Pigs, sheep, and cows: getting them down and back up again
Large animals offer registered veterinary technicians special anesthetic challenges when compared to small animal species. Their size and anatomy make anesthesia particularly risky, especially with ruminating animals, due to the increased risk of bloating. Repeated rolling can create a displaced stomach, which if not corrected, can be fatal. Regurgitation and aspiration are also a constant threat particularly before and after intubation while the animal is in lateral recumbency.
The danger of regurgitation can be minimized by:
1. Fasting 12-24 hours prior to anesthesia, depending on the age of the animal.
2. Depriving water for approximately 8 hours prior to anesthesia.
3. Elevating the neck (in lateral recumbency) to avoid regurgitation. Position the head sloped down to facilitate drainage of saliva and other intraoral materials.
4. Passing a stomach tube to allow drainage of ruminal materials (and gas) during recumbency.
5. Clearing solid materials from the mouth at the end of anesthesia, and leaving the endotracheal tube with the cuff inflated until the animal is in sternal recumbency, is swallowing, and is able to withdraw its tongue into the mouth.
Creating an anesthetic protocol
Medication selection should take into account the type of procedure (painful vs. painless), health status of the animal, and side effects of specific medications. Controlling pain management prior to pain stimulus is vital and starts with premedication. This must be addressed for every patient, in accordance with animal welfare regulations and our veterinary oaths.
Species such as cattle, sheep, and goats tend to be docile creatures, and prior sedation may not be necessary for anesthetic induction. Placing an IV catheter facilitates an easier induction, however IV injection of induction agents can be done without an indwelling IV catheter. IV catheterization in large animals is fairly simple in ruminants, and more complicated in other species, like pigs. In ruminants the jugular vein is easily catheterized either awake or sedated using an 18g, 16g, or 14g long IV catheter. In awake animals, such as cattle, that are less easily restrained, a small local block may be necessary. The cephalic and saphenous vein are accessible in ruminants, but caution must be taken if attempting to place in an awake animal due to kicking.
Sedation
Pigs require sedation or tranquilization prior to induction. Combinations of an alpha-2 adrenergic agonist and ketamine or Telazol IM are often used. The neck muscle groups, lumbar, and gluteal regions work well for intramuscular access. An IV catheter can be placed while the animal is under gas anesthetic via a tight fitting mask. Induction via gas anesthetics is not uncommon in pigs, but IV induction is preferable due to quicker intubation and decreased anesthetic gas exposure for staff.
Pigs pose a challenge when it comes to IV access; the ears are the easiest site for catheterization. The cephalic and saphenous vein can be used but these sites are challenging requiring a blind attempt with a large amount of soft tissue to penetrate. A surgical cut down to the jugular vein may be appropriate in certain instances. In all large animal species a sheath in a femoral vein for fluid or medication delivery and artery for blood collection or invasive blood pressure monitoring is most practical if the patient is in ventral or lateral recumbency.
Anticholinergics, such as glycopyrrolate and atropine, are common practice, but tend to thicken the saliva of animals making expulsion difficult if aspiration occurs. Atropine is usually avoided because it can slow gastrointestinal motility, which can be devastating in ruminant species. Unless the animal is very young, a balanced anesthesia approach should alleviate the need for anticholinergics in premedication cocktails. Anticholinergics should be at hand in the event of arrhythmias, most commonly bradycardia, but not be used to correct bradycardia associated with dexmedetomidine.
A crystalloid fluid is most commonly used at a rate of 5-20ml/kg/hr depending on the animal’s hydration status, expected or observed blood loss, and blood pressure.
Intubation
Immediately after induction, intubation is recommended to reduce the risk of aspiration in ruminants. Intubation of small ruminants such as sheep or goats will require two people; one to support the body in a ventral position while holding the mouth open, and the other to press firmly on the back of the tongue to visualize the epiglottis using a long laryngoscope. This is best achieved by applying lidocaine to the tracheal opening using a red rubber catheter or an atomizer. Then, a stylet or the endotracheal tube can be placed. The cuff should be inflated to prevent fluids from dripping into the lungs.
For larger ruminants, like cattle, intubation is often performed in lateral recumbency. A mouth gag is placed so that an operator can insert an arm into the animal’s mouth to palpate the epiglottis and guide the tube down the trachea. Young animals, depending on their size, can be treated like sheep or goats. In some difficult instances an endoscope may be used for visualization.
Pigs are intubated most easily like a dog, however difficulties arise with the limited ability to open the mouth Pig intubation videos can be viewed at the link below.
Patient monitoring
Anesthetic depth assessment, as in small animals, involves eye position, jaw tone, palpebral reflex, and stimulus response. Slight changes in vital signs are expected with an increase in heart rate or blood pressure, but should plateau without causing tachycardia or hypertension.
Blood pressure monitoring is vital in any anesthetic case; the gold standard is via the arteries, which is fairly easy in large animals. Maintaining the mean arterial pressure above 70mmHg is recommended. Bradycardia can be tolerated, especially when alpha-2 adrenergic agonists have been used, as long as the blood pressure is stable. Assessing limb perfusion under pressure is necessary for longer procedures, and plenty of soft padding should be provided. Hypotensive animals can be treated by assessing the anesthetic depth, followed by fluid boluses of 10-15ml/kg.
Keeping the patient warm is imperative under anesthesia. If a large body cavity is open for an extended period of time prior to closing, a warm saline lavage may help increase a hypothermic patient’s core body temperature.
Complications such as hypoglycemia are not uncommon in young animals, especially after fasting. A balanced crystalloid fluid like LRS or NaCL should be used with 50% dextrose.
Since the viscera can push against the diaphragm, interfering with normal tidal volume, a mechanical ventilator is highly recommended to alleviate this effect and prevent atelectasis. Normal capnography readings have a normal end tidal CO2 reference range of 35-45mmHg. Large animals, whose respiratory system is not appropriately maintained, can suffer from acid-base disorders and prolonged recoveries.
Some surgical cases may require total IV anesthesia. Fentanyl and diprivan combinations or morphine, lidocaine, and fentanyl can be used. A reliable and cost-effective method is to use guaifenesin/ketamine/xylazine CRI. This combination, run at low rates in addition to local blocks, is also ideal for standing anesthesia, commonly performed in cows.
Recovery
Prolonged procedures can create pressure ulcers, poor limb perfusion, and/or organ and nerve damage. Following surgery, the anesthetist should ensure that all appropriate analgesics have been administered to help prevent pain wind-up. Extubation of ruminant species is achieved with the cuff semi-inflated, acting like a squeegee. While in lateral recumbency, elevating the head and neck slightly decreases the risk of aspirating saliva and rumen fluid. The tube should be removed as soon as the patient can swallow or chew. Regular TPR checks should be performed until the animal is able to stand. Prolonged recoveries may require staff to massage or provide passive range of motion with the limbs to increase circulation and reduce post-op stiffness. Eating soon after an anesthetic event is encouraged to promote GI health.
Three useful pig intubation videos can be found at:
www.k2publishing.ca/cital1.mov
www.k2publishing.ca/cital2.mov
www.k2publishing.ca/cital3.mov
This article is based on Mr. Cital’s presentation at the Ontario Association of Veterinary Technicians Conference in Niagara Falls, ON.CVT
