Improved cardiac cell excitation with symmetrical biphasic defibrillator waveforms

Area of Science:

• Cardiovascular Physiology
• Electrophysiology
• Cell Biology

Background:

• Ventricular defibrillation is hypothesized to result from exciting a critical mass of ventricular cells.
• Monophasic waveforms' defibrillation threshold correlates with cell excitation threshold in models.
• Biphasic waveforms reduce defibrillation thresholds in vivo, but their effect on cell excitation is less understood.

Purpose of the Study:

• To investigate if symmetrical biphasic waveforms decrease cellular excitation thresholds.
• To compare the excitation thresholds of biphasic and monophasic waveforms in a cell culture model.
• To determine the influence of extracellular potassium and pacing rate on waveform effectiveness.

Main Methods:

• Experiments utilized 100- to 250-microns cell aggregates from chick embryos.
• Excitation threshold strength-duration curves were measured for monophasic and biphasic waveforms.
• Transmembrane potentials were recorded during stimulation under varying conditions (potassium concentration, pacing interval).

Main Results:

• Symmetrical biphasic waveforms significantly reduced excitation thresholds compared to monophasic waveforms for durations >3 ms.
• The threshold ratio (biphasic/monophasic) at rheobase was 0.63.
• Excitation occurred during the second phase of biphasic stimulation at subthreshold intensities for monophasic waveforms.
• Biphasic waveform effectiveness increased under simulated 'fibrillation conditions' (high potassium, short pacing interval).

Conclusions:

• Symmetrical biphasic waveforms lower the cellular excitation threshold, supporting their use in defibrillation.
• The reduction in excitation threshold by biphasic waveforms is dependent on extracellular potassium levels and cellular beat rate.
• These findings validate the cell excitation hypothesis for defibrillation and inform waveform optimization.