Essentials of Small Animal Anesthesia and Analgesia

Kurt Grimm, William J. Tranquilli, Leigh Lamont, Kurt A. Grimm, William J. Tranquilli, Leigh A. Lamont, Kurt A. Grimm, William J. Tranquilli, Leigh A. Lamont

• Publisher: Wiley-Blackwell
• Year: 2011
• Print ISBN: 9780813812366
• eBook ISBN: 9781119946182
• Edition: 2
• Topic: Medicine
• Subtopic: Veterinary Medicine

Essentials of Small Animal Anesthesia and Analgesia, Second Edition presents the fundamentals of managing small animal anesthesia patients in a clinically relevant, accessible manual. The bulk of the book is distilled from Lumb and Jones' Veterinary Anesthesia and Analgesia to provide authoritative information in a quick-reference format, with references to Lumb and Jones' throughout for easy access to further detail. Logically reorganized with an easy-to-use structure and an increased focus on pain management, this new edition features new chapters on equipment and managing specific conditions.

The Second Edition has been updated to reflect current practices in anesthesia and analgesia, and a new companion website offers review questions and answers, video clips, and an image bank with additional figures not found in the printed book. Essentials of Small Animal Anesthesia and Analgesia, Second Edition provides veterinary care providers and students with key information on anesthetic and analgesic pharmacology, physiology, patient assessment, and clinical case management.

Chapter 1

Patient evaluation and risk management

William W. Muir, Steve C. Haskins, and Mark G. Papich

Introduction

The purpose of anesthesia is to provide reversible unconsciousness, amnesia, analgesia, and immobility for invasive procedures. The administration of anesthetic drugs and the unconscious, recumbent, and immobile state, however, compromise patient homeostasis. Anesthetic crises are unpredictable and tend to be rapid in onset and devastating in nature. The purpose of monitoring is to achieve the goals while maximizing the safety of the anesthetic experience.

Preanesthetic evaluation

All body systems should be examined and any abnormalities identified. The physical examination and medical history will determine the extent to which laboratory tests and special procedures are necessary. In all but extreme emergencies, packed cell volume and plasma protein concentration should be routinely determined. Contingent on the medical history and physical examination, additional evaluations may include complete blood counts; urinalysis; blood chemistries to identify the status of kidney and liver function, blood gases, and pH; electrocardiography; clotting time and platelet counts; fecal and/or filarial examinations; and blood electrolyte determinations. Radiographic and/or ultrasonographic examination may also be indicated.

Following examination, the physical status of the patient should be classified as to its general state of health according to the American Society of Anesthesiologists (ASA) classification (Table 1.1). This mental exercise forces the anesthetist to evaluate the patient’s condition and proves valuable in the proper selection of anesthetic drugs. Classification of overall health is an essential part of any anesthetic record system. The preliminary physical examination should be done in the owner’s presence, if possible, so that a prognosis can be given personally. This allows the client to ask questions and enables the veterinarian to communicate the risks of anesthesia and allay any fears regarding management of the patient.

Table 1.1. Classification of physical status^a

Source: Muir W.W. 2007. Considerations for general anesthesia. In: Lumb and Jones’ Veterinary Anesthesia and Analgesia, 4th ed. W.J. Tranquilli, J.C. Thurmon, and K.A. Grimm, eds. Ames, IA: Blackwell Publishing, p. 17.

Category	Physical status	Possible examples of this category
I	Normal healthy patients	No discernible disease; animals entered for ovariohysterectomy, ear trim, caudectomy, or castration
II	Patients with mild systemic disease	Skin tumor, fracture without shock, uncomplicated hernia, cryptorchidectomy, localized infection, or compensated cardiac disease
III	Patients with severe systemic disease	Fever, dehydration, anemia, cachexia, or moderate hypovolemia
IV	Patients with severe systemic disease that is a constant threat to life	Uremia, toxemia, severe dehydration and hypovolemia, anemia, cardiac decompensation, emaciation, or high fever
V	Moribund patients not expected to survive 1 day with or without operation	Extreme shock and dehydration, terminal malignancy or infection, or severe trauma

^a This classification is the same as that adopted by the ASA.

