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.

Sequential overlapping pulse shock waveforms facilitate defibrillation compared with single pulse shocks of the same total energy. This is due at least in part to the changing orientation of the electrical vector during the multiple pulse shock.

Wider applicability of an implantable automatic defibrillator depends on achieving internal cardiac defibrillation consistently with the lowest possible energy. In animal studies, we have found that the cardiac defibrillation threshold could be reduced when sequential shocks separated in time and spacially arranged were delivered to the heart. We compared internal cardiac defibrillation using a single pulse shock delivered through an intravascular catheter with this new method for internal cardiac defibrillation in patients undergoing cardiac surgery for the correction of arrhythmias. For the single pulse shock and the first pulse of the sequential pulse shock, current was passed through an intravascular catheter with the catheter cathode at the apex of the right ventricle and the anode at the superior vena cava-atrial junction region. The second pulse of the sequential pulse countershock was delivered between the catheter cathode in the right ventricular apex and an oval plaque electrode secured on the laterobasal left ventricular epicardium as anode. With the single pulse alone for shock delivery, 12 patients could be defibrillated with an average of 20.1 ± 16.8 J, with a corresponding leading-edge peak voltage and current of 836 + 319 V and 9.4 + 4.5 A, respectively. However, two of the patients could not be defibrillated with energies below 50 J. With the sequential pulse shock delivery, a significant reduction in all values were recorded. Mean total energy for defibrillation averaged 7.7 + 6.0 J. Leading-edge peak voltage and current from the catheter averaged 430 + 148 V and 5.0 ± 2.8 A, respectively. In addition, all patients could be defibrillated with less than 23 J, and nine of the 12 patients (75%) could be defibrillated with less than 7.5 J. In contrast, only two of these same 12 patients (17%) could be defibrillated with less than 7.5 J using the single pulse alone (binomial exact test, p = .0156). We conclude that sequential pulse defibrillation provides a pronounced reduction in the total energy necessary for defibrillation compared with the single pulse delivered through a catheter alone. Furthermore, the sequential pulse system provides a reduction in the current density at the electrodes, potentially reducing myocardial damage. This system may be important in the design of a totally implantable automatic defibrillator. Circulation 73, No. 3, 484-491, 1986. THE DEVELOPMENT of the automatic internal defibrillator by Mirowski and co-workers'-' has provided the impetus for a search for the optimum electrode system4 5 for internal cardiac defibrillation. We

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.

Objective-To compare the defibrillation eYcacy of a novel lead system placed in the middle cardiac vein with a conventional non-thoracotomy lead system. Methods-In eight pigs (weighing 35-71 kg), an electrode was advanced transvenously to the right ventricular apex (RV), with the proximal electrode in the superior caval vein (SCV). Middle cardiac vein (MCV) angiography was used to delineate the anatomy before a three electrode system (length 2 × 25 mm + 1 × 50 mm) was positioned in the vein. An active housing (AH) electrode was implanted in the left pectoral region. Ventricular fibrillation was induced and biphasic shocks were delivered by an external defibrillator. The defibrillation threshold was measured and the electrode configurations randomised to: RV→AH, RV+MCV→AH, MCV→AH, and RV→SCV+AH. Results-For these configurations, mean (SD) defibrillation thresholds were 27.3 (9.6) J, 11.9 (2.9) J, 15.2 (4.3) J, and 21.8 (9.3) J, respectively. Both electrode configurations incorporating the MCV had defibrillation thresholds that were significantly less than those observed with the RV→AH (p < 0.001) and RV→SCV+AH (p < 0.05) configurations. Necropsy dissection showed that the MCV drained into the coronary sinus at a location close to its orifice (mean distance = 2.7 (2.2) mm). The MCV bifurcated into two main branches that drained the right and left ventricles, the left branch being the dominant vessel in the majority (6/7) of cases. Conclusions-Placement of specialised defibrillation electrodes within the middle cardiac vein provides more eVective defibrillation than a conventional tight ventricular lead. (Heart 2000;84:425-430)

Defibrillation in the middle cardiac vein (MCV) has been shown to reduce ventricular defibrillation thresholds (DFTs). Low amplitude auxiliary shock (AS) from an electrode sutured to the left ventricle at thoracotomy have also been shown to reduce DFT if delivered immediately prior to a biphasic shock (between the ventricular RV and superior vena caval (SVC) electrodes). This study investigates the impact on DFT of an AS shock from a transvenously placed MCV lead system. A standard defibrillation electrode was positioned in the RV in eight anesthetized pigs (35-43 kg). A 50 x 1.8-mm electrode was inserted in the MCV through an 8 Fr angioplasty guide catheter. A 150-V (leading edge) monophasic AS was delivered (95 microF capacitor) from the MCV-->Can with three different pulse widths (3, 5, 7 ms). A primary biphasic shock (PS) (95 microF capacitor, phase 1: 44% tilt, 1.6-ms extension and phase 2: 2.5-ms fixed duration) was delivered from the RV-->Can +/- AS. The four configurations were randomized and DFTs (PS + AS) assessed using a modified binary search. Ventricular fibrillation (VF) was induced with 60 Hz AC followed 10 seconds later by the test shock. The DFTs were compared using repeated measures analysis of variance (ANOVA). All configurations incorporating AS produced significant (P < 0.05) reduction in the DFT compared to no AS (13.8 +/- 7.4 J). There was no difference in the efficacy of differing pulse widths (P > 0.05); 3 ms (11.0 +/- 5.4 J), 5 ms (11.5 +/- 6.0), and 7 ms (10.6 +/- 5.3 J). In conclusion, delivering an AS from a transvenous lead system deployed in the MCV reduces the DFT by 23% compared to a conventional RV-->Can shock alone.

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.

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.