Use of the Anderson Sling suspension system for recovery of horses from general anesthesia

OBJECTIVE: To describe a sling recovery system (Anderson Sling) for horses and to evaluate outcome of high-risk horses recovered from general anesthesia by a sling. STUDY DESIGN: Retrospective study. SAMPLE POPULATION: Horses (n=24) recovered from general anesthesia. METHODS: Complete medical and anesthetic records (1996-2003) for horses recovered from general anesthesia using the Anderson Sling system were evaluated retrospectively. Information retrieved included anesthetic protocol, surgical procedure, recovery protocol, recovery time, and quality of the recovery. Horses were recovered from anesthesia supported by the Anderson Sling in a standing position within a traditional padded equine recovery stall. RESULTS: Twenty-four horses had 32 assisted recoveries; 31 events were successful. No complications associated with the sling or recovery system protocol occurred. One horse was intolerant of the sling's support and was reanesthetized and recovered successfully using head and tail ropes. CONCLUSION: The Anderson Sling recovery system is an effective and safe way to recover horses that are at increased risk for injury associated with adverse events during recovery from general anesthesia. CLINICAL RELEVANCE: The Anderson Sling system should be considered for assisted recovery of equine patients from general anesthesia.     

Anesthetic management of the head trauma patient

Objective: To describe the optimal anesthetic management of patients with brain injury, with emphasis on the support of oxygen delivery to the brain, and the effects of anesthetic agents on cerebral perfusion. Data sources: Clinical and experimental studies from both the human and veterinary neuroanesthesia literature. Summary: The management of patients following primary traumatic brain injury (TBI) significantly impacts outcome. Outcome can be improved by strategies that improve oxygen delivery to the brain and prevent cerebral ischemia. Anesthetic agents have widely variable effects on the blood supply to the brain and, therefore, choice of anesthetic agent can influence neurological outcome. Although in the past, anesthetic agents have been selected for their neuroprotective properties, it is increasingly being recognized that the support of cerebral perfusion during anesthesia contributes more significantly to a positive outcome for these patients. Support of cardiorespiratory function is, therefore, highly important when anesthetizing patients with TBI. Conclusion: Choice of anesthetic agent is determined by the extent of brain injury and intracranial pressure (ICP) elevation. Factors that should be considered when anesthetizing head trauma patients include the effects of anesthetic agents on the cardiac and respiratory systems, their effects on cerebral blood flow (CBF), ICP, and possible neuroprotective benefits offered by certain agents.     

Isoflurane as an inhalational anesthetic agent in clinical practice

Isoflurane is the most recently available inhalational anesthetic agent on the market today. Although there have been few clinical trials comparing its use to halothane and methoxyflurane, the pharmacology of the agent suggests certain situations in which it may be the preferable agent. These include avian anesthesia, geriatric patients, patients with cardiovascular disease or hepatic disease, critically ill and unstable patients, cases such as brachiocephalics where upper airway obstruction is a concern during recovery, patients where increases in intracranial pressure should be avoided, and cesarean section. In addition, the rapid recoveries seen with isoflurane may be an advantage for outpatient surgeries.     

Sedation and anesthesia of pet rabbits

Pet rabbits frequently become stressed when handled and may require sedation or chemical immobilization for procedures such as blood collection, IV catheter placement, radiography, deep ear cleaning, and dentistry. Common surgical procedures requiring general anesthesia include spay, castration, gastrotomy, cystotomy, and orthopedic procedures. Rabbits may be difficult to safely sedate or anesthetize. Individual rabbits may have varying sensitivity to the depressant effects of anesthetics. The apparent sensitivity of the rabbit's respiratory center to anesthetic drugs and the narrow range between anesthetic and toxic doses in this species add to the unpredictable character of rabbit anesthesia. Furthermore, mortality following anesthesia and surgery in sick rabbits is common. Strategically, safe anesthesia of rabbits must include the planning of procedures so that anesthetic time is minimized. Clinicians must be on guard for individual variation in response to drugs. Minimizing the use of cardiovascular depressant agents, use of agents with a high therapeutic index, and careful titration of doses to effect, along with thorough cardiorespiratory monitoring, will permit attainment of appropriate anesthetic depth with the widest margin of safety. This article presents several injectable and inhalant anesthetic protocols that may assist in effective management of many types of rabbit patient.     

