increased lung and chest wall impedance [85]. The observed decrease in lung compliance and volumes, such as functional residual capacity and total and vital lung capacity [71, 72, 86, 87], leads to further increase in CO2 tensions [87–90]. Therefore, the use of mechanical ventilation is important to maintain CO2 levels within acceptable limits and to avoid decreases in arterial oxygen tensions [88, 91]. In animals and in humans, the Trendelenburg position is associated with greater respiratory depression, particularly regarding oxygenation [72, 85, 88, 92]. These respiratory effects seem to be species dependent with dogs and cats less impacted than horses and sheep. In addition, horses with a higher body weight seem to be more affected, with higher arterial CO2 and lower oxygen tensions [92].
Complications Associated with Pneumoperitoneum
These are in addition to cardiovascular and respiratory changes expected with insufflation of a gas into the peritoneal space. They include retroperitoneal or subcutaneous emphysema both of which are generally the result of insufflation of gas outside of the peritoneal cavity. The occurrence of subcutaneous emphysema has been reported in dogs after laparoscopic gastropexy, nephrectomy, and ovariectomy [35,93–95]. A marked increase in CO2 tension is typically observed in the face of extraperitoneal insufflation and should alert the anesthetist for this complication [96]. If the insufflation gas is CO2, this tends to resolve fairly quickly after insufflation of gas is discontinued; one may also tap this virtual space and remove some gas to improve animal comfort. Additional analgesic therapy should be considered as pain is frequently observed in these animals. More serious complications include pneumomediastinum [97], pneumothorax [98–102], and pneumopericardium [103, 104], which are all thought to be a result of insufflation gas (creating positive pressure) tracking through embryonic remnants or alternatively through potential diaphragmatic defects or weak points. Alveoli rupture, associated with the use of high peak inspiratory pressures to maintain minute ventilation, should also be considered as a possible cause. These complications may be life threatening and the anesthetist should be ready to intervene rapidly should they occur. In a tension pneumothorax caused by alveoli rupture due to high peak inspiratory pressures used during laparoscopy, cardiac output is compromised and severe hypotension, oxygen desaturation, and a decrease in the partial pressure of end‐tidal (but not arterial) CO2 are commonly observed. In these animals, discontinuation of mechanical ventilation and thoracocentesis/chest tube placement should be performed quickly. On the other hand, when the pneumothorax is caused by migration of the CO2 used for abdominal insufflation, similar cardiorespiratory depression is typically seen, except for the partial pressure of end‐tidal CO2, which might rise as the CO2 is absorbed via the pleural surface [32, 100]. In most of those cases, a chest drain may be avoided as the CO2 can be removed from the chest via suction through the abdominal cavity [98]. CO2 is absorbed much faster than air and any residual pneumothorax is spontaneously absorbed. Mechanical ventilation with the addition of positive end expiratory pressure has been advocated to minimize CO2 accumulation in the chest and to maintain adequate ventilation and oxygenation [32, 98, 100]. Spontaneous pneumothorax has been reported in two dogs during laparoscopic ovariectomy and gastropexy, which was suspected to be associated with potential excessive traction on the esophagus due to gastric manipulation [101]. Three cases of tension pneumothorax due to CO2 migration during laparoscopic hiatal hernia repair have recently been reported in dogs [102]. Cardiovascular collapse developed suddenly, but good communication with the surgery team with quick termination of CO2 insufflation and deflation of the abdomen allowed for the return of adequate cardiac function. A chest tube was placed and continuous or intermittent suction was performed as needed to allow for the procedure to be concluded without conversion to laparotomy. The potential for tension capnothorax has also been described in laparoscopic hiatal hernia repair in humans [98, 105], and this recent short case series [102] suggest a high risk of this complication in dogs. The anesthetist needs to be aware of the potential for this complication, as prompt recognition and management are essential to a positive outcome.
Perhaps the most concerning complication is the potential for gas embolism. This results when insufflation gas enters the vasculature and is transported to the heart and lungs. In small amounts, the gas is delivered to the lungs and usually cleared without much consequence to the patient. As volumes of gas increase, hypotension, tachycardia, arrhythmias, and a decrease in end‐tidal CO2 tension may be noted [106, 107]. If a larger (patient size dependent) volume of gas were to reach the heart, there is the potential for the gas to occupy one of the chambers of the heart and prevent blood flow that ultimately results in cardiac arrest [107–110]. Several studies using transesophageal echocardiography have demonstrated a very high incidence of subclinical gas emboli during different laparoscopic surgical procedures in both animals and humans [111–114]. The fact that no adverse effects were reported in those studies may be related to the use of CO2 for abdominal insufflation. The potential for embolization exists with CO2 but is lower than for air, nitrous oxide, or helium. This is why despite not being inert (and so contributing to a rise in arterial tensions of CO2 more than caused by distention alone), CO2 is still preferred as the insufflation gas for laparoscopic interventions [32, 111, 112, 115]. Despite its relative safety, fatal embolism has been reported with CO2 use in both human and animals [107, 109]. If gas embolism is suspected, abdominal insufflation should be immediately discontinued. When possible, the animal should be placed in left lateral recumbency and cardiac massage started in an attempt to dislodge the emboli from the heart. Placement of a central venous or pulmonary artery catheter may allow aspiration of the gas emboli [32, 84]. One of the authors had experienced a fatal suspected CO2 embolism event in a small dog undergoing laparoscopic cholecystectomy. While most severe cases of CO2 embolism typically occur at the beginning of the procedure, due to inadvertent placement of the Veress needle into a vessel or organ parenchyma [116], it can also occur later in the procedure as was the case in this dog. Later occurrence of CO2 embolism is thought to be due to smaller amounts of CO2 entraining the circulation through injured vessels in the abdominal wall or surgical site [112, 116]. Attempts to dislodge the emboli (as described above) were unsuccessful. Severe CO2 embolism is extremely uncommon, but consequences can be devastating. The mortality rate associated with severe gas embolism in humans has been reported at 28% [117].
Insufflation with CO2 gas is typically performed at 22 °C and 0% relative humidity. The use of a warmed humidified CO2 insufflation (37 °C, 97% relative humidity) has been proposed as a manner to minimize postoperative discomfort, perioperative hypothermia, as well as peritoneal injury and adhesion formation [118–120]. However, studies on potential benefits of using warmed humidified CO2 insufflation have provided conflicting results. While various studies have reported less pain with the use of warmed humidified CO2 insufflation in human patients [118,121–123], others have failed to detect a benefit [124, 125]. A recent study in dogs showed that the ones insufflated with warmed humidified CO2 had higher pain scores than the ones insufflated with the cold dry gas, although no dogs required rescue analgesia [126]. In addition, no benefit regarding maintenance of core body temperature in the perioperative period has been demonstrated from the use of warmed humidified CO2 in humans [121, 124, 125, 127] or dogs [126]. A potential benefit may be the reduction in peritoneal injury that has been shown with the use of warmed humidified CO2 in rats [119, 120] and in dogs [126]. These might include less adhesion formation [120] and lower susceptibility to implantation of cancer cells and metastasis at portal sites [128, 129], but further studies are needed to confirm its clinical significance.
Anesthesia Management
While there are some unique aspects to laparoscopic intervention as noted in the preceding text, the basic principles of anesthesia must first be applied. General anesthesia should be a reversible event that provides amnesia, analgesia, unconsciousness, and muscle relaxation while supporting thermoregulation, cardiovascular, respiratory, neurologic, hepatic, and renal functions. To meet these basic principles, care should be individualized for the animal with consideration given to the reason for the presentation, the
