Small Animal Anesthesia Techniques

Preface

The purpose of this book is to provide an easily accessible guide to the veterinary professional, for development of a balanced anesthesia protocol with appropriate analgesia. The authors assume the reader is familiar with concepts of veterinary medicine; this book outlines the anesthetic process, gives example protocols for specific patients, and helps to anticipate anesthetic complications. The information, while supported by research where appropriate, is also reflective of the authors’ personal preferences and experiences. It is recommended the anesthetist become familiar with healthy patients for routine procedures before attempting anesthesia in the critical patient.

Amanda M. Shelby

Carolyn M. McKune

Acknowledgments

While many veterinary professionals have and continue to be influential, I would like to give special acknowledgment to the following individuals who helped inspire my interests in anesthesia and the development of this work: L. Pablo, for your patience and guidance; J. Bailey, for encouraging improvement; A. Shih, for inspiring awe and speediness; S. Robertson, for your humility and compassion; T. Torres, for believing in me before anyone else; M. Fitzgerald, for your contributions, writing, and patience; C. McKune, for encouragement, reassurance, and professional guidance; A. daCunha and P. Queiroz-Williams, for capturing the great shots; and Thumbwars, for bathing the kids. Thank you.

A.S.

The efforts of many people resulted in the creation of this book, starting with the influence of Dr. Mike Mison, who reintroduced me to an academic setting where I would eventually go on to meet Ms. Amanda Shelby, veterinary technician extraordinaire, who would involve me in this project. However, a few folks along the way deserve a special acknowledgment: all of the anesthesia technicians and anesthesiologists at UC Davis College of Veterinary Medicine (particularly Dr. L. Barter, Dr. R. Brosnan, and Dr. P. Wong), who directly shaped the anesthesiologist I am today; Ms. Amanda Shelby, a brilliant mind with a hardworking mother's ethics; and Dr. Sheilah Robertson, a selfless mentor who is truly beyond her time. In addition, all of the students I have had the pleasure of working with since 2003 have taught me more about what it means to be a compassionate veterinarian than all of the formal instruction I've ever had.

However, no professional endeavor will succeed without a strong personal foundation. My rock in that respect is my wonderful husband, Dr. Michael J. Dark, without whom this would not have been possible. If I have to dedicate this book to someone, it is to the joys of my life: my children, Michael D. Dark and Elspeth L. Dark. You are the future and this book is for you.

C.M.

About the companion website

This book is accompanied by a companion website:

www.wiley.com/go/shelbyanesthesia

The website includes:

Videos

Images

Worksheets for calculations

Chapter 1

Anesthetic process

Step 1: Preanesthetic assessment

The patient's primary veterinarian performs a complete physical examination (PE) and history, which is made available to the anesthetist. The anesthetist then reviews the patient's history, performs his or her own preanesthetic PE, and reviews or requests additional diagnostic information (such as blood work [BW] or radiographs). The preceding information is obtained and reviewed within 24 hours of anesthesia, to ensure the most recent and therefore most pertinent assessment of the patient. When all the necessary information is collected, the anesthetist assigns the anesthesia candidate an American Society of Anesthesiologists' (ASA) score.

A. History

Important components of patient history include identification of the chief complaint(s), supporting diagnostic information, time of last meal, previous anesthetic complications, known allergies, vaccination records, and current medications. Each of these components plays a vital role in the anesthetist's ability to advocate for the patient. For example, identifying the chief complaint of the patient will ensure the appropriate analgesia is selected, correct area is prepared for surgery, appropriate fluids are chosen, necessary supportive measures (such as inotropes) are prepared in advance, and so on. The time of the last meal is a key determinant if fasting duration is appropriate (although this is not always possible in the emergent situation), to reduce the incidence of regurgitation and possible esophageal stricture. Previous anesthesia concerns and known allergies allow the anesthetist to appropriately tailor drugs to avoid combinations that would result in serious consequences (such as a patient that has allergies to eggs receiving propofol, which contains an egg lecithin). Vaccination records are important to control contagious disease and to protect from zoonotic diseases (such as rabies). Current medications also influence the anesthetic protocol; a good rule of thumb is to ask a client what passes through the pet's mouth other than food or water. Often, clients forget herbs or vitamins that are considered medications.

