Small Animal Laparoscopy and Thoracoscopy. Группа авторов
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upon insufflation and generally plateaus in ventilated patients. Spontaneously breathing animals tend to fatigue and may not be able to maintain the plateau [138]. Hence, for procedures where insufflation will be sustained for greater than 20–30 minutes, use of positive pressure ventilation is recommended to minimize hypercapnia and a consequent reduction in pH and potential adverse effects on intracranial pressure and anesthesia depth. Typically, minute ventilation will need upward adjustment to compensate for the delivery of CO2 with the insufflator. One should follow the basic principles of ventilation and consider the change in chest wall compliance when making these adjustments.
Because of the potential to cause barotrauma and compromise venous return (and so cardiac output) further with a significant increase in tidal volume, it is common to first increase respiratory rate and then if needed adjust the airway pressure/tidal or minute volume depending on the type of ventilator being utilized. Both pressure and time‐cycled (volume limited) ventilators may be used, but the user should be familiar with the special considerations for each of these in the face of reduced pulmonary compliance caused by insufflation and potential postural changes. While some elevation in arterial CO2 tension is acceptable in a healthy patient, a general recommendation in patients without intracranial disease is to maintain this value at or below 60 mmHg. In the absence of blood gas monitoring, the end‐tidal CO2, which may be up to 10 mmHg (but is usually 0–5 mmHg) lower than arterial CO2 tensions, should be maintained below 55 mmHg. Ventilation may also help prevent atelectasis of the lung resulting from cranial displacement of the diaphragm and so help maintain oxygenation. The use of positive end‐expiratory pressure (PEEP) has been shown to be beneficial in improving gas exchange and pulmonary mechanics in healthy dogs [139] as well as in humans [140]. Alveolar recruitment maneuvers in addition to PEEP may further improve gas exchange, particularly when a steep Trendelenburg position is required [139, 141]. However, it is important to consider the potential negative impact on cardiovascular function associated with these ventilatory techniques.
Along with providing respiratory support, it is important to monitor respiratory function. Carbon dioxide may be monitored in the airway with a capnograph or in the blood using a blood gas analyzer. The former has the benefit of being continuous and noninvasive. Additionally, the capnograph will alert the anesthetist to low cardiac output states as may occur for a number of reasons including that resulting from an intravascular gas (CO2) embolus.
A pulse oximeter similarly is easily applied and provides continuous measurement of oxygen saturation while also recording pulse rate. In most dogs and cats undergoing laparoscopic procedures, the use of pulse oximetry is considered sufficient for monitoring of oxygenation, as hypoxemia is not a likely complication even with insufflation in patients breathing high fraction of inspired oxygen (FiO2). However, in patients breathing a lower FiO2, hypoxemia is possible and likely to occur more rapidly if a complication (e.g., pneumothorax) occurs. If N2O is used as part of the anesthetic protocol, oxygen saturation monitoring is especially important.
In the high risk or critically ill animal, arterial blood gas analysis is useful and provides information about CO2 and oxygen tensions. Blood pH and other parameters including electrolytes, blood glucose, and lactate are frequently included with blood gas results and provide additional information to facilitate appropriate management of the animal.
Summary
As laparoscopic interventions become well established in veterinary medicine, it is important that the veterinary team is well versed in both surgical and anesthetic aspects of management to ensure a successful outcome.
References
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