Читать книгу Complications in Equine Surgery - Группа авторов - Страница 291

Definition

Unexpected movement (e.g. of the limbs or head) during anesthesia may be considered a complication, as it may result in injury to the horse or personnel and damage to surgical and anesthesia equipment.

Risk factors

 Inadequate dosing of anesthetic drugs

 Lack of experience monitoring or attention to anesthetic depth

 Ophthalmic procedures

Pathogenesis

Movement occurs primarily as a result of inadequate depth of anesthesia. However, it is the authors’ experience that some horses will move without warning, even when maintained at what appears to be an appropriate plane of anesthesia based on objective and subjective monitoring parameters.

Prevention

While it seems obvious that anesthesia depth influences movement on the surgical table and the answer is to keep the horse at a deeper plane of anesthesia, this is confounded by the negative effects of the inhalation agents on cardiorespiratory function and the potential for other complications (even death) with an anesthetic overdose.

The use of adjunctive medications with analgesic or inhaled anesthetic sparing properties may be beneficial [10–13]. Infusions of ketamine, lidocaine, alpha‐2 adrenergic agonists (e.g. xylazine, detomidine, romifidine, medetomidine, dexmedetomidine), guaifenesin, benzodiazepines, and propofol have been used for this purpose. When considering the use of any adjunctive drug, the cardiorespiratory effects must be weighed against the overall health status of the horse. For example, while alpha‐2 agonists have been shown to provide analgesia, reduce the minimum alveolar concentration (MAC) of inhalant anesthetics, and improve recovery quality in horses, they also cause significant decreases in cardiac output. Other drugs may have better cardiovascular effects but negatively influence recovery quality. Benefits and risks of individual drugs should be evaluated in context of the individual case.

The literature is inconclusive with regard to the benefit of systemically administered opioids as an anesthetic adjunct [14]. Mu opioids largely tend to increase MAC for inhaled anesthetics or do not enhance inhalant anesthetic sparing properties of other infusions [15–17]. Provision of regional opioids (e.g. intra‐articular or epidural administration) has been definitively shown to be beneficial for analgesia [18–20].

The use of regional anesthesia techniques (e.g. intra‐testicular block for castration [21], distal limb blocks) can minimize the potential for horse movement while also providing analgesia and a reduced need for systemically administered anesthetics.

For procedures in patients where involuntary movement is difficult to manage, neuromuscular blocking drugs could be administered to prevent further movement. The anesthetist is cautioned to ensure that horses are concurrently administered appropriate doses of anesthetic and analgesic drugs, as neuromuscular blocking drugs do not have either of these properties. The use of neuromuscular blockade alone to immobilize a horse should be considered inhumane. Additionally, the ability to provide positive pressure ventilation is critical.

Monitoring

Generally, the responsibility falls to the individual managing the horse’s anesthesia care to determine depth based on a combination of behavioral and physiological parameters. With inhalant anesthetics, a light plane of anesthesia is typically dictated by a brisk palpebral reflex, lacrimation, spontaneous blinking, and rapid nystagmus [22]. Heart rate, blood pressure, and respiratory rate can increase during a light plane of anesthesia and before movement occurs.

The use of anesthetic agent analyzers may be useful to guide inhalation anesthesia dose if available. Anesthetic depth assessment can be challenging when injectable anesthetic agents are used in combination with inhalants or as the sole means of maintaining recumbency as reflexes used to assess depth as described above are better maintained [23].

Much has been written on the use of adjunctive anesthetic techniques, and the reader is referred to in‐depth reviews of partial or total intravenous anesthesia in horses [24]. Documentation regarding movement during anesthesia however is sparse, but it is reported that horses undergoing ocular surgery are more likely to move during the procedure as compared to horses undergoing orthopedic procedures. This is perhaps related to the fact that ocular signs commonly used to monitor anesthetic depth are hidden from the anesthetist in these types of surgeries. Interestingly, the use of a gas analyzer improved the odds that horses would remain motionless, though horses undergoing enucleation still tended to move even when end‐tidal inhalant concentrations were monitored [25].

Some work has been done evaluating bispectral index (BIS) monitoring in anesthetized horses. This type of monitoring was developed to attempt to provide an objective measure reflecting the level of hyponosis of the patient based on electroencephalogram tracings, predominantly in human medicine where neuromuscular blockade is commonly used and accidental awareness is a particular problem. Data in horses is conflicting, but studies indicate that depending on the drug, BIS might be somewhat predictive of depth of anesthesia but not always of intraoperative movement.

Treatment

Adjustments can be made to anesthetic depth as described above, keeping in mind that should a horse become light enough to move during a general anesthetic maintained with inhalants, a rapid bolus of an injectable anesthetic (e.g. ketamine, thiopental) is required prior to subsequent adjustment of the vaporizer setting. This is because changes in the amount of inhalant anesthetic delivered to the horse are slowed several minutes by the large reservoir volume of large animal anesthetic circuits.