B. PE guidelines

The primary veterinarian performs a complete PE prior to electing anesthesia for a patient. However, the anesthetist systematically performs his or her own PE on every patient in his or her care. Developing a consistent approach to the PE increases the likelihood of recognizing abnormal findings. If possible, this exam is performed prior to any administration of drugs. A quiet, stress-limited environment for patient evaluation is ideal. The following is a rough guide for the anesthetic PE:

1. Overall observation

(a) Disposition:

Is the patient nervous, calm, anxious, or aggressive?

(b) Gait:

Is the patient lame or neurologic? Does the patient have a head tilt?

(c) Level of consciousness:

Is the patient bright or quiet? Is it alert and responsive or depressed and obtunded?

(d) Pain assessment:

Does the patient display any outward signs of discomfort or pain? A preemptive pain evaluation will assist in postoperative pain assessment and includes assessment of the patient's demeanor, attention and response to palpation of areas considered painful, desire or reluctance to move, and posture (1).

(e) Observation of respiratory effort and abdomen:

Does the abdomen look distended? Is there an abdominal component to the patient's breathing?

2. Head to tail evaluation

(a) Head:

Starting at the head, look at the overall symmetry of the eyes, nose, lips, mandible, and maxilla. Is discharge from the eyes or nose present? Are the eyes sunken in? Determine capillary refill time (CRT) by lifting the patient's lip and pressing on a section of pink gingiva. Take note of the mucous membrane color (MMC) and moistness. If possible, open and examine the mouth. Does anything prohibit straightforward intubation?

(b) Neck:

Palpate the size of the trachea to assist with endotracheal (ET) tube selection.

(c) Body:

Auscultate the thorax. Listen for lung and heart sounds. Determine the heart rate (HR) and rhythm. Palpate the abdomen for fluid, enlarged organs, or masses that might make ventilation under anesthesia challenging. Palpate femoral pulses while ausculting heart sounds to feel for pulse variations. Perform skin tenting to establish hydration status. Obtain body temperature (Table 1.1).

Table 1.1 Physical examination: normal findings for the canine and feline.

C. Blood work guidelines

BW is important in helping the anesthetist determine the patient's hydration status as well as roughly predicting drug metabolism and elimination. Minimum BW for young, healthy patients undergoing routine/elective procedures includes packed cell volume (PCV), total solids (TS), and an Azo-Stick to assess renal function within 24 hours of anesthesia. A complete blood count (CBC) and chemistry (CHEM) within 2 weeks of the anesthetic assessment are recommended for nonelective procedures, geriatric patients, and patients with any abnormal findings on PE. If the patient has any indication of changes in PE or abnormalities on previous BW, the CBC and CHEM are repeated prior to anesthesia. Additional BW such as clotting profiles, blood typing, cross matching, and blood gas and electrolyte analysis are indicated based on the procedure or disease process (Table 1.2 and Table 1.3).

Table 1.2 Normal blood work for canine and feline patients.

Source: Reference 2.

aReference ranges are specific to the analyzer and laboratory being used.

Table 1.3 Normal blood gas values for canine and feline patients.a

Source: Reference 2.

Note: Reference values will vary slightly with the analyzer and laboratory being used.

aFelines are more acidic when compared with canines, with slightly lower pH normal ranges and lower HCO3.

bNormal range for PaO2 is dependent on inspired oxygen content (FiO2). This table lists the value of PaO2 for a patient breathing room air, with a FiO2 of 21%. Under anesthesia, FiO2 increases to 100%, bringing up PaO2 to 400–500 mmHg.

D. Physical status

Assignment of physical status is based on the patient's overall signalment, PE, diagnostics, and anesthetic urgency. The ASA's assignment of physical status is used to classify patients based on the preanesthetic assessment of intraoperative and postoperative risks (Table 1.4).

Table 1.4 Physical status classification system (3).

