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Notes: Capicitance for Biphasic Waveform. Introduction: Current implantable cardioverter defibrillators (ICDs) use relatively large capacitance values. Theoretical considerations suggest, however, that improved defibrillation energy requirements may he obtained with smaller capacitance values. Methods and Results: We compared the energy requirement for defibrillation in a porcine model using a biphasic waveform generated from two capacitance values of 140 μF and 85 μ K. Phase 1 reversal of the shock waveform occurred at 65% tilt. Phase 2 pulse width was equal to phase 1. Shocks were delivered through epicardial patch electrodes after 10 seconds of induced ventricular fibrillation. The defibrillation threshold (DFT) was determined by a “down-up” technique requiring three reversals of defibrillation success or failure. The DFT was defined as the average of the values obtained with all trials starting from the successful shock prior to the first failure to defibrillate to the last successful defibrillation. In eight experiments, the measured parameters at DFT were as follows. The average stored and delivered DFT energies for the 85 μF capacitor were 6.1 ± 2.1 and 6.0 ± 2.0 J, respectively, compared to 7.5 ± 1.3 and 7.4 ± 1.3 J for the 140 μF capacitor (P 〈 0.04). The phase 1 pulse widths were significantly shorter for the 85 μF capacitor (5.1 ± 0.8 msec vs 9.2 ± 1.3 msec) and the impedances were lower (54.4 ± 5.8 Ω vs 59.9 ± 6.3 Ω). The mean leading edge voltage was trending higher for the 85 μF capacitor, but this difference did not reach statistical significance (374± 63 V vs 326 ± 30 V; P =0.055). Conclusion: Smaller capacitance values do result in lower energy requirements for the biphasic waveform, at a possibly higher leading edge voltage and a much shorter pulse width. Smaller capacitance values could represent a significant enhancement of well-established benefits demonstrated with the biphasic waveform.

Notes: The shape of the shock waveform influences defibrillation efficacy. However, the optimal combination between capacitance size and truncation/tilt which can determine monophasic waveform's shape, has not been determined for external defibrillation. The purpose of this study was to assess the effects of varying capacitance and tilt on external defibrillation using exponential monophasic waveforms. In a pig model of external defibrillation (n = 10, 30 ± 6 kg), nine exponential monophasic waveforms combining three capacitance values (30 μF, 60 μF, and 120 μF) and three tilt values (55%, 75%, and 95%) were tested randomly. The energy and leading edge voltage at 50% defibrillation success (E50 and V50) were used to evaluate defibrillation efficacy. E50 and V50 were determined by the Bayesian technique. The lowest stored E50 for the 30μF, 60 μF, and 120 μF waveforms were 90 ± 12 J (95% tilt), 106 ± 45 J (55% tilt), and 107 ± 52 J (75% tilt), respectively. The lowest V50 for the 30 μF, 60 μF, and 120 μF waveforms were 2,439 ± 166 V (95% tilt), 1,849 ± 375 V (55% tilt), and 1,301 ± 322 V (75% tilt), respectively. The average current at external defibrillation threshold demonstrated a strength versus pulse duration relationship similar to that seen with pacing. Reducing capacitance has the same effect as truncating the waveform. The E50 is more sensitive to tilt values changes in larger capacitance waveforms. This study suggests that the optimal combination between capacitance and tilt may be 120 μF and 55%–75% for external defibrillation. (PACE 2003; 26:2213–2218)

Notes: The optimal electrode configuration for endocardial defibrillation is still a matter of debate. Current data suggests that a two pathway configuration using the right ventricle (RV) as cathode and a common anode constituted by a superior vena cava (SVC) and a pectoral can (C) is the most effective combination. This may be related to the more uniform voltage gradient created by shocks delivered using this configuration. We hypothesized that more effective waveforms could be obtained by varying the distribution of the shock current between the two pathways of a three electrode endocardial defibrillation system. In 12 pigs, we compared the characteristics and the defibrillation efficacy of six biphasic waveforms discharged using either a two (RV → C) or a three (RV → SVC + C) electrode combination with the following configurations: Configuration 1 (W1): the RV apical coil was used as a cathode and the subcutaneous C as anode (RV → C). Configuration 2 (W2): The RV was used as cathode and the combination of the atriocaval coil (SVC) and the subcutaneous C as anode (RV → SVC + C). Configuration 3 (W3): The RV → C was used for the first 25% off + and RV → SVC + C for the remainder of the discharge including f 2. Configuration 4 (W4): The RV → C was used for the first 50% off + and RV → SVC + C for the remainder of the discharge including f 2. Configuration 5 (W5): The R V → C was used for the first 75% off + and RV → SVC + C for the remainder of the discharge including f 2. Configuration 6 (W6): The RV → C was used for f + and RV → SVC + C for f2. As an increasing fraction of the waveform was discharged using the RV → SVC + C pathways, the impedance and the pulse width decreased while the tilt, the peak, and the average current significantly increased. The waveforms delivered using the RV → SVC + C configuration for 100% or 75% of their duration had significantly lower stored energy DFT than the other waveform. Current distribution between three endocardial electrodes can be altered during the shock and generates waveforms with different characteristics. Shocks with 75% or more of the current flowing to the RV → SVC + C required the lowest stored energy to defibrillate. This method of energy steering could be used to optimize current delivery in a three electrodes system.

