William J. Tranquilli1, Kurt A. Grimm2 and Leigh A. Lamont3 1 Professor Emeritus, Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois at Urbana‐Champaign, Urbana, Illinois, USA 2 Veterinary Specialists Services, PC, Conifer, Colorado, USA 3 Department of Companion Animals, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada Veterinary anesthesia continues to evolve as a science and specialty within the veterinary profession. The major drivers of change are advances in medical technology, development of evidence‐based guidelines for patient care, and socioeconomic and demographic changes in countries where animals serve evolving roles. One thing that remains certain is that veterinary anesthesiologists will continue to be advocates for patient safety, humane care, and quality of life and serve as frontline educators for best practices in anesthesia, analgesia, and pain management. Proper use of anesthetics, sedatives, and analgesics can alleviate pain, create amnesia, and produce muscle relaxation essential for safe and humane patient care [1]. Important uses include facilitation of immobilization for various diagnostic, surgical, and therapeutic procedures; safe transportation of wild and exotic animals; and euthanasia and the humane slaughter of food animals. Anesthesia, sedation, and analgesic drug administration are not without significant patient risk and are not recommended for trivial reasons. The continued development of better techniques and drugs along with continuing efforts to educate veterinary care providers has minimized the overall risk of anesthesia and pain alleviation in an ever‐increasing and more sophisticated patient care environment. Any discussion with the animal‐owning public, such as that occurring with owners when obtaining informed consent, requires the use of proper terminology and clear communication to convey the issues central to the safe delivery of veterinary anesthesia and pain therapy. The term anesthesia, derived from the Greek term anaisthaesia, meaning “insensibility,” is used to describe the loss of sensation to the entire body or a specific portion of it. Anesthesia is induced by drugs that depress the activity of nervous tissue locally, regionally, or within the central nervous system (CNS). From a pharmacological viewpoint, there has been a significant redefining of the term general anesthesia [2], and both central nervous stimulants and depressants can be useful general anesthetics [3]. Management of pain in patients involves the use of drugs that are often called analgesics. The term is derived from an, which implies “negative” or “without,” and alges(is), meaning “pain” [4]. Clinical management of pain often results in varying degrees of effectiveness that represent states of hypoalgesia or decreased sensation of pain. It is important to understand that the administration of an analgesic drug does not necessarily create the state of analgesia. The diverse uses for anesthesia (as it relates to immobilization, muscle relaxation, and antinociception) and the requirements peculiar to species, age, and disease state necessitate the use of a variety of drugs, drug combinations, and methods. Throughout this text and elsewhere, various terms are used to describe the effects of anesthetic drugs, pain‐inhibiting drugs, and various techniques and routes of administration. Commonly used terms are defined below, and the reader is referred to other chapters for more details where appropriate. Analgesia is the absence of pain in response to stimulation, which would normally be painful. The term is generally reserved for describing lack of pain in a conscious patient [5]. Acupuncture is a system of therapy using long, fine needles to induce hypoalgesia. Additional modalities of acupuncture point stimulation have been utilized, including mechanical and electrical stimulation. See Chapter 49. Balanced anesthesia is achieved by the simultaneous use of multiple drugs and techniques. Different drugs contribute variably to specific components of the anesthetic state: amnesia, antinociception, muscle relaxation, and alteration of autonomic reflexes. Dissociative anesthesia is induced by drugs (e.g., ketamine) that dissociate the thalamocortic and limbic systems. This form of anesthesia is characterized by a cataleptoid state, in which eyes remain open and swallowing reflexes remain intact. Skeletal muscle hypertonus persists unless a strong sedative or peripheral or central muscle relaxant is co‐administered. See Chapter 27. Electronarcosis, electroanesthesia, or electrosleep refers to the passage of electrical currents through the cerebrum to induce deep narcosis. Even though there have been successful studies, this form of anesthesia has never gained popularity and is rarely used in veterinary practice. Electronarcosis should not be confused with the inhumane practice of electroimmobilization. General anesthesia is drug‐induced unconsciousness that is characterized by controlled but reversible depression of the CNS and perception. In this state, the patient is not arousable by noxious stimulation. Sensory, motor, and autonomic reflex functions are attenuated to varying degrees, depending upon the specific drug(s) and technique(s) used. Hypnosis is a condition of artificially induced sleep, or a trance resembling sleep, resulting from