Estimation of Irreversible Electroporation (IRE) Electric Field Threshold (EFT), Area and associated Adiabatic Heating (AH) dependent on Pulsed Electric Field (PEF) waveform parameters

Context of Use

Pulsed Field Ablation (PFA) is a novel method for cardiac ablation, relying on irreversible electroporation induced by high-energy pulsed electric fields (PEFs) to create localized lesions in the heart atria. A significant challenge in optimizing PFA treatments is determining the lethal electric field threshold (EFT), which governs ablation volume and varies with PEF waveform parameters. The lack of standardized nonclinical testing methods has left optimal EFTs for cardiac ablation uncertain.

The impact of a range of clinically relevant biphasic pulse parameters on lethal EFT and adiabatic heating (AH) was evaluated on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) monolayers [1]. Cell death areas were assessed using fluorescent dyes and confocal microscopy, while lethal EFTs were quantified through comparison with electric field numerical simulations.

Fitting of these results through machine learning algorithms were used to develop this open-source online calculator.

This calculator receives as input:

• No. of Pulses, number of pulses in 1 train
• Phase duration $[\mu s]$, time duration of one phase composing the biphasic pulse
• PRF $[kHz]$, Pulse Repetition Frequency: inverse of the time elapsed between the beginning of a pulse and the beginning of the following pulse
• Interphase duration $[\mu s]$, time between the two phases with different polarity
• Density $[g/cm^3]$, mass density of the sample under test. Use values from your experimental settings. If measurements are not available consider using literature, e.g, the density database from IT'IS.
• Electrical conductivity $[mS/cm]$, electrical conductivity of the sample under test. Use values from your experimental settings. If measurements are not available, consider using literature, e.g., the dialectric properties database from IT'IS.

And provides as outputs:

• Lethal EFT $[kV/cm]$, lethal Electric Field Threshold estimated from in vitro experiments on hiPSC-CMs
• AD $[mJ/g]$, Absorbed Dose at the lethal EFT computed using analytical formulas from [2], Eq \eqref{eq:AD}
• AH [C], Adiabatic Heating at the lethal EFT computed using analytical formulas from [2], Eq \eqref{eq:AH}
• Cell death area $[mm^2]$, area estimated from lethal EFT and pre-computed numerical modeling using COMSOL Multiphysics and specific to this electrode geometry

Analytical fromulas for adiabatic heating estimation

\begin{equation} AD= \sigma (E^2/\rho) . \tau . 10^6 \label{eq:AD} \end{equation} \begin{equation} AH = AD \times 0.24/1000 \label{eq:AH} \end{equation}

where $\tau = 2tp$ is the pulse duration $[s]$, $E$ is the electric field $[kV/cm]$ at the cell death threshold, $\rho$ is the density $[g/cm^3]$ and $\sigma$ is the electrical conductivity $[mS/cm]$ of the sample under testing.

Limitations of use

While this study provides valuable insights into lethal EFT in hiPSC-CMs and its dependence on various PEF parameters, it has some limitations.

1. The calculator can be used only in the range of parameters studied (i.e., biphasic pulses, phase duration 0.2-10 $\mu s$, interphase delay $1 \mu s$, number of biphasic pulses 50-400, pulse repetition frequency $2-200 kHz$, train number 1). The log-log fitting is carried out to be used only for interpolation. The fit was NOT tested for extrapolation and extension of the range of parameters of the study.
2. The log-log fitting is an attempt to best fit the experimental results by minimizing the residual sum of the squares. It should be noted that this fitting was not designed from physics’ first principles, and a model that explains the intricate dependencies of the lethal EFT based on first principles is yet to be proposed.
3. This method does not take into account the possibility of neuromuscular stimulation, which could potentially influence the choice of specific parameters, such as the PRF.
4. The use of hiPSC-CMs, while minimizing species and biological disparities, does not fully replicate the complex in vivo environment of the heart. Further studies in animal models are necessary to validate the efficacy of the developed open-source calculator in real-world applications.

References

[1]	M. Casciola, T. K. Feaster, M. J. Caiola, D. Keck, and K. Blinova, "Human in vitro assay for irreversible electroporation cardiac ablation," Frontiers in physiology, vol. 13, p. 1064168doi: 10.3389/fphys.2022.1064168
[2]	B. L. Ibey, S. Xiao, K. H. Schoenbach, M. R. Murphy, and A. G. Pakhomov, "Plasma membrane permeabilization by 60- and 600-ns electric pulses is determined by the absorbed dose," Bioelectromagnetics, vol. 30, no. 2, pp. 92-9, Feb 2009, doi: 10.1002/bem.20451.