Factors that determine the most appropriate gas for pneumoperitoneum are type of anesthesia, physiologic compatibility, toxicity, ease of use, safety, delivery method, cost, and non-combustibility.  Gases used for pneumoperitoneum include carbon dioxide (CO2), air, oxygen, nitrous oxide (N2O), argon, helium and mixtures of these gases.
 

CO2 gas insufflation is preferred by most laparoscopists because it has a high diffusion coefficient and is a normal metabolic end product rapidly cleared from the body.  Also, CO2 is highly soluble in blood and tissues and does not support combustion.  The risk of gas embolism is lowest with CO2.  Cardiac arrhythmias can occur with CO2 pneumoperitoneum. Because of possible CO2 induced hypercarbia, N2O may be preferred in patients with cardiac disease. With prolonged procedures, CO2 retention is possible as evidenced by tachycardia and acidosis.
 

Pneumoperitoneum is usually initiated by use of a needle or trocar device to transverse the abdominal wall and distend the peritoneal cavity. Another method of access is with an open incision and entering by direct vision through the peritoneum.  Caution is required with any method of abdominal entrance or distention.  Abdominal penetration complications and incorrect gas placement may result in bleeding or gas dissection within the abdominal wall.  Bowel injury, puncture of intra-abdominal vessels, dissection of the fascia or omentum can occur.
 

After peritoneal access, a gas delivery system is used to inflate and maintain the abdominal distention.  Preset pressures of 15 mm Hg or less are safest to maintain pneumoperitoneum and allow performance of laparoscopic techniques.  Intra-abdominal pressures in excess of 25 mm Hg are associated with increased airway pressure, increased intrathoracic pressure, increased femoral venous pressure and signs of cardiovascular stimulation with tachycardia and hypertension. Large patients and those who have had multiple abdominal surgeries present a challenge to establish a pneumoperitoneum. Patient selection for laparoscopic procedures and surgical judgment concerning the appropriateness of laparoscopic versus open surgery should be individualized for each circumstance.
 

Gas delivery systems are composed of a containment cylinder, insufflator (gas throttling down pressure regulating unit), tubing, filter and abdominal entry device or port.  The gases used for medical purposes have their production regulated by the Food and Drug Administration.  Acceptable limits of contamination are listed in The U.S. Pharmacopoeia.  Gas cylinders are made of ferrous alloy that meets Department of Transportation specifications which ensure safe transport.  The cylinders contain the gas as a liquid under pressure.  Over time, the cylinders build up inorganic and organic contamination.  This occurrence requires filtration of the gas prior to insufflation of a patient’s abdomen. The pressure change from the containment cylinder to insufflator and into the patient’s abdomen causes cooling by the Jewel-Thompson effect.

The temperature of carbon dioxide gas is about 20.1° C as it enters the abdomen.  The cool gas causes hypothermia if the gas is not pre-conditioned. Gas flow also contributes to hypothermia by convection effects.  There is enhanced evaporation from the bowel surface due to gas turbulence from pressurized delivery.  Additionally, general anesthesia causes patients to be unable to maintain thermal stability.  The net effect is a loss of 0.3 degree C per 60 liters of gas insufflated. In addition, hypothermia may cause decreased gastrointestinal motility and lead to increased potential for ileus.
 

When the laparoscope is first introduced into the abdominal cavity lens fogging often occurs.  This phenomenon is due to the relatively cold dry lens being introduced into a warm moist environment causing the dew point to be reached.  This results in condensation forming on the internal lens surface.  When the insufflation gas is heated and hydrated or a surface wetting agent is used, no lens fogging occurs and the visual field is clear.
 

The gases used for pneumoperitoneum have low water content.  CO2 has less than 200 parts per million of water.  Dry insufflation gases cause drying of the peritoneum and result in intact mesothelial cells being lost or desiccated from the peritoneum surface.  To preserve peritoneal surface integrity and decrease the tendency to adhesion formation continuous or intermittent moistening should be performed.
 

All mechanical systems have inherent weaknesses.  Insufflators require proper calibration and maintenance.  Insufflator pressure accuracy depends on the quality of the gauges used in the insufflator.  Wide ranges of variation are seen due to gauge inaccuracy. Pressure testing should be done regularly to assure proper readings.
 

Over time, insufflators become contaminated on their internal and external surfaces.  Germicidal cleaning of external ports is important.  Gas filtration to 0.3 microns prior to abdominal entry assures reduction of quantitative exposure of the peritoneal cavity from these organic and inorganic materials.
 

