Advantages and guidelines for using alpha-2 agonists as anesthetic adjuvants.

Xylazine and medetomidine produce reliable sedation, muscle relaxation, and analgesia in dogs and cats. In addition, alpha-2 agonists have proved very effective as sedative-analgesic adjuncts when coadministered with benzodiazepine or opioid agonists. Alpha-2 agonists should not be classified as monoanesthetics. They are excellent anesthetic adjuncts when combined with dissociatives and opioids. Because of the acute alterations in cardiopulmonary function commonly induced by alpha-2 agonists, it is suggested that their use be restricted to the young healthy patient undergoing routine surgical or diagnostic procedure. The development of more specific and selective alpha-2 agonists will continue to enhance the safety and reliability of this novel class of compounds. The unique spectrum of anesthetic properties induced by alpha-2 agonists has assured them of an increasingly prominent role in the development of new and sophisticated ways of achieving anesthesia.     

Perioperative use of selective alpha-2 agonists and antagonists in small animals

Alpha-2 agonists are the only single class of anesthetic drugs that induce reliable, dose-dependent sedation, analgesia, and muscle relaxation in dogs and cats. Used at low doses, as adjuncts to injectable and inhalational anesthetics, selective alpha-2 agonists dramatically reduce the amount of anesthetic drug required to induce and maintain anesthesia. This reduction in anesthetic requirements is achieved without significant depression of pulmonary function and with limited effects on cardiovascular function. Selective alpha-2 agonists can also be used postoperatively to potentiate the analgesic effects of opioids and other drugs. Given the nearly ideal pharmacodynamic profile and reversibility of alpha-2 agonists, these drugs will play a central role in balanced approaches to anesthesia and the management of perioperative pain in healthy dogs and cats.     

Anesthesia techniques in sheep and goats

A variety of techniques can be used to anesthetize and restrain sheep and goats safely and humanely both in the clinic and in the field. The use of inhalational, injectable, and local anesthetic agents is discussed. Nontraditional agents (opioids and alpha-2 adrenergic agonists) for epidural analgesia also are reviewed because of their promising clinical application.     

Sedation and chemical restraint of deer

This review examines the properties of chemical immobilising agents of use or potential use in deer for velvet antler removal. The alpha-2-adrenergic agonist, xylazine, is the most commonly used agent for chemical restraint of stags for this purpose in New Zealand. This compound is often combined with an opioid or ketamine to enhance its effectiveness. Concerns about the carcinogenic potential of a xylazine metabolite have led to a call by the deer industry to limit the use of this drug and stimulated enquiry into alternatives. The potent pure opioid agonists, fentanyl, carfentanil and etorphine, are less effective than xylazine when used alone, and their use presents greater risk to operators and animals and is subject to more stringent legal restrictions. Newer alpha-2-adrenergic agonists, detomidine and medetomidine, are more potent than xylazine and unlikely to result in carcinogenic metabolites. Availability of specific alpha-2-adrenergic antagonists further enhances the usefulness of these drugs. The dissociative agent, ketamine, has proven useful for immobilisation of various domestic and wildlife species and captive species of deer but undesirable side effects and the low concentration, relative to the effective dose, of currently available formulations limit its usefulness. Combination of a dissociative agent with an alpha-2-adrenergic agent reduces the required dose of both and enhances their efficacy. A commercially available combination of the dissociative anaesthetic agent, tiletamine, and the benzodiazepine, zolazepam, is more potent than ketamine and induction times are faster. This combination can be manipulated to varying concentrations by dissolution with alpha-2-adrenergic agents. A long-acting neuroleptic agent, zuclopenthixol, has recently been investigated for calming deer and may be an alternative to classical sedative-immobilising agents. Most chemical immobilisation agents used alone or in combination produce respiratory depression and supplemental oxygen has been recommended. All would likely result in some form of chemical residues in velvet antler.     

Primate anesthesia.

This article provides an update on some of the recent advances in primate anesthesia. It focuses in particular on some of the newest information available regarding the effects of opioids and alpha-2 agonists in primates, and how these effects are different from what we might expect in other companion animals. It reviews the important properties of the latest induction and inhalation agents, and stresses the need for continuous monitoring of the anesthetized patient.     

Epidural anesthesia and analgesia in horses.

Intercoccygeal, or caudal, epidural injection of local anesthetics is a convenient method of producing analgesia and local anesthesia of the tail and perineal structures in conscious standing horses. This technique has been further developed to provide long duration analgesia and anesthesia by placement of catheters into the epidural space of horses. More recently, opioid, alpha-2 adrenergic agonists, ketamine and other analgesic agents have been administered by caudal epidural injection, providing pain relief in both conscious, standing and anesthetized, recumbent horses. This chapter describes the development of different anesthetic and analgesic epidural techniques in horses, methods for epidural injection and catheterization, and reviews the current literature related to epidural analgesia and pain control in horses.     

Results of the confidential enquiry into perioperative small animal fatalities regarding risk factors for anesthetic-related death in dogs

OBJECTIVE: To identify major risk factors associated with anesthetic-related death in dogs. DESIGN: Case-control study. ANIMALS: 148 dogs that died or were euthanized within 48 hours after undergoing anesthesia or sedation and for which anesthesia could not be reasonably excluded as a contributory factor (cases) and 487 control dogs that did not die within 48 hours after undergoing anesthesia or sedation (controls). PROCEDURES: Details of patient characteristics, preoperative evaluation and preparation, procedure, anesthetic and sedative agents used, monitoring, postoperative management, and personnel involved were recorded. Mixed-effects logistic regression modeling was used to identify factors associated with anesthetic-related death. RESULTS: An increase in physical status grade, urgency of the procedure, age, or intended duration of the procedure; a decrease in body weight; anesthesia for a major versus a minor procedure; and use of injectable agents for anesthetic induction and halothane for maintenance or use of inhalant anesthetics alone (compared with use of injectable agents for induction and isoflurane for maintenance) were associated with increased odds of anesthetic-related death. CONCLUSIONS AND CLINICAL RELEVANCE: The results suggested that specific factors could be associated with increased odds of anesthetic-related death in dogs. Knowledge of these factors should aid the preoperative assessment and perioperative management of dogs undergoing anesthesia and sedation.     

Medetomidine and dexmedetomidine: a review of cardiovascular effects and antinociceptive properties in the dog

Alpha(2)-adrenoreceptor agonists (alpha(2)-agonists) are commonly used in small animal anaesthesia for their potent sedative and analgesic properties, although concerns about their cardiovascular effects have prevented their full adoption into veterinary practice. Research into alpha(2) adrenoreceptor agonists and their clinical use is extensive, therefore this review focuses on the use of dexmedetomidine and medetomidine in dogs. Emphasis is given to the cardiovascular effects and antinociceptive action of these agents.     

Other potentially useful new injectable anesthetic agents.

Ultrashort barbiturates are not ideal injectable anesthetic agents, and new agents continue to be released as investigators pursue the goal of finding a more ideal agent. Of the new injectable agents discussed, propofol seems to be the most promising drug. Propofol should find a place in veterinary practice as an outpatient anesthetic agent because it has a rapid, smooth, and complete recovery even after repeated or continuous administration. Midazolam does not induce anesthesia in healthy, small animals and, as such, can only be used in combination with other injectable agents, such as ketamine or the thiobarbiturates. In our practice, Telazol has found a place in the anesthetic management of feral cats and aggressive dogs, where it is used for heavy sedation or to induce anesthesia. The role of flumazenil, as a reversal agent, in veterinary practice remains to be determined; however, the role in small domestic animals is unlikely to be significant.     

Alpha-2 adrenergic agonists as analgesics in horses.

Administration of alpha-2 agonists to horses produces a variety of behavioral effects (sedation, somnolence, analgesia), and physiological effects. One of the most significant beneficial effects of administering alpha-2 agonists is the degree of analgesia they provide. Alpha-2 agonists have been the mainstay of analgesia for colic pain in horses since their introduction to clinical veterinary medicine. The increased potency of the more recently introduced alpha-2 agonists allows the provision of analgesia for conditions not previously relieved by other drugs. Unfortunately, there are significant side effects associated with alpha-2 agonist administration. Studies are underway to identify the physiologic effects associated with the stimulation of each alpha-2 receptor subtype, in hopes of developing subtype-specific alpha-2 agonists and antagonists.     

Parenteral Anticholinergics in Dogs With Normal and Elevated Intraocular Pressure

Dogs given parenteral anticholinergic drugs have been thought to be at risk for development or exacerbation of elevated intraocular pressure (IOP). In a randomized, blinded, placebo-controlled study, we evaluated the effect of intramuscular glycopyrrolate (0.01 mg/kg) on pupil diameter and IOP in unanesthetized normal dogs. Treatment with glycopyrrolate did not change pupil diameter or IOP from baseline, nor were there differences between glycopyrrolate and saline-treated (control) dogs. In addition, the authors retrospectively reviewed the medical records of 2,828 dogs undergoing general anesthesia between April 1987 and September 1990 to determine if there was an association between parenteral anticholinergic medication and postanesthetic elevation in IOP. The authors also determined the frequency of bradycardia requiring anticholinergic therapy during anesthesia in dogs with glaucoma. Of the 2,828 cases reviewed, the records of 46 dogs coded for glaucoma were examined in detail. The 46 dogs underwent 62 episodes of anesthesia, with 23 episodes including exposure to an anticholinergic drug. An increase in IOP from preanesthetic to postanesthetic measurement occurred in three dogs. One of these dogs received anticholinergic medication for bradycardia during anesthesia. The postanesthetic elevation in IOP in this dog was probably not drug related. Preanesthetic anticholinergic administration did not affect the incidence of anticholinergic administration for bradycardia during the anesthetic episode. Anticholinergic therapy during anesthesia was more frequent when the preanesthetic medication included an opiate drug. These studies do not indicate an association between parenteral anticholinergic administration and elevations in IOP.     

Propofol anesthesia.

Although questions may still remain regarding the use of this unique sedative-hypnotic drug with anesthetic properties in high-risk patients, our studies have provided cardiopulmonary and neurological evidence of the efficacy and safety of propofol when used as an anesthetic under normal and selected impaired conditions in the dog. 1. Propofol can be safely and effectively used for the induction and maintenance of anesthesia in normal healthy dogs. Propofol is also a reliable and safe anesthetic agent when used during induced cardiovascular and pulmonary-impaired conditions without surgery. The propofol requirements to induce the safe and prompt induction of anesthesia prior to inhalant anesthesia with and without surgery have been determined. 2. The favorable recovery profile associated with propofol offers advantages over traditional anesthetics in clinical situations in which rapid recovery is important. Also, propofol compatibility with a large variety of preanesthetics may increase its use as a safe and reliable i.v. anesthetic for the induction and maintenance of general anesthesia and sedation in small animal veterinary practice. Although propofol has proven to be a valuable adjuvant during short ambulatory procedures, its use for the maintenance of general anesthesia has been questioned for surgery lasting more than 1 hour because of increased cost and marginal differences in recovery times compared with those of standard inhalant or balanced anesthetic techniques. When propofol is used for the maintenance of anesthesia in combination with a sedative/analgesic, the quality of anesthesia is improved as well as the ease with which the practitioner can titrate propofol; therefore, practitioners are able to use i.v. anesthetic techniques more effectively in their clinical practices. 3. Propofol can induce significant depression of respiratory function, characterized by a reduction in the rate of respiration. Potent alpha 2 sedative/analgesics (e.g., xylazine, medetomidine) or opioids (e.g., oxymorphone, butorphanol) increase the probability of respiratory depression during anesthesia. Appropriate consideration of dose reduction and speed of administration of propofol reduces the degree of depression. Cardiovascular changes induced by propofol administration consist of a slight decrease in arterial blood pressures (systolic, mean, diastolic) without a compensatory increase in heart rate. Selective premedicants markedly modify this characteristic response. 4. When coupled with subjective responses to painful stimuli, EEG responses during propofol anesthesia provide clear evidence that satisfactory anesthesia has been achieved in experimental dogs. When propofol is used as the only anesthetic agent, a higher dose is required to induce an equipotent level of CNS depression compared with the situation when dogs are premedicated. 5. The propofol induction dose requirement should be appropriately decreased by 20% to 80% when propofol is administered in combination with sedative or analgesic agents as part of a balanced technique as well as in elderly and debilitated patients. As a general recommendation, the dose of propofol should always be carefully titrated against the needs and responses of the individual patient, as there is considerable variability in anesthetic requirements among patients. Because propofol does not have marked analgesic effects and its metabolism is rapid, the use of local anesthetics, nonsteroidal anti-inflammatory agents, and opioids to provide postoperative analgesia improves the quality of recovery after propofol anesthesia. 6. The cardiovascular depressant effects of propofol are well tolerated in healthy animals, but these effects may be more problematic in high-risk patients with intrinsic cardiac disease as well as in those with systemic disease. In hypovolemic patients and those with limited cardiac reserve, even small induction doses of propofol (0.75-1.5 mg/kg i.v.) can produce profound hypotens     

Effects of alpha-2 adrenoceptor agonists during recovery from isoflurane anaesthesia in horses

REASONS FOR PERFORMING STUDY: Recovery from inhalant anaesthesia in the horse is a critical and difficult period to manage; however, several factors could help to obtain a calm recovery period including choice of anaesthetic and analgesic procedure used and the conditions under which anaesthetic maintenance and recovery occur. OBJECTIVES: The objective of this study was to evaluate and compare the quality of recovery in horses administered saline, xylazine, detomidine or romifidine during recovery from isoflurane anaesthesia. METHODS: Six mature and healthy horses were premedicated with i.v. xylazine and butorphanol, and anaesthesia induced using ketamine. After 2 h of inhalant anaesthesia with isoflurane vaporised in oxygen, saline solution, xylazine (0.1 mg/kg bwt), detomidine (2 microg/kg bwt) or romifidine (8 pg/kg bwt) were administered. The quality of recovery of each horse and the degree of sedation and ataxia were evaluated. Cardiovascular and respiratory parameters were recorded, and arterial blood samples obtained and analysed for pH, PO2 and PCO2 during recovery. RESULTS: Quality of recovery was better in groups treated with alpha-2 adrenergic receptors agonists, showing less ataxia. Degree of sedation was greater in the romifidine group. CONCLUSIONS: We concluded that the administration of alpha-2 adrenoceptor agonists during recovery from isoflurane anaesthesia in horses prolonged and improved the quality of recovery without producing significant cardiorespiratory effects. POTENTIAL CLINICAL RELEVANCE: Administration of alpha-2 adrenoceptor agonists after inhalent anaesthesia could prevent complications during the recovery period.     

Survey of utilization of medetomidine and atipamezole in private veterinary practice in Quebec in 2002

This survey evaluates early perceptions about the use of medetomidine and atipamezole among veterinary practitioners in Quebec in 2002. Response rate was 23.5%; 71.1% of the practitioners did not use these products because of lack of information (69.3%), unavailability of the drugs in the practice (23.3%), or other reasons (7.3%), including concerns about the safety of alpha-2 agonists. Most veterinarians who used these products (70.4%) used them only rarely. Sedation by medetomidine was qualified as good (44.2%) or excellent (36%), and analgesia as good (46.5%) or average (32.7%). Waking up after atipamezole was qualified as good (47.5%) or excellent (40.9%). These perceptions indicate an opportunity for wider use of the products in veterinary practice. With more education and experience, practitioners could find medetomidine hydrochloride alone or in combination with an opioid useful for sedation, analgesia, and premedication for healthy animals. Reversal with atipamezole hydrochloride is considered effective, when residual sedation is undesirable.     

Epidural anesthesia and postoperatory analgesia with alpha-2 adrenergic agonists and lidocaine for ovariohysterectomy in bitches

The aim of this study was to determine the viability and cardiorespiratory effects of the association of epidural alpha-2 adrenergic agonists and lidocaine for ovariohysterectomy (OH) in bitches. Forty-two bitches were spayed under epidural anesthesia with 2.5 mg/kg body weight (BW) of 1% lidocaine with adrenaline (CON) or in association with 0.25 mg/kg BW of xylazine (XYL), 10 μg/kg BW of romifidine (ROM), 30 μg/kg BW of detomidine (DET), 2 μg/kg BW of dexmedetomidine (DEX), or 5 μg/kg BW of clonidine (CLO). Heart rate (HR), respiratory rate (f_R) and arterial pressures were monitored immediately before and every 10 min after the epidural procedure. Blood gas and pH analysis were done before, and at 30 and 60 min after the epidural procedure. Animals were submitted to isoflurane anesthesia if they presented a slightest sign of discomfort during the procedure. Time of sensory epidural block and postoperative analgesia were evaluated. All animals in CON and DEX, 5 animals in ROM and CLO, 4 animals in XYL, and 3 in DET required supplementary isoflurane. All groups, except CLO, showed a decrease in HR. There was an increase in arterial pressures in all groups. Postoperative analgesia lasted the longest in XYL. None of the protocols were totally efficient to perform the complete procedure of OH; however, xylazine provided longer postoperative analgesia than the others.     

The use of atropine to control heart rate responses during detomidine sedation in horses

Detomidine is a sedative-analgesic which has a pharmacological profile similar to xylazine. There is evidence that the sedative effects are mediated through alpha-2 adrenoceptors.Cardiopulmonary responses were determined using detomidine as the principal agent and as a preanesthetic prior to the induction of general anesthesia. Compatibility with guaifenesin, sodium thia-mylal and halothane were determined.As in the case of xylazine, detomidine produces a slowing of heart rates. This was found to be either sinus bradycardia or heart block. There may be a corresponding increase in systolic blood pressures. The respiratory pattern is altered through the arterial blood gases and pH data supported evidence of adequate ventilation. The heart rate response to detomidine without anticholinergic treatment was transient and related to he duration of drug action.Atropine sulfate, 0.02 mg/kg i.v. was effective in preventing or treating bradycardia or heart block from detomidine. Heart rates also increased during the administration of guaifenesin and sodium thia-mylal when given 50 min poisit-detomidine.     

Clinical pain management techniques for cats

Although pain management is an emerging and popular topic in veterinary medicine, use of analgesics in cats has received little attention relative to their canine counterparts. Some of the difficulty lies in assessment of whether or not a cat is in pain. Simple observation of a cat in a cage relies upon overt expression of pain, and is often inaccurate. Pain scales have been developed that allow a semiquantitative evaluation of the degree of pain an animal may be experiencing. However, treating pain based upon observation of the painful state is less effective than anticipating and preemptively treating pain. This article reviews specific methods for preemptively treating and alleviating pain in the cat. The traditional approach to pain management involves drug administration. Specific categories of agents used in cats include opioids, nonsteroidal anti-inflammatories, or alpha-2 agonists. Other modalities of pain management, which are also reviewed, include use of local anesthetic drugs for local and regional analgesia, as well as acupuncture.     

Neuromuscular blocking agents in equine anesthesia

In summary, neuromuscular blocking agents can be used safely and to advantage in equine anesthesia. Muscle-relaxant use in equine anesthesia has been helped by the development of new relaxants such as atracurium, which has a reliable and reproducible duration of action. There are certain cases that benefit particularly by the use of relaxants but their use is not limited to these cases. These cases involve horses that experience persistent movement and hypotension during anesthesia, are undergoing ophthalmic or abdominal surgery or fracture repair, or are severely ill. Horses receiving muscle relaxants during anesthesia require mechanical ventilation, and neuromuscular blockade should be monitored with a peripheral-nerve stimulator.     

Standing musculoskeletal surgery.

Indications for performing orthopedic surgery on the standing horse include inability to tolerate general anesthesia, risk of worsening an injury during recovery from anesthesia, and cost. The surgeon should be aware that performing surgery in the standing horse can be more demanding and require more experience than the same procedures when the time and convenience of general anesthesia are available. Improved sedatives and analgesics have allowed more latitude because the horses now are more tolerant than when older agents were used. Common sense should be applied to each situation before the decision is made to do a procedure, because every horse and problem is unique.     

The pharmacology of local anesthetics.

Understanding of the pharmacology of local anesthesia is important for selection of a local anesthetic for use in equine standing surgery. In general, the action potential is inhibited by local anesthetics by preventing the influx of sodium ions across the axonal membrane. The physicochemical properties of each local anesthetic determine the onset of action, potency, and duration of action. Procaine, chlorprocaine, lidocaine, and mepivacaine are the local anesthetics still used clinically in horses; lidocaine is the most widely used. The future of equine local anesthesia may see the introduction of longer acting, more potent drugs currently used clinically in humans and dogs as well as drugs not classified as local anesthetics--alpha-2 agonists and opioids--for use in epidural anesthesia.