Assisted Recovery in Horses Awakening from General Anesthesia

Summary

Many methods that have been developed over the past decades to assist horses during recovery from general anesthesia have decreased the incidence and/or severity of postanesthetic complications. Yet, none of them has completely eliminated the risk of severe or fatal trauma to the patient or injury to the personnel who assist in recovery. While no systematic outcome studies comparing some or all of the described techniques of assisted recovery in a selected equine patient population has been undertaken to date, it appears that sling and pool recovery systems, though not completely fail-safe, offer the best option for safeguarding surgical success (particularly after major orthopedic surgery) and minimizing the risk of injury in recovering horses.

Introduction

Due to significant advances in equine anesthesia and analgesia over the past decades the majority of equine patients recover from general anesthesia in unremarkable manner after uncomplicated elective surgery and anesthesia [1]. Nevertheless, recovery from general anesthesia remains a potentially life-threatening event for the horse and a challenge for the anesthetist, even when anesthetic and surgical management is routine. To a large part this is due to the horse’s flight behavior often prompting the animal to stand prematurely, i.e., before anesthetic agents and their depressant effects on mental, proprioceptive, and motor function have worn off. Based on previous enquiries into perioperative morbidity and mortality [2-6], anesthesia-related complications are significantly more common in the horse than in small animals with reported fatality rates reaching on average 1% for elective equine surgeries. When patients undergoing emergency procedures are included, mortality rates associated with surgery and anesthesia rise to 4 - 8% [6,7]. Injuries to the musculoskeletal apparatus (e.g., fractures, myopathy) rank among the top three complications associated with perioperative death [6]. Contributing factors include the temperament, age, size and body weight of the animal, duration of anesthesia, anesthetic protocol used, perioperative cardiopulmonary complications, degree of postoperative pain, and type of surgical procedure performed, with fracture surgery carrying a particularly high risk for complications during recovery [3,6].

While an appropriate environment (i.e., an adequately designed recovery stall) is a prerequisite to prevent injuries in the post-operative period additional techniques may be applied to support the animal during this phase. A wide variety of recovery protocols have evolved over time intended to assist the horse when awakening from general anesthesia. This chapter will provide an overview of the systems and methods in use, criteria based upon which they are selected, and their advantages and potential disadvantages.

Recovery Stalls

Features of recovery stalls designed to optimize conditions for safe recovery of horses from anesthesia have been described previously [8-11] and include:

• Close proximity to, but separate from, surgery room
• Size: 4 - 5 m2 (~ 43 - 54 ft2) for average sized adult horses (avoid oversize)
• Well padded walls and doors (at least 2.5 - 3.0 m [~ 8 - 10 ft] high) using robust surface material that can withstand high impacts and can easily be cleaned
• Soft floor with non-slip surface that provides sufficient traction even when wet, has a drainage system, and can readily be cleaned and disinfected
• Lockable front and hind doors that are secured by transverse bars and floor/ceiling bolts
• Holes high in front and back doors allowing head and tail ropes to be passed to the outside to facilitate assistance in recovery
• Light source that can be dimmed
• Heating/AC system that allows separate temperature control within the recovery stall

Additional features that should be considered to allow for various methods of assistance during recovery are:

• Door windows, cameras or observation platform to allow monitoring of the patient during recovery
• Recessed wall outlets for oxygen supply, suction lines, and electrical power
• Stout metal rings recessed into the wall of each corner of the stall well above a horse’s head height (approx. 2.5 m [~ 8 ft] above floor)
• Ceiling hook centered over the stall to hold a hoist or overhead rail hoist system
• Escape route for personnel attending the horse within the stall during recovery
• Movable 30 - 40 cm (12 - 16 inch) thick, vinyl-covered foam mattress
• Box with emergency drugs and surgical tracheostomy set to allow immediate assistance in cases of sudden airway obstruction or other crisis situations requiring re-anesthetizing of the patient

Pre-recovery Preparation

The horse, being a flight animal, tends to escape any "adverse condition" that frightens or hurts it, often leading to premature and violent attempts to stand when awakening from general anesthesia [10,12]. This applies particularly to recovery from inhalant anesthetics. Hence, any post-anesthetic or better pre-recovery treatment that will render the horse calm and reduce its pain will help improving the quality of recovery and thus reducing the risk of injuries, not the least because it allows the recumbent horse more time to exhale residual anesthetic gases before attempting to stand up [8-11]. Where appropriate, local anesthetics and/or opioids or α2 agonists may be administered systemically, intra-articularly, epidurally, or locally prior to or during surgery to provide significant pain relief that lasts into the recovery period [10,13-15]. Table 1 lists drugs used post-operatively for pre-recovery sedation and analgesia. Dependent on dose and drug (or drug combination) chosen, recumbency may be prolonged to such an extent that it becomes necessary to place the patient on a thick recovery mattress to prevent pressure injuries to neuronal and musculoskeletal structures.

It is essential to ensure a patent airway and adequate blood oxygenation as hypoxemia induces rapid muscle fatigue and causes restlessness both of which compromise the horse’s ability to safely stand up after anesthesia. Hence, it is recommended to leave the patient orally or nasally intubated throughout the recovery period or at least till the patient is actively swallowing, and to provide supplemental oxygen if necessary [8,10,11].

Dependent on the length of the anesthetic, type and volume of pre- and intraoperatively infused fluids, and drugs administered during premedication and anesthesia, it might be necessary to empty the horse’s bladder before moving the animal into the recovery stall. Not only does a full bladder lead to restlessness, more so in males than females, but it also increases the likelihood of urine discharge during recovery leading to a slippery recovery stall floor.

Padded bandages may be indicated in certain situations to avoid additional trauma in unsteady horses. Likewise padded hoods or leather helmets maybe required to protect surgery sites in the head from further injury (e.g., after eye, head trauma, or plastic surgery).

Techniques of Assisted Recovery

Most equine hospitals offer a variety of options to support horses during recovery from general anesthesia (Table 2). They may include more simple techniques such as use of personnel within the stall to assist the horse manually and/or the use of ropes to steady head and tail while the animal is lifting from the floor, or modified flooring (e.g., deflating air pillow system). While those recovery methods appear to be adequate for the vast majority of equine patients waking up from general anesthesia, they may not sufficiently safeguard repair of a major orthopedic injury as the studies by Young & Taylor [3] and Johnston et al., [6] demonstrate. Particularly for this group of high-risk patients more sophisticated techniques such as recovery within a sling or a pool have been developed to better protect the horse from injuries during recovery that jeopardize the success of the surgical procedure. However, each recovery method has its own advantages and disadvantages and none is suitable for every situation. Many factors must be considered when selecting the appropriate recovery system for an individual patient (Table 3).

Personnel within Recovery Stall Assisting Patient Manually

This technique is based on the concept that the horse should be kept in lateral recumbency as long as possible, i.e., until various signs (e.g., cessation of nystagmus, return of normal tongue tone, chewing, return of normal responses to environmental stimuli) indicate that the patient has regained sufficient mental function and muscle strength [10,13]. For this purpose, an experienced attendant kneels behind the horse’s head with one knee pushing on its neck and both hands holding its head, raising the horse’s nose and stretching it backwards. This prevents the horse from swinging its head ventrally, which is essential for it to move into sternal recumbency and then to get up. If the horse is still weak it will not be able to overcome the resistance of the attendant. A second person, if available, may stand behind the croup of the horse and will grasp the tail the moment the horse is allowed to get up. Pre-recovery sedation and analgesia (Table 1) is often helpful to minimize struggling in the early post-anesthetic period and lasts usually between 15 and 40 min. Most horses stand within 1 hr of discontinuing anesthetic drug administration. Placing the animal on a mattress should be considered if a longer period of recumbency (> 1 hr) is anticipated. Once the horse is judged as having regained adequate consciousness and muscle strength, it should be allowed to roll sternal and then stand-up when it tries next. In this situation, the two attendants may or may not be able to control the movement of the horse. Once standing, they should immediately take hold of the halter and tail to keep the animal from moving until it has regained sufficient coordination to walk safely, thereby preventing any further harm. Only persons familiar with equine recoveries should be allowed to "hand-recover" horses.

Head and Tail Rope Recovery

This method is very similar to the manual assistance and is based on the same concept (Fig. 1) [8,10,11,13]. It requires that the corners (or opposite walls) of the recovery stall are equipped with recessed rings and/or front and hind doors of the recovery stall have holes through which head and tail ropes can be passed. Ropes should be strong enough but not too stiff to allow tying of knots for secure attachment to head (or halter) and tail and to ensure smooth gliding through rings and/or door holes. Once the decision has been made that the horse is awake and strong enough to get up, the attendant leaves the position at the head. Both (or more) helpers can assist the horse in rising from lateral to sternal and then standing position by pulling on the ropes while keeping a safe distance from the recovering animal and yet providing some balance support to the horse. Tension on the tail rope reduces the forward momentum of the horse as it tries to stand up. Once the horse is standing head and tail ropes help keep it steady. With head and tail ropes attached, the horse can only use about one third of the recovery stall, which may be particularly helpful if the recovery box is rather large.

Figure 1. Use of a foam rubber mattress and head and tail ropes to assist a horse during recovery from anesthesia. The bottom front leg is pulled forward to lessen pressure on the dependent limb and all lower limbs are protected with gaiters against injury. Tubing entering one nostril provides oxygen supplementation. The horse is positioned so that it will roll off the mattress as it moves into sternal recumbency.

Deflating Air Pillow

The recovery of horses on an air cushion has previously been described as significantly improving recovery quality [9]. The rationale behind this concept is to keep the animal recumbent until adequate mentation and muscle strength have returned, yet avoiding any risk of injury to personnel. The system utilizes a rapid inflation-deflation air pillow made of robust vinyl that is placed on top of a 5 cm (2 inch) thick high-density foam mat covering the entire floor of the recovery stall. When fully inflated the cushion is 46 cm (18 inch) thick and completely filling the floor of the recovery box. Elastic ropes attached to each corner of the vinyl cover top are attached to the corners of the recovery stall, keeping the top surface under tension when the pillow is later deflated. Following surgery, the anesthetized patient is placed in the center of the recovery stall on the deflated air mattress and then doors to the recovery box closed. The pillow is rapidly filled with air using a fan outside the recovery stall that continuously pumps air through a hose-like duct into the mattress keeping it equally inflated. The soft pillow hinders the horse from rolling sternal - even during aggressive attempts to rise - while at the same time protecting it from injuring itself. In addition, the soft cushion provides uniform support to the dependent part of the body preventing pressure injury to nerves and muscles. Once the horse is judged adequately awake, the fan is switched off and disconnected from the air hose which is folded underneath the pillow, and two 92 cm (36 inch) zippers along the sides of the air mattress are opened to allow for rapid deflation. As soon as the pillow is flat, the horse can attain sternal recumbency and safely stand.

Large Animal Vertical Lift

The large animal lift (LAL) was developed very recently as a robust but simple and lightweight device for assisting horses to stand and which can be easily put on a recumbent horse. Two versions of this support system are commercially available. In both the main component is an aluminum spread bar, to which chest and body slings are attached and which can handle body weights of at least up to about 1000 kg (2200 lbs) (Fig. 2). In the postanesthetic period, vertical animal lifts are particularly helpful to assist very heavy and/or muscle fatigued horses that are unable to rise on their own. As the horse attempts to move from lateral to sternal recumbency it is lifted off the ground into a standing position (see www.largeanimallift.com for details). A head and tail rope helps to stabilize the patient during this process.

