Canine and Feline Anesthesia

• Unless the procedure must be performed immediately for the patient’s well-being, correct significant physiologic abnormalities such as dehydration, hypotension, and anemia before anesthesia.

• Base the anesthetic protocol on results of the minimum database. Do not use a single, standard protocol for all patients.

• Use a balanced protocol consisting of multiple agents. This approach reduces the required dose of any one agent, thus minimizing the adverse effects of all agents.

• Double-check all injectable drug doses before administration, and ensure that the concentration of an agent drawn into a syringe is the same as that used for the drug calculations.

• Label all syringes containing injectable anesthetic agents with the name of the patient, the name of the drug, and the drug concentration if more than one concentration is available.

• Administer no more than the minimum dose of drug needed to achieve the desired level of anesthesia.

• Unless told otherwise, administer all IV agents “to effect.”

Anesthetic Induction with an Intramuscular Agent or Combination

When anesthetics are administered by IM injection, anesthetic depth gradually increases after injection (typically over 5 to 20 minutes) and, after peak effect, gradually decreases as the agent is metabolized (typically over 30 to 60 minutes or longer). Once the injection has been given, the anesthetist has little control over changes in depth or the peak effect. If the depth is inadequate, the anesthetist can give additional anesthetic, but if depth is excessive, the anesthetist can only monitor and support the patient until the agent is metabolized and the patient recovers naturally. The only exception to this is when using opioids, benzodiazepines, and alpha₂-agonists. In these cases the anesthetist can decrease anesthetic depth rapidly by administering the corresponding antagonist (reversal agent) (Figure 8-1).

Anesthetic Induction with an Intravenous Injection of an Ultra–Short-Acting Agent to Effect

Anesthetic induction with an intravenous injection of an ultra–short-acting agent to effect is a technique used for short procedures that require less than 10 minutes of anesthesia, such as removal of a Steinman bone pin, examination of the pharynx and larynx, or changing of a bandage. Propofol, methohexital, thiopental sodium, or etomidate may be used this way. When anesthesia is induced by IV injection, anesthetic depth typically increases rapidly over 15 seconds to a few minutes after the initial injection, then decreases gradually over 10 to 20 minutes. When using this technique, the anesthetist controls the peak effect and can increase anesthetic depth rapidly by giving additional boluses of the drug. However, as with IM anesthesia, there is little control over duration, and anesthetic depth cannot be decreased (Figure 8-2).

Total Intravenous Anesthesia by Intravenous Boluses of an Ultra–Short-Acting Agent

With total intravenous anesthesia (TIVA) by IV boluses of an ultra–short-acting agent, anesthesia is induced as previously described and then is,maintained by giving additional boluses every 3 to 5 minutes as needed to maintain surgical anesthesia. This technique is acceptable for noninvasive procedures of short to moderate length but is somewhat cumbersome for major surgeries because it is somewhat challenging to keep the patient at an optimum anesthetic depth even with constant monitoring. Propofol is the agent most commonly used to provide TIVA, although methohexital and etomidate are alternatives. With this technique the anesthetist can increase the depth rapidly by giving incremental boluses, but—as with IM administration—if the patient is deeply anesthetized, the anesthetist can only support the patient and wait until the agent is metabolized and the anesthetic depth naturally decreases (Figure 8-3).

Total Intravenous Anesthesia by Constant Rate Infusion

TIVA by constant rate infusion (CRI) is similar to TIVA by bolus injections, except that anesthesia is maintained by infusing small amounts of anesthetic constantly with a syringe pump. Although similar in effect to maintenance with bolus injections, this technique moderates and slows down changes in depth by avoiding sudden infusion of a large amount of drug. Instead, only the amount needed to maintain anesthesia is infused on a continuous basis for the duration of the procedure (Figure 8-4).

Induction and Maintenance with an Inhalant Agent

The use of an inhalation agent for induction and maintenance differs from injection techniques in several respects. Anesthetic induction with the commonly used inhalant agents is usually faster than IM induction but slower than IV induction (about 5 to 8 minutes with isoflurane or sevoflurane). Also, the anesthetist has excellent control over depth and can either increase or decrease depth relatively rapidly by changing the vaporizer dial setting. With inhalant agents, however, there is a delay between the time the dial setting is changed and the time it takes for patient anesthetic depth to change, because it takes at least several minutes for the new concentration to fill the breathing circuit, reach the patient’s lungs, and equilibrate with the blood and CNS.

The time required for anesthetic depth to change is influenced by a number of factors including the patient’s respiratory drive, the agent used, the carrier gas flow rate, the type of breathing circuit, and the volume of the breathing circuit. For this reason, vaporizer setting adjustments must be anticipated as much as possible through close monitoring. In general, if a patient’s anesthetic depth is significantly light or deep, larger dial changes are warranted, whereas if the patient is slightly too lightly or deeply anesthetized, more subtle changes are needed. The anesthetist will develop a “feel” for the exact amount to change the dial setting in any given circumstance through experience (Figure 8-5).

Intravenous Induction and Maintenance with an Inhalant Agent

Intravenous induction and maintenