Nanosecond pulsed electric field is an emerging locoregional electric ablative therapy that can be used to treat a variety of solid tumors, including melanoma(15), breast cancer(16), colon cancer(9), osteosarcoma(17), pancreatic cancer(18) and hepatocellular carcinoma(19). Non-thermal and ignorance-of-heatsink effect are two of the most significant features of the nsPEF ablation, which made it possible to be applied on the tumors located near the vital structures like porta hepatis. The operations on this area is recognized as complicated operations for surgeons and considered as relative contraindications for RFA(20, 21).
In this study, we demonstrated for the first time that nsPEF could achieve complete ablation on liver tissues and preserve the large vessels within ablation areas well. Conventional thermal ablations like RFA are based on excessive heat energy to cause necrosis of liver tissues, without discrimination of parenchymal hepatic tissues or matrix tissue like vascular fibers(22). However, the nsPEF relies on electric pulses to generate irreversible nanopores on the cell membrane and does no damage to other types of molecules, which explains why it has the potential to preserve cellular matrix of the large vessels. As the short-term and mid-term pathological follow up shows, the large vessels including veins and bile ducts in the ablated area kept a complete vascular structure and the perivascular tissues were ablated accurately with no sparing. It is confirmed that the large vessels were protected from the electric damage. Although,some of the large vessels occurred mild muscularis layers damage and endothelial loss༌the presence of the extracellular matrix greatly facilitated the reendothelialization process within 14 days. It could also explain the significant immune responses and rapid tissue healings observed in a series of nsPEF-treated researches(14, 16, 19, 23). Another interesting phenomenon is that compared with veins, bile ducts maintained better with a more complete structure and received less damage on the biliary epithelial cells. According to previous studies about RFA, it is assumed that bile ducts are more sensitive to thermal damage and less vulnerable to electric stimulations(7, 24, 25).
The protective effects for vasculature systems, especially for bile ducts, could be validated by the blood test results. Serum bilirubin including TB and DB showed no obvious changes during the whole procedure, indicating that the nsPEF treatment didn’t cause biliary complications like cholestasis secondary to bile duct strictures that resulted from the thermal damage, which was of high incidence after RFA(26). Besides, TBA underwent mild changes immediately after the treatment but then kept steady, suggesting that nsPEF may slightly disturb the metabolism of lipid. Notably, the number of blood platelet, namely PLT, went up at 2 days post-treatment and last to 7days, which raises the concern of thrombosis formation. Although the thrombosis was not found in histopathologic results and the PLT went down at 14 days post-treatment, the increasing trend of PLT indicated a procoagulant effect of nsPEF. Previous studies reported that thrombogenicity is one of the risk factors after thermal ablation, which lead to the administration of heparin(27). For the above reasons, the lesson and problem in thermal ablation worth further researches in nsPEF.
Aminotransferases (ALT and AST) of the liver function results underwent temporary increase, which was assumed to be released by the dead hepatic parenchymal cells. Their subsequent recovery confirmed the safety of nsPEF. Also, the results of TP and Alb proved that nsPEF doesn’t impair the capacity of albumin synthesis for the liver. RBC and Hb from blood routine results kept at the same level, proved that no massive hemorrhage occurred during the treatment. In the meantime, the inflammatory results like the percentages of neutrophils didn’t change significantly, which indicated the strict asepsis operation could effectively avoid infectious risks during ultrasound guided puncture process.
The effect of the electric ablation on the cardiovascular and skeletal muscle systems were investigated in this study. Previous studies proved the release of high voltage powers increases cell membrane permeability and opens a path for ion transport, which can induce cardiac arrythmias and defibrillation, leading to some unpredictable cardiac accidents(28–30). In addition, the electric stimulation on excitable tissues like motor nerves can cause the involuntary contraction of the muscles of the subjects which may impede the proceed of the treatment(31–33). However, in this study, we adopted the synchronization pulse generating system, which would automatically stop if the ECG detects abnormal heart activities, effectively protected cardiovascular muscles from the electric damage. We assumed that the increase of the myocardial enzymes of CK and CK-MB was caused by muscle puncture rather than myocardial injury and the level of Cardiac troponin I confirmed this assumption. CK-MB-related muscle damage and the mild increase of LDH and HBDH is affordable in liver cancer patients. In addition, the general anesthesia and insulated electrode needles prevented the contraction of skeletal muscles. These results proved that the appropriate operation of synchronization pulse generating system and general anesthesia in nsPEF treatment is effective and necessary to prevent the possible side effect on cardiac and skeletal muscles.
However, there are certain problems remained to be solved in the future experiments: (1) It is difficult to find the accompanying arteries of the large veins in hepatic hilar area under ultrasound guidance and even harder to include the three kinds of vessels (hepatic veins, arteries and bile ducts) in the ablation zone at one time. Therefore, our research mainly focused on the effect of nsPEF ablation on large hepatic veins and bile ducts and some of the minor arteries. However, the effect of nsPEF on large arteries, which have faster speed of blood flow requires further verification. (2) The risk of thrombosis formation reflected by PLT should be evaluated in the long run. Such evaluation requires not only the reexamination of blood coagulation functions, but also the regular monitoring of the large vessels by ultrasound.