Pressure testing the anesthesia machine to detect leaks is an important task that should be done prior to each anesthetic event. Another might be checking that the hobbles are right where they are needed and that they are clean and not frayed. (Hobbles and ropes might be subject to a weight strain test every year or more frequently to certify that they will not fail under a known stress.)
Being familiar with the drugs that have useful properties in horses is critical. Sedatives, analgesics, induction drugs, and maintenance drugs are the general categories of drugs used in large animal anesthesia (see specific drug discussions later). Newer drugs have been found to have properties that can be used in special applications. Recently, for example, the use of medetomidine (or dexmedetomidine) CRIs during general anesthesia have been helpful in reducing MAC in “balanced anesthesia” cases. Newer, fast-acting inhalants such as desflurane and sevo-flurane may be useful as alternative inhalant anesthetics. Safe application of these newer drugs, however, requires adequate familiarity and regular use by the anesthetist in order to be used wisely. Familiarity breeds confidence and success.
Last, but not least, planning is what makes everything work. For example, the equine anesthetist must have a plan for a potentially dangerous excitement phase during the induction of anesthesia in the horse. Besides making good drug combination choices for induction of anesthesia, the anesthetist must be in full charge of everyone’s safety. Besides the fully trained anesthetist, properly trained and experienced assistant personnel should be present to minimize risk of potential injury to the patient or the caretakers. All participants present in an induction should know what they should and should not be doing during those very intense few moments of induction and early minutes of general anesthesia in the horse.
General Considerations Regarding Anesthesia Drugs
The anesthetist must develop and use finely honed skills of observation when giving a drug to an equine patient. The drug effects should be assessed in each individual horse and the anesthetist must have an alternative plan of action if the effects are not what is expected.
Equine patients may differ widely in their sensitivities to anesthetic drugs and to different noxious stimuli. Differences in breed, occupation, and environment can change how a drug can affect individual drug response in equine patients.
A relatively broad range of doses may be used to produce a desired effect. These dose ranges may vary significantly even when giving the same drug to the same horse on different occasions. A drug may have “typical” effects on any one day and different, opposite effects on another. Variations in cardiac output, for example, may change how a drug is processed within the body, requiring either much more or much less drug for the same effect. (See “Uptake” and “Distribution.”)
The dose providing an adequate effect in one set of circumstances (i.e., a castration) may be inadequate or excessive for another set of circumstances (compare a fracture repair versus taking radiographs). Environment is one of the keys to understanding why the “recommended dose” is not always the dose required to get the job done safely or effectively.
Virtually all inhalant anesthetics, induction agents, and sedatives alter cardiovascular function in some way or another, so it is imperative to monitor key indicators of these drug effects. This is why a watchful eye, good record-keeping habits, and concentration are essential during the entire perianesthetic period. Today’s standards of equine monitoring now include invasive blood pressure, ECG, pulse oximetry, capnometry, and temperature. The addition of cardiac output and agent monitoring continue to gain popularity as cost of these modes of monitoring comes within reach of more veterinarians. (See “monitoring”.)
The Five Phases
There are five phases of general anesthesia:
- Patient physical exam
- Laboratory results
- Emptying of the GI tract
- Preanesthetic sedatives
- Muscle relaxants
- General anesthetics
- Balanced anesthesia
- Sedative antagonists
- Patient attitude/sedation
Preanesthetic Phase of General Anesthesia
Patient physical examination
Preparation of the equine patient for anesthesia requires a physical examination by the anesthetist, usually some laboratory testing, partial emptying of the gastrointestinal (GI) tract, and the use of preanesthetic medications.
The physical exam is important, in particular an exam of the cardiovascular and respiratory systems. However, other systems, such as the renal and hepatic functions, are of similar interest since they involve the metabolism and excretion of the anesthetic drugs. A systemic approach should be used to evaluate each patient.
Knowing the patient’s health history is so important. The physical exam may never reveal as much as a thorough past history. Asking the owner key questions will likely reveal most serious predispositions including pneumonia, history of heaves, reaction to drugs, lameness problems, etc. Knowing the history may change the nature of the physical exam as well. If the horse has had previous anesthesia, knowing how it went and what the nature of the recovery was can be very helpful in planning subsequent anesthesia events.