moderate depression of the CNS from which the patient is readily aroused. Hypothermia refers to a decrease in body temperature, induced either locally or generally, to supplement insensitivity and decrease anesthetic drug requirements and reduce metabolic needs. It is primarily used in neonates or in patients undergoing cardiovascular surgery. See Chapter 34. Inhalation or inhalant anesthesia refers to the practice of administering anesthetic gases or vapors via inhalation in combination with oxygen. See Chapter 28. Injectable anesthesia refers to the practice of administering anesthetic solutions via intravenous, intramuscular, or subcutaneous injection. Other injectable routes include intraperitoneal and intrathoracic but, except for some laboratory animal species, these are not generally recommended. See Chapters 27 and 54. Local and regional analgesia/anesthesia refers to loss of sensation, notably pain, in a particular area or region of the body, usually defined by the pattern of innervation of the affected nerve(s). Anesthetic drug may be applied topically or injected locally into or around the surgical site (variably referred to as “field block,” “incisional block,” or “infiltrative block”), perineurally around peripheral nerve(s), between fascial planes, or neuraxially (into the epidural or subarachnoid space). See Chapters 60, 63, and 66. Narcosis is a drug‐induced state of deep sleep from which the patient cannot be easily aroused. Narcosis may or may not be accompanied by antinociception, depending on the techniques and drugs used. Nociception is the neural (physiologic) process of encoding noxious stimuli [5] that underlies the conscious perception of pain. Nociception does not require consciousness and can continue unabated during general anesthesia if techniques that interrupt or inhibit the transduction, transmission, and modulation of nociceptive stimuli are not utilized. Oral (enteral) or rectal administration routes may be used to administer certain anesthetic or analgesic agents. There is often a greater degree of interspecies and interindividual variability in the dose–response relationship of orally administered drugs due to differences in absorption and first‐pass hepatic metabolism. Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage [5]. See Chapter 46. Sedation is a state characterized by CNS depression accompanied by drowsiness and some degree of centrally induced relaxation. The patient is generally unaware of its surroundings but can become aroused and is responsive to noxious stimulation. Sedatives are not recommended to immobilize a patient when painful stimuli are likely to occur (e.g., surgery). See Chapter 22. Surgical general anesthesia is the state/plane of anesthesia that provides unconsciousness, amnesia, muscle relaxation, and hypoalgesia sufficient for painless surgery. Total intravenous anesthesia (TIVA), partial intravenous anesthesia (PIVA), and targeted controlled infusion (TCI) describe anesthetic techniques that utilize intravenous infusion of one or more drugs to produce a suitable anesthetic state. Automated infusion systems are available that allow the input of patient parameters and pharmacokinetic information for specific drugs and allow the anesthesiologist to target a predetermined plasma drug concentration (TCI). Tranquilization results in behavioral change wherein anxiety is relieved and the patient becomes relaxed but remains aware of its surroundings. Tranquilizers are drugs that result in tranquilization when administered; however, many prefer to use the term “anxiolytic” or “anti‐anxiety” when describing drugs that decrease anxiety and induce relaxation. See Chapter 22. Transcutaneous electrical nerve stimulation (TENS, TNS, or TES) is a technique that induces local analgesia by low‐intensity, high‐frequency electrical stimulation of the skin through surface electrodes. TENS has many similarities to electroacupuncture. See Chapter 49. Twilight anesthesia is a state of heavy sedation where the patient is still conscious, but cooperative, and has limited or no recall (amnesia). This technique is popular for outpatient anesthesia in human medicine for diagnostic procedures and for minor surgical procedures when combined with local anesthetics and additional analgesic drugs. Twilight anesthesia is a term in common use by laypeople to connote heavy sedation and does not refer to a specific anesthetic procedure or technique. While there are accounts in both ancient western and eastern historical texts chronicling various drugs and techniques used to achieve insensibility in humans and animals, little appears in the formal literature until the 19th century. Not surprisingly, the history of human and veterinary anesthesia is tightly interwoven, and the early timeline includes contributions from chemists, physicians, dentists, and veterinarians alike. Other authors have chronicled the early history and evolution of veterinary anesthesia at various points over the past 65 years, and a number of interesting reviews are available elsewhere for readers looking for more detail [6–14]. By taking the time to reflect on the history of our specialty, we are better able to appreciate its continued evolution. In 1800, Humphrey Davy administered nitrous oxide to a guinea pig and suggested that it may have anesthetic