Preanesthetic pain evaluation

The diagnosis and treatment of pain require an appreciation of its consequences, a fundamental understanding of the mechanisms responsible for its production, and a practical appreciation of the analgesic drugs that are available. Semiobjective and objective behavioral, numerical, and categorical methods have been developed for the characterization of pain and, among these, the visual analog scale (VAS) has become popular. Ideally, pain therapy should be directed toward the mechanisms responsible for its production (multimodal therapy), with consideration, when possible, of initiating therapy before pain is initiated (preemptive analgesia). The American Animal Hospital Association (AAHA) has developed standards for the assessment, diagnosis, and therapy of pain that should be adopted by all veterinarians (Table 1.2).

Preanesthetic stress evaluation

Both acute and chronic pain can produce stress. Untreated pain can initiate an extended and potentially destructive series of events characterized by neuroendocrine dysregulation, fatigue, dysphoria, myalgia, abnormal behavior, and altered physical performance. Even without a painful stimulus, environmental factors (loud noise, restraint, or a predator) can produce a state of anxiety or fear that sensitizes and amplifies the stress response. Distress, an exaggerated form of stress, is present when the biologic cost of stress negatively affects the biologic functions critical to survival. Pain, therefore, should be considered in terms of the stress response and the potential to develop distress.

Increased central sympathetic output causes increases in heart rate and arterial blood pressure, piloerection, and pupil dilatation. The secretion of catecholamines from the adrenal medulla and spillover of norepinephrine released from postganglionic sympathetic nerve terminals augment these central effects. Ultimately, changes in an animal’s behavior may be the most noninvasive and promising method to monitor the severity of an animal’s pain and associated stress.

Table 1.2. AAHA pain management standards (2003)

Sources: Muir W.W. 2007. Considerations for general anesthesia. In: Lumb and Jones’ Veterinary Anesthesia and Analgesia, 4th ed. W.J. Tranquilli, J.C. Thurmon, and K.A. Grimm, eds. Ames, IA: Blackwell Publishing, p. 19, and the AAHA, Lakewood, CO.

1. Pain assessment for all patients regardless of presenting complaint

2. Pain assessment using standardized scale/score and recorded in the medical record

3. Pain management is individualized for each patient

4. Practice utilizes preemptive pain management

5. Appropriate pain management is provided for the anticipated level of pain

6. Pain management is provided for the anticipated duration of pain

7. Patient is reassessed for pain throughout potentially painful procedure

8. Patients with persistent or recurring disease are evaluated to determine their pain management needs

9. Analgesic therapy is used as a tool to confirm the existence of a painful condition when pain is suspected but cannot be confirmed by objective methods

10. A written pain management protocol is utilized

11. When pain management is part of the therapeutic plan, the client is effectively educated

Patient preparation

Preanesthetic fasting

Too often, operations are undertaken with inadequate preparation of patients. With most types of general anesthesia, it is best to have patients off feed for 12 hours previously. Some species are adversely affected by fasting. Birds, neonates, and small mammals may become hypoglycemic within a few hours of starvation, and mobilization of glycogen stores may alter rates of drug metabolism and clearance. Induction of anesthesia in animals having a full stomach should be avoided, if at all possible, because of the hazards of aspiration.

Preanesthetic fluid therapy

In most species, water is offered up to the time that preanesthetic agents are administered. It should be remembered that many older animals have clinical or subclinical renal compromise. Although these animals remain compensated under ideal conditions, the stress of hospitalization, water deprivation, and anesthesia, even without surgery, may cause acute decompensation. Ideally, a mild state of diuresis should be established with intravenous fluids in nephritic patients prior to the administration of anesthetic drugs.