Heart and pulse
Heart auscultation will allow the anesthetist to hear murmurs and other abnormal heart sounds if present. Abnormal rhythm disturbances may be noticed. Palpation of arterial pulses will similarly give the anesthetist an appreciation of heart strength and regularity.
Lungs and upper airway
Auscultation of healthy lungs should be difficult because air moving through normal lungs makes very little sound. Otherwise, the anesthetist is listening for sounds of fluid in each lung field, sounds that might indicate the presence of pulmonary edema. Lung sounds are checked bilaterally for indications of airway constriction, such as wheezes that might indicate pneumonia. The upper airway is also checked for signs of obstruction. The respiratory rate, respiratory pattern, and depth of chest excursions should be noted as well. The quantity of airflow being exhaled out of each nostril should be the same; otherwise, an obstruction might exist. Flaring nostrils while at rest may be an indication of excessive effort and likely represents some problem that should be addressed.
Skin is checked for normal sweating and skin turgor. In some places on a horse, “tenting” or skin turgor indicates the degree of dehydration of the patient. A healthy coat is also an indicator of general good health.
Dryness of mucous membranes (MM) may indicate another reason to suspect dehydration in a patient whereas moist membranes tend to indicate adequate hydration. Pink color of MM indicates good peripheral perfusion, indirectly giving the anesthetist a rough idea of oxygen saturation and cardiac output.
After the physical exam, the decision about what laboratory testing should be performed needs to be made. The minimum lab testing that should be done for each patient is packed cell volume (PCV) or hematocrit, total plasma solids (TS), and white blood cell count (WBC) with fibrinogen (Muir and Hubbell, 2009). Horses with inflammatory, traumatic, or neoplastic disorders may have an elevated fibrinogen. Blood should be drawn once the horse is calm because PCV may be elevated in the excited horse (contraction of the spleen).
If the physical exam sheds light on other possible problems, other tests might be added, including electrolytes (sodium, potassium, or calcium), creatinine, hepatic enzymes, and others. Electrolyte imbalances may contribute to postoperative muscle weakness, cardiac dysrhythmias, and acid-base imbalance. Elevated creatinine may indicate possible urinary dysfunction, while elevated hepatic enzymes may predict potential problems with anesthetic drug clearance (Muir and Hubbell 2009).
The PCV is a measure (%) of the cellular portion of the blood after being centrifuged in a microhematocrit device. High values indicate possible dehydration; low values indicate anemia. Total solids (TS) is a measure of the refractive index of plasma proteins, which indicates either hemodilution (low value) or hemoconcentration (high value).
A dehydrated patient would have a high PCV and high TS. A patient with hemolytic anemia would have a low PCV and normal TS. The presence of a “left shift” in white cell count (lots of immature emerging new white cells) and a high fibrinogen might indicate a festering pulmonary infection such as pneumonia. In the presence of a fever, these three things might be enough for a surgeon to consider postponing general anesthesia.
Emptying of the GI tract
A horse can rupture its stomach on induction of general anesthesia. This is almost always a fatal complication and can happen if the stomach is full of ingesta or fluids. Horses are normally kept off feed for several hours before induction of general anesthesia, some for as long as 6 to 12 hours and others much less. Free access to water is allowed. If horses have not been muzzled for some reason and ate only hay, the surgery is not cancelled. However, if the patient ate grain in any quantity, gas formation in the stomach is possible, and elective surgery should be postponed 24 hours to allow for gas reabsorption.
The anesthetist relies upon these drugs to decrease fear and anxiety in horse patients, making them safer and easier to work with. Preanesthetics facilitate smooth inductions and recoveries from anesthesia. They constitute an integral part of the anesthesia sequence of induction. By controlling the excited or painful or violent patient we can help protect the people who handle the patient. Preanesthetics can have the following effects:
- Reducing the amount of general anesthetic used
- Calming the patient and reducing pain
- Providing muscle relaxation
- MAC reduction
- Blocking the vasovagal reflex (although considered controversial, anticholinergics such as atropine have been used to treat bradycardias resulting from vagal stimulation or alpha-2 agonist effects).
- Smooth recovery
The more debilitated or ill the patient is, in general, the less preanesthetic medication required. Conversely, the more excited, painful, or active the patient, the lower is the effect of the preanesthetic, and preanesthetic drug requirements go up. In every individual patient, the preanesthetic should be tailored to the patient and not just given routinely. The following is a list of commonly used preanesthetic drugs used in horses.
