Precautions during Anesthesia
Judging the depth of general anesthesia requires assessment of several reflexes and, more importantly, response to surgical manipulations (Gray and McDonell 1986b). Some jaw tone persists under deep anesthesia, but swallowing motions usually mean that the anesthetic depth is lightening.
Palpebral reflexes are usually lost during surgical anesthesia, but the corneal reflex is maintained at all levels. The pupils dilate during light anesthesia or when cerebral hypoxia occurs during deep anesthesia. Some authors report that the globe does not rotate in response to the depth of anesthesia of the goat (Riebold 2015), while others claim that it does (Clarke et al. 2014).Arterial blood pressure can be monitored as an indicator of circulatory adequacy during anesthesia (Wagner and Brodbelt 1997). The normal mean arterial pressure during anesthesia is 75-100 mm Hg (Riebold 2015). Hypotension (defined as mean arterial pressure less than 60 mm Hg) results in underperfusion of the brain and kidneys. An over- the-needle Teflon catheter is inserted into an auricular, saphenous, or common digital artery for direct blood pressure measurement, or an ultrasonic flow detector can be placed over a limb artery (Wagner and Brodbelt 1997; Clarke et al. 2014; Riebold 2015). Administering IV fluids and decreasing the depth of anesthesia usually improve blood pressure. When available, a pulse oximeter attached to tongue, ear, interdigital skin, teat, rectum, or tail permits monitoring of pulse rate and hemoglobin saturation (Hodgkinson and Dawson 2007; Snyder 2007).
When anesthesia is apt to be prolonged, lateral recumbency is preferred to dorsal recumbency. Left lateral recumbency reduces the risk of regurgitation, compared with positioning on the other side (Trim 1987). While the animal is in dorsal recumbency, the weight of the rumen and intestinal contents (and the pregnant uterus) compresses the aorta and posterior vena cava and restricts movement of the diaphragm.
The function of the lungs is also markedly impaired in lateral recumbency, so efficiency during surgery becomes important to the survival of the goat. Intermittent positive pressure ventilation via use of an Ambu bag and oxygen supply is helpful for correcting respiratory acidosis when general anesthesia is prolonged, especially when in dorsal recumbency.If an endotracheal tube is not used (e.g., when abdominal surgery is performed under tranquilization and local anesthesia), the goat should then be placed on a slanted surface so that the abdomen is below the thorax. This reduces the pressure of abdominal viscera on the lungs. Rumen fluid is less likely to rise to the mouth and be inhaled. At the same time, the head should be placed lower than the neck to permit saliva to drain from the mouth (Seddighi and Doherty 2016). Ruminal tympany can be controlled by passing a stomach tube or by centesis of the rumen with a 14-gauge needle or trocar. After surgery is complete, the goat should be propped in sternal recumbency to facilitate eructation.
During prolonged general anesthesia (longer than 2 hours) the goat may lose a large quantity of saliva (up to 500 mL/hour). This leads to acidosis unless IV replacement fluids containing sodium bicarbonate are supplied. Care must be taken to support ventilation if bicarbonate is administered parentally. Serial blood gases can help determine any acid-base disturbances, the etiology thereof, and potential treatments necessary. It is also possible to collect the saliva in a bucket and return it to the goat via stomach tube after surgery (Linzell 1964). Barbiturate anesthesia is associated with a greater loss of saliva and hence of bicarbonate than is inhalant anesthesia with halothane or methoxyflurane (Edjtehadi and Howard 1978). Hypothermia is another serious problem associated with general anesthesia, especially in neonates or if the animal has been held off feed. When surgery is performed on the farm, the goat should be kept in a heated building or supplied with a heating pad, heating blanket, or lamp, or with hot water bottles, until it is up and actively feeding again.
If the goat is pregnant, care should be taken to minimize the risk to the fetus (Ludders 1988). This means reducing the dosage of preanesthetics and anesthetics to the minimum necessary for the procedure, avoiding hypoxemia and hypovolemia of the dam, using increased oxygen flow and intra-operative IV fluid administration, and keeping the patient warm to speed recovery. If good placental perfusion and oxygenation are not maintained, the fetus may be aborted after the anesthetic episode.