A direct comparison of epicardial and nonthoracotomy defibrillation using monophasic and biphasic shocks

Aortic input impedance and ventriculoarterial coupling following cardioversion/defibrillation

Defibrillation shocks are commonly used after cardiac surgery or during defibrillator implantation. The hemodynamic consequences of countershocks on circulatory dynamics are not completely understood, and there is a lack of information concerning the effects on ventriculoarterial interaction. The study presented here was performed to assess the influence of defibrillation shocks on arterial hemodynamics and ventriculoarterial coupling. Eight mongrel dogs (weight 15-18 kg) were anesthetized and median thoracotomy was performed. Pressure in the ascending aorta and the left ventricle and flow in the ascending aorta were continuously monitored. After induction of atrial or ventricular fibrillation, termination was achieved by epicardial low energy shocks (atrium, 3J; ventricle, 51). In an additional attempt ventricular fibrillation was terminated by a high energy shock (34J). Aortic input impedance was calculated by fast-Fourier-transformation of aortic flow and pressure signals, while ventriculoarterial coupling was expressed by the ratio of aortic and ventricular end systolic elastance (Ea/Ees). Defibrillation by low energy shocks of atrial or ventricular fibrillation did not result in changes of the aortic impedance spectrum, and ventriculoarterial coupling remained unaltered compared to control conditions. High energy defibrillation, however, resulted in a marked rise in total peripheral resistance (P < 0.03). The ratio of Ea/Ees increased significantly (P < 0.005). These effects were reversible within 15 minutes. Low energy defibrillation does not alter arterial hemodynamics and ventriculoarterial coupling in this experimental setup. High energy defibrillation, however, results in a temporary increase of ventricular load. This finding may be of particular interest in patients with poor left ventricular function.

Biphasic waveforms for ventricular defibrillation: optimization of total pulse and second phase durations

Waveform parameters may affect the efficacy of ventricular defibrillation. Certain biphasic pulse waveforms are more effective for ventricular defibrillation than monophasic waveforms, but the optimal biphasic waveform parameters have not been identified. The purpose of this study was to investigate the effects of total pulse duration and the duration of the second (negative) phase on voltage and energy defibrillation requirements using biphasic waveforms. Defibrillation efficacy was evaluated in an isolated rabbit heart model using the Langendorff technique. The biphasic waveform was a truncated exponential with the initial voltage of the second phase equal to 50% of the final voltage of the first phase. An up/down protocol was used to determine the 50% probability-of-success levels (E50) for delivered energy and initial voltage. First, using pulse waveforms with equal positive and negative phase durations, test waveforms with total durations of 4 ms (2 ms positive + 2 ms negative), 6 ms (3 + 3 ms), and 16 ms (8 + 8 ms) were compared to the control waveform of 8 ms (4 + 4 ms) in 30 experiments. Defibrillation voltage requirements with 4 ms (174 +/- 56 V) were higher (P = 0.001) compared to 8 ms (127 +/- 49 V). Defibrillation voltage requirements for the 6-ms and 16-ms waveforms were similar to the 8-ms control waveform. Delivered energies tended to be higher with the 4-ms waveform. A second series of 40 experiments were performed to compare monophasic (4 + 0 ms) and three asymmetric biphasic waveforms (4 + 2 ms, 4 + 8 ms, and 4 + 16 ms) to the symmetric control waveform (4 + 4 ms). The monophasic (2.15 +/- 1.21 J) and the 4 + 16 ms waveform (1.86 +/- 1.09 J) required higher energies (P < or = 0.05) than the control waveform (1.24 +/- 0.41 J and 0.87 +/- 0.7 J, respectively). The monophasic waveform also resulted in greater voltage requirements (223 +/- 64 V) compared to the control waveform (160 +/- 26 V) (P = 0.02). Energy and voltage requirements were similar for the 4 + 2 ms and 4 + 8 ms waveforms compared to the control. Defibrillation requirements with biphasic waveforms were affected by total and second phase duration. For waveforms with equal phase durations, total durations between 6-16 ms resulted in the lowest values for defibrillation. For waveforms with variable second (negative) phase durations, durations ranging from 50%-200% of the first phase did not affect defibrillation efficacy.

Simulated internal defibrillation in humans using an anatomically realistic three-dimensional finite element model of the thorax

INTRODUCTION

Determination of the optimal electrode configuration during implantable cardioverter defibrillator (ICD) implantation remains largely an empirical process. This study investigated the feasibility of using a finite element model of the thorax to predict clinical defibrillation metrics for internal defibrillation in humans. Computed defibrillation metrics from simulations of three common electrode configurations with a monophasic waveform were compared to pooled metrics for similar electrode and waveform configurations reported in humans.

METHODS AND RESULTS

A three-dimensional finite element model was constructed from CT cross-sections of a human thorax. Myocardial current density distributions for three electrode configurations (epicardial patches, right ventricular [RV] coil/superior vena cava [SVC] coil, RV coil/SVC coil/subcutaneous patch) and a truncated monophasic pulse with a 65% tilt were simulated. Assuming an inexcitability threshold of 25 mA/cm2 (10 V/cm) and a 75% critical mass criterion for successful defibrillation, defibrillation metrics (interelectrode impedance, defibrillation threshold current, voltage, and energy) were calculated for each electrode simulation. Values of these metrics were within 1 SD of sample-size weighted means for the corresponding metrics determined for similar electrode configurations and waveforms reported in human clinical studies. Simulated myocardial current density distributions suggest that variations in current distribution and uniformity partially explain differences in defibrillation energy requirements between electrode configurations.

CONCLUSION

Anatomically realistic three-dimensional finite element modeling can closely simulate internal defibrillation in humans. This may prove useful for characterizing patient-specific factors that influence clinically relevant properties of current density distributions and defibrillation energy requirements of various ICD electrode configurations.