Anesthesia of the sighthound

The sighthounds are an ancient group of dog breeds that have been selectively bred for high-speed pursuit of prey by sight. Probably as a consequence of this selection process, these dogs have a number of idiosyncrasies that can potentially adversely affect their anesthetic management. These include (1) nervous demeanor which can lead to stress-induced clinical complications, such as hyperthermia; (2) lean body conformation with high surface-area-to-volume ratio, which predisposes these dogs to hypothermia during anesthesia; (3) hematological differences such as a higher packed cell volume and lower serum protein compared with other dog breeds which may complicate interpretation of preanesthetic blood work; (4) Impaired biotransformation of drugs by the liver resulting in prolonged recovery from certain intravenous anesthetics, especially thiopental; and increased risks of drug interactions. Safe anesthetic management of sighthounds should include sedative premedication and appropriate use of analgesic drugs to minimize perioperative stress. Thiopental, or any other thiobarbiturate, should not be used in these dogs. Propofol, ketamine/diazepam combination, and methohexital are recommended alternative intravenous anesthetics. Avoid coadministration of agents that inhibit drug biotransformation, such as chloramphenicol. Inhalation anesthesia using isoflurane is the preferred anesthetic maintenance technique. Core body temperature should be monitored closely and techniques to minimize hypothermia should be employed both during anesthesia and into the recovery period.     

AAHA anesthesia guidelines for dogs and cats

Safe and effective anesthesia of dogs and cats rely on preanesthetic patient assessment and preparation. Patients should be premedicated with drugs that provide sedation and analgesia prior to anesthetic induction with drugs that allow endotracheal intubation. Maintenance is typically with a volatile anesthetic such as isoflurane or sevoflurane delivered via an endotracheal tube. In addition, local anesthetic nerve blocks; epidural administration of opioids; and constant rate infusions of lidocaine, ketamine, and opioids are useful to enhance analgesia. Cardiovascular, respiratory, and central nervous system functions are continuously monitored so that anesthetic depth can be modified as needed. Emergency drugs and equipment, as well as an action plan for their use, should be available throughout the perianesthetic period. Additionally, intravenous access and crystalloid or colloids are administered to maintain circulating blood volume. Someone trained in the detection of recovery abnormalities should monitor patients throughout recovery. Postoperatively attention is given to body temperature, level of sedation, and appropriate analgesia.     

General clinical considerations for anesthesia of the horse

The peculiarities of the equine species present a number of unique situations that must be addressed when horses are anesthetized. Perhaps the most troublesome situation is related to the horse's size. Though the horse's large lungs are responsible in part for its sustainable athletic ability, they are detrimental to effective ventilation when the horse is anesthetized and placed in a recumbent position. Of major concern is depression of ventilation and cardiovascular function. Hypercapnia and hypoxemia usually result from hypoventilation, and with time all anesthetized horses suffer from some degree of cardiovascular depression. Decreased blood flow coupled with the horse's weight pressing downward on the undermost tissues frequently disturbs microcirculation and causes injury to muscle tissue. Of major importance is the product of anesthetic depth and anesthetic time. Only through careful observation and initiation of supportive measures can injuries related to anesthesia or surgery be kept to a minimum. Because of the horse's nature, safe anesthesia cannot always be assured, even when state-of-the-art anesthetic techniques are practiced.     

Anesthesia for the acute abdomen patient

Patients with acute abdomen often have marked physiologic and pathologic changes, making anesthesia both challenging and potentially hazardous for the patient. A thorough understanding of the pathophysiologic mechanisms of cardiovascular function under anesthesia and selection of appropriate anesthetic protocols are critical to a successful anesthetic outcome. The goal is to produce anesthesia while minimizing depression of the cardiovascular system. Monitoring and management of acid-base and cardiovascular function serve to ensure appropriate oxygen delivery to the tissues during anesthesia. Postoperative management can significantly influence patient outcome following anesthesic recovery, and must therefore be considered in the anesthetic plan. Finally, pain management in all patients is an important aspect of case management, and should not be overlooked. This article serves to educate the clinician in the above-described areas in regard to the acute abdomen patient.     

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     

Anesthesia in the Llama

Anesthesia for llamas is similar to other domestic species, although adjustments in technique are required to allow for species variations. Xylazine (0.4-0.6 mg/kg) is well tolerated for sedation. The thiobarbiturates (8-10 mg/kg), ketamine (2.5-5.0 mg/kg), or combinations of guaifenesin and thiobarbiturates or guaifenesin and ketamine (to effect) can be used for induction of anesthesia. In juvenile or debilitated animals, anesthesia can be induced with halothane or isoflurane administered by mask. After tracheal intubation, anesthesia can be maintained with the inhalation agents, usually halothane or isoflurane. Supportive therapy and many anesthetic monitoring techniques used in domestic animals can be used in llamas. While under marginal planes of anesthesia, llamas can have more active physiologic responses to pain, including bradycardia and vasoconstriction, than domestic animals. Llamas are more prone to airway obstruction after tracheal extubation than domestic ruminants but otherwise recover as well from general anesthesia as domestic ruminants.     

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.     

Anesthetic protocols for common emergencies.

Anesthesia, sedation, and pain management should be taken seriously in the emergency patient. Proper knowledge of the drugs available and their pharmacokinetics and pharmacodynamics are necessary to administer anesthesia safely to critical patients. A proactive approach regarding monitoring, titration of anesthetic drugs, and anticipation of life-threatening complications helps in achieving successful anesthetic outcomes.     

Regional anesthesia and analgesia for oral and dental procedures.

Regional anesthesia and analgesia benefit the client, the patient, and the practitioner, and their use is becoming the standard for care. Familiarity with the processes involved in the generation of pain aids in understanding the benefits of preemptive and multimodal analgesia. Local anesthetic blocks should be a key component of a treatment plan, along with opioids, nonsteroidal anti-inflammatory drugs, N-methyl-D-aspartate receptor antagonists, and other therapies. Nerve blocks commonly used for dentistry and oral surgery include the infraorbital, maxillary, mental,and mandibular blocks.     

Anesthesia of the intensive-care patient

Mortality rates for healthy horses undergoing general anesthesia for routine procedures have been reported to range from 0.6% to 1.8%; these numbers increase as high as 5% when systemically ill horses are included in calculations. Anesthesia of intensivecare equine patients should first include critical assessment of the need for versus risk of anesthetic intervention. When general anesthesia cannot be avoided, the following measures should be considered and addressed throughout the perianesthetic period: preanesthetic assessment and cardiopulmonary stabilization, perianesthetic oxygen supplementation and ventilatory support, vigilant monitoring of the cardiopulmonary system, supportive therapy (ie, inotropic agents, fluids) to promote adequate cardiac output, use of perioperative adjunct systemic and/or regional agents to reduce inhalant requirements, close observation and assistance during recovery, and postoperative analgesia.     

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.     

Comparative cardiovascular and pulmonary effects of sedatives and anesthetic agents and anesthetic drug selection for the trauma patient