Figure 2. Model of a large animal vertical lift. (Courtesy of Häst, Inc.)

Sling Recovery

In horses, the high risk of injury to the musculoskeletal system during recovery has been known since the early days of equine anesthesia and has lead to the design of a broad variety of different large animal sling systems. The history of this development has recently been described in detail [16]. Sling systems are particularly useful for horses waking up following surgical bone repair by internal fixation. In these patients, any misstep or excessive weight loading of the affected limb during recovery may provoke implant failure or disruption of the fracture. In addition, they are advantageous in patients in which an external skeletal fixation device or transfixation cast has been applied, those with neurological or musculoskeletal dysfunction, or those in poor body condition.

Any sling should be placed on the horse when it is still anesthetized or very sedate. Should the decision to use a sling system be made after the animal has already tried to stand but was not successful, it should be re-anesthetized or at least heavily sedated unless the horse is very calm and cooperative. Injection or infusion of an α2-agonist (detomidine, medetomidine) may be considered to keep the patient deeply sedated and pain-free, allowing sufficient time for elimination of residual anesthetic. After 30 - 60 min, the patient should be allowed to slowly wake up from sedation. If recovery becomes prolonged, one may administer an α2-antagonist (yohimbine 0.05 - 0.10 mg/kg or atipamezole 0.05 - 0.10 mg/kg) intramuscularly or intravenously [11]. Once the horse looks bright and is making an attempt to get up, the sling should be lifted to raise all four legs just off the ground and then the animal lowered again. If the horse is awake enough it will stand as soon as its hoofs touch the floor.

a. Sling-Shell Recovery System

The sling-shell recovery system was developed at the University of Berne for assisting horses during recovery from inhalant anesthesia and is a sling suspension system for which retrospective data have been reported [12]. The apparatus consists of two customized glass-fiber enhanced plastic shells which fit adult horses (280 - 685 kg [616 - 1500 lbs]) and are connected to each other by a short girth (Fig. 3). One shell matches the contour of the front breast while the other fits around the ventral part of the thorax just caudal to the elbows. The purpose of the shells is to support most of the animal’s weight in the front without causing pressure damage to blood vessels or nerves or impairing expansion of the rib cage during breathing. Transverse girths passing in front and behind the thighs support the hind part of the body.

Figure 3. Sling-Shell-System. The system is first suspended on four hoists to ensure equal girth lengths on each side (A) and then put on the patient while still recumbent (B). Once considered sufficiently awake, the horse is lifted in the sling into standing position (C) (Courtesy of Prof. U. Schatzmann, University of Berne, Switzerland).

These and two straps attached to the edges of the shells are hooked on four hoists (250 kg (550 lbs) lifting capacity each) suspending the sling system. The head is supported by a leather halter affixed to the ceiling of the recovery stall by means of a rope and also attached to a ring in the wall. To mount the recovery system the anesthetized patient is turned into dorsal recumbency and the shells put on chest and ventral thorax. Subsequently the horse is rolled back into lateral recumbency and the straps attached to the four hoists and tightened. A tail rope is then attached to a fifth hoist (500 kg (1100 lbs) lifting capacity) and the halter put on. As soon as the horse is reasonably awake and attempts to roll sternal, it is lifted up by all 5 hoists into a standing position with the head height being readjusted. Once the patient regains orientation and is steady, the sling system is removed.

Sling recovery was well tolerated in 72% of the 104 horses studied, associated with minor complications in 22 % of animals, and had to be aborted in 6% of cases, in which patients did not tolerate the sling at all despite sedation with xylazine.

b. Liftex Large Animal Sling

The Liftex sling is widely used in many veterinary hospitals and follows a common design that has emerged over time (Fig. 4). It is commonly put on horses with major musculoskeletal trauma already prior to surgery for safe induction of anesthesia and safe animal transport from the induction/recovery stall to the surgery table and vice versa or to and from a recovery pool. It has previously also been successfully used to assist horses to stand, similarly to large animal lift systems. The sling design has several advantages. The rear piece of the sling shall prevent the animal from backing out of the sling or dog-sitting in it. The breathable nylon fabric results in less sweating and can be adjusted to fit horses of different sizes. The fulcrum of suspension from the lifting rings can be adjusted to promote sternal over abdominal support, with the point of lifting of the sling being over the withers area. The design also offers flexibility in handling horses with injured shoulders or humerus fractures.