Xylazine is an alpha-2 adrenergic agonist drug that is used for sedation and analgesia. Its commercial name is Rompun ®. Xylazine is not an ideal drug for sick or debilitated animals because of its negative cardiopulmonary effects, which include potent peripheral vasoconstriction and bradycardia with possible 2nd degree AV block. Xylazine can also cause relaxation of the equine airways and occasional upper airway obstruction (both vagal and alpha-2 effects) (Muir and Hubbell 2009).
- Equine Dose: 0.5–1.1 mg/kg IV/IM
- Time to peak effect: 5–10 minutes
- Length of sedation: 15–30 minutes (typical)
- Length of analgesia (visceral): 45–60 minutes (typical)
- Antagonists: Yohimbine, atipamizole, tolazoline
This widely used drug causes sedation, ataxia, and analgesia, especially visceral analgesia. It will initially cause a transient increase in blood pressure due to vasoconstriction, commonly resulting in compensatory atrioventricular block and bradycardia. This initial hypertension is followed by hypotension. Xylazine can occasionally cause personality changes manifested as ear pinning, tail switching, biting, and kicking, thought to be due to a decrease in inhibition. Arousal is possible in response to loud noises or touch. Although the horse appears well sedated, it may be stimulated and awakened enough to deliver a powerful kick if startled.
Xylazine is supplied as either 20 mg/mL (small animal) or as 100 mg/mL (large animal).
- Category: alpha-2 adrenergic agonist
- Major uses: sedation and analgesia
- Cardiopulmonary effects: similar to xylazine
- Equine dose: 0.01–0.02 mg/kg IV/IM, 0.02 mg/kg sublingual (Taylor and Clarke 2007); CRI 0.01–0.02μg/kg/min following initial IV dose of 0.01–0.02 mg/kg
- Length of sedation: 20–40 minutes (typical)
- Length of analgesia (visceral): 120–180 minutes (typical)
- Antagonists: same as xylazine
- Supplied as 1.0 mg/mL
This drug, although more expensive than xylazine, is being used widely for both equine sedation and pain control because its analgesic effect (and other side effects) lasts longer and sedation is slightly more potent.
- Category: alpha-2 adrenergic agonist
- Major uses: sedation, analgesia, and balanced anesthesia (MAC reduction)
- Cardiopulmonary effects: similar to xylazine, potent peripheral vasoconstriction
- Equine dose: 5–20μg/kg IV/IM (0.0005–0.0020 mg/kg); CRI 3.5μg/kg/min (Muir and Hubbell 2009)
- Antagonists: same as xylazine
- Category: phenothiazine derivative
- Major use: tranquilizer, preoperative sedative
- Cardiopulmonary: potent peripheral vasodilation, lowering blood pressure. Clinical doses can lower blood pressure 15–20 mm Hg (Muir and Hubbell 2009) and so should not be used in patients that are anemic, hypotensive, shocky, or seizuring. Vasodilation effects can abolish thermoregulation and heat loss can be significant. Ace can cause penile retractor muscle paralysis (paraphymosis), and for this reason it is not recommended for use in breeding stallions.
- Equine dose: 0.03–0.10 mg/kg IV/IM (Muir and Hubbell 2009)
- IV tranquilization onset is about 20–30 minutes, lasting an hour or more for sedation; vasodilation effects may last from 6 to 12 hours. Ace can also be given orally. Acepromazine can be used to reduce the excitement effects seen with opioids such as morphine in the horse (Muir and Hubbell, 2009).
- Category: anticholinergic
- Major use: decreasing sympathetic tone, increasing heart rate (for treating bradycardia—horses have high vagal tone at rest, and drugs [alpha-2] and surgical manipulations [pulling on an optic nerve, for example] may lead to bradyarrhythmias or even cardiac arrest), bronchial dilation, decreasing salivation.
- Cardiopulmonary: see earlier in this chapter.
