Sharon B. Melnick

Defibrillation threshold and cardiac responses using an external biphasic defibrillator with pediatric and adult adhesive patches in pediatric-sized piglets

Before recommendations for using an automatic external defibrillator on pediatric patients can be made, a protocol for the energy of a biphasic waveform energy dosing needs to be determined that will allow ventricular defibrillation of 8 year olds while causing only a minimal amount of cardiac damage to infants. Pediatric- and adult-sized electrode patches were alternately applied to 10 isoflurane-anesthetized piglets weighing 3.8-20.1 kg to approximate the body weights of newborns to children < 8 years old. The defibrillation threshold (DFT) was determined for biphasic truncated exponential waveform shocks. Additional shocks, varying from the DFT to 360 Joules (J), were delivered during sinus rhythm or following 30 s of ventricular fibrillation (VF). The DFT was 2.4+/-0.81 and 2.1+/-0.65 J/kg for pediatric and adult patches, respectively (P = N.S.). The change in left ventricular (LV) dP/dt from baseline as a function of shock strength was significantly different at 1 and 10 s after shocks of increasing energy that were delivered in sinus rhythm, and 1, 10, 20, and 30 s after defibrillation shocks. There was no significant difference in LV dP/dt with increasing shock energy at 60 s with either patch size. The time to return of sinus rhythm, ST-segment deviation, and cardiac output were also not significantly different from baseline 60 s following shocks of up to 360 J delivered during sinus rhythm or VF with either patch. The same amount of energy delivered with a biphasic external defibrillator successfully defibrillated VF whether adult or pediatric patches were used. Cardiac rhythm and hemodynamic variables were unaltered at 60 s after shocks delivered at energies of up to 360 J. These data suggest that there is a substantial safety margin above a DFT strength shock for this biphasic waveform in piglets.

Fiberglass needle electrodes for transmural cardiac mapping

We developed a new method for fabricating plunge needle electrodes for use in cardiac mapping. The needles have 12 electrodes with 1-mm spacing, are 0.5 mm in diameter, and are fabricated from fiberglass reinforced epoxy. They are stiff enough to be easily inserted into beating hearts and durable enough to be reused many times. We found that these new needles elicit smaller, more quickly resolving injury potentials, and when inserted in a row with 2-mm spacing, disrupt ventricular fibrillation activation patterns less than traditional steel needles.

Relative Efficacy of Monophasic and Biphasic Waveforms for Transthoracic Defibrillation After Short and Long Durations of Ventricular Fibrillation

BACKGROUND Recently, interest has arisen in using biphasic waveforms for external defibrillation. Little work has been done, however, in measuring transthoracic defibrillation efficacy after long periods of ventricular fibrillation. In protocol 1, we compared the efficacy of a quasi-sinusoidal biphasic waveform (QSBW), a truncated exponential biphasic waveform (TEBW), and a critically damped sinusoidal monophasic waveform (CDSMW) after 15 seconds of fibrillation. In protocol 2, we compared the efficacy of the more efficacious biphasic waveform from protocol 1, QSBW, with CDSMW after 15 seconds and 5 minutes of fibrillation. METHODS AND RESULTS In protocol 1, 50% success levels, ED50, were measured after 15 seconds of fibrillation for the 3 waveforms in 6 dogs. In protocol 2, defibrillation thresholds were measured for QSBW and CDSMW after 15 seconds of fibrillation and after 3 minutes of unsupported fibrillation followed by 2 minutes of fibrillation with femoral-femoral cross-circulation. In protocol 1, QSBW had a lower ED50, 16.0+/-4.9 J, than TEBW, 20.3+/-4.4 J, or CDSMW, 27.4+/-6.0 J. In protocol 2, QSBW had a lower defibrillation threshold after 15 seconds, 38+/-10 J, and after 5 minutes, 41.5+/-5 J, than CDSMW after 15 seconds, 54+/-19 J, and 5 minutes, 80+/-30 J, of fibrillation. The defibrillation threshold remained statistically the same for QSBW for the 2 fibrillation durations but rose significantly for CDSMW. CONCLUSIONS In this animal model of sudden death and resuscitation, these 2 biphasic waveforms are more efficacious than the CDSMW at short durations of fibrillation. Furthermore, the QSBW is even more efficacious than the CDSMW at longer durations of fibrillation.

Sustained Reentry in the Left Ventricle of Fibrillating Pig Hearts

Abstract— It has been proposed that ventricular fibrillation (VF) is driven by sustained reentry. However, mapping studies have not detected such “mother rotors” in large mammalian hearts. We mapped VF from three 21×12 unipolar electrode arrays in 6 pigs. Two of the arrays were adjacent to each other on the left-ventricular epicardium. Electrode spacing was 2 mm. The third array consisted of 21 needles (0.5-mm diameter, 12 electrodes, 1-mm spacing) inserted in a row (2-mm spacing) between the epicardial arrays. A total of 88 5-second VF epochs were analyzed with automatic reentry detection algorithms. Although intramural reentry was sporadically present (29 total occurrences), it was always short-lived with a mean life span of 127±57 ms. However, in 3 of the 6 animals, sustained epicardial reentry (ie, reentry persisting for more than a few cycles) was consistently present, often lasting for several seconds. For each epoch, we computed indices characterizing (1) the relative duration of reentry on the two epicardial arrays (R), (2) the flow of wavefronts between epicardial arrays (W), and (3) the relative activation rates of the two epicardial arrays (F). R did not correlate with either W or F indicating that rotor-containing regions did not produce a net outflow of wavefronts and were not faster than neighboring regions. Thus, sustained epicardial, but not intramural, rotors were consistently present in some large animal hearts during VF. However, we found no evidence that these rotors were responsible for sustaining VF through the mechanisms outlined in the mother rotor hypothesis.

Comparison of six clinically used external defibrillators in swine

BACKGROUND External defibrillation has long been practiced with two types of monophasic waveforms, and now four biphasic waveforms are also widely available. Although waveforms and clinical dosing protocols differ among defibrillators, no studies have adequately compared performance of the monophasic or the biphasic waveforms. This is the first study to compare defibrillation efficacy among biphasic external defibrillators, and does so as part of a study comparing all commonly available waveforms using their respective manufacturer-provided and clinically used doses. METHODS AND RESULTS Efficacy of six waveforms was tested in 852 short-duration ventricular fibrillation episodes in 14 swine. Protocol 1: 200-J monophasic damped sine (MDS) and monophasic truncated exponential (MTE) shocks were compared to 150-J biphasic shocks in six swine at the low-impedance of these animals. Protocol 2: Four commercially available biphasic defibrillators were compared using their respective manufacturer-recommended dose protocols in eight swine at low and simulated high-impedance. At low-impedance, all biphasic shocks achieved near-perfect success, while efficacy was significantly lower for MDS (67%) and MTE (30%) shocks. In protocol 2, first-shock success rates of the four biphasic defibrillators were uniformly high (97, 100, 100, and 94%) for low-impedance shocks, and decreased for high-impedance shocks (62, 92, 82, and 64%). There were statistically significant differences in efficacy among devices. CONCLUSIONS Commonly used MDS and MTE waveforms provide markedly dissimilar efficacies. Despite impedance-compensation schemes in biphasic defibrillators, impedance has an impact on their efficacy. At high-impedance, modest efficacy differences exist among clinically available biphasic defibrillators, reflecting differences in both waveforms and manufacturer-provided doses.

Evaluation of acute cardiac and chest wall damage after shocks with a subcutaneous implantable cardioverter defibrillator in Swine.

A subcutaneous implantable cardioverter defibrillator (S‐ICD) could ease placement and reduce complications of transvenous ICDs, but requires more energy than transvenous ICDs. Therefore we assessed cardiac and chest wall damage caused by the maximum energy shocks delivered by both types of clinical devices.

Transmural recording of shock potential gradient fields, early postshock activations, and refibrillation episodes associated with external defibrillation of long-duration ventricular fibrillation in swine.

BACKGROUND Knowledge of the shock potential gradient (nablaV) and postshock activation is limited to internal defibrillation of short-duration ventricular fibrillation (SDVF). OBJECTIVE The purpose of this study was to determine these variables after external defibrillation of long-duration VF (LDVF). METHODS In six pigs, 115-20 plunge needles with three to six electrodes each were inserted to record throughout both ventricles. After the chest was closed, the biphasic defibrillation threshold (DFT) was determined after 20 seconds of SDVF with external defibrillation pads. After 7 minutes of LDVF, defibrillation shocks that were less than or equal to the SDVF DFT strength were given. RESULTS For DFT shocks (1632 +/- 429 V), the maximum minus minimum ventricular voltage (160 +/- 100 V) was 9.8% of the shock voltage. Maximum cardiac nablaV (28.7 +/- 17 V/cm) was 4.7 +/- 2.0 times the minimum nablaV (6.2 +/- 3.5 V/cm). Although LDVF did not increase the DFT in five of the six pigs, it significantly lengthened the time to earliest postshock activation following defibrillation (1.6 +/- 2.2 seconds for SDVF and 4.9 +/- 4.3 seconds for LDVF). After LDVF, 1.3 +/- 0.8 episodes of spontaneous refibrillation occurred per animal, but there was no refibrillation after SDVF. CONCLUSION Compared with previous studies of internal defibrillation, during external defibrillation much less of the shock voltage appears across the heart and the shock field is much more even; however, the minimum nablaV is similar. Compared with external defibrillation of SDVF, the biphasic external DFT for LDVF is not increased; however, time to earliest postshock activation triples. Refibrillation is common after LDVF but not after SDVF in these normal hearts, indicating that LDVF by itself can cause refibrillation without requiring preexisting heart disease.

