Expected Outcome
Under extreme conditions, hypothermia alters blood viscosity and coagulation pathways and will increase the likelihood of myocardial fibrillation. Smaller decreases in body temperature as likely to be observed in the horse will affect anesthetic dose requirements (MAC is reduced 5–8% per degree centigrade decrease in body temperature) and rate of clearance of anesthetic drugs [106, 107]. This has clinical relevance in that an individual may unknowingly over‐anesthetize a patient and likely prolong recovery from anesthesia.
Much attention is given to inadvertent perioperative hypothermia in human medicine as it is associated with increased morbidity (e.g. wound infection, coagulopathy) and prolonged hospital stay [108]. It is also associated with shivering in the recovery period, which not only increases metabolic oxygen demand but is also reported to be extremely uncomfortable 109. Although not much work has been done with respect to the complications associated with hypothermia in horses, hypothermia does occur routinely in anesthetized horses and is correlated with both increasing number of attempts and time it takes for a horse to stand in recovery [102].
Complications During Anesthetic Recovery
Poor Recovery Quality
Definition
Poor recovery quality could be defined in a number of ways, from simply uncoordinated to involving minor or even serious injury. While historically presumed cardiovascular events during the anesthesia period contributed to the mortality rate, recent information suggests that injury in the recovery period is the primary reason for peri‐anesthetic mortality in adult horses [59, 110, 111].
Risk factors
Length of procedure
Temperament of the horse
Physiologic status (e.g. systemically compromised colic, lactating mare)
Painful procedure
Pre‐existing long bone fracture
Placement of heavy bandages, splints, or casts
Slippery or uneven recovery surfaces
Light plane of anesthesia on transport to recovery stall
Pathogenesis
It is often said that if an animal has a poor induction, the recovery too will be poor. In the authors’ experience, neither a good nor poor induction has been a consistent predictor of recovery quality. Poor recovery quality or catastrophic injury in recovery likely does not have a single causative factor, and in some cases catastrophic injury can occur in the absence of an otherwise poor recovery (e.g. a horse stands in one relatively quiet attempt but suffers a long bone fracture upon standing).
Clearly the temperament of the horse may play a role [112] and learning with repeated anesthesia may play a role in improving recovery [113], but other factors, such as the general well‐being of the animal, nature of the procedure, use of analgesic and supportive medications, drainage of the urinary bladder, placement of a cast or heavy bandage, the environment and footing, assistance provided in recovery, etc. may all influence the recovery from anesthesia.
Prevention
Risk of catastrophic injury in recovery has resulted in the increasing prevalence, perhaps even routine, use of providing a sedative or tranquilizer to horses recovering from inhalation anesthesia. Studies have compared doses of injectable agents, different injectable agents [114, 115], transitioning from inhaled to injectable agents [116, 117], and more recently reducing inhaled anesthesia dose during procedures by concurrent use of injectable agents [23, 118, 119]. While largely favorable results support the use of these techniques, poor recoveries sometimes with disastrous consequences to the horse and injury to personnel still occur.
Assistance in recovery can take many forms, ranging from basic assistance on the tail to stabilize the animal and helping it rise during attempts to stand to recovery using a pool, air pillow, or sling. While much has been written on these techniques, there are no comprehensive studies to support use of any one method when other factors surrounding anesthesia management and logistical considerations (e.g. experience of personnel with the system) are applied. In the authors’ experience, the most broadly applied system and one that can be learned fairly quickly seems to be the use of head and tail ropes to help support and assist the horse to standing.
Treatment
The anesthetist should always be prepared for the catastrophic injury in recovery. A dose of sedation and anesthetic induction drugs should be readily available until the horse is safely standing, as treatment might include re‐sedating or anesthetizing the horse to facilitate diagnostic testing (e.g. radiographs) and intervention to manage the condition. Humane euthanasia may be necessary, depending on the situation.
Expected Outcome
Many horses can experience poor recovery quality, such as stumbling, flipping over, or making multiple attempts to stand without apparent harm or only minor injuries. There is obviously potential for long bone fractures to occur and outcome thus depends on the nature and location of the fracture as well as the owner’s willingness to pursue treatment.
Numerous studies have described complication rates during equine recovery, which are generally considered better following injectable medications than after inhalation anesthesia [40, 125, 126]. However, the overall reporting of mortality does not seem to have changed significantly, despite newer medications and advances in other aspects of anesthesia management [48].
The reader is referred to Chapter 16: Complications During Recovery from General Anesthesia.
Other Complications Associated with Sedative and Anesthetic Drugs
Increased Urine Output
Definition
Not necessarily considered a complication in its own right but one that might influence management of standing sedation and anesthesia is the notable increase in urine production following administration of alpha‐2 agonist medications to horses.
Risk factors
Use of alpha‐2 adrenergic agonists
Pathogenesis
Xylazine and detomidine have both been shown to increase urine production multi‐fold over normal in standing horses [127–129]. The mechanism by which alpha‐2 agonists increase urine production is primarily related to inhibition of anti‐diuretic hormone [37]. General anesthesia tends to reduce production [130] but it still remains above normal values of approximately 0.5 ml/kg/hour, even in water deprived animals [127].
Monitoring
Urine production can be assessed via the placement of a urinary catheter and measurement of urine output over the anesthetic period.
Treatment
There is no specific treatment, but diuresis of this magnitude may contribute to dehydration and should be considered when calculating intravenous fluid administration rates during sedation and anesthesia. In addition, horses sedated for standing procedures with alpha‐2 adrenergic agonist drugs will often shift body position or attempt to posture to void. Therefore, catheterization of the urinary bladder can be helpful for longer procedures and is especially recommended in female horses when procedures involve rectal or vaginal manipulation or when