Extended charge banking model of dual path shocks for implantable cardioverter defibrillators

Abstract

Background: Single path defibrillation shock methods have been improved through the use of the Charge Banking Model of defibrillation, which predicts the response of the heart to shocks as a simple resistor-capacitor (RC) circuit. While dual path defibrillation configurations have significantly reduced defibrillation thresholds, improvements to dual path defibrillation techniques have been limited to experimental observations without a practical model to aid in improving dual path defibrillation techniques.

Methods: The Charge Banking Model has been extended into a new Extended Charge Banking Model of defibrillation that represents small sections of the heart as separate RC circuits, uses a weighting factor based on published defibrillation shock field gradient measures, and implements a critical mass criteria to predict the relative efficacy of single and dual path defibrillation shocks.

Results: The new model reproduced the results from several published experimental protocols that demonstrated the relative efficacy of dual path defibrillation shocks. The model predicts that time between phases or pulses of dual path defibrillation shock configurations should be minimized to maximize shock efficacy.

Discussion: Through this approach the Extended Charge Banking Model predictions may be used to improve dual path and multi-pulse defibrillation techniques, which have been shown experimentally to lower defibrillation thresholds substantially. The new model may be a useful tool to help in further improving dual path and multiple pulse defibrillation techniques by predicting optimal pulse durations and shock timing parameters.

Figure 1

Overview of the Extended Charge Banking Model for dual-path defibrillation. (A) The Charge Banking Model estimates the response of the heart to a shock as an RC circuit model. (B) The effect of a shock on many small sections of the heart may be estimated by modeling the response of each section as a separate RC circuit. (C) A weighting function approximating the current distribution through the heart for single path and dual path shocks is combined with the sectioned heart model to predict the response of the whole heart to a shock. See Figures 3 and 4 for additional labels and text for explanation. (D) Successful defibrillation is predicted by determining the effectiveness of single and dual path shocks in causing a minimum threshold model voltage in a critical mass of the heart. See Figure 5 for coordinate labels and text for further explanation.

Figure 2

Responses of tissue receiving varying strength shocks from a sequentially pulsed waveform delivered between two electrode sets (A and B) as the shock strengths for each pulse is varied from 0 to 100% of full strength. The normalized sequentially pulsed waveform is shown on the top right. The maximum response values for each combination of Shock A and Shock B are shown. Examples of the model response to Shock A, Shock B, and the added responses are shown for 3 combinations of shock strength.

Figure 3

Weighting function based on published field gradient measurements. a) Tissue receiving 20% of the shock received the highest weighting (1) while tissue receiving 10% and 100% of the shock received fixed weightings of 0.1. The cumulative weighting distribution (b) demonstrates that while approximately 80% of the tissue receives at least 24% of the full shock strength, only 20% of the tissue receives 60% of the full shock strength.

Figure 4

Alternate three dimensional views of the switched dual path shock waveform weighting function. The side view (b) shows the same shape as the control waveform weighting function. The bottom figure (c) shows a contour map of the weighting function.

Figure 5

Critical mass criteria combined with a control shock weighting function. (A) For a single path shock and a critical mass of 80%, or for 80% of the area under the curve to achieve threshold, tissue that receives 24% of the maximum shock strength must reach threshold for the shock to successfully defibrillate. The area under the dashed line represents the 20% of tissue that would have a subthreshold response to a shock delivered at the DFT. (B) Critical mass criteria combined with a switched shock weighting function. For a critical mass of 80%, the combined response of the tissue must be at least 31% of the combined maximum response for the shock to successfully defibrillate. The removed section represents the 20% of tissue with a response of less than 31% of the maximum response. (C) A contour map of the dual path weighting function (shown in B) with the lowest model voltage 20% response is shown.

Figure 6

Comparison of Extended Charge Banking Model to experimental results to experimental findings. The new model results are compared against Dosdall et al.[36] and Reighard et al.[72] for sequentially pulsed waveforms in terms of leading edge voltage (A) and delivered energy (B). The model results are compared against the findings of KenKnight et al.[31], which consisted of a biphasic waveform preceded by an auxiliary shock from an LV electrode with 1–20 ms between the shocks in terms of leading edge voltage (C) and delivered energy (C). See text for further description of electrode configuration and waveform used.