Comparison of Low-Energy Versus High-Energy Biphasic Defibrillation Shocks Following Prolonged Ventricular Fibrillation

Abstract Introduction. Since the initial development of the defibrillator, there has been concern that, while delivery of a large electric shock would stop fibrillation, it would also cause damage to the heart. This concern has been raised again with the development of the biphasic defibrillator. Objective. To compare defibrillation efficacy, postshock cardiac function, and troponin I levels following 150-J and 360-J shocks. Methods. Nineteen swine were anesthetized with isoflurane and instrumented with pressure catheters in the left ventricle, aorta, and right atrium. The animals were fibrillated for 6 minutes, followed by defibrillation with either low-energy (n = 8) or high-energy (n = 11) shocks. After defibrillation, chest compressions were initiated and continued until return of spontaneous circulation (ROSC). Epinephrine, 0.01 mg/kg every 3 minutes, was given for arterial blood pressure < 50 mmHg. Hemodynamic parameters were recorded for four hours. Transthoracic echocardiography was performed and troponin I levels were measured at baseline and four hours following ventricular fibrillation (VF). Results. Survival rates at four hours were not different between the two groups (low-energy, 5 of 8; high-energy, 7 of 11). Results for arterial blood pressure, positive dP/dt (first derivative of pressure measured over time, a measure of left ventricular contractility), and negative dP/dt at the time of lowest arterial blood pressure (ABP) following ROSC were not different between the two groups (p = not significant [NS]), but were lower than at baseline. All hemodynamic measures returned to baseline by four hours. Ejection fractions, stroke volumes, and cardiac outputs were not different between the two groups at four hours. Troponin I levels at four hours were not different between the two groups (12 ± 11 ng/mL versus 21 ± 26 ng/mL, p = NS) but were higher at four hours than at baseline (19 ± 19 ng/mL versus 0.8 ± 0.5 ng/mL, p < 0.05, groups combined). Conclusion. Biphasic 360-J shocks do not cause more cardiac damage than biphasic 150-J shocks in this animal model of prolonged VF and resuscitation.

Effect of a Passive Endocardial Electrode on Defibrillation Efficacy of a Nonthoracotomy Lead System

OBJECTIVES We investigated the impact of an inactive endocardial lead on the 50% effective dose (ED50%) for successful ventricular defibrillation. BACKGROUND The presence of abandoned epicardial mesh patch electrodes detrimentally affects the defibrillation efficacy of an endocardial lead system. It is not known whether abandoned endocardial electrodes produce a similar effect. METHODS An endocardial lead system (ENDOTAK, model 0062, Cardiac Pacemakers, Inc.) was implanted in eight dogs (mean +/- SD weight 23.7 +/- 1.0 kg). The ED50% for each of seven lead configurations was determined by a three-reversal point protocol in a balanced-randomized order with and without a second electrically passive endocardial lead system in the right ventricle (power 0.97 to detect a 50-V difference). Biphasic shocks with 80% tilt were delivered 10 s after the induction of ventricular fibrillation. In one configuration the active electrode made contact with the passive electrode in the right ventricular (RV) apex. In another configuration the active electrode was placed in a more proximal position to avoid contact. Additionally, the ED50% was determined for the endocardial lead system with a passive pacing lead positioned in the RV apex. RESULTS ED50% values for peak voltage, peak current and delivered energy were not significantly different with or without a passive RV electrode, and this was true whether or not the active electrode touched the passive electrode. However, ED50% values were significantly higher when the active electrode was slightly proximal than when it was positioned at the apex. CONCLUSIONS Physical contact between active and passive endocardial electrodes does not significantly alter defibrillation efficacy in this dog model. An increase in ED50% energy was caused by a slightly proximal position. Therefore, a good electrode position within the right ventricle is a more important determinant of defibrillation efficacy than is avoidance of the electrode touching a passive electrode.

Reentry site during fibrillation induction in relation to defibrillation efficacy for different shock waveforms.

Reentry Site and Defibrillation Waveform Efficacy. Introduction: Unsuccessful defibrillation shocks may reinitiate fibrillation by causing postshock reentry.