energy (P greater than 0.20), but there were significant differences in both current (P less than 0.01) and impedance (P less than 0.001). It is concluded that despite alterations in impedance and current flow, voltage and energy DFT are not significantly different between open and closed chest animals. This suggests that intraoperative testing of implantable defibrillators is predictive of postoperative performance.

Implantable defibrillator electrode systems: a brief review.

Since the first report of a defibrillation attempt with an intracardiac catheter electrode nearly 30 years ago, investigators have developed implantable electrode systems consisting of metal disks, endocardial catheters, and epicardial patches. These early efforts demonstrated the feasibility of low-energy reversion of ventricular tachyarrhythmias, and also provided some insight into the mechanisms of fibrillation and defibrillation. This review describes the evolution of implantable defibrillator electrode systems. Early investigators attempted defibrillation with submuscularly implanted metal disks or a disk electrode paired with an endocardial catheter electrode. Electrode design emphasis turned to transvenous catheter systems with electrodes placed in the right ventricle and right atrium. A more successful configuration placed the proximal electrode in the superior vena cava. In an effort to ensure proper placement of the distal electrode in humans, the catheter was replaced with an epicardial patch. More recently, a combination of electrodes and multiple pulses has substantially reduced the energy required to defibrillate. Effective electrode systems that can convert lethal arrhythmias with a minimum of energy will aid in making implantable cardioverters and defibrillators the therapy of choice in patients at high risk of sudden coronary death.

Bronchial-arterial interdependence in isolated dog lung.

The bronchus and artery, embedded in the lung parenchyma, were modeled as adjoining cylindrical tubes in an elastic continuum. Solutions using finite-element analysis of nonuniform stress and strain occurring from an initial uniform state were computed for a reduction in arterial pressure. Maximal nonuniform principal and shear stresses in the parenchyma, equal to 2.5 times the mean periarterial stresses, occurred in the region adjacent to the bronchial-arterial joint. Bronchial cross section became oval and elongated along the line passing through the centers of the tubes, whereas arterial cross section elongated at right angles to this line. These predicted changes in shape of bronchus and artery were verified by radiographic measurements in isolated lobes, held at constant transpulmonary pressures of 4 and 25 cmH2O while arterial pressure was varied. Results suggest that peribronchovascular interstitial fluid pressure may be nonuniform and that the bronchial-arterial joint may be the preferential site for emphysematous perivascular lesions, which may occur on lung hyperinflation.

Lung deformations at minimal volume.

Minimal volume (MV, gas plus tissue volume at zero transpulmonary pressure) was determined in excised dog lobes after inflation with air and with saline. MV was significantly greater in saline-filled lobes than air-filled lobes. Air inflation was performed with the lobe supported in two different ways, which were chosen to produce different deformations: 1) supported on the cephalic surface and 2) hung by strings attached to the cephalic surface. MV measured in hung lobes was significantly greater than MV measured in supported lobes. The problem of excised lobes deformed by gravity was analyzed in the two configurations using linear elasticity. The saline volume was assumed to be the stress-free state for the lobe, and the volume change was computed from the stress-free state due to the gravitational deformation. The analysis suggests that the difference between saline and air MV is due primarily to the gravitational deformation of air-filled lungs.

Inspiration induced by phrenic nerve stimulation increases internal defibrillation energy requirements.

INTRODUCTION: The purpose of this study was to systematically evaluate the effects of active inspiration induced by phrenic nerve stimulation on the energy required for 50% successful defibrillation (E50). METHODS AND RESULTS: Shocks (95-microF biphasic waveform) were delivered after 10 seconds of ventricular fibrillation between a right ventricular coil and left pectoral test can in ten anesthetized pigs (25 to 37 kg). Using a 1-J step size, the E50 was determined with an up/down, three-reversal method. Positive-pressure ventilation was halted just before fibrillation, and shocks were delivered during expiration or at the end of 2 seconds of bilateral phrenic stimulation (50 Hz, 0.3 msec, 5 to 6 V). Phrenic stimulation produced inspiratory volumes that were 15.3 +/- 1.7 mL/kg (mean +/- SD). The E50 was 9.8 +/- 1.9 J during expiration and increased to 13.0 +/- 1.7 during inspiration (P = 0.001). The leading-edge voltage at the E50 was 451 +/- 46 V during expiration and 519 +/- 33 V during inspiration (P = 0.001). The leading-edge current at the E50 was 9.7 +/- 1.0 A during expiration and increased to 11.3 +/- 1.4 A during inspiration (P = 0.002). The average impedance was 47.8 +/- 2.7 omega during expiration and 47.3 +/- 3.3 omega during inspiration (P = 0.12). CONCLUSION: Inspiration induced by phrenic stimulation results in a 31% increase in the E50 compared with expiration. The decrease in shock efficacy occurs in the absence of a change in impedance. Active inspiration may alter the distribution of the electrical field leading to a decrease in shock efficacy.

Temporal stability of sequential pulse defibrillation threshold.