- Equine dose: 7–10μg/lb IV/IM (0.007–0.010 mg/lb)
Pretreatment of a horse with atropine may not eliminate bradycardia, but it may prevent cardiac arrest (Taylor and Clarke 2007). Presurgical treatment of sinus bradycardia may include reduction/reversal of the anesthetic drug, administration of atropine, and administration of catecholamine such as dobutamine or epinephrine (Muir and Hubbell 2009). Atropine may take as long as 5 minutes to take effect and may cause an initial bradycardia before heart rate increases. Atropine can cause decreased gut motility, resulting in postoperative colic, and so routine use remains controversial.
- Category: benzodiazepine
- Major use: sedation, muscle relaxant, anticonvulsant
- Used as a premedicant mainly only in foals because it does not provide adequate sedation in healthy adult equines
- Can be combined with ketamine to provide muscle relaxation for induction (in place of guaifenesin)
- Cardiopulmonary: little effect
- Equine dose: 0.10–0.25 mg/kg
- Antagonist: flumazenil
Diazepam increases cough reflexes and laryn-gospasm; it is inexpensive and is dissolved in propylene glycol, which can cause it to sting on administration.
Another popular benzodiazepine is midazolam, commercial name, Versed ®. Midazolam is dissolved in water (faster IM/SQ dosing) and has 1/2 the half life of diazepam and 1/2 the dose (0.0045–0.01 mg/kg), but it is also more expensive per dose (Adams 2001).
- Category: synthetic opioid agonist antagonist
- Major use: used synergistically with alpha-2 sedatives for sedation and visceral analgesia
- Equine dose: 0.01–0.02 mg/kg IV/IM
Butorphanol has fewer negative side effects as with morphine but provides good analgesia in horses as compared to canines. Butorphanol can cause ataxia and “twitching” or head jerking. It is a kappa agonist mu antagonist with a duration of about 60 minutes (IV).
- Category: opioid agonist
- Major use: analgesia
- Cardiopulmonary: may increase heart rate, cardiac output, and blood pressure at clinical doses (Muir and Hubbell 2009)
- Equine dose: 0.1–0.3 mg/kg IV/IM (NOTE: Give IV morphine very slowly after a tranquilizer. Duration of effects is 3–5 hours. Convulsions are possible.)
- Antagonist: naloxone (0.05 mg/lb)
Besides potent analgesia, morphine can produce excitement in the horse in the absence of a tranquilizer, although using the drug IM can minimize this effect. Morphine can be a potent respiratory depressant and can increase GI sphincter tone (constipating effect) (Muir and Hubbell 2009).
Induction a gents for horses
- Category: centrally acting skeletal muscle relaxant
- Known as “GG” or “GGE,” for its old name (glyceryl guaiacolate ether), it is a mild sedative with little or no analgesic properties.
- Cardiopulmonary: little cardiopulmonary depression in the dose range below
- Equine dose: 25–50 mg/kg to effect (duration 20 minutes, longer in stallion ponies)
- Supplied most often as 5% guaifenesin in 5% dextrose and typically given under pressure (pressure infusor) to effect until signs of ataxia are seen, at which time IV bolus of general anesthetic (thiopental or ketamine) is given. Halves the induction dose of thiopental necessary, and masks the seizurelikeb excitement of typical ketamine inductions. Smoother inductions and recoveries than without GG. Some common problems include:
- Residual muscle weakness in recovery
- Tissue irritant if GG goes perivascularly (sloughing)
- Hemolysis if concentration is over 5%
- Mild cardiovascular depressant at higher doses
- Residual muscle weakness in recovery
- Category: ultra–short-acting thiobarbiturate
- Major use: general anesthetic, causes loss of conscience.
- Equine dose: 4–6 mg/kg IV
- Duration: 4–10 minutes
Thiopental is a potent dose-dependent cardiovascular and respiratory depressant. Problems include the following:
- Excitement during induction and recovery
- Little analgesia or muscle relaxation
- Narrow margin of safety
- Tissue irritant if injected perivascularly
- Highly protein-bound (reduce dose in hypoproteinemia)
Caution: Do not use for patients in shock, dehydrated, or otherwise hypovolemic. Geriatrics are more sensitive to its effects.