properties. Twenty‐four years later, Henry Hickman demonstrated that pain associated with surgery in dogs could be alleviated by inhaling a mixture of nitrous oxide and carbon dioxide. He reasoned that the latter increased the rate and depth of breathing, thus enhancing the effects of nitrous oxide. Much later, in the early 1990s, studies confirmed that unconsciousness could be induced in 30–40 seconds in piglets breathing carbon dioxide (50%) in oxygen (50%) [15]. It was not until 1842 that diethyl ether was used for human anesthesia, and within a year of William Morton’s famous public demonstration of “etherization” at Massachusetts General Hospital in 1846, others began using ether to produce unconsciousness in animals. The Boston physician, Charles Thomas Jackson, was among the first to publish his findings about the use of ether in animals in 1853 [16]. In 1844, a dentist from Connecticut named Horace Wells rediscovered the anesthetic properties of nitrous oxide and recognized its potential for dental practice. While his work was neglected for a number of years, nitrous oxide was ultimately introduced to human anesthesia in 1862. Chloroform was discovered by Justus Liebig in 1831, and in 1847, Marie Jean Pierre Flourens used it to induce anesthesia in animals. Around this time, British physician George H. Dadd had immigrated to the United States and begun practicing veterinary medicine where he routinely employed general anesthesia in his animal patients. He was one of the first veterinarians in the United States to advocate for the humane treatment of animals, including the use of anesthesia in veterinary surgery, and he vigorously promoted the application of sound scientific principles to the practice of veterinary medicine [17]. In 1875, Oré published the first monograph on intravenous anesthesia using chloral hydrate and, three years later, Humbert described its use in horses. Pirogoff was the first to attempt rectal anesthesia with chloral hydrate in 1847, and intraperitoneal injection was first described in 1892 in France. Thus, by the end of the 19th century, various routes of anesthetic administration had been identified, and rudimentary investigations into the safety and efficacy of multiple anesthetics had been undertaken. After the initial isolation of cocaine by Albert Niemann in Germany in 1860, Anrep suggested the possibility of using cocaine as a local anesthetic in 1878. In 1884, Koller used cocaine for local anesthesia of the eye, and Halsted described cocaine regional anesthesia a year later. Its use was subsequently popularized by Frederick Hobday, an English veterinarian. In 1885, James Leonard Corning was credited for using cocaine for spinal anesthesia in dogs. From his description, however, it would appear that he induced epidural anesthesia. In 1898, August Bier induced true spinal anesthesia in animals and then in himself and an assistant [18]. While local infiltration was first popularized by Reclus in 1890 and Schleich in 1892, conduction regional anesthesia had been earlier introduced by Halsted and Hall in New York in 1884. These techniques increased in popularity with the discovery of local anesthetics less toxic than cocaine. Local anesthetics were used by Cuille and Sendrail in 1901 in France to induce subarachnoid anesthesia in horses, cattle, and dogs. That same year, Cathelin reported epidural anesthesia in dogs, but it remained for Retzgen, Benesch, and Brook to utilize this technique in larger species during the 1920s. Although paralumbar anesthesia was employed in humans by Sellheim in 1909, it was not until the 1940s that Farquharson and Formston applied this technique in cattle. Despite all these promising advances in the latter half of the 19th century, and presumably due in large part to many unfavorable side effects, general anesthesia was not broadly adopted by the veterinary profession until well into the 20th century. Unfortunately, a “heavy hand,” without analgesia/anesthesia or even sedation, was the stock‐in‐trade of many “large animal” practicing veterinarians well into the latter half of the 20th century. Although diethyl ether and chloroform were utilized in pets in the early part of the 20th century, general anesthesia was not widely accepted until the discovery of barbiturates in the late 1920s, in particular, the introduction of pentobarbital in 1930 and thiopental in 1934. Because of rough, prolonged recoveries, the acceptance of barbiturate general anesthesia in larger species was delayed until phenothiazines were introduced by Charpentier in France in 1950. General anesthesia of large farm animals was further advanced by the discovery of fluorinated hydrocarbons and the development of “large animal” inhalant anesthetic equipment. Since the 1970s, the introduction of newer classes of drugs together with techniques for their safe co‐administration (e.g., phenothiazines, benzodiazepines, α2‐adrenergic receptor agonists, opioids, guaifenesin, and dissociatives) has further advanced the safety and utility of anesthesia for both large and small animal species [8].
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Overview, History, and Current Issues in Veterinary Anesthesia and Analgesia
Overview
Terminology
History of veterinary anesthesia
Early milestones
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