We have shown that sequential pulse defibrillation threshold voltage and total delivered energy do not change with maturation of the electrode tissue interface for up to 12 weeks after implantation of two different electrode configurations. This result is important to predict the future performance of an implantable defibrillator that is tested only at implant.

Sequential pulse countershock between two transvenous catheters: feasibility, safety, and efficacy.

We evaluated the feasibility, safety, and efficacy of sequential pulse countershock (SqCS) delivered solely through two endocardial catheters for the termination of ventricular tachycardia (VT) and fibrillation (VF) in patients undergoing electrophysiology studies (EPS). Thirty-four patients (31 men, 3 women) with a mean age of 56.8 +/- 10.1 years were studied. Etiology of VT/VF was ischemic heart disease (n = 26), cardiomyopathy (4) repaired tetralogy of Fallot (n = 1), heart transplant (n = 1), and no identifiable heart disease (n = 2). Catheters were positioned successfully in 29 patients. These were positioned in the right ventricular apex (RVA) and the coronary sinus (CS), respectively. The RVA electrode served as the common cathode for both pulses. The two electrodes located near the right atrium/superior vena cava junction served as anode for pulse 1 while the distal CS electrodes served as anode for pulse 2. Twenty-nine induced VT episodes with cycle length (CL) 220-370 msec were treated. SqCS successfully terminated 15 VT (100-500V) while 14 were accelerated or degenerated to VF. VTCL was longer in successful SqCS episodes than in those that were accelerated (285 +/- 17.3 vs 245 +/- 30.8 msec, P less than .003). Of 26 VF episodes, 21 were terminated with SqCS (500-900V) and 5 were terminated by transthoracic rescue shocks. On 2 occasions, failure to defibrillate was attributable to poor catheter position at the time of shock. No complications occurred. We conclude that SqCS delivered solely between endocardial catheter electrodes is feasible and effective using energy doses within the range of existing implantable cardioverter defibrillators.

Axial distortion of airways in the lung.

Axial loads were applied around the circumference of an airway lumen by pulling on a cup-shaped anchor that embedded itself in the airway wall. Axial displacements were measured as a function of distance from the load, and the data were compared to the results of mathematical analyses of continuum mechanics models. In the modeling it was assumed that the elastic tube representing the airway is bonded to the surrounding elastic continuum representing the parenchyma and that axial forces are transmitted between the tube and the continuum by shear stresses at the interface. The agreement between the measured and computed axial displacements supports the hypothesis that the shear stresses are the dominant coupling mechanism. The following quantitative relations between force and displacement were obtained. The axial displacement produced by the load L was approximately 0.05 L/pi alpha mu, where alpha is the airway radius and mu is the shear modulus of the parenchyma. The displacement decayed to approximately one-half this maximal value at two diameters from the load.

Low voltage direct current delivered through unipolar transvenous leads: an alternate method for the induction of ventricular fibrillation.

The induction of VF during testing of an ICD may not always be possible using either burst pacing or high energy T wave shocks. The purpose of this study was to evaluate the effectiveness of low energy DC stimulation for inducing VF in a porcine model. The VFT was measured using constant voltage stimuli and a step-up method in ten anesthetized pigs (25-30 kg). Stimuli of different durations (0.5, 1.0, 2.0 s) were delivered (unsynchronized) between a right ventricular apical coil and a subcutaneous test can. Current was measured from the voltage drop across a series resistor (10 omega). With anodal stimulation, VF required 6.4 +/- 0.2 V compared to 13.8 +/- 0.6 V with cathodal stimulation (P < 0.001). The current required to induce VF (measured 10 ms after the stimulus onset) was 58.3 +/- 2.2 mA with anodal stimulation and 119.3 +/- 4.7 mA with cathodal stimulation (P < 0.001). Stimulus duration did not significantly influence the voltage or current required for VF induction. In 6 of the 10 pigs, synchronizing a 0.5-second stimulus to the R wave did not significantly alter the VFT compared to same stimulus synchronized to mid-upslope of the T wave. The results indicate that VF can be consistently induced through transvenous electrodes by passing unsynchronized DC for 0.5-2 seconds. The induction of VF required about 50% less current and voltage with anodal stimulation. It should be possible to induce VF with the DC voltage available from the internal battery source of an ICD.

Transthoracic defibrillation: effect of dual-pathway sequential pulse shocks and single-pathway biphasic pulse shocks in a canine model.

To determine whether dual-pathway sequential shocks and single-pathway biphasic shocks improved the efficacy of transthoracic defibrillation, we delivered single or sequential truncated waveform shocks of variable duration, voltage, and direction (polarity) to three groups of closed-chest dogs. Dual-pathway sequential shocks were assessed in group 1 (eight animals), biphasic shocks with a single pathway were compared in 11 dogs (group 2), and the effect of varying the duration of the biphasic shocks was assessed in group 3 (four animals). There was no improvement in success rates of the intervention shocks compared with a standard single "control" shock at any energy level. In this experimental model unidirectional or biphasic sequential shocks given over single or dual pathways were not superior to standard single-pulse transthoracic defibrillation.