Jule N. Wetherbee

Comparative efficacy of monophasic and biphasic truncated exponential shocks for nonthoracotomy internal defibrillation in dogs.

Pentobarbital-anesthetized dogs were studied to determine the relative efficacy of monophasic and biphasic truncated exponential shocks employing a nonthoracotomy internal defibrillation pathway that consisted of a right ventricular catheter electrode (cathode) and a subcutaneous chest wall patch electrode (anode). In part 1 of the experiments, six dogs (19.6 +/- 1.1 kg) were utilized. Monophasic pulses of 5, 7.5, 10 and 12.5 ms duration were compared with biphasic pulses of the same total duration. The biphasic pulses had an initial positive phase (P1) followed by a terminal negative phase (P2) with the initial voltage equal for each phase. For each biphasic total pulse width, five relative P1 versus P2 durations were tested (50 and 50%, 75 and 25%, 90 and 10%, 25 and 75%, 10 and 90%). Ventricular fibrillation was induced by alternating current and pulse configurations were tested randomly to determine the minimal voltage and energy for defibrillation (threshold). Biphasic shocks with P1 longer than P2 were associated with significantly lower (p less than 0.01) energy thresholds than were monophasic shocks. Additionally, there was no significant relation between pulse width and voltage or energy thresholds. In part 2 of the experiments, six dogs (20.2 +/- 1.6 kg) were studied. Monophasic shocks were compared with biphasic shocks with P1 versus P2 durations of 75 and 25% and 90 and 10% for total pulse widths of 7.5, 10 and 12.5 ms. Threshold determinations were performed as in part 1. Subsequently, five initial voltages clustered about threshold were randomly tested four times and dose-response curves constructed for each pulse configuration with the use of stepwise logistic regression. Biphasic shocks resulted in significantly lower energy (p less than 0.0001) and voltage (p less than 0.001) requirements than did monophasic shocks.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of left ventricular mass to defibrillation threshold for the implantable defibrillator: A combined clinical and animal study

Defibrillation results when a critical mass of myocardium is depolarized. The relationship between echocardiographic determinations of left ventricular mass, volume, and cavity radius to wall thickness ratio and defibrillation threshold for the implantable defibrillator was examined. Ten patients with two large patch defibrillating lead systems were studied. Defibrillation threshold was determined intraoperatively as the lowest energy terminating ventricular fibrillation. Left ventricular mass, volume, and radius/posterior wall thickness ratio were calculated from two-dimensional echocardiograms. A significant correlation was found between left ventricular mass and defibrillation threshold (r = 0.78, p less than 0.01). The correlations between defibrillation threshold and left ventricular volume (r = 0.59) and radius/wall thickness ratio (r = 0.55) were not significant. Subsequently, 11 dogs undergoing defibrillation trials with a transvenous catheter and a chest wall patch were studied. Defibrillation threshold was defined as the lowest energy-terminating ventricular fibrillation (four separate attempts). Subsequently, the heart was dissected, and the left ventricle (including the septum) was weighed. The correlation between left ventricular weight and defibrillation threshold (r = 0.76) was significant (p less than 0.01). We conclude that noninvasive assessment of left ventricular mass and direct measurement of left ventricular weight are significantly correlated with defibrillation threshold and consistent with the critical mass hypothesis.

Strength‐Duration Curves of Fixed Pulse Width Variable Tilt Truncated Exponential Waveforms for Nonthoracomy Internal Defibrillation in Dogs

Six anesthetized dogs (wgt 19.6 + 1.1 kg) underwent defibrillation trials using truncated monophasic pulses of 2.5—20 msec in duration. The current pathway consisted of a 4 cm2 RV catheter electrode (cathode) and a 13.9 cm2 subcutaneous chest wall patch (anode). Fibrillation was induced by alternating current and defibrillation attemped 10 seconds later. Only one test shock was assessed for each fibrillation episode. The various durations were tested randomly, and the minimum peak voltage and energy resulting in defibrillation was determined for each. Shorter pulse durations were associated with lower energies with pulses of 2.5—15 msec having significantly lower energy thresholds than shocks of 20 msec (P < 0.05). The relationship between duration and voltage threshold is hyperbolic with minimum voltage between 7.5 and 12.5 msec while the shortest and longest pulses were associated with the highest voltage thresholds. Shocks of 5 to 15 msec were associated with significantly lower voltage threshold than 2.5 msec pulses (P < 0.05). The threshold average current (Iav) reached a nadir at 10 msec. Shocks in the midrange of those tested resulted in the best combination of low average current and energy requirements for defibrillation using this nonthoracotomy lead system.

Comparison of monophasic with single and dual capacitor biphasic waveforms for nonthoracotomy canine internal defibrillation.