Objective: To integrate and compare the effects of tranquilizer/sedatives and anesthetic drugs on various parameters of cardiovascular function in normal dogs and in dogs stressed by hypovolemia, anemia, and endotoxemia, and to discuss the relative merits and appropriate precautions of anesthetic drugs with respect to specific patient physiologic complications. Data sources: Personal data and experiences in conjunction with veterinary and human clinical and research studies. Human and veterinary data synthesis: Drugs that produce calming, sedation, muscle relaxation, analgesia, and loss of consciousness have the potential to produce marked cardiorespiratory effects particularly in hemorrhaged, hypovolemic‐traumatized animals. Acute but key cardiovascular components that are affected by sedative and anesthetic drugs include heart rate and rhythm, venous return (preload), systemic vascular resistance (afterload), and myocardial contractile (inotropic) and relaxation (lusitropic) properties. In addition, all sedative and anesthetic drugs alter or depress normal baroreceptor reflex activity, thereby inhibiting or eliminating the animal's normal physiologic response to decreases in arterial blood pressure and predisposing to tissue hypoperfusion, decreased oxygen delivery, and oxygenation. Oxygen delivery needs to be adequate to meet the metabolic (oxygen) requirements of the patient. Decreases in oxygen delivery to tissues increases oxygen extraction, thereby maintaining tissue oxygenation (supply‐independent oxygen consumption phase) until compensatory processes reach their limit and any further decrease in oxygen delivery causes a decrease in oxygen consumption (supply‐dependent oxygen consumption phase). The critical oxygen delivery that defines the transition between these 2 phases is generally higher in the anesthetized state than in the awake state. The effect of anesthetics on critical oxygen delivery at comparable anesthetic dosages is pentobarbital=ketamine>alfentanil>etomidate=propofol>inhalational anesthetics. Anesthetics generally decrease oxygen consumption from the awake, baseline state; exceptions are ketamine and ether. Ketamine, however, increases oxygen delivery and oxygen extraction. Conclusions: The transition from the awake to the anesthetized state is a huge imposition on the physiology of animals and, therefore, should be accomplished with great care and proper vigilance. Rapid, ‘crash’ induction of anesthesia should be avoided in hypotension‐prone animals and slow, prolonged induction should be avoided in animals with respiratory disorders. It is not recommended to implement an unfamiliar protocol in critical patients, even if it might be pharmacologically preferable. Familiarity with an anesthetic drug is a very important reason for its selection.     

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.     

Respiratory reflexes in response to upper-airway administration of sevoflurane and isoflurane in anesthetized, spontaneously breathing dogs

OBJECTIVE: To evaluate the respiratory effects occurring during administration of sevoflurane or isoflurane to the upper airway in dogs. STUDY DESIGN: A prospective, randomized study. ANIMALS: Twelve healthy adult beagles (6 males, 6 females). METHODS: At least 2 weeks after undergoing permanent tracheostomy, dogs were premedicated with acepromazine-buprenorphine, and anesthesia was induced with thiopental and maintained with alpha-chloralose. The upper airway was functionally isolated so that the inhalant could be administered to the upper airway while dogs were breathing 100% O2 via the tracheostomy. Respiratory reflexes in response to the administration of sevoflurane or isoflurane at concentrations of 1.2, 1.8, and 2.4 times the minimal alveolar concentration (MAC) (administered in 100% O2 at a flow rate of 5 L/min) were recorded. Reflexes in response to administration of each anesthetic were also recorded following upper-airway administration of lidocaine. RESULTS: Respiratory reflexes elicited by upper-airway administration of each anesthetic were characterized by a dose-dependent increase in expiration time, with a resultant decrease in respiratory minute ventilation and increase in end-tidal PCO2. The magnitude of these responses was greater with isoflurane than with sevoflurane at 1.8 and 2.4 MAC. These reflexes were abolished after lidocaine nebulization into the upper airway. CONCLUSION: Isoflurane induces greater reflex inhibition of breathing than does sevoflurane when the anesthetic is inhaled into the upper airway at concentrations used for mask induction.     

Feline anesthetic deaths in veterinary practice

Anesthetic complications appear relatively rare, though recent work suggests they are more common in cats than dogs. Current estimates indicate that approximately 0.11% (1 in 895 anesthetics) of healthy cats die of an anesthetic-related death, which is more than twice as frequent as has been recently reported in dogs (0.05% or 1 in 1849). Most of these deaths occurred in the postoperative period. A number of risk factors have been associated with death, including patient health status, age, weight, and procedure type and urgency. Endotracheal intubation and fluid therapy have been reported to be associated with increased odds of anesthetic death in cats and may reflect higher risk techniques in cats compared with dogs. Monitoring patient pulse and the use of a pulse oximeter were also recently reported to be associated with reduced risk of anesthetic death. These data can help veterinarians care for their patient under anesthesia and address greater attention to patient assessment and management before anesthesia, as well as more careful fluid administration and patient monitoring during and after anesthesia, which could reduce perioperative complications in cats.