Initial abdominal entry pressure readings should be low—less than 2-3 mm Hg.  Elevated initial pressures indicate improper placement.  Increased intra-abdominal pressures after proper access can impede venous return and result in potential anesthesia complications.  Pressure on intra-abdominal surfaces due to the pneumoperitoneum can inhibit bleeding giving a false sense of security regarding hemostasis.  Prior to concluding any procedure, surgical sites need to be observed with reduced pressure to assure appropriate hemostasis.
 

During laparoscopic procedures the abdominal cavity can become contaminated with smoke from the lasers or electrosurgical device used.  On a toxicologic basis, tissue combustion within the closed abdomen at laparoscopy is an iatrogenic smoke poisoning incident.  Toxic chemicals produced by pyrolysis of human tissue.  These chemicals effect peritoneal cells and other cellular components (i.e. activation of macrophages and increased production of tumor necrosis factor).  Absorption of these chemicals occurs via the peritoneum.  Combustion processes that occur in low oxygen environments cause elevated CO emissions and are common in the laparoscopic situation.  Peritoneal absorption of CO causes carboxyhemoglobin formation.  Carbon monoxide has 200-240 times greater affinity for hemoglobin than oxygen.  The half-life of CO is 5.33 hours in room air.  Depending on the amount of smoke produced, anesthetic oxygen concentration and whether smoke evacuation was performed during the procedure, determines the postoperative effects of CO and how much time is required to return to preoperative levels. Carbon monoxide (CO) is known to cause cardiac arrhythmias and can initiate or exacerbate many intra- and postoperative complications.  For these reasons smoke within the pneumoperitoneum should be continuously or intermittently evacuated.
 

Methemoglobinemia may occur during laparoscopic procedures when abdominal tissue combustion occurs.  Methemoglobin is the oxidative product of hemoglobin causing the reduced ferrous (Fe2+) to be converted to the ferric (Fe3+) form.  The difference between methemoglobin and oxyhemoglobin in the ferric state is that methemoglobin is formed from unoxygenated hemoglobin and is not capable of carrying oxygen or carbon dioxide.  This property shifts the oxyhemoglobin dissociation curve to the left, inhibiting oxygen delivery to tissues, and may lead to anoxia.  The eventual concentration of smoke and subsequent physiologic changes that occur depend on the amount tissue pyrolized, duration of smoke exposure and effectiveness of smoke evacuation.  It must be noted that pulse oximetry does not give a proper evaluation of oxygen saturation in the presence of dyshemoglobinemias (carboxyhemoglobin and methemoglobinemia).

Peritoneal defenses are also effected by irrigation and suction.  Irrigation serves to separate tissue surfaces and remove debris and clotted material.  However, irrigation also causes dilution washout of resident peritoneal macrophages.  Macrophages direct host defense mechanisms that result in recognition, phagocytosis and destruction of foreign substances.
 

As a result of irrigating the peritoneal cavity with 1 liter of fluid, 60-80% of the original number of macrophages are washed out.  It has been shown in the murine model and in patients undergoing peritoneal dialysis that restoration of 90% of the original complement of macrophages requires 72-84 hours. Postoperatively, macrophages are intimately involved in protecting the peritoneum and abdominal cavity from foreign material, bacteria and foreign bodies.  They are also involved with the initiation of reperitonealization.
 

Tissue combustion generates 284 mg of particulates from each gram of tissue pyrolized or 0.3-3.0 x 109 particles per gram of tissue vaporized.  These particles range from 0.1-1.0 microns in size clustering between 0.2-0.5 microns.  This material is phagocytosed by macrophages, chemically digested, and causes macrophage activation, alteration in chemotaxis and increased cytokine production.
 

The seemingly inactive invisible pneumoperitoneum is not a static condition and must not be ignored in laparoscopic surgery.  The pneumoperitoneum is a dynamic space that affects the patient’s general well being, and specific physiologic cellular processes.  The insufflation gas needs to be filtered to reduce contamination, heated to reduce hypothermia and hydrated to preserve cellular integrity and reduce adhesion formation.  It is important to recognize the affects of intra-abdominal therapy and the consequences of surgical devices.  This includes tissue particles, aerosol production, the by-products of combustion, and their effect on peritoneal tissues locally and the body chemistry and metabolism as a whole.
 

Conclusion
 

The pneumoperitoneum creates a complex dynamic set of changing conditions having pathophysiologic consequences during laparoscopy. The effects of unseen tissue touching due to gas flow, cellular stress and inflammation caused by lack of water in the gas combined with increased intra-abdominal pressure can be minimized and corrected. Maintaining intra-abdominal pressure below 12 mmHg, using humidified warmed gas, and adhering to microsurgical principles of tissue handling can be controlled and result in improved clinical outcomes.