Monophasic and single capacitor and dual capacitor biphasic truncated exponential shocks were tested in pentobarbital-anesthetized dogs with use of a nonthoracotomy internal defibrillation pathway consisting of a right ventricular catheter electrode and a subcutaneous chest wall patch electrode. Seven dogs weighing 20.2 +/- 0.5 kg were utilized. Monophasic pulses of 10 ms duration were compared with three biphasic pulses. All biphasic waveforms had an initial positive phase (P1) followed by a terminal negative phase (P2) and the total duration of P1 plus P2 was 10 ms. The dual capacitor biphasic waveform (P1 9 ms, P2 1 ms) had equal initial voltages of P1 and P2. Two simulated single capacitor biphasic waveforms were also tested, the first designed to minimize the magnitude of P2 (P1 9 ms, P2 1 ms with initial voltage of P2 equal to 0.3 of the initial voltage of P1) and the second to maximize P2 (P1 5 ms, P2 5 ms with initial voltage of P2 = 0.5 P1). Alternating current was used to induce ventricular fibrillation and four trials of eight initial voltages from 100 to 800 V were performed for each of the four waveforms. Stepwise logistic regression was utilized to construct curves relating probability of successful defibrillation and energy. In the logistic model, the dual capacitor biphasic and single capacitor biphasic waveforms that maximized P2 were associated with significantly (p less than 0.001) lower energy requirements for defibrillation than those of the monophasic waveform. The single capacitor biphasic waveform that minimized P2 was not significantly better than the monophasic waveform.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonthoracotomy internal defibrillation in dogs: Threshold reduction using a subcutaneous chest wall electrode with a transvenous catheter electrode

The efficacy of truncated exponential waveform shocks using a cardioverter-defibrillator catheter with and without a 13.9 cm2 subcutaneous thoracic patch electrode was examined in 10 pentobarbital-anesthetized dogs. The cardioverter-defibrillator catheter was positioned through the external jugular vein with the distal 4 cm2 shocking electrode located in the right ventricular apex and the 8 cm2 proximal electrode located in the superior vena cava. Four electrode configurations were tested: 1) distal electrode (cathode) to proximal electrode and chest wall patch (common anodes), 2) distal electrode (cathode) to chest wall patch (anode), 3) distal electrode (cathode) to proximal electrode (anode), and 4) chest wall patch (cathode) to proximal electrode (anode). The lowest randomized energy resulting in termination of alternating current-induced ventricular fibrillation on four trials at that energy was 20.2, 21.3, 27.4 and greater than 40 J, respectively, for configurations 1 through 4. The energy requirements for configurations 1, 2 and 3 were significantly lower than for configuration 4 (p less than 0.001). Additionally, configurations incorporating the distal electrode and the patch electrode (configurations 1 and 2) were significantly better than the catheter alone (configuration 3; p less than 0.05). There was no significant difference between configurations 1 and 2. In conclusion, the addition of a subcutaneous chest wall electrode to the cardioverter-defibrillator catheter significantly lowered energy requirements for defibrillation, suggesting that a nonthoracotomy approach for the automatic implantable cardioverter-defibrillator is feasible.

Long-term internal cardiac defibrillation threshold stability.

The automatic implantable cardioverter‐defibrillator is tested intraoperatively with defibrillation trials to ensure effectiveness. It is unknown if the energy requirement for internal defibrillation remains stable and that once demonstrated effective, if the device will continue to be effective in terminating lethal ventricular arrhythmias. In this study, the defibrillation energy requirement was compared in 56 patients at the time of lead implantation to that obtained at the time of generator replacement. Mean time to generator replacement was 17. ± 6.6 months. The defibrillation threshold was stable over that time (11. 9 ± 6.7 joules compared to 12.7 ± 8.4 joules, NS). There was no relation between transmyocardial impedance and defibrillation threshold. In addition, no effect on defibrillation threshold was demonstrated by the use of various cardiac medications, concomitant surgery or the occurrence of clinical shocks during follow‐up.

Electrode Polarity Is an Important Determinant of Defibrillation Efficacy Using a Nonthoracotomy System

Experimental and clinical data using epicardial patch electrodes and monophasic waveform suggest that electrode polarity may be an important determinant of defibrillation efficacy. Our objective was to examine the effect of electrode polarity in an animal model using a nonthoracotomy system and monophasic and biphasic waveforms for defibrillation. We examined the effect of lead polarity in 14 pentobarbital anesthetized dogs (21.1 ± 2.4 kg) using monophasic and biphasic shocks and a nonthoracotomy system. Monophasic and single capacitor biphasic shocks of 10‐msec total duration were used. The lead system consisted of a right ventricular catheter electrode with 4‐cm2 surface area and a left chest wall subcutaneous patch electrode with 13.9‐cm2 surface area. Electrode polarities RV(−)‐Patch(+) and RV(+)Patch(−) were tested using both monophasic and biphasic waveforms. Alternating current was used to induce ventricular fibrillation and test shocks were delivered after 10 seconds of ventricular fibrillation. Each polarity configuration for monophasic and biphasic waveforms was tested four times at five different capacitor voltage levels (200–600 V, in 100‐V increments). Defibrillation efficacy curves were constructed using logistic regression analysis for each animal and energies associated with 80% probability of successful defibrillation (E80) were determined. The mean E80 ± SD values were as follows. Monophasic waveform: RV(−)Patch(+) 23.4 ± 7.5 J; RV(+)Patch(−) 20.9 ± 7.9 J(P <0.03). Biphasic waveform: RV(−)Patch(+) 15.8 ± 6.8 J; RV(+)Patch(−) 12.5 ± 6.0 J (P < 0.03). The mean impedance values for both waveforms using either polarity ranged from 65.4 to 67 ohms and were not significantly different. Biphasic waveforms were superior to monophasic (P < 0.01), regardless of lead polarity. For either waveform, reversal of lead polarity in some animals resulted in improved defibrillation efficacy and worsening in others, butasagroup, the RV(+)Patch(−) electrode configuration was superior. Conclusions: These observations suggest that electrode polarity is an important determinant of defibrillation efficacy for nonthoracotomy defibrillation. The optimal electrode configuration cannot be determined a priori, suggesting that alternate polarity configurations should be tested to maximize the defibrillation safety margin.