Step 2: Premedication

Premedication is performed with the goals of providing sedation to facilitate restraint and intravenous (IV) catheterization, providing preemptive analgesia, and reducing dosage requirements of induction and maintenance drugs. Depending on the drug(s) selected, premedication is typically given intramuscularly (IM) (see Figure 1.1), but may be administered subcutaneously, orally/buccally, or IV. Additionally during this phase of the anesthetic process, the patient is catheterized (see Table 1.6), preoxygenated for at least 3–5 minutes with 100% oxygen up to the point of induction and intubation, and pre-instrumented if deemed necessary. See Table 1.5.

Figure 1.1 IM premedication in the epaxial muscles of a dog. Palpate dorsal spinal processes of lumbar spine; epaxial muscles lie laterally on either side. Insert needle and aspirate to ensure placement in muscle. Inject. Courtesy of Patricia Queiroz-Williams.

Table 1.5 Premedication drugs.

Table 1.6 IV catheter placement.

Figure 1.2 (a) Cephalic IV catheter. (b) Saphenous IV catheter in a canine patient.

Figure 1.3 Taping a cephalic catheter in place. With tape sticky side up placed under the catheter hub, fold tape over the catheter hub, allowing the tape to adhere sticky side to sticky side. Wrap the remaining length of tape securely around patient's limb. Courtesy of Anderson da Cuhna.

Step 3: Induction

Induction is the process of initiating anesthesia to facilitate intubation. Typically, this involves giving an induction drug intravenously either as a bolus or to effect. See Table 1.7 for a list of commonly used drugs.

Table 1.7 Induction drugs.

A. Selecting an ET tube

There are several ways to select an appropriately sized ET tube. One can palpate the trachea, using the thumb and index finger to feel the lateral edges to gauge width. Another method is to assess the diameter of the space between the nares and to select an ET tube of that width. This method may over- or underestimate ET tube size depending on species and breed variation. One may also use previous experience to choose an appropriate size of ET tube. Whatever method is chosen, prior to inducing the patient, have three ET tubes available (the size believed to fit, a size smaller, and a size larger). Brachycephalic breeds (i.e., pugs or bulldogs) often use smaller ET tubes than expected. Sight hounds and dachshunds typically use larger ET tubes than expected. Cats will typically use 3.5- to 5-mm internal diameter (ID) ET tubes. Determining the length of ET tube is also important prior to inducing the patient. Hold the ET tube next to the patient, with the tip at the thoracic inlet (see Figure 1.4). This measurement helps to avoid endobronchial intubation. ET tubes are commonly reused in the veterinary setting. The ET tube is cut to a suitable length; excessively long ET tubes increase resistance and dead space. ET tubes come with two different types of cuff systems: high-pressure/low-volume cuffs or low-pressure/high-volume cuffs. The high-pressure/low-volume cuff adheres well to the ET tube, resulting in little additional diameter to the tube. However, it requires a high intracuff pressure to achieve a seal; in addition, this high pressure occurs with relatively little volume delivered into the pilot balloon, which is deceiving for the anesthetist, who may administer more volume to fill the pilot balloon, resulting in tracheal damage. This seal is in contact with only a small portion of the tracheal wall, forming a tight, high-pressure seal only at a pinpoint location. These cuffs are associated with adverse events such as tracheal rupture and tears in cats (4). The authors discourage their use. Low-pressure/high-volume cuffs add some bulk to the ET tube, making them potentially more difficult to place. However, they exert a much lower pressure while still achieving an adequate seal. In addition, the seal is formed along the entire length of the cuff, rather than at one specific point. In all, this type of cuff, while more cumbersome, provides the safest seal. See Table 1.8 for information on intubating a patient.

Figure 1.4 Sizing the length of ET tube on a patient prior to inducing. Courtesy of Anderson da Cuhna.

Table 1.8 Intubation of canine or feline patient.

Note: Although typically the preferred position for intubation is sternal, patients are also intubated in lateral or dorsal recumbency. For patients in lateral recumbency, the head is hyperextended until it forms a straight line with the spine. Dorsal recumbency is a difficult position for intubation and is performed only if necessary (e.g., a patient is accidentally extubated during a surgical procedure).

Figure 1.5 Canine larynx. Courtesy of Anderson da Cuhna.

Figure 1.6 Sternal positioning for intubation in a feline patient. Courtesy of Anderson da Cuhna.

Step 4: Maintenance phase

This phase typically involves gas inhalants or total intravenous anesthesia (TIVA). Close, continual patient monitoring is key. See Table 1.9 for agents routinely used to maintain anesthesia.

Table 1.9 Routine maintenance anesthetics.

Step 5: Recovery/postoperative phase

The recovery phase begins with the discontinuation of the maintenance anesthetic agent and includes extubation and at least the first 3 hours postextubation. The postoperative phase includes any period during which the patient requires continual management and observation. A patient is extubated only when it can maintain its own airway. Typically, this occurs when the patient swallows and lifts its head. The ET tube cuff is deflated only after the patient can swallow (unless otherwise indicated). Most anesthetic mortalities that occur in the recovery phase will happen in the first 3 hours (6). Continuous observation is recommended during this phase, and it is important to provide adequate analgesia and pain assessment. Pain scoring techniques are helpful in assessing the patient (see Appendix A).

References

1. Morton C, Reid J, Scott E, Holton L, Nolan A. Application of a scaling model to establish and validate an interval-level pain scale for assessment of acute pain in dogs. Am J Vet Res. 2005;66(12):2154–66.

2. Silverstein D, Hopper K. Small Animal Critical Care Medicine. St. Louis, MO: Saunders Elsevier; 2008.

3. ASA Physical Status Classification System (April 9, 2013). Available at: http://www.asahq.org/Home/For-Members/Clinical-Information/ASA-Physical-Status-Classification-System.

4. Mitchell SL, McCarthy R, Rudloff E, Pernell RT. Tracheal rupture associated with intubation in cats: 20 cases (1996–1998). J Am Vet Med Assoc. 2000;216(10):1592–5.

5. Muir WW, Mason DE. Side effects of etomidate in dogs. J Am Vet Med Assoc. 1989;194(10):1430–4.

6. Brodbelt D. Perioperative mortality in small animal anaesthesia. Vet J. 2009;182(2):152–61.

Chapter 2

Anesthesia equipment and monitoring

This chapter details proper anesthesia equipment and how to safely set up an anesthetic machine (1). In addition, it outlines tools to assist the anesthetist in monitoring patient vital signs and anesthetic depth. Descriptions of monitoring equipment, equipment uses, advantage and disadvantages of the devices, and how to properly place equipment on the patient are provided. Normal wave forms and vital parameters are depicted.

I. Anesthesia machine

This critical piece of equipment functions to produce a gas mixture with variable composition of precisely selected gases (2). The machine has three systems:

A. High pressure system

1. Receives gases at cylinder pressure and then serves to decrease that pressure and make it constant.

2. Yoke or yoke block connects the machine to the compressed gas source (pipeline or cylinder).

3. Cylinder pressure gauge uses a bourdon tube to indicate how much pressure the remaining compressed gas in the tank exerts, as an indicator of how much supply is available (see Equation 2.1).

(2.1a) c2-math-0001a

(2.1b) c2-math-0001b

Equation 2.1 is used to determine the volume of oxygen remaining in the cylinder from pounds per square inch of pressure. Equation 2.1a is for E cylinder, and Equation 2.1b is for H cylinder.

4. Pressure regulator reduces pressure (which fluctuates with temperature and content) to 50 psi and maintains constant flow of compressed gas to the anesthetic system.

B. Intermediate pressure system

1. Receives gases from the regulator or pipeline, which are made constant. From there gases pass to the flowmeter and oxygen flush valve.

2. Pipeline inlet allows access to a hospital pipeline system if present.

3. Oxygen pressure failure device attempts to protect the patient in case of delivery of a hypoxic gas mixture, and will either alarm or cut off non-oxygen gases should supply of oxygen pressure drop considerably.

4. Flowmeter assembly controls and measures the amount of gas flow in liters per minute (L/min). Gas flow is measured at the largest point on the indicator (e.g., if a ball is used, it is measured at the center of