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.
Hypotension
Definition
Blood pressure values from calm, unsedated horses range from 120–140, 80–100, and 100–120 mmHg for systolic, diastolic, and mean arterial pressure, respectively [29]. A target mean arterial blood pressure between 70 and 90 mmHg is suggested for anesthetized horses, depending on the horse’s size and corresponding muscle mass, padding, and anticipated duration of anesthesia. In foals, blood pressure may be maintained at lower values (mean arterial blood pressure between 50 and 65 mmHg) depending on their age and size, in keeping with values considered normal for them [30, 31].
Risk factors
Use of inhalant anesthetics, especially in the absence of inotropic support
Patients with systemic disease or compromise (e.g. endotoxemia, hypovolemia)
Pathogenesis
Hypotension is a common, even expected, complication with use of inhaled anesthetics in the horse as they dose dependently resulting in reduction of myocardial contractility. In medically compromised horses, inhaled anesthetics may also induce vasodilation as has been reported in human beings [32]. Data from horses in which inhalation anesthetics were administered in absence of other medications shows blood pressure and cardiac output reaching about half of normal awake values described in the horse at a surgical plane of anesthesia [33, 34].
Prevention
Ability to monitor blood pressure as well as knowledge and availability of the drugs used to support blood pressure during anesthesia maintenance are important in preventing hypotension and its consequences.
Monitoring
While indirect monitoring (e.g. cuff, Doppler) may be used for short procedures in healthy horses with anesthesia times of less than 1 hour, direct arterial blood pressure monitoring is generally recommended for inhalation anesthesia. Arterial catheters are commonly placed percutaneously in the facial, transverse facial, or dorsal metatarsal arteries.
Treatment
The positive inotrope dobutamine is ideal for treatment of hypotension as it counters the decrease in cardiac contractility caused by the inhalation agents and improves both cardiac output and blood pressure. Dobutamine also increases intramuscular blood flow in both the dependent and non‐dependent limbs of anesthetized horses [35]. Dobutamine is used as a constant rate infusion due to its short duration of action.
Ephedrine also increases blood pressure, cardiac output, and muscle blood flow in horses [36]. Due to its relatively longer duration of action, it is given as an intravenous bolus for the treatment of hypotension. The anesthetist should be aware that ephedrine is a central nervous system stimulant and its provision may result in a lightened plane of anesthesia. Tachyphylaxis (progressively less drug effect with subsequent dosing) is also seen, as ephedrine’s mechanism of action involves release of stores of endogenous catecholamines that eventually become depleted [37].
In patients with volume depletion, replacement of volume (e.g. crystalloid or colloid bolus, blood transfusion) should be attempted. Foals with anesthetic‐induced hypotension will also often respond positively to a crystalloid fluid bolus (5–10 ml/kg). It may be challenging to provide the appropriate volume rapidly in an adult horse, and fluid pumps can aid in providing large volume replacement.
Vasoconstrictive drugs such as norepinephrine, phenylephrine, or vasopressin may also be used if the cause of hypotension is