ICD Capacitors and Defibrillation

ICD Capacitors

  • The IBHRE exam requires basic knowledge of electrical components and their function.

  • Electrical engineering is a complex field, so this is an introductory overview.

Capacitor

  • A capacitor stores electrical charge on closely spaced conductors (plates).
  • When voltage is applied, electric charges of equal magnitude but opposite polarity accumulate on each plate.
  • A capacitor can be visualized as a "tank full of electrons."
Capacitance (C)
  • Capacitance measures the amount of electric charge (Q) stored (or separated) for a given electric potential.
  • Formula: Capacitance=charge/voltageCapacitance = charge/voltage or C=Q/VC = Q/V
  • Where:
    • C = Capacitance
    • Q = Charge
    • V = Voltage
  • The farad is the unit of capacitance, defined as the capacitance for which a potential difference of one volt results in a static charge of one coulomb.
  • ICD capacitors are generally rated between 90 and 150 microfarads.
Dielectric
  • A dielectric (electrical insulator) is a substance highly resistant to electric current.

Short Biphasic Pulses and Defibrillation Threshold

  • Study: Short Biphasic Pulses from 90 Microfarad Capacitors Lower Defibrillation Threshold by Charles D. Swerdlow et al. (PACE 1996; 19:1053-1060).
  • Theoretical models suggest that smaller output capacitors may result in lower defibrillation thresholds (DFTs) compared to the 120-150 µF capacitors commonly used in ICDs.
  • Capacitors around 90 µF may balance the benefits of smaller capacitors with the need for high voltages.
  • The study compared DFTs for:
    • 120 µF -65% tilt pulses
    • 90 µF -65% tilt pulses
    • 90 µF -50% tilt pulses
  • The 90 µF -50% tilt pulse had half the duration of the 120 µF -65% tilt pulse.
  • The time constant of ICD pulses is the product of the defibrillation pathway's resistance and the output capacitor's capacitance.
Implications
  • Smaller capacitors require greater voltage to deliver an equal charge to the tissue.
  • Higher voltage results in faster charge delivery (voltage = electrical pressure).
  • Faster discharge is closer to the "membrane time constant" and thus more effective.
  • Example:
    • A 155 µF capacitor might require 600 volts to deliver 29 joules.
    • An 86 µF capacitor might require 830 volts to deliver 30 joules.
  • Because the 86 µF capacitor has a higher voltage, the charge is delivered more quickly.

Mark Kroll, PhD - Internal Communication

  • Compares an optimal waveform to the waveform used in the MADIT 2 trial.
  • Optimal waveform:
    • Peak voltage: 830 volts
    • Phase one duration: 4 ms
    • Phase two duration: 2 ms
  • MADIT 2 waveform:
    • Low voltage: ~600 volts
    • Fixed durations with a 60% phase one tilt and a 50% phase two tilt.
    • For a 50 ohm patient, durations are 7 and 6 ms respectively (approximately twice as long as optimal).
Defibrillation Efficacy
  • A 21 joule output from an 830 V device can deliver equivalent defibrillating efficacy compared to a 27 J output from a 600 volt unit.
  • The optimized waveform's effectiveness can be confidently calculated based on cellular, animal, and human research.

Recommendation

  • Become familiar with electronic principles as they apply to pacing and defibrillation.