Figure 4. Horse in Liftex Large Animal Sling. Once awake, the animal is lowered to have initial ground contact with its feet (top) and then encouraged to stand firmly on the floor (bottom). Tension on head and tail rope keep the horse steady during this process.

c. Anderson Sling

This type of sling (Fig. 5) has been described in detail elsewhere [17-19]. It is a specially designed sling system attached to a metal frame that can be affixed to either an overhead hoist or a hydraulic apparatus with power supply that lifts and stabilizes a horse waking up from general anesthesia after surgery. Unlike most slings which support the horse by its chest and abdomen, the Anderson Sling supports the animal from the skeletal system, distributing its weight evenly and comfortably without interference with muscles, nerves, or respiratory function. In addition, the hydraulically controlled overhead frame provides the option for fine adjusting the weight distribution between front and rear as well as left and right limbs based on specific needs of an individual patient, i.e., allows a horse to bear weight on only the front or hind limbs, or even to adjust it in a manner to have only three legs bear weight while one limb is kept suspended and restrained. Finally, the design and additional padding makes this system significantly more comfortable allowing horses to stay in the sling for extended periods of time post surgery (weeks to months).

A previous retrospective study of 28 horses recovered in the Anderson sling-suspension system described recovery as satisfactory (i.e., "quiet and smooth" to "good" or "very good" to "excellent") in 93% of the cases without any horse sustaining direct injury or suffering any complication as a result of recovery in the Anderson sling [19].

Figure 5. Horse in Anderson Sling (Courtesy of Mr. Charles Anderson).

Pool Recovery

Waking horses up in a pool system may offer invaluable advantages over recovery in a sling system and can be lifesaving in cases that are at extremely high risk of unstable recovery due to the nature of the original surgical trauma (primarily long bone fractures and/or extensive soft tissue trauma), prolonged and complicated surgical procedures (mostly fracture repair by internal or external skeletal fixation techniques, major soft tissue procedures), central or peripheral neuronal disease, and/or the temperament of the patient [11,20]. The two pool recovery methods currently in use are described below.

a. Hydro-Pool

Recovery of horses in a Hydro-Pool system (for photo and video images visit www.vetmed.wsu.edu/depts-vth/images/pool_horse.jpg and www.pferdeklinik.unizh.ch/Inh_Klinik/aufwach.html, respectively) has previously been described in detail [8,20]. The rectangular water pool measures approximately 3.7 m long, 1.2 m wide, and 2.6 m deep (12 x 4 x 8.5 ft) and has a hydraulic, stainless steel grate floor that can be raised and lowered rapidly. Grate-covered drains that span along either side of the pool allow rapid water drainage as the mobile floor is raised. The pool water is heated close to body temperature (32 - 37°C; 90 - 100°F).

After surgery, the still anesthetized and intubated patient is lifted into the pool using a large animal sling or net and an overhead rail and hoist system. An air-filled inner tire tube is placed around the upper neck, or preferably an air-filled flotation device placed in front of the neck, allowing the horse to rest its head without risk of drowning and water aspiration. Two ropes are attached to the halter for subsequent cross-ties to support the head and restraining it during the recovery phase. The sling remains in place while the entire body is submerged in the water pool but does not support weight. While in the pool the animal has to be carefully monitored for signs of respiratory distress and cardiovascular dysfunction. It may be mechanically ventilated with oxygen using a demand valve and stays intubated at least until swallow reflexes have returned. A short-acting α2-agonist (xylazine) maybe administered at this time to calm the animal. Once the horse is considered adequately awake, the floor of the pool is initially raised until it touches the horse’s hoofs allowing the patient to bear some weight. If the patient demonstrates sufficient weight bearing capability and independently supports his head the pool floor is raised further until the water level reaches the mid-thorax. At this point, the inner tube or air-filled flotation device, cross-tie head ropes and sling or net are removed and the floor lifted to ground level and the patient walked from the recovery pool to a warm stall where it can be dried before leaving to the barn.

