Is there an optimal capacitance for defibrillation?

Implantable ultrasonic imaging assembly for automated monitoring of internal organs

An implantable miniaturized imaging device can be attractive in many clinical applications. They include automated, periodic, high-resolution monitoring of susceptible organs for early detection of an anomalous growth. In this paper, we propose an implantable ultrasonic imager capable of online high-resolution imaging of a region inside the body. A feasibility analysis is presented, with respect to design of such a system and its application to online monitoring of tumor growth in deep internal organs. We use ultrasound (US) imaging technology, as it is safe, low-cost, can be easily miniaturized, and amenable for long-term, point-of-care (POC) monitoring. The design space of the proposed system has been explored including form factor, transducer specifications and power/energy requirements. We have analyzed the effectiveness of the system in timely detection of anomalous growth in a case study through software simulations using a widely-accepted ultrasonic platform (Field II). Finally, through experimental studies using medical grade phantoms and an ultrasound scanner, we have evaluated the system with respect to its major imaging characteristics. It is observed that interstitial imaging under area/power constraints would achieve significantly better imaging quality in terms of contrast sensitivity and spatial resolution than existing techniques in deep, internal body parts, while maintaining the automated monitoring advantages.

Comparison between two defibrillation waveforms

Two defibrillation waveforms, the chopped biphasic pulses and the constant current pulses, were assessed and compared. Two indices are introduced. The first one is the ratio between the delivered energy W and the energy W(0) of a rectangular pulse with the same duration and electric charge. The second index η(C) = W(0)/W(C0) stands for the level of utilizing the initially loaded capacitor energy W(C0). Some design considerations are also discussed. Another aspect of the study is the choice of appropriate capacitor for pulse generation. The results obtained show that there is no outstanding optimal waveform. The W/W(0) ratio is higher for the known constant current shapes but specifically with a patient resistance lower than 80 Ω. On the other hand, the implementation of these shapes would face several difficulties. The chopped biphasic waveforms are obtained by relatively simple technical solutions leading to very small in size and weight portable instruments.

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

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.