- Category: cyclohexlamine, dissociative anesthetic
- Major use: general anesthetic, analgesic
- Commercial name: Ketaset, Vetalar
- Equine dose: 1.5–2.2 mg/kg IV, duration 5–10 minutes
- Cardiopulmonary: minimal cardiovascular and respiratory depression
Allows for very brisk reflexes, muscle rigidity, and myotonic contractions. Used without preanesthetic muscle relaxants (not recommended), rigidity is pronounced. May cause an increase in blood pressure and heart rate. Inductions (in well-sedated horses) and recoveries are smooth, but repeated doses tend to increase the incidence of rough recoveries. Poor surgical conditions when used alone. Muscle rigidity and swallowing may make intubation difficult. Use with diazepam, midazolam, or guaifenesin for good muscle relaxation. Good choice for anesthesia for shocky patients and those with hypovolemia.
- Category: paired drugs—a dissociative anesthetic and a benzodiazepine
- Major use: general anesthesia (imitation of ketamine-valium mixture)
- Commercial name: Telazol ®
- Cardiopulmonary: similar to ketamine
- Equine dose: 0.7–1.0 mg/kg IV
- Dried powder mixed with water is good for 2 weeks.
Induction and maintenance of anesthesia is often performed using inhalational agents delivered by an anesthesia machine of some kind. These agents are potent and rapid acting. In addition, they allow rapid adjustment of the depth of anesthesia during the perioperative period. Recovery from these inhalants involves “blowing off” the anesthetic agent into the environment. Induction and recovery can be rapid, even in neonatal foals with immature liver function. All of the inhalants below are potent vasodilators, profound respiratory depressants, nonexplosive and nonflammable.
No longer in production
- Vapor pressure 240 mm Hg
- Equine MAC 1.31–1.64%
- Cost per mL $0.08
- Saturation in oxygen 32%
- Percent metabolized 1.7%
- Good analgesia; peripheral vasodilation and hypotension
- Potent respiratory depressant (usually requiring a ventilator); short cases recover relatively smoothly, longer cases can be rougher.
- Vapor pressure 160 mm Hg
- Equine MAC 2.31–2.84%
- Cost per mL $0.48
- Saturation in oxygen 21%
- Percent metabolized 3%
- Good analgesia, rapid change in depth
- Compound “A” can produce nephrotoxins in absorbent, but no clinical evidence of kidney damage has been seen.
- Reaction with dry KOH can produce CO.
- Quick recoveries, requires aid of sedatives
- Vapor pressure 664 mm Hg
- Equine MAC 7.02–8.06%
- Cost per mL $0.60
- Saturation in oxygen 87%
- Percent metabolized 0.02%
- Good analgesia, very rapid change in depth
- Quick recoveries; may require aid of postoperative sedatives to avoid too rapid recovery (Muir and Hubbell 2009)
Total Intravenous Anesthesia (TIVA) is popular for short surgical procedures. Most recommendations limit TIVA to under 2 hours due to the cumulative effects of IV drug metabolism. This involves the use of “triple drip,” which is most commonly made with 1 L 5% guaifenesin, 500 mg xylazine, and 1.5–2.0 g of ketamine all mixed together. Sedation and induction are accomplished in the usual manner, and triple drip may be used for maintenance of anesthesia at a rate of about 2 mL/kg/hr to effect. Horses will maintain a brisk palpebral reflex and may blink spontaneously under a surgical (minor) plane of anesthesia.
Balanced anesthesia refers to the combination of multiple drugs to reduce the amount of any one particular drug to produce general anesthesia. In the horse, we may balance the effects of a number of drugs, such as isoflurane plus lidocaine infusion, with a medetomidine CRI and induction with ketamine/diazepam.
One of the most important differences between small animals and horses is that there is a significant potential for developing neuropathy and myopathy with a horse on the surgical table while under anesthesia.
Simply stated, the horse’s weight combined with hypotensive effects of general anesthetics could reduce the flow of blood to large portions of the musculature. This could result in serious nerve and muscle damage unless the anesthetist takes appropriate preventative measures, such as maintaining adequate blood pressure and proper positioning and padding. Most surgical tables for horses use some form of foam padding to spread the weight of the horse over a larger area (Fig. 35.3).
Others may use water-filled mattresses and inflated air bladders to accomplish the same result. Attention to the details of positioning is required to prevent a number of classic neuropathies such as radial nerve paresis/paralysis. In this case, the horse in lateral recumbency can develop radial nerve damage unless the lower leg is properly pulled forward, either on the table or in the recovery stall.