Dehydrated animals should be treated with fluids and appropriate alimentation prior to operation; fluid therapy should be continued as required. An attempt should be made to correlate the patient’s electrolyte balance with the type of fluid that is administered. Anemia and hypovolemia, as determined clinically and hematologically, should be corrected by administration of whole blood or blood components and balanced electrolyte solutions. Patients in shock without blood loss or in a state of nutritional deficiency benefit by administration of plasma or plasma expanders. In any case, it is good anesthetic practice to administer intravenous fluids during anesthesia to help maintain adequate blood volume and urine production, and to provide an available route for drug administration.

Prophylactic antibiotic administration

Systemic administration of antibiotics preoperatively is a helpful prophylactic measure prior to major surgery or if contamination of the operative site is anticipated. Antibiotics are ideally given approximately 1 hour before anesthetic induction.

Oxygenation and ventilation

Several conditions may severely restrict effective oxygenation and ventilation. These include upper airway obstruction by masses or abscesses, pneumothorax, hemothorax, pyothorax, chylothorax, diaphragmatic hernia, and gastric distention. Affected animals are often in a marginal state of oxygenation. Oxygen administration by nasal catheter or mask is indicated if the patient will accept it. Intrapleural air or fluid should be removed by thoracocentisis prior to induction because the effective lung volume may be greatly reduced and severe respiratory embarrassment may occur on induction. Anesthetists should be prepared to carry out all phases of induction, intubation, and controlled ventilation in one continuous operation.

Heart disease

Decompensated heart disease is a relative contraindication for general anesthesia. If animals must be anesthetized, an attempt at stabilization through administration of appropriate inotropes, antiarrhythmic drugs, and diuretics should be made prior to anesthesia. If ascites is present, fluid may be aspirated to reduce excessive pressure on the diaphragm.

Hepatorenal disease

In cases of severe hepatic or renal insufficiency, the mode of anesthetic elimination should receive consideration, with inhalation anesthetics often preferred. Just prior to induction, it is desirable to encourage defecation and/or urination by giving animals access to a run or exercise pen.

Patient positioning

During anesthesia, patients should, if possible, be restrained in a normal physiological position. Compression of the chest, acute angulation of the neck, overextension or compression of the limbs, and compression of the posterior vena cava by large viscera can all lead to serious complications, which include hypoventilation, nerve and/or muscle damage, and impaired venous return.

Tilting anesthetized patients alters the amount of respiratory gases that can be accommodated in the chest (functional residual capacity [FRC]) by as much as 26%. In dogs subjected to hemorrhage, tilting them head-up (reverse Trendelenburg position) was detrimental, producing lowered blood pressure, hyperpnea, and depression of cardiac contractile force. When dogs were tilted head-down (Trendelenburg position), no circulatory improvement occurred. In most species, the head should be extended to provide a free airway and to prevent kinking of the endotracheal tube.

Selection of an anesthetic and analgesic drugs

The selection of an anesthetic is based on appraising several factors, including:

(1) The patient’s species, breed, and age.

(2) The patient’s physical status.

(3) The time required for the surgical (or other) procedure, its type and severity, and the surgeon’s skill.

(4) Familiarity with the proposed anesthetic technique.

(5) Equipment and personnel available.

In general, veterinarians will have greatest success with drugs they have used most frequently and with which they are most familiar. The skills of administration and monitoring are developed only with experience; therefore, change from a familiar drug to a new one is usually accompanied by a temporary increase in anesthetic risk.

The length of time required to perform a surgical procedure and the amount of help available during this period often dictate the anesthetic that is used. Generally, shorter procedures are done with short-acting agents, such as propofol, alphaxalone-CD, and etomidate, or with combinations using dissociative, tranquilizing, and/or opioid drugs. Where longer anesthesia is required, inhalation or balanced anesthetic techniques are preferred.

Drug interactions

When providing anesthesia and analgesia to animals, veterinarians often administer combinations of drugs without fully appreciating the possible interactions that may and do occur. Many drug interactions, both beneficial (resulting in decreased anesthetic risk) and harmful (increasing anesthetic risk), are possible. Although most veterinarians view drug interactions as undesirable, modern anesthesia and analgesic practice emphasizes the use of drug interactions for the benefit of the patient (multimodal anesthesia or analgesia).

A distinction should be made between drug interactions that occur in vitro (such as in a syringe or vial) from those that occur in vivo (in patients). Veterinarians frequently mix drugs together (compound) in syringes, vials, or fluids before administration to animals. In vitro reactions, also called pharmaceutical interactions, may form a drug precipitate or a toxic product or inactivate one of the drugs in the mixture. In vivo interactions are also possible, affecting the pharmacokinetics (absorption, distribution, or biotransformation) or the pharmacodynamics (mechanism of action) of the drugs and can result in enhanced or reduced pharmacological actions or increased incidence of adverse events.

Nomenclature

Commonly used terms to describe drug interactions are addition, antagonism, synergism, and potentiation. In purely pharmacological terms that have underlying theoretical implications, addition refers to simple additivity of fractional doses of two or more drugs, the fraction being expressed relative to the dose of each drug required to produce the same magnitude of response; that is, response to X amount of drug A = response to Y amount of drug B = response to 1/2XA + 1/2YB, 1/4XA + 3/4YB, and so on. Additivity is strong support for the assumption that drug A and drug B act via the same mechanism (e.g., on the same receptors). Confirmatory data are provided by in vitro receptor-binding assays. Minimum alveolar concentration (MAC) fractions for inhalational anesthetics are additive. All inhalants have similar mechanisms of action but do not appear to act on specific receptors.

Synergism refers to the situation where the response to fractional doses as described previously is greater than the response to the sum of the fractional doses (e.g., 1/2XA + 1/2YB produces more than the response to XA or YB).

Potentiation refers to the enhancement of action of one drug by a second drug that has no detectable action of its own.

Antagonism refers to the opposing action of one drug toward another. Antagonism may be competitive or noncompetitive. In competitive antagonism, the agonist and antagonist compete for the same receptor site. Noncompetitive antagonism occurs when the agonist and antagonist act via different receptors.

The way anesthetic drugs are usually used raises special considerations with regard to drug interactions. For example, (1) drugs that act rapidly are usually used; (2) responses to administered drugs are measured, often very precisely; (3) drug antagonism is often relied upon; and (4) doses or concentrations of drugs are usually titrated to effect. Minor increases or decreases in responses are usually of little consequence and are dealt with routinely.

Commonly used anesthetic drug interactions

Two or more dif...

Table of contents

• Cover
• Title page
• copyright
• Contributors
• Contributors
• Preface
• Chapter 1: Patient evaluation and risk management
• Chapter 2: Anesthetic physiology and pharmacology
• Chapter 3: Pain physiology, pharmacology, and management
• Chapter 4: Chronic pain management
• Chapter 5: Anesthesia equipment
• Chapter 6: Patient monitoring
• Chapter 7: Acid–base balance and fluid therapy
• Chapter 8: Anesthesia management of dogs and cats
• Chapter 9: Anesthesia and immobilization of small mammals
• Chapter 10: Local anesthetics and regional analgesic techniques
• Chapter 11: Anesthesia for patients with cardiovascular disease
• Chapter 12: Anesthesia for patients with respiratory disease and/or airway compromise
• Chapter 13: Anesthesia for patients with neurological disease
• Chapter 14: Anesthesia for small animal patients with renal disease
• Chapter 15: Anesthesia for patients with liver disease
• Chapter 16: Anesthesia for patients with gastrointestinal disease
• Chapter 17: Anesthesia for patients with endocrine disorders
• Chapter 18: Anesthetic considerations for special procedures
• Chapter 19: Anesthetic emergencies and accidents
• Index