Notes: The impedance of defibrillation pathways is an important determinant of ventricular defibrillation efficacy. The hypothesis in this study was that the respiration phase (end-inspiration versus end-expiration) mayalter impedance and/or defibrillation efficacy in a “hot can” electrode system. Defibrillation threshold (DFT) parameters were evaluated at end-expiration and at end-inspiration phases in random order by a biphasic waveform in ten anesthetized pigs (body weight: 19.1 ±2.4 kg; heart weight: 97 ± 10g). Pigs were intubated with a cuffed endotracheal tube and ventilated through a Drager SAVrespirator with tidal volume of 400–500 mL. A transvenous defibrillation lead (6 cm long, 6.5 Fr) was inserted into the right ventricular apex. A titanium can electrode (92-cm2 surface area) was placed in the left pectoral area. The right ventricular lead was the anode for the first phase and the cathode for the second phase. The DFT was determined by a “down-up down-up” protocol. Statistical analysis was performed with a Wilcoxon matched pair test. The median impedance at DFT for expiration and inspiration phases were 37.8 ±3.1 Ω and 39.3 ± 3.6 Ω, respectively (P = 0.02). The stored energy at DFT for expiration and inspiration phases were 5.7 ± 1.9 J and 6.0 ± 1.0 J, respectively (P = 0.594). Shocks delivered at end-inspiration exhibited a statistically significant increase in electrode impedance in a “hot can” electrode system. The finding that DFT energy was not significantly different at both respiration phases indicates that respiration phase does not significantly affect defibrillation energy requirements.

Notes: The polarity of a monophasic and biphasic shocks have been reported to influence DFTs in some studies. The purpose of this study was to evaluate the effect of the first phase polarity on the DFTofa biphasic shock utilizing a nonthoracotomy “hot can” electrode configuration which had a 90-μF capacitance. We tested the hypothesis that anodal first phase was more effective than cathodal ones for defibrillation using biphasic shocks in ten anesthetized pigs weighing 38.9 ± 3.9 kg. The lead system consisted of a right ventricular catheter electrode with a surface area of 2.7 cm2 and a left pectoral “hot can” electrode with 92.9 cm2 surface area. DFT was determined using a repeated “down-up” technique. A shock was tested 10 seconds after initiation of ventricular fibrillation. The mean delivered energy at DFT was 11.2 ± 1.7 J when using the right ventricular apex electrode as the cathode and 11.3 ± 1.2 J (P = NS) when using it as the anode. The peak voltage at DFT was also not significantly different (529.0 ± 41.3 and 531.8 ± 28.6 V, respectively). We concluded that the first phase polarity of a biphasic shock used with a nonthoracotomy “hot can” electrode configuration did not affect DFT.

Notes: “Sawtooth First Phase” Biphasic Waveform. Introduction: A major limitation in a conventional truncated exponential waveform is the rapid drop in current that results in short duration of high current or longer duration with a lower average current. We hypothesized that increasing the first phase average current by boosting the decaying waveform prior to phase reversal may improve defibrillation efficacy. Methods and Results: To better simulate a “rectangular” waveform during the first phase, a “sawtooth” defibrillation waveform was constructed using “parallel-series” switching of capacitances (each 30 μF) during the first phase. This permitted a boost in the voltage late in the first phase. This sawtooth hiphasic waveform (sawtooth) was compared to two clinical waveforms: a 135-μF capacitance (control-1) and a 90-μF capacitance (control-2) waveform. Defibrillation threshold (DFT) parameters were evaluated in 13 anesthetized pig models using a system consisting of a transvenous right ventricular apex lead (anode) and a left pectoral ldquo;hot can” electrode (cathode) system. DFT was determined by a “down-up down-up” protocol. The stored energy for sawtooth, control-1, and control-2 was 10.5 ± 2.8 J, 12.3 ± 3.7 J*, and 12.2 ± 2.8 J*, respectively (*P 〈 0.01 vs sawtooth). The average current of the first phase for sawtooth, control-1, and control-2 was 7.6 ± 1.3 A, 4.7 ± 0.9 A*, and 6.2 ± 0.9 A*, respectively (*P = 0.0001 vs sawtooth). Conclusion: A sawtooth biphasic waveform utilizing a “parallel-series” switching system of smaller capacitors can improve defibrillation efficacy. A higher average current in the first phase generated by such a waveform may contribute to more efficient defibrillation by facilitating myocyte capture.