Small Animal Laparoscopy and Thoracoscopy. Группа авторов

head up – also known as Fowler or reverse Trendelenburg – (Figure 7.3) positioning can compromise venous return and cardiac output due to gravitational effects. This is of greater consequence during anesthesia due to the blunting of baroreceptor reflexes. During Trendelenburg positioning, there is an increase in venous return from the pelvic limbs. However, cardiac output again decreases, but the reasons differ and include decreases in heart rate and vasomotor tone [64, 65]. In anesthetized dogs, both body positions have further compromised cardiac output during pneumoperitoneum, with the reverse Trendelenburg position having the most significant impact [47]. There is also increasing concern regarding changes in intracranial pressure, which will compound those seen with carbon dioxide pneumoperitoneum. A recent study has shown a correlation between laparoscopic insufflation pressures and intracranial pressure in human patients undergoing laparoscopic ventriculoperitoneal shunt placement [66]. While unlikely to be serious in healthy patients, this could be of great significance in patients with intracranial disease.

      Peritoneum distension due to abdominal insufflation may increase vagal tone and cause bradyarrhythmias, with a reported incidence between 14 and 27% in healthy young humans [67, 68]. Bradycardia should be addressed quickly as it may be an early indication of cardiac arrest [69, 70].

      Respiratory Effects

Respiratory function is also altered during laparoscopic intervention. The increase in abdominal pressure and volume limits diaphragmatic excursion and reduces pulmonary compliance, functional residual capacity, and vital capacity of the lung and may lead to ventilation/perfusion mismatch [42, 48,71–73]. Hence, it is not surprising that the effects tend to be proportional to insufflation pressures as is shown in young swine [74] and adult dogs [48, 75]. In spontaneously breathing dogs, respiratory rate remained unchanged with abdominal insufflation, but a significant reduction in tidal volume was reported [75]. With volume‐controlled ventilation maintenance of tidal volume results in an increase in peak inspiratory pressure [48], and hypercapnia and hypoxemia might still occur. Similarly, when using pressure‐controlled ventilation, the peak inspiratory pressure must be increased, to overcome the decrease in lung compliance and avoid a reduction in tidal volume. The resulting positive pressure in the chest has an additional impact on reducing venous return and thus cardiac output could be further compromised.

      In spontaneously breathing animals, the decrease in tidal volume and increase in end‐tidal carbon dioxide are proportional to increasing insufflation pressure and the negative impact lasts longer in animals exposed to the higher pressures [75]. This reflects fatigue on the part of the patient and has led to the common recommendation for mechanical ventilation in patients in whom the procedure is anticipated to last longer than 15–30 minutes.

      The inability for a patient to compensate for the elevation in CO₂ by adjusting their ventilation is even more notable when CO₂ is used as the insufflation gas as is common practice. This is because CO₂ is highly diffusible and enters the blood stream contributing to a rise in arterial tension. Hence, the impact on ventilation is greater than insufflation with an inert gas, such as helium, or with other gases such as nitrous oxide (N₂O) or air (albeit those gases have other disadvantages) [76–78]. An increase in arterial CO₂ tensions may initially be cardiovascularly supporting [41, 76], but will ultimately result in a concurrent decrease in blood pH which in turn has a potential to impact cellular metabolic processes. The cardiac rhythm may also be affected by the increase in CO₂ tensions and resulting acidosis and increased sympathetic tone, which can lead to tachycardia, premature ventricular contractions, and in rare occasions ventricular tachycardia and fibrillation [79].

      Elevated CO₂ tensions are also associated with increased cerebral blood flow [80, 81], but additional mechanisms may exist [82]. In compromised patients or those breathing a low inspired oxygen tension, excessive CO₂ levels may contribute to hypoxemia. In addition, CO₂ tensions greater than 90 mmHg have anesthetic effects in their own right [83].

      As for the cardiovascular system, additional factors such as positioning may further impact respiratory effects [32, 71, 72, 84]. Both Trendelenburg and reverse Trendelenburg positions have a negative impact on lung expansion due to 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