Background
The peritoneal cavity with its peritoneal fluid is a specific environment different from that of plasma. The mesothelial cell lining of the peritoneal cavity and its organs facilitates the gliding of the bowels and actively regulates homeostasis and transport of fluids, molecules and cells. In males, the volume of peritoneal fluid is small. In women of reproductive age, follicular exudation increases the volume and adds high concentrations of steroid hormones. The peritoneal cavity is not vascularised and constitutes a sterile cavity that does not belong to the body homeostasis. Any trauma in the peritoneal cavity causes an inflammatory reaction and a mesothelial cell retraction, exposing the basal membrane. This abolishes the blood-peritoneal fluid barrier and permits the entry of immunocompetent cells and facilitates diffusion of larger molecules as immunoglobins, which is an efficient defence mechanism to intruders [
1,
2].
The large and flat mesothelial cells react within seconds to any trauma by retraction and bulging [
1,
2] causing an acute inflammation [
3] which increases with the duration and severity of the trauma. Identified traumas are surgical manipulation, mesothelial cell hypoxia by CO
2 pneumoperitoneum, deeper ischaemia at an intraperitoneal pressure of more than 8 mmHg and ischaemia-reperfusion at desufflation [
4], oxidative stress [
5] or reactive oxygen species (ROS) induced by exposure to air with 20% of oxygen, desiccation and saline as irrigation liquid. The severity and the duration of this acute inflammation of the entire peritoneal cavity create an inversely proportional reduction of fibrinolysis. This, in turn, increases the potential of adhesion formation through a reduction in tissue plasminogen activator (tPA) and an increase in plasminogen activator inhibitor (PAI) [
6,
7]. During laparoscopic surgery, the retraction and bulging of mesothelial cells cause a progressive increase in CO
2 resorption. The acute peritoneal inflammation increases postoperative C-reactive protein concentrations (CRP) and causes postoperative pain [
2].
During laparoscopic surgery, prevention of the mesothelial cell retraction and the subsequent acute inflammation effectively prevents or decreases the associated consequences including postoperative adhesion formation and postoperative pain. In addition, it accelerates recovery and in animal experiments decreases tumour metastasis. The most effective preventive factors are the addition of more than 5% of nitrous oxygen to the CO
2 pneumoperitoneum, cooling of the peritoneal cavity below 31 °C, minimalising mechanical trauma and ROS production, using Ringer’s lactate instead of saline and administering one or two doses dexamethasone postoperatively [
2]. If used together with a barrier [
8], this approach results in virtually adhesion free surgery [
9].
The similarity between our current knowledge derived largely from animal experiments, and the microsurgical tenets developed in the early 1970s empirically, but controlled by systematic second-look laparoscopy, 8–12 weeks after the initial operation, is striking. These principles were developed for open surgery and soon after applied in laparoscopic surgery [
10]. These microsurgical principles indeed are a combination of gentle tissue handling, judicious use of electrical and/or laser energy, use of inert sutures, continuous irrigation with Ringer’s lactate at room temperature during the procedure to avoid desiccation, shielding the bowels from the ambient air, thorough lavage of the peritoneal cavity at the end of the procedure, instillation of Ringer’s lactate solution containing a minimum of 500 mg of hydrocortisone succinate into the peritoneal cavity before closure and administration of one or two doses of dexamethasone after surgery.
Microsurgical tenets were proven to decrease adhesion formation and to increase pregnancy rates in open and laparoscopic surgery [
10,
11]. The relative importance of each of these factors that decrease acute inflammation and adhesion formation was investigated only recently in a laparoscopic mouse model with proof of concept trials in human [
9]. However, the addition of low doses of N
2O which is the single most effective factor was investigated during laparoscopic surgery with an insufflation pressure only. Since there is no insufflation pressure in open surgery, we, therefore, decided to evaluate the effect of N
2O in a mouse model for open surgery before undertaking a trial in human.
Discussion
These experiments confirm that the prevention of adhesion formation by conditioning is similar in both open and laparoscopic surgery. The main damaging effect of CO2, thus, is caused by mesothelial cell hypoxia and retraction, and less by tissue and ischaemia-reperfusion. Adhesions indeed increase with the duration of exposure to CO2 and with desiccation. N2O in concentrations of more than 5% appears to be the single most effective factor with a marginal beneficial effect when 4% of oxygen is added to this gas mixture. Although not all observations made during laparoscopic surgery were repeated in open surgery, we conclude that the mechanisms involved are similar in both open and laparoscopic surgery. The key factors are mesothelial cell damage and acute inflammation in the entire peritoneal cavity, as a reaction to trauma, hypoxia, ROS, oxidative stress and desiccation.
The exact mechanisms involved in the peritoneal cavity that enhance or prevent adhesion formation are not fully understood. The half maximal effect around 2.5% of N
2O indicates that N
2O has a drug-like effect, the mechanism of which is unknown. It is also not understood why mortality is 100% after exposure for 60 min to non-humidified CO
2 and no mortality when non-humidified N
2O is used, although the desiccated aspect of the bowels is the same. We only can speculate that mortality is not only caused by the desiccation but mainly by the severity of the inflammatory process since N
2O strongly and O
2 slightly decrease the inflammatory reaction [
18].
