Regarding WSES Jerusalem guidelines for treatment of acute appendicitis no difference in several methods of mesoappendix dissection was reported. But stating the danger of leaving foreign bodies in the abdominal cavity, utilization of energy devices was preferable over clipping of appendicular artery. Moreover guidelines suggest energy devices in case of inflamed and oedematous mesoappendix consider ME as “the most costeffective method” among them .
This study has shown that the duration of ME was the key factor affecting the extent of thermal spread during appendectomy, not the power. Regardless of the power settings (30 or 60 W), heat spread more than 2 cm laterally along the mesoappendix when application time exceeded 3 s. Perforation correlated with long-lasting single exposure both in LP and SP groups. In all animals with perforation a single exposure of monopolar energy lasted over 1.5 seconds thereby causing rising of the temperature of the small intestine to a critical threshold through the entire thickness of the intestinal wall (2 mm).
It is difficult to predict and measure thermal spread resulting from ME application . Several authors  have hypothesized or reported that lower power settings would lead to less thermal spread. In fact, surgeons who use ME devices use different power settings, modes and activation times depending on the task . To our knowledge, no standardized curriculum for surgeons has so far addressed the safe use of ME for the dissection of the mesoappendix during laparoscopic appendectomy. The majority of manuals recommend applying “the lowest possible power setting” , others suggest setting the power to 70 to 90 W in the “pure cutting” mode or to 50 W in the “coagulation” mode . Jones et al., showed in a randomized clinical trial that thermal injury occurred more frequently in the coagulation mode compared to the blend mode at the level of the trocar incision sites . The WSES Jerusalem guidelines for diagnosis and treatment of acute appendicitis provide no recommendations regarding ESU power and voltage for appendectomy . In our study, “coagulation” and “cutting” modes were fixed at 30 and 60 W because general surgeons tend to use such combinations for mesentery dissection.
Previous studies have demonstrated that the area of living tissue affected by a thermal burn gradually grows in size after the initial impact, reaching its maximum by days 3 to 7, and does not start to shrink until day 14 . Damage to nearby structures typically occurs on day 4–5 following the surgery due to unrecognized energy transfer [18, 20]. Although the cecum had been heated up to 42 °C (“critical temperature”), no clinical manifestations were noticed on postoperative day 5. At the same time, histological examination revealed signs of moderate inflammation and reactive edema affecting all tissue layers, attesting to thermal exposure. However, at the time of necropsy, the inflammation was aseptic in nature: there was no neutrophil infiltration and MPO levels in the cecal tissue were low. This could be explained by the design of our experiment: appendectomy was performed on the non-inflamed appendix. A study of lateral thermal damage to the mesoappendix and the appendiceal base during laparoscopic appendectomy in children demonstrated that the postoperative pain syndrome and the duration of hospital stay directly depend on the temperature of the heated tissue and the size of the thermal lesion .
The cut-off value for what we called “critical temperature” for possible cell damage in our study was set at 42.C, as others [6, 21], However, in fact, there is no consensus as to what temperature should be considered critical in terms of cell damage. Experimental studies have found that cells heated in excess of 20 ⁰С die within 15 seconds; heated in excess of 25 ⁰С, in 4 seconds, and heated in access of 30 ⁰С, in 2 seconds . We found no differences in the histopathological changes to the cecal tissue between the studied groups. Total ME application times and surgery duration were longer in the LP group than in the SP group. This could be explained by the fact that in order to achieve the same effect at low power settings, the duration of a single application needs to be increased. Longer application times lead to extensive lateral thermal spread; as a result, the tissues are exposed to the temperatures that cause cell damage for longer periods of time. This means that the severity of thermal injury does not change significantly when low power settings are applied, as compared to standard settings. The maximum temperature was the same at any power settings. In addition, three rabbits from both groups demonstrated a histological picture of thermal lesions in different layers of the cecal wall following the longest exposure of the cecum to T ≥ 42 °C. Previously, Hefermehl  conducted an experiment in order to investigate a relationship between thermal spread and power settings/application time. He discovered that at 60 W the area of thermal spread increased by one-third (in comparison with 30 W). Longer application times (2 s) led to a multifold increase in the lesion area from 3.5 to more than 20 mm. The majority of recommendations on the use of coagulation devices were developed based on ex vivo experiments on bovine and porcine musculofascial strips that had no blood flow in them [10, 11]. This is in accordance with our findings, duration of application was the key factor leading to injury. This fact should be taken into account when elaborating surgical guidelines.
