Ann M. Donohoo

Large Change in Voltage at Phase Reversal Improves Biphasic Defibrillation Thresholds Parallel-Series Mode Switching

BACKGROUND Multiple factors contribute to an improved defibrillation threshold of biphasic shocks. The leading-edge voltage of the second phase may be an important factor in reducing the defibrillation threshold. METHODS AND RESULTS We tested two experimental biphasic waveforms with large voltage changes at phase reversal. The phase 2 leading-edge voltage was twice the phase 1 trailing-edge voltage. This large voltage change was achieved by switching two capacitors from parallel to series mode at phase reversal. Two capacitors were tested (60/15 microfarads [microF] and 90/22.5 microF) and compared with two control biphasic waveforms for which the phase 1 trailing-edge voltage equaled the phase 2 leading-edge voltage. The control waveforms were incorporated into clinical (135/135 microF) or investigational devices (90/90 microF). Defibrillation threshold parameters were evaluated in eight anesthetized pigs by use of a nonthoracotomy transvenous lead to a can electrode system. The stored energy at the defibrillation threshold (ion joules) was 8.2 +/- 1.5 for 60/15 microF (P < .01 versus 135/135 microF and 90/90 microF), 8.8 +/- 2.4 for 90/22.5 microF (P < .01 versus 135/135 microF and 90/90 microF), 12.5 +/- 3.4 for 135/135 microF, and 12.6 +/- 2.6 for 90/90 microF. CONCLUSIONS The biphasic waveform with large voltage changes at phase reversal caused by parallel-series mode switching appeared to improve the ventricular defibrillation threshold in a pig model compared with a currently available biphasic waveform. The 60/15-microF capacitor performed as well as the 90/ 22.5-microF capacitor in the experimental waveform. Thus, smaller capacitors may allow reduction in device size without sacrificing defibrillation threshold energy requirements.

Sawtooth first phase biphasic defibrillation waveform: a comparison with standard waveform in clinical devices.

“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.

Effects of Polarity on Defibrillation Thresholds Using a Biphasic Waveform in a Hot Can Electrode System

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.

Energy steering of biphasic waveforms using a transvenous three electrode system.

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.

External Exponential Biphasic Versus Monophasic Shock Waveform: Efficacy in Ventricular Fibrillation of Longer Duration

Ventricular fibrillation (VF) duration may be a factor in determining the defibrillation energy for successful defibrillation. Exponential biphasic waveforms have been shown to defibrillate with less energy than do monophasic waveforms when used for external defibrillation. However, it is unknown whether this advantage persists with longer VF duration. We tested the hypothesis that exponential biphasic waveforms have lower defibrillation energy as compared to exponential monophasic waveforms even with longer VF duration up to 1 minute. In a swine model of external defibrillation (n = 12, 35 ± 6 kg), we determined the stored energy at 50% defibrillation success (E50) after both 10 seconds and 1 minute of VF duration. A single exponential monophasic (M) and two exponential biphasic (B1 and B2) waveforms were tested with the following characteristics: M (60 μF, 70% tilt), B1 (60/60 μF, 70% tilt/3 ms pulse width), and B2 (60/20 μF, 70% tilt/3 ms pulse width) where the ratio of the phase 2 leading edge voltage to that of phase 1 was 0.5 for B1 and 1.0 for B2. E50 was measured by a Bayesian technique with a total of ten defibriilation shocks in each waveform and VF duration randomly. The E50 (J) for M, B1, and B2 were 131 ± 41, 57 ± 18* and 60 ± 26* with 10 seconds of VF duration, respectively, and 114 ± 62, 77 ± 45* and 72 ± 53* with 1 minute of VF duration, respectively (*P < 0.05 vs M). There was no significant difference in the E50 between 10 seconds and 1 minute of VF durations for each waveform. We conclude that (1) the E50 does not significantly increase with lengthening VF durations up to 1 minute regardless of the shock waveform, and (2) external exponential biphasic shocks are more effective than monophasic waveforms even with longer VF durations.

EFFECTS OF RESPIRATION PHASE ON VENTRICULAR DEFIBRILLATION THRESHOLD IN A HOT CAN ELECTRODE SYSTEM

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.

Dual Level Sensing Significantly Improves Automatic Threshold Control for R Wave Sensing in Implantable Defibrillators

ICDs must sense R waves over a range of amplitudes without sensing P or T waves. Automatic threshold control (ATC) is an accepted sensing method for that task. ATC sensing levels are from 25%‐75% of the electrogram (EGM) peak, decreasing with an exponential decay. A high sensing level for a time after peak detection may better allow ATC to pass over a T wave, while a lower sensing level thereafter may better allow ATC to sense the next R wave. An A TC was designed with two sensing levels and time constants (T), using a 58% level (T = 1,75 s) for 325 ms after peak detection switching to 33% (T = 1.1 sj thereafter, and was compared to a single level ATC (sensing level = 50%, T = 1.4 s). The two ATC circuits were tested with 22 arrhythmia EGMs to determine sensitivity and specificity rates at ± 1‐, 2‐, 5‐, 10‐, and 20‐mV amplitudes. It was confirmed that a dual level ATC significantly improves the sensitivity rate without degrading the high specificity rate of a standard sensing circuit.