The observed effects of 5 to 10% of N
2O in open surgery at atmospheric pressure shed new light on the pathophysiology of adhesion formation. CO
2 pneumoperitoneum at an insufflation pressure of 15 mmHg decreases peritoneal oxygenation, triggers hypoxemia inducible factor (HIF) and decreases tissue plasminogen activator and upregulates PAI for several days. These effects of CO
2 pneumoperitoneum are less and/or of shorter duration at lower insufflation pressures and disappear at insufflation pressures below 8 mmHg in human and 2 mmHg in mice [
3,
6‐
8]. Taking into account the differences in size between man and mice and Pascal’s law, these pressures are considered the pressures at which vascular compression of the peritoneum, hypoxia and oxidative stress start. Since at atmospheric pressure N
2O still decreases adhesion formation caused by the CO
2 environment, we must conclude that key mechanism driving the subsequent events is mesothelial hypoxia and retraction. In addition, the observations that with laparoscopic surgery in the presence of a 10% N
2O environment at 15 mmHg pressure, the extent of adhesion formation is similar to open surgery at atmospheric pressure strongly suggests that 10% of N
2O prevents mesothelial cell oxidative stress hypoxia and its consequences including mesothelial cell retraction and decreased fibrinolysis enhanced adhesion formation and postoperative pain. However, whether N
2O also has a protective effect on oxidative stress caused by partial oxygen pressures higher than 75 mmHg (or more than 10% O
2 at atmospheric pressure) as in air remains to be investigated.
Despite the differences that exist between oxidative stress caused by 20% CO
2 in ambient air in open surgery and the detrimental effect of the CO
2 pneumoperitoneum and insufflation pressure, the prevention of adhesion formation and postoperative pain are similar in both open and laparoscopic surgery. Beside the use of a proper atraumatic surgical technique and precise haemostasis, the important adhesion preventive factors in open surgery are to avoid ROS formation, caused by the 20% oxygen concentration in ambient air; the use of N
2O in concentrations of 5% or more; cooling the peritoneal cavity; avoiding desiccation; the use of Ringer’s lactate solution instead of saline; being toxic for mesothelial cells [
19‐
25] for intraoperative irrigation and terminal thorough lavage and administration of one or two doses of dexamethasone after surgery. Although, it has not been investigated as yet whether N
2O can prevent the damaging effects of exposure to 20% of oxygen concentration, the flooding of the surgical site in open surgery with 5 to 10% of N
2O will require a carrier gas for which both CO
2 and nitrogen (N
2) seem suitable. The importance of cooling in open surgery has indirectly been confirmed in rats using cold saline infusions [
26,
27]. Adhesions were also less when the abdominal cavity was exposed to the atmosphere of the operating theatre (21% O
2, 21 °C, 40–47% relative humidity) than to CO
2 + 4% of oxygen and 95–100% relative humidity at 37 °C [
28]. Prevention of desiccation is much more important in open surgery than in laparoscopic surgery considering the 100% mortality of mice when exposed to dry CO
2 for 60 min. The toxicity of saline for the peritoneum was known since the early 1970s [
19,
20] and has recently been confirmed [
21‐
25]. The use of dexamethasone and another tenet of microsurgery was only proven to be effective after conditioning in laparoscopic surgery in an animal model. In any case, adhesion formation following open and laparoscopic surgery appears to be remarkably similar in an atmosphere of 10% N
2O in CO
2 without desiccation.
The implementation of these principles to open surgery should be carried out judiciously. That saline should be abandoned, and a richer solution should be used for irrigation is obvious. Prevention of desiccation can be achieved by continuous irrigation as done in microsurgery, by covering bowels with moistened inert towels and/or by flooding the operative field with humidified CO
2 [
29,
30]. The latter indeed decreased adhesion formation in open cardiac surgery. The instillation of humidified CO
2 deep into the surgical field also decreased oxidative stress since the organs were no longer exposed to the 20% of O
2 in ambient air. A similar effect was achieved by shielding the organs in microsurgery. The exposure of the surgical field to the temperature of the operating theatre has never been an issue in open surgery. We can be happy today that cooling unexpectedly has a beneficial effect. The administration of dexamethasone after surgery, eventually at the end of surgery, may be beneficial to reduce inflammation and adhesion formation and accelerate recovery, while its use might aggravate an eventual infection. The proven very strong beneficial effect of 5 to 10% of N
2O with no explosion risk demands a trial in open surgery. As described for humidified CO
2 in cardiac surgery, [
29,
30] the deep instillation of gases heavier than air will fill and flood progressively the operation field. CO
2 seems obvious as a carrier gas since it is heavier than air with minimal irritative effect at atmospheric pressure. N
2O, which fortunately also is heavier than air, should be used in concentrations of 5 to 10% of N
2O. For this reason, we used the same combination for these experiments, the efficacy of which had furthermore already been proven in animal models. It will obviously be necessary to prevent or reduce contamination of the operating theatre with N
2O. The suggested upper threshold for N
2O is 25 ppm [
31]. We speculate that this can be achieved with aquarium-like drapings extending above the operating field with aspiration at the borders to prevent overflow; the opening of the draping would be a compromise between being sufficiently large to permit surgery but small enough to prevent mixture with the ambient air. Indeed even without aspiration contamination with 2 L/min with 10% of N
2O would result in only 15 ppm N
2O in a normal sized (e.g. 40 m
3) and ventilated (e.g. refresh rate of 20 cycles/h) operating room.