When horses shall recover in a Hydro-Pool certain precautions and concerns apply that must be taken into account:

1. Wounds should be closed in a multilayered fashion to allow for watertight closure.
2. Use of cyanoacrylate glue spray enhances effective water-proof closure of wounds, particularly when water-proof bandage material cannot be applied.
3. Water-repellant bandage and cast material should be used whenever possible.
4. Pulmonary edema development (17% risk) is the most common complication reported.
5. The horse may suffer skin abrasions when paddling vigorously along the pool walls.
6. The horse may try to exit the pool on its own if anxious, intolerant to the pool or awake.
7. The horse must be carefully dried after removal from the pool.

b. Pool-raft System

Protocol and clinical experiences with recovery of equine patients in the high risk category using a Pool-raft System (Fig. 6) have recently been reported [20]. The only system of this kind is located at the University of Pennsylvania using a round recovery pool (6.7 m (22 ft) in diameter and 3.4 m (11 ft) deep) that is surrounded by a cantilevered deck. The circumferential overhang ensures that the horse’s limbs are not hitting the pool wall when the animal paddles with its legs while floating in the raft. The pool is filled with water heated to a constant temperature of 36°C (96°F).

Figure 6. Pool-raft recovery of a horse. After surgery, the horse is sling-lifted into the raft (A) and then both raft and horse lowered into the water pool. The animal is floating in the raft while awakening from recovery (B) with its head resting on an air cushion attached to the raft (C). Once fully awake, the patient is lifted out of the raft to the adjacent recovery stall.

Following surgery, the anesthetized and intubated patient is lifted using a large animal sling and overhead monorail/hoist system into the pool room, where it is lowered in to a large (6-man) raft modified with four long rubber sleeves to accommodate the limbs of the horse and an additional air-cushion attachment at the front that is later inflated to protect the head from striking the pool deck or sinking into water. Sling and raft are suspended on independent hoists (1 ton load capacity each). Once correctly positioned in the raft, horse and raft are lowered into the pool. The raft is secured to rings on the pool deck, while the horse’s head is secured using cross-tied head ropes. At this point any tension is taken off the sling and a tail rope attached for later transfer of the horse to the recovery box. Oxygen supplementation and mechanical ventilation are provided at the discretion of the anesthesiologist. Sedation or tranquilization with xylazine or acepromazine may become necessary if the patient shows signs of severe anxiety or excitement early during the pool recovery phase or immediately before removal of the patient from the pool. Attendants observe the horse carefully while waking up. Return of normal mentation and tongue tone, cessation of nystagmus, and vigorous activity including forward striking with one or more extremities indicate full recovery. Immediately following one of those water treading activities that are tiring to the animal, the patient is blind folded and lifted out of the raft and transferred to the nearby recovery box using the sling. Head and tail ropes are passed through rings on opposite walls of the recovery stall to stabilize front and rear end of the animal when it is lowered in a standing position until its hoofs merely touch the floor, at which point the blindfold is removed and the horse encouraged to stand on its feet.

There are distinct advantages of this specific recovery system over the Hydro-Pool system:

1. Use of a flotation device avoids complications associated with complete or partial submersion of the animal’s body.
2. Wounds and wound bandages/casts stay dry better, reducing risk of incisional infection.
3. Significantly lower, if any, risk for pulmonary edema development.
4. Faster average recovery time.
5. Little risk of acquiring skin abrasions while in the pool raft.