Comparison of efficacy of automatic implantable cardioverter defibrillator in patients older and younger than 65 years of age

PURPOSE The efficacy of the automatic implantable cardioverter defibrillator (AICD) was compared in elderly patients and younger patients with life-threatening ventricular tachyarrhythmias. Clinical characteristics, surgical complications, and long-term survival rates were compared between the two age groups. PATIENTS AND METHODS A retrospective study was conducted of 54 elderly patients (greater than 65 years) and 79 younger patients (less than 65 years) who had had AICDs implanted for recurrent symptomatic ventricular tachycardia and/or ventricular fibrillation. RESULTS In 85% of elderly patients and 78% of younger patients, coronary artery disease was the underlying disease (NS). The mean left ventricular ejection fraction was 31.4 +/- 14.3% in the elderly patients and 35.7 +/- 17.6% in the younger patients (NS). Concomitant myocardial revascularization was performed in 37% of elderly patients and 29% of younger patients (NS); however, only 4% of elderly patients had concomitant left ventricular resection or cryoablation, compared with 15% of younger patients (p less than 0.001). Two patients in each age group died perioperatively (4% versus 3%, NS), and no significant difference in surgical morbidity or length of hospital stay following AICD implantation was noted between the age groups. In conjunction with AICD, elderly patients more commonly received antiarrhythmic drugs, with 54% of elderly patients taking amiodarone at the time of hospital discharge compared with 29% of the younger patients (p less than 0.008). In contrast, beta-blockers were more commonly used in younger patients (16% versus 2%, p less than 0.03). At a mean follow-up of 25 months, 11 (20%) elderly patients and 16 (20%) younger patients had died. Six deaths in elderly patients and five deaths in younger patients were classified as arrhythmic deaths (NS); however, only one younger patient and three elderly patients died suddenly (NS). Calculated survival curves demonstrated similar survival rates in the two age groups with approximately 90%, 87%, and 80% of the patients alive at 1, 2, and 3 years, respectively. Theoretic survival curves calculated from appropriate AICD shocks demonstrated significantly lower survival compared with actual survival. CONCLUSION It is concluded that AICD is a very effective treatment for life-threatening ventricular tachyarrhythmias, and this benefit applies to elderly patients as well as younger patients.

Long-term complications of implantable cardioverter defibrillator lead systems.

Over a period of 8.5 years in 255 patients with full-system implantable cardioverter defibrillators, lead-specific complications requiring reoperation developed in 32 patients. A total of 36 leads were affected. Lead fracture was the cause of failure in half of these patients. Refinement and improvement in structural designs of these leads, use of endocardial leads, and attention to technical details during implantation should decrease the incidence of these complications.

Comparison of ST segment depression in upright treadmill and supine bicycle exercise testing

Significant differences in the hemodynamic response to upright and supine exercise have been reported in patients with coronary artery disease. The purpose of the present study was to compare the degree of myocardial ischemia as assessed by ST segment depression during upright treadmill and supine bicycle exercise in 98 patients with coronary artery disease and in 34 patients with normal coronary arteries. The amount of ST segment depression at maximal exercise in patients with coronary artery disease was 0.90 +/- 0.80 mm for treadmill and 1.34 +/- 1.09 mm for supine bicycle (p less than 0.001). The amount of ST segment depression during treadmill and supine bicycle exercise tests was also compared at highest similar heart rates (0.68 +/- 0.77 versus 1.17 +/- 1.01, p less than 0.001), at highest similar rate-pressure products (0.71 +/- 0.77 versus 1.08 +/- 1.04, p less than 0.001), at highest similar metabolic equivalents of oxygen consumption (MET) levels (0.69 +/- 0.75 versus 1.20 +/- 1.05 mm, p less than 0.001) and at the onset of angina (0.84 +/- 0.73 versus 1.18 +/- 0.88 mm, p less than 0.001). The rate-pressure product achieved at maximal exercise was similar in both tests (18.74 +/- 5.80 x 10(3) versus 18.81 +/- 5.17 x 10(3), p = NS). The occurrence of angina during treadmill and supine bicycle exercise tests was similar (47 of 98 versus 48 of 98, respectively, p = NS). For the detection of coronary artery disease, the sensitivity was 50.0% for treadmill and 63.3% for supine bicycle (p less than 0.05) and the specificity was 73.5 versus 70.6%, respectively (p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)