Effects of Interphase and Interpulse Delays on Tissue Impedance and Pulsed Field Ablation

Abstract

Purpose: High-frequency irreversible electroporation (H-FIRE) is a pulsed field ablation (PFA) technique that employs a series of high-voltage, microseconds-long positive and negative pulses, separated by interphase (d1) and interpulse (d2) delays to non-thermally ablate tissue. Previous experimental and computational data suggest an impact of delays on nerve excitation and electrochemical effects. However, the impact of delays on PFA outcomes, such as change in resistance and ablation generation, has only recently started to be elucidated.

Methods: While recording the applied voltage and currents, we delivered a series of increasing voltages, termed voltage ramps, into tuber and cardiac tissues using both needle electrode pairs and flat plate electrodes. Tissues were stained for metabolic activity to measure irreversible electroporation areas following treatment.

Results: Our findings support previous in vitro data that delays do not significantly affect ablation areas. While there were significant differences in applied current, resistance, and conductivity between different pulse widths at sub-electroporation electric fields, we found no significant differences after inducing electroporation between different delays and pulse widths. Consequently, since delays do not affect ablation areas or local conductivity, the data suggests that delays should not affect the electric field threshold or Joule heating within the tissue.

Conclusion: The findings presented here provide critical insights into electroporation-dependent tissue conductivity changes from H-FIRE with implications for improving H-FIRE parameterization and computational models for treatment planning in cancer and cardiac pulsed field ablation.

Keywords: Cardiac; Delays; High-frequency irreversible electroporation; Impedance; Pulsed field ablation.

Fig. 1

A Experimental Setup. The voltage and current are generated by a (i) custom high-voltage generator and are monitored using an (ii) oscilloscope attached to a (iii) 1000x attenuated high-voltage probe and (iv) 10x attenuated current probe. A (v) high-voltage splitter box directs the current either to the oscilloscope or to the (vi) treated tissue (i.e., potato or heart) with two monopolar NanoKnife^® probes inserted. B Voltage and current for 2 cycles of a representative 2–5-2–5 [positive pulse—interphase delay—negative pulse—interpulse delay], recorded on the oscilloscope. C Representative voltage and current data for 1 voltage ramp. After inserting the probe, the voltage ramp applies a series of 3 bursts, increasing the voltage each ramp. D Resistance measurements for the final applied voltage within a voltage ramp or for that voltage alone, with no preceding ramps. Impedance distribution of E a 1 µs, F a 2 µs, G a 5 µs, and H a 10 µs pulse width H-FIRE waveform with delays varying from 1 µs to 100 µs. (Panel D: n = 8)

Fig. 2

There is not a significant difference in tissue resistance when varying delays. Resistance was measured using 2 monopolar probes with a 1.0 cm center-to-center spacing. Resistance measurement from 50 V/cm to 2000 V/cm for A 1 µs, B 2 µs, C 5 µs, and D 10 µs pulse width H-FIRE waveforms with delays varying from 1 µs to 100 µs. (Panels A–D: n = 8)

Fig. 3

There is only a significant difference in tuber tissue resistance at low applied distance-normalized voltages. A At sub-electroporation thresholds, there is a significant difference for resistances between pulse widths, with higher pulse widths having higher resistances. B As the tissue begins to electroporate, there is still a significant difference between different pulse widths, but the overall resistance significantly drops C at 250 V/cm, there is only a significant difference in resistance between the 1 µs and 10 µs pulse widths. D–J At high applied distance-normalized voltages, there is not a significant difference in resistance between pulse widths. (Panels A–J: n = 24; *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001)

Fig. 4

There is no significant difference when varying delays in ex vivo cardiac tissue. A Two fixed-spacing 5-mm exposure needle electrodes were inserted 10 mm into fresh porcine heart at 4 locations (#1–4) within the left ventricle (LV). Measured resistance for the B 1 µs pulse width and C 10 µs pulse width. D Resistances for 1 and 10 µs at different distance-normalized voltages. E Metabolic 2,3,5-Triphenyl tetrazolium chloride (TTC) staining of cardiac tissue slices ~ 4 hours after treatment with 10 µs pulsed field ablation; red indicates metabolically active tissue. #3 is a 10-1-10-1 treatment and #4 is 10–100-10–100 treatment. F Measured lesion areas for various to 10 µs pulsed field ablation treatments. (Panels B-D: n = 8; Panel F: n ≥ 5; *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001)

Fig. 5

Tissue ablation areas are not affected by interphase and interpulse delays. A Representative image of ablation demarcated with brown melanin formation. B Ablation areas after either voltage ramp and treatment or treatment alone show no significant difference. B Measured ablation areas 10 minutes, 1 hour, 4 hours, and 24 hours after treatment. C Ablation areas for each applied pulse width. Measured ablation areas for different delays with A 1 µs, B 2 µs, C 5 µs, and D 10 µs pulse width H- FIRE waveforms. B, C, and E–H: n = 8; D: n = 24; ***p < 0.001, ****p < 0.0001)

Fig. 6

There is not a significant difference in local tissue conductivity between different delays. A Cylindrical tissue slices were placed between parallel flat plate electrodes to deliver a uniform electric field. Measured tissue conductivity for the B 1 µs pulse width at different delays and C 10 µs pulse width at different delays. D There is a significant difference in measured conductivity at 100 V/cm. E–L There is no significant difference in measured conductivity from 250 to 2000 V/cm applied. (Panels B–C, n = 8; Panels D–L, n = 24; ****p < 0.0001)