The dynamics of thermal spread may be influenced by the difference in blood supply to healthy and inflamed tissues or their hydrophilicity. However, Pogorelić reported no significant differences in lateral thermal spread during appendectomy in patients with acute appendicitis and in the absence of inflammation . In patients with appendicitis, the diameter of the inflamed appendix changes along its course, which means that its regions can heat up to different temperatures as the current passes through it. In our experiment, the temperature of the appendix rose only after the heat reached the mesoappendix, and that rise was not very pronounced. We also observed the so-called “jump-over” effect indicating that thermal processes in deeper tissues can only be evaluated indirectly and sometimes can go unnoticed. Infrared camera captures a temperature rise in the serosa occurring after the appendix gets heated up through its entire thickness from inside. Probably the mesoappendix-appendix borderline played a role in preventing the appendix from heating. This observation calls into question the role of the appendix diameter in thermal spread.
In theory, thermal injury involves two major zones during appendectomy with ME: the mesoappendix and the cecum. Interestingly, however, three animals from both groups developed perforation of the small intestine, and another rabbit developed transmural necrosis of the small intestine. Several underlying mechanisms are possible. First, current channeling through the mesenteric vessels might provoke substantial lateral thermal spread. Diamantis  compared the effects of different electrosurgical modalities (mono- and bipolar coagulation, impedance-controlled bipolar vessel sealing and US shears) on the short gastric vessels (1 mm in diameter) on rabbits. Four of 20 animals treated with ME developed perforation of the greater curvature of the stomach at the coagulation site on post-operative day 3. Bipolar coagulation resulted in stomach perforation in two rabbits (10%). Khan et al.  studied the effect of thermal spread on the prostatic nerve plexus during robotic prostatectomy in vivo. He demonstrated that interposition of the vessels significantly reduced thermal spread to distal tissues. In our case, the structures in the proximity to the coagulation site were affected because the appendix and the small intestinal loop share common blood vessels  and because heat travels through tubular structures rapidly.
Another possible cause of intestinal perforation is the pedicle effect described by Humes et al. . This occurs when the electrical current goes through a tubular structure to the point where the latter enters another tubular structure of a larger diameter.. Humes et al. demonstrated the pedicle effect in a clinical series of three patients with similar common bile duct perforations discovered during laparoscopic cholecystectomy and occurred at the junction of the cystic and common bile duct. A similar effect was observed in our study when the energy spread along the mesenteric vessels that entered the small intestinal loop. Our hypothesis would be that more heat was emitted at the vasculo-intestinal junction than along the vessels.
Another phenomenon observed in our study was the clamp effect (the heating of tissues grasped by forceps or a clamp through which the electrical current flows). It can be explained in terms of the physical fundamentals of electricity. When current passes through a conductor, it heats it up according to the Joule-Lenz law. As the conductor diameter decreases, resistance and local heat production increase. The clamp effect can be critical for tubular structures, such as the ureters or the intestinal wall, leading to their heating at a distance from the site of coagulation.
Further detailed investigation is necessary to identify the conditions under which preformation can occur, to assess its severity and the risk of perforation when performing coagulation in proximity to tubular structures.
Summing up, all of the listed effects may play a significant role in postoperative complications.
Limitations of this study. We recognize that all laparoscopic appendectomies are not performed with ME, but this practice might be more prevalent than admitted. The major limitation of our study was that we used healthy animals with non-inflamed appendix, and therefore the potential enhancement of inflammation was not studied. No antimicrobial prophylaxis was administered to the animals in our study, so we do not know the effects of antibiotic on thermal spread. As heavy postoperative complications were not expected we didn’t plan to investigate the influence of antibiotics on their developing because it didn’t correlate with the purpose of the study. Another limitation of this study is the small sample size. Moreover we did not compare the effects of other hemostatic devices (bipolar, LigaSure, EnSeal, ultrasound etc). We recognize that while monopolar equipment is less expensive than the other modalities, there are ample data that show that ME might be more dangerous.
In addition, because of anatomical differences between species, the results obtained in this study might not be directly extrapolated to humans. Moreover, the duration of application of ME in these animals may not be the same that is required in humans. Last, we examined two groups of rabbits which had the "standard" electrocautery surgical management; there was no true control group which had no electrocautery. Therefore we did not study the effect of tissue handling and dissection on the inflammatory process. Either a sham group without appendectomy could have been use or appendectomy performed without electrocautery.