Most horses in which anesthesia is maintained with inhalant anesthetics receive over 90% oxygen from the anesthetic machine. Providing room air (e.g., 0.21 FiO 2) results in low arterial oxygen levels that could impair oxygen delivery to tissues in some horses therefore an FiO 2 of at least 30% to 50% is recommended for prolonged inhalant or intravenous general anesthesia. Unfortunately, without blood gas analysis it is difficult to accurately assess an individual horse's oxygen status. The optimum FiO 2 in adult spontaneously breathing or ventilated horses that provides optimum oxygenation (PaO 2) while minimizing the development of atelectasis has not been determined. 13, 19 Unlike the spontaneously breathing horse, oxygenation and the degree of shunt remain constant over time in horses that are mechanically ventilated unless abdominal tympany develops. 31, 39 Mechanical ventilation, when instituted early in the anesthetic period, results in a lower shunt fraction than in spontaneously breathing horses by minimizing the development of airway closure and atelectasis. 38, 39 Once atelectasis develops in the spontaneously breathing horse, it will likely persist even after return to a lower FiO 2 unless some method of alveolar recruitment is used ( Box 17-6). Experimentally, oxygen supplementation resulting in an FiO 2 >85% in anesthetized spontaneously breathing horses increases the degree of intrapulmonary shunt. Nitrogen in the alveoli normally has somewhat of a protective (“splinting”) effect since it is not readily absorbed by the blood. Increased FiO 2 values administered with inhalant anesthesia “wash out” alveolar nitrogen (80% of room air) and the replacement of nitrogen with an easily absorbed gas (oxygen) is likely responsible for time-related increases in shunt fraction. 10, 13, 19, 26, 38 Atelectasis and the increase in ventilation-perfusion inequalities are responsible for the progressive time-related decrease in PaO 2 and increase in pulmonary shunt fraction during elevated FiO 2. 10 Specifically PaO 2 values remain stable over time or may decrease in laterally recumbent anesthetized horses spontaneously breathing >90% oxygen, whereas the PaO 2 decreases with time in horses placed in dorsal recumbency. Temporal changes in the response to oxygen supplementation on PaO 2 have been reported in the spontaneously breathing horses administered an inhalant anesthetic and breathing >90% oxygen. Nonetheless, PaO 2 generally increases to values that result in a saturation of oxygen (SpO 2) of greater than 95%. 38, 39 The PaO 2 does not increase in direct proportion to FiO 2 as a result of ventilation-perfusion inequalities and intrapulmonary shunt. ![]() 18, 26, 38, 39 The PaCO 2 also increases slightly during oxygen supplementation in spontaneously breathing horses because of the removal of the stimulation of the respiratory center by low PaO 2. 13, 15 Supplementation of the inspired gases with oxygen (increased FiO 2) in spontaneously breathing or mechanically ventilated anesthetized horses consistently increases PaO 2 and the oxygen content of arterial blood (CaO 2). Recumbency in anesthetized adult horses consistently results in a decrease in PaO 2 values as a result of a reduction in lung volume, ventilation-perfusion inequalities, and an increase in the intrapulmonary shunting of blood. ![]() McDonell, in Equine Anesthesia (Second Edition), 2009 Respiratory Effects of Oxygen Supplementation and Mechanical Ventilation To further improve visibility, use of surgical telescopes and a head lamp is beneficial, particularly for surgeries involving the caudal mandible or maxilla.Ĭarolyn L. In dorsal recumbency, the neck should be fully extended and the head end of the table slightly lowered. ![]() In addition, having continuous suction available is very helpful. The use of a cuffed endotracheal tube and pharyngeal pack is necessary to prevent aspiration. 14,27 The main hazard of dorsal and sternal recumbency is fluid aspiration. 14,48,50 For mandibulectomy procedures, the authors prefer sternal recumbency, with the head elevated and the maxillas suspended between IV poles or secured to an anesthesia screen. Therefore, dorsal recumbency is recommended for maxillectomy. 48 Although lateral recumbency offers good exposure of the buccal surfaces of the uppermost teeth and jaws, it only provides fair visualization of the palate and lingual surfaces of the opposite quadrants. Lateral recumbency is preferred by most veterinarians for mandibulectomy and maxillectomy procedures. Verstraete, in Oral and Maxillofacial Surgery in Dogs and Cats, 2012 Patient positioning
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