Introduction

Successful anesthetic management of dogs and cats requires an understanding of basic physiology and pharmacology integrated with various clinical competencies relating to anesthesia and pain management. The approach to anesthetic management should always be systematic and follow a stepwise plan as outlined in Table 8.1. In some situations, a standardized form may be useful to organize relevant patient-related data (Figure 8.1). Similarly, the details of the anesthetic plan can also be itemized in a standardized worksheet (Figure 8.2).

At first glance, this approach may seem unnecessarily complicated and impractical. In reality, though, experienced veterinarians work through this process every day. They simply are able to move through the steps very quickly and efficiently when dealing with young, healthy patients presenting for routine elective procedures. By adopting this approach or something similar for every animal requiring anesthesia, the process will become second nature. Then, when presented with more challenging and complex cases, the veterinarian will be better equipped to identify the key issues and formulate a rational plan.

Rather than attempting to address all of the steps outlined in Table 8.1, this chapter will focus primarily on considerations for drug protocol and clinical technique selection. The other steps are covered elsewhere in this text and the reader is referred to chapters addressing patient evaluation, physiology, pharmacology, airway management, equipment, monitoring, fluid therapy, pain management techniques, and anesthesia for special procedures or diseases for more detailed information.

Drug protocol selection

Based on Table 8.1, it is apparent that drug protocol selection constitutes only one component of the overall anesthetic management picture. While selection of an inappropriate drug protocol may undoubtedly contribute to a poor outcome, the reverse is not necessarily true. The selection of a perfectly reasonable drug protocol does not ensure that complications will not occur. Regardless of the drugs selected, vigilant monitoring of the anesthetized patient is required and supportive interventions need to be initiated when indicated.

Figure 8.1. Preanesthetic patient evaluation form. NSF, no significant findings.

Table 8.1. Systematic approach to anesthetic management of dogs and cats

Figure 8.2. Anesthetic plan worksheet.

Numerous drugs are available to today’s practitioner and they may be combined and delivered in many ways. It is not necessary for most veterinarians to have all of these drugs at their disposal, and the use of many different drug protocols does not necessarily imply superior anesthetic care. However, familiarity with only a single anesthetic technique will, at best, limit the veterinarian’s ability to perform all of the diagnostic and surgical procedures common in today’s modern practice and, at worst, may compromise the care of certain patients.

Balanced anesthesia

A flowchart outlining the thought process for designing preanesthetic protocols in dogs and cats can be found in Figure 8.3. Dosages for commonly used preanesthetic drugs from each of the three main categories (sedatives, opioids, and anticholinergics) are provided in Tables 8.2 and 8.3. For more detailed information on the pharmacology, indications, and contraindications of all of these drugs, please refer to Chapter 2.

Induction of anesthesia

The goals of optimal anesthetic induction in dogs and cats are summarized in Box 8.2. The transition from the conscious to the anesthetized state is usually accomplished by the administration of one or more injectable drugs. In specific situations, induction with an inhalant agent such as isoflurane or sevoflurane administered via a face mask or in an induction chamber may also be indicated. Inhalant inductions are not recommended for routine use because it is difficult to precisely control the delivered concentration of drug, and pollution of the local atmosphere with anesthetic vapor is significant.

Figure 8.3. Steps in selecting preanesthetic medications in dogs and cats.

Table 8.2. Common preanesthetic medications^a in dogs

^a Refer to Figure 8.1 for suggestions on how to combine drugs from each of the three categories; an injectable NSAID (carprofen, meloxicam) may also be administered as part of the preanesthetic medication if indicated.

^b The maximum total dose is often capped at approximately 7 mg to prevent overdosing large breed dogs.

^c The maximum total dose is often capped at approximately 3 mg to prevent overdosing large breed dogs.

^d The maximum total dose is often capped at approximately 0.5 mg to prevent overdosing large breed dogs.

Table 8.3. Common preanesthetic medications^a in cats

^a Refer to Figure 8.1 for suggestions on how to combine drugs from each of the three categories; an injectable NSAID (carprofen, meloxicam) may also be administered as part of the preanesthetic medication if indicated.

Injectable induction protocols may use either the IM or IV route. The IM route is technically straightforward and may be preferred in certain situations where large numbers of relatively young, healthy patients are being anesthetized for short routine procedures (e.g., spay/neuter programs in animal shelters). In most cases, higher total doses of drug are required with this route, and there is no possibility to titrate to effect. While the IV route usually requires placement of a catheter, it offers the advantage of dosing to effect and is usually associated with more rapid inductions and recoveries. Since IV access is recommended for most patients undergoing general anesthesia anyway, catheter placement prior to induction is simply good practice. For these reasons, IV induction protocols are preferred for most dogs and cats requiring general anesthesia.

Injectable IM induction agents commonly used in dogs and cats include ketamine combinations, tiletamine–zolazepam (Telazol^®, Pfizer Animal Health, New York), and alphaxalone-CD. There are numerous IM protocols that have been reported for general anesthesia in dogs and cats and most combine the preanesthetic medication and the induction agent(s) in a single IM injection. Most of these cocktails involve various combinations of dissociatives, benzodiazepines, alpha₂ agonists, and opioids. A few selected protocols are presented in Tables 8.4 and 8.5, though this list is by no means exhaustive.

