Anesthetizing animals: Similar to humans yet, peculiar?

From time immemorial, animals have served as models for humans. Like humans, animals too have to undergo several types of elective and emergency surgeries. Several anesthetic techniques and drugs used in humans are also used in animals. However, unlike humans, the animal kingdom includes a wide variety of species, breeds, and sizes. Different species have variable pharmacological responses, anatomy, temperament, behavior, and lifestyles. The anesthetic techniques and drugs have to suit different species and breeds. Nevertheless, there are several drugs and many peculiar anesthetic techniques used in animals but not in human beings. Keeping this in mind, literature was hand searched and electronically searched using the words "veterinary anesthesia," "anesthetic drugs and techniques in animals" using Google search engine. The interesting information so collected is presented in this article which highlights some challenging and amazing aspects of anesthetizing animals including the preanesthetic assessment, preparation, premedication, monitoring, induction of general anesthesia, intubation, equipment, regional blocks, neuraxial block, and perioperative complications.

INTRODUCTION

“If all the animals were gone, men would die from a great loneliness of spirit, for whatever happens to animals also happens to man. All things are connected.”

Chief Seattle of the Duwamish Tribe in his letter to President Franklin Pierce.[1] Preanesthetic assessment, grading, investigations, optimization, preparation, many anesthetic drugs, and techniques are used in animals as in humans.[23] The contribution of animal experiments played a significant role in the evolution of anesthesia.[4] Nevertheless, several drugs and techniques not used in humans are being used in veterinary anesthesia. Knowledge of these aspects would be useful to us because often, drugs and techniques used in animals serve as models for humans. Anesthesia in some animals can be quite challenging because of their variable anatomy, lifestyle and aggressive behavior.[5] Keeping this in mind, literature was hand searched and electronically searched using the words “veterinary anesthesia,” “anesthetic drugs and techniques in animals” using Google search engine. The information so collected is presented in this article, which highlights some interesting, peculiar aspects of veterinary anesthesia.

PECULIARITIES OF PREANESTHETIC ASSESSMENT IN ANIMALS

The patient's signalment (species, breed, sex, and age) is assessed. E.g., brachycephalics often have upper airway obstructive disease. Doberman dogs are likely to have von Willebrand's disease.[6] Diabetes mellitus (DM) and dilated cardiomyopathy are more common in certain breeds of dogs.[7] Species variation in drug biotransformation and pharmacological response may be encountered, e.g., horses, dogs, and cats require 10 times the dose of xylazine than cattle.[8]

A history of exercise intolerance or syncope especially in a dog would indicate cardiomyopathy. Polydypsia, polyuria, and weight loss would suggest DM in dogs.[7] Chronic renal disease, common in dogs[9] is the most common cause of hypertension; in cats renal disease and hyperthyroidism can cause hypertension.[10] The myxomatous degenerative valvular disease is the most common cardiac disease in dogs.[11]

There are some peculiarities regarding clinical examination in animals. In rats and rabbits, the color of light reflected from the eye is useful to detect pallor and cyanosis.[8] The hydration status can be assessed by performing a skin tent in between the scapulae and by assessing the “slipperiness” or the moisture on the gums.[12] It is important that the temperament of the animal should be assessed. The blood pressure (BP) cuff may be placed on a limb or the tail. Edema appears in different regions in different species in the limbs and along the ventral body wall in horses, in the brisket region in cattle and the abdominal cavity in dogs and cats.[9] There can be interbreed and interspecies differences in BP.[13]

PREOPERATIVE FASTING

In large ruminants, food is withheld for 24–48 h and water for 12 h[14] whereas fasting is not necessary for small mammals as vomiting during induction does not occur in them.[15]

In horses, a full stomach may rupture when the horse falls to the ground on induction. In ruminants, a few hours of starvation reduce the fermentation rate within the stomach, thus delaying the development of tympany when eructation is suppressed by general anesthesia. Prolonged fasting in horses predisposes to postanesthetic colic.[9]

PREOPERATIVE AND ANESTHETIC MEDICATIONS

Various drugs which are not used by human beings are used in animals preoperatively [Table 1].[16171819202122] Behavioral, manual, and mechanical restraint methods are used in some animals before premedication.[23]

Table 1

Anesthetic drugs used only in animals

Drugs can be injected by direct delivery systems, handheld injections, pole syringes for cooperative animals and by remote delivery systems with air activated darts and blow pipe projectors for noncooperative animals.[1623] For injections, thick skin can be a problem in many animals.[5] Intramuscular absorptions are most rapid in the neck, shoulder, and hip; in that order in animals.[16]

MONITORING IN ANIMALS

Though similar to humans, there are some special features. There are very peculiar signs used to assist in monitoring depth of anaesthesia in animals [Table 2].[82224] Though similar to humans, there is some special feature. The peripheral artery used for counting heart rate by palpation includes the femoral artery in dogs and cats, the lingual artery on the ventrum of the tongue in dogs, the facial or metatarsal arteries in horses, the femoral, median or auricular arteries in ruminants and pigs. The lowest acceptable heart rates during anesthesia are 50–65 beats/min for most animals and 26 beats/min for horses [Figure 1].[9]

Table 2

Signs used to assist in monitoring depth of anesthesia in animals

Figure 1

Counting heart rate by palpation of lingual artery on the ventrum of the tongue (in a dog)

Doppler flow meter technique can be readily performed for arterial BP measurement in any sized animal.[2526] The auscultatory technique is not generally useful in animals as the amplitude of sounds associated with limb blood flow is difficult to ascertain with just a stethoscope.[26] Oscillometry is unreliable for animals weighing <5 kg.[25]

In most animals, the cuff is wrapped around an extremity proximal to the probe or around the tail base.[27] The pulse oximeter probe placement sites include the tongue, earlobe, lip folds, toe pads, axillary or inguinal skin fold, prepuce or vulva.[25] Some monitors using infrared absorption spectrometry principles cannot be used for horses or ruminants as they will measure exhaled methane and record the concentrations as halothane.[9]

SOME PECULIAR METHODS OF ANESTHESIA INDUCTION IN ANIMALS

The right intraperitoneal injection route is used especially in rodents and rabbits.[1928] Induction chambers made from plexiglass boxes or plastic bags are used for small mammals like cats.[16] They can cause stress, delayed airway control, and environmental contamination [Figure 2].[29]

Figure 2

An induction chamber

Almost every horse undergoing anesthesia will be standing at induction. Free standing wherein the horse is induced and allowed to fall completely unsupported, support by personnel, tilt tables, squeeze box, swing door, slings, and belly bands are some methods used to aid consciously standing horses to a state of unconsciousness [Figures 3 and 4].[30]

Figure 3

Induction of general anesthesia in a horse using tilting table top

Figure 4

Induction of general anesthesia in a horse using tilted table top

Xylazine is delivered using small darts, which result in accurate delivery in conjunction with ketamine, etorphine, and carfentanil in capturing wild animals.[31]

Many minor surgical procedures in animals like horses can be performed in the field under sedation or general anesthesia.[32]

For aquatic animals like fish, anesthetics like tricaine methanesulfonate, clove oil and isoflurane are mixed in water in the fish tank for induction. For long duration surgeries, a submersible recirculating water pump is used to pump the anesthetic diluent water into the fishes' mouth, past the gills and out through the operculum.[15]

AIRWAY MAINTENANCE

Ruminants are mostly intubated for general anesthesia.[16] Due to a wide variety in beak and bill sizes, air tight face mask placement is difficult [Figure 5]. In many animals, copious, viscid oral, and nasal secretions can cause difficulty in airway maintenance.[15]

Figure 5

Animal face masks

In small animals, the patient is intubated in the sternal recumbency position. After induction, an assistant holds the patient's head, opens the mouth and extends the head and neck. With the other hand, the assistant pulls the tongue out of the mouth and down. The anesthetist does a laryngoscopy and does the endotracheal intubation. Intubation in lateral and dorsal recumbency is more difficult than in sternal recumbency. The most reliable method to judge the appropriate endotracheal tube (ETT) diameter is by gentle tracheal palpation. Another subjective method involves using the width of the nose between the nares as an approximation of tracheal diameter.[33] Regarding the tracheal size, there will be some variation by breed.[34]

Camelids, cervids, bovids, rabbits, and ferrets have long, narrow oral cavities so that the glottis becomes difficult to visualize. A long laryngoscope blade and tube exchangers are useful in them.[16] In some penguins, the median tracheal septum can interfere with intubation. In crocodiles, the ETT cuff cannot be completely inflated because of the completeness of tracheal rings.[16]

Large animal intubation is done using a mouth gag to open and keep the mouth open. The tube is passed into the larynx with a metal rod acting as a stiletto inside it. Later, the metal rod is removed.[35]

Intubation by palpation involves the insertion of the anesthetist's arm into the animal's mouth with the mouth gag applied and fingers separating arytenoids thus facilitating ETT passage.[35] 12–18 gauge plastic intravenous catheters for endotracheal intubation are used in guinea pigs, hamsters, rats, and mice. In a tracheal obstruction in birds, caudal thoracic air sacs can be cannulated.[16]

RECOVERY FROM GENERAL ANESTHESIA/SEDATION

During emergence from anesthesia, ruminants, and brachycephalic breeds are positioned in sternal recumbency.[816] The ideal setting for reversal in animals is a quietened, darkened area.[32] Horses are especially subject to excitement and struggling during recovery. Large animals such as horses and ruminants should be recovered in an open grassy area or be removed to a stall which should be padded to minimize injury during emergence from the anesthetic.[8] A halter and lead shank are attached to the horse, which is recovering outdoors because some horses can rise quickly and attempt to run away.[32] An anesthetized animal is never placed in the same cage with a conscious one because the former cannot protect itself. The water pan is removed from the cage to prevent accidental drowning of the semiconscious patient. Postepidural anesthesia, the hind legs are hobbled to a degree sufficient to prevent abduction until muscle control is regained.[8]

LOCAL ANESTHETICS, REGIONAL BLOCKS, CENTRAL NEURAXIAL BLOCK IN ANIMALS

The safe use of loco-regional anesthesia techniques in animals requires a thorough understanding of the relevant anatomy of the species of interest.[36] Local anesthetic techniques are primarily used in cattle, buffaloes, and equinus.[37] Both epidural and spinal blocks are useful. The muscle relaxation produced by central neuraxial block restricts movements and causes recumbency. The caudal epidural technique is most commonly used in animals such as horses and cattle for obstetric manipulations, recto-vaginal, and tail surgeries. Morphine, xylazine, and detomidine, are some adjuncts used. Proximal and distal paravertebral blocks are used in cattle for standing laparotomies including cesarean sections. A fully developed unilateral block produces a curvature of the spine, the convexity of which is toward the analgesic side [Figure 6].[32437]

Figure 6

Curvature of the spine toward the analgesic side after a paravertebral/lumbar epidural block.

VETERINARY ANESTHETIC EQUIPMENT

Pole syringes used in animals are handheld injections with the added safety of long poles.[1623]

Magill ETT (plain or cuffed) are most commonly used and are available in a variety of sizes from 3 to 40 mm internal diameter. Plain tubes are used in very small animals such as cats and birds.[38] The cole tube is commonly used in birds because they have complete tracheal rings that do not expand to accommodate a cuff and also in pediatric patients where tracheal rupture from cuff inflation is a concern. Reinforced tubes are used for some procedures.[33] The laryngeal mask airway is useful in animals prone to difficult intubation.

Nonrebreathing circuits such as Bain and Ayre's T-piece are used for patients weighing <7 kg and rebreathing systems like closed/semi-closed for patients weighing more than 7 kg. Bags up to 30 liters are available.[3439] For mobile inhalant anesthesia, tripod units that can be set up anywhere including hillsides are available. Specially insulated inhalational anesthesia units are available.[40]

PERIOPERATIVE COMPLICATIONS

Animals like pigs can develop hypothermia when sedated or anesthetized because of little body hair. Long haired animals with thick undercoats can become hyperthermic.[9] Some postoperative complications occurring specially in large well-muscled horses and ruminants include myopathy especially when intraoperative hypotension occurs, radial, facial and femoral neuropathy, colic and laryngeal paralysis; if the neck is over-extended during surgery.[1430] The self-inflicted injury commonly occurs upon recovery in horses due to their size, temperament, and flight instinct when they try to stand before they are fully ready. In wild animals, capture myopathy can occur with exhaustive endurance.[1641] When large species are restrained in recumbency, restraining ropes, and bands may restrict respiration or compress nerves and muscle groups severely, and colonic rupture can occur.[1641]

Bloat, regurgitation, and aspiration can occur after general anesthesia in ruminants because of their physioanatomic peculiarities.[14] Species characteristics can contribute to perioperative risks e.g., an increased risk of narrow tracheas and airway obstruction in brachycephalic dogs, malignant hyperthermia in some strains of swine, laryngospasm in cats. Unfamiliarity with the species in exotic animals results in high clinical mortality.[15]

CONCLUSION

Anesthetizing animals involves using a wide variety of drugs, positions, techniques, and equipment, which has to suit the wide variation in animal species, breed, size, anatomy, lifestyle, temperament, and location. Hence, it is more challenging than in humans.

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Conflicts of interest

There are no conflicts of interest.