Irreversible electroporation (IRE) is a non-thermal ablation technology that creating irreversible nanoscale pores in the cell membrane and leading to cell death through apoptosis [1]. Based on the application of high voltage and short monopolar pulses with two electrodes placed across the target site, IRE has been implemented to ablate primary and metastatic hepatic or pancreatic tumors near thermosensitive structures. IRE was known to cause muscle contractions during treatment. Sedative and neuroparalytic agents should be used in patients undergoing invasive mechanical ventilation [2]. Moreover, IRE adhere to the heart can be cardiac arrhythmia when the electrical pulse delivered outside of the absolute refractory period of the cardiac cycle [3, 4]. Thus, synchronization was recommended to use during the operation of IRE in case of fatal cardiac arrhythmia [5, 6].

High-Frequency Irreversible Electroporation (H-FIRE) is the next-generation nanoknife ablation technology to conquer many of the technical barriers of conventional IRE as mentioned above [7]. Previous studies have showed that H-FIRE, which delivered ultra-short bipolar electrical pulses, minimized the stimulation of nerve and muscle. There is widespread speculation that H-FIRE could be used without ECG monitoring and taking no account of synchronization with R wave on ECG [8]. However, it is well-known that the principle of electroporation is permeabilization of the cell membrane around the electrode. The pore produced by electroporation affected the transportation of ion across the cell membrane. In reality, whether H-FIRE affects the cardiac electrophysiology is still unknown.

There are two wave-forms of H-FIRE. In theory, asymmetric waveform is more effective than symmetrical waveform [9]. Es et al. had demonstrated an asymmetric waveform for H-FIRE creates deeper lesions than symmetric waveform of the same energy and frequency [10]. However, the cardiac safety did not evaluate. The purpose of the present study is to evaluate cardiac safety in vivo with asymmetric waveform of H-FIRE in contrast to the conventional IRE.

In the present study, we evaluated the cardiac safety of a novel asymmetrical waveform for H-FIRE, and compared it to IRE of the equivalent energy. In this study, the ablation efficacy of H-FIRE was similar to IRE of liver. IRE group showed an increase in cTnI after four hours of the ablation. However, the concentration of cTnI after 72 h of the ablation was lower to baseline. Whereas, the level of cTnI showed no significant change in H-FIRE group during the ablation. In consistent with previous study, these results were due to the maximum current and the intensity of energy release applied of IRE was higher than H-FIRE [11].

Cardiac depolarization and repolarization are depended on ion flux alteration inside and outside of the cardiac cells. External electrical stimuli can alter cell membrane potential, leading to a localized depolarization [12]. The fundamental of electroporation is permeabilization of the cell, which can induce cardiac arrhythmia [3]. Previous studies suggested that the current electricity may trigger cardiac arrhythmia during and after IRE ablation near to the heart. Therefore, IRE was modified delivering electric impulses with ECG synchronization [3, 4]. Different from IRE, H-FIRE delivers bipolar pulses of 1–5 μs in a rapid burst with the benefits in reducing nerve and muscle stimulation [7]. These properties allow the administration of H-FIRE without the requirement of paralytics. Theoretically, it is speculated that H-FIRE obviates the requirement of cardiac synchronization. There are two waveform for H-FIRE. Moreover, asymmetric H-FIRE is more effective than symmetrical H-FIRE in vivo [10]. Previous report showed that no EKG abnormalities during symmetrical H-FIRE ablating the hepatic tissue in swines [13]. Thus, specific safety issues on the heart with asymmetric H-FIRE should be discussed. It is well known that distance is key to avoid cardiac arrhythmia. Previous study of IRE suggested that synchronization may be necessary at distances greater than 1.7 cm from the myocardium (as measured on an axial CT scan) [5]. We speculated that the results in our study might due to the electric pulses on liver was a stimulus of remote field.

It is worthy that the effective refractory period was a fundamental prerequisite for the maintenance of fibrillation conduction. If electrical stimuli fall on the vulnerable period, it can induce cardiac malignant arrhythmia. In contrast, if electrical stimuli fall during the absolute refractory period, arrhythmogenic potential is minimized. Refractory period of the ventricular myocardium refers to the starting of QRS complex to the peak of T wave. Vulnerable period of the ventricular myocardium refers to the peak of T wave to the ending of T wave (Tpeak–Tend). In our study, most of the electrical stimuli detected in ECG arises on QRS complex of the swine’s autonomous cardiac rhythm. Although H-FIRE pulse was arised on the vulnerable period, no existing dysrhythmia was seen of H-FIRE. ECG is the tracing of cardiac electrical activity. Electrical pulses of H-FIRE detected on ECG did not mean the stimuli conducted to the cardiac cell. It is the limitation of our study that the intra-cardiac electrical activity did not examine. We did not know whether the electrical signal of H-FIRE delivered on vulnerable period could cause cardiac arrhythmia. QT interval, QTc interval, and Tpeak–Tend interval are markers of cardiac arrhythmia. Both prolonged QT and QTc [14] and shortened QT interval and QTc [15] are associated with malignant ventricular arrhythmia (VAs). Prolonged Tpeak–Tend is associated with an elevated arrhythmic risk [16]. Compared with the ECG before H-FIRE ablation, QT interval, QTc and Tpeak-Tend did not change in the present study. These results suggested that no electrocardiogram abnormalities were observed during H-FIRE for asymmetrical waveform. Whereas, our results were contrast to a meta-analysis, which showed that the incidence of cardiac arrhythmias related to IRE on liver was 16% (21 of 129 patients) [17]. Due to the patient with cardiac comorbidity was prone to cardiac arrhythmia. In addition, we found the piglets had an increase in cTnI after 4 h of IRE ablation. However, the concentration of cTnI decreased to baseline after 72 h of IRE ablation, indicating cardiac injury is temporary. Moreover, level of cTnI had no significant change during the administration of H-FIRE. That suggested H-FIRE maybe safe than conventional IRE. Symmetric H-FIRE is safer than IRE and asymmetric H-FIRE for the pluses cancelled each other out. But the ablation efficiency also could be impacted. Asymmetric H-FIRE is the balance between safe and efficiency.

Our study has many limitations. First, the number of animals used for this study was small. Second, whether stimulation on liver with asymmetrical H-FIRE has an effect on the heart, future studies on H-FIRE should incorporate measurements of intra-cardiac potential. Third, this study was based on normal porcine model, future studies about cardiac safety of H-FIRE needs to evaluate in patients with liver tumor.

In conclusion, these results showed H-FIRE with asymmetrical waveform seems relatively safe without major cardiac complications on the normal liver without tumors and the piglets without any potential cardiac diseases.