Injectable IV induction agents commonly used in dogs and cats include ketamine, Telazol, propofol, thiopental, etomidate, and alphaxalone-CD. Other supplemental drugs may also be administered concurrently and are referred to as coinduction agents. In the case of ketamine, this practice is well recognized, as a benzodiazepine (diazepam or midazolam) is almost always coadministered to counteract ketamine-induced muscle rigidity and potential seizure activity. Though not mandatory, coinduction agents can also be used with propofol, thiopental, etomidate and alphaxalone-CD to minimize their dose requirements. This benefits patients by smoothing out the induction phase and reducing the potential for adverse cardiopulmonary side effects. Diazepam, midazolam, and lidocaine (in dogs only) are the most commonly used coinduction agents in small animals. The combination of a coinduction agent with propofol, thiopental, etomidate, or alphaxalone-CD tends to be most useful in patients that are not profoundly sedated after the preanesthetic period and where higher doses of IV induction drug are anticipated. Note that if midazolam has been given IM as part of the preanesthetic medication, it may be appropriate to reduce or eliminate the dose of benzodiazepine given IV as a coinduction agent. Dosages for commonly used injectable and inhalant induction and coinduction drugs in dogs and cats can be found in Tables 8.6 and 8.7. For more detailed information on the pharmacology, indications, and contraindications of all of these drugs, please refer to Chapter 2.

Table 8.4. Selected IM anesthetic protocols^a for dogs

^a Recovery times may be prolonged with any dissociative-based IM protocol.

^{b Lower end of dose range will result in heavy sedation/immobilization, while the higher end will produce general anesthesia of variable duration.}

In debilitated and/or hemodynamically unstable patients, induction with an opioid (usually fentanyl, remifentanil, oxymorphone, or hydromorphone) in combination with a benzodiazepine (midazolam or diazepam) given in alternating IV increments to effect often produces a smooth induction and facilitates transfer to the maintenance phase of anesthesia. This approach will be discussed in more detail in an upcoming section.

Maintenance of anesthesia

Goals of the anesthetic maintenance phase in dogs and cats are summarized in Box 8.3. Unless an IM injectable induction protocol has been used, most procedures lasting more than 15 minutes will require the administration of additional anesthetic agents to extend or maintain anesthesia. This can be accomplished with either injectable or inhalant anesthetics. Any of the IV injectable agents listed in the previous section can be used in smaller “top-up” doses given to effect if the duration of anesthesia only needs to be extended for a few minutes. Repeated redosing with thiopental is not recommended, as redistribution sites may become saturated, leading to a prolonged recovery.

Table 8.5. Selected IM anesthetic protocols^a for cats

^a Recovery times may be prolonged with any dissociative-based IM protocol.

^b The lower end of the dose range will result in heavy sedation/immobilization while the higher end will produce general anesthesia of variable duration.

If longer-duration injectable maintenance is required, propofol is considered the drug of choice and can be administered as a continuous rate infusion (CRI) for extended periods of time. After appropriate preanesthetic medication and IV induction, propofol CRI doses for anesthetic maintenance in dogs and cats range from 0.2 to 0.8 mg/kg/min and are often supplemented with ketamine, midazolam, dexmedetomidine, and/or opioids as indicated.

Most contemporary small animal veterinary hospitals are equipped with inhalant anesthetic machines, so the most common means of maintaining anesthesia in dogs and cats is to use isoflurane or sevoflurane. For longer and/or more invasive procedures, it is also common practice to supplement inhalant anesthesia with other drugs during the maintenance phase to reduce inhalant requirements once preanesthetic medications have worn off. This may be accomplished in several ways, including the use of local and regional anesthetic or analgesic techniques, or the administration of supplemental systemic drugs such as hydromorphone, oxymorphone, morphine, fentanyl, remifentanil, lidocaine, ketamine, midazolam, and dexmedetomidine. These techniques will be discussed again later in this chapter.

Table 8.6. Anesthetic induction and co-induction agents in dogs

^a The lower end of dose range is usually feasible if preanesthetic medication has been administered or coinduction agents are to be included.

^b Can substitute diazepam at the same dose.

^c It is usually recommended to administer the coinduction agent(s) and immediately titrate the primary induction drug to effect.

Recovery from anesthesia

The ideal anesthetic recovery should be rapid, complete, and stress free for the patient. In selected cases, drug reversal may be indicated to hasten recovery. Commonly used reversible drugs include dexmedetomidine (reversed with atipamezole), midazolam and diazepam (reversed with flumazenil), and opioid agonists (reversed with pure antagonists such as naloxone or the agonist–antagonist butorphanol). In other situations, supplemental sedation may be required with low doses of dexmedetomidine, acepromazine, or midazolam to prevent or treat dysphoria or agitation. In all cases, management of pain must be a priority and the most commonly used postanesthetic analgesics are opioids and NSAIDs. Occasionally, adjunctive analgesic agents may also be indicated as part of the pain management plan. Please refer to Tables 8.8 and 8.9 for injectable and oral postoperative analgesic dosage recommendations.

Table 8.7. Anesthetic induction and coinduction agents in cats

^a The lower end of the dose range is usually feasible if preanesthetic medication has been administered or coinduction agents are to be included.

^b Can substitute diazepam at the same dose.

^c It is usually recommended to administer the coinduction agent(s) and immediately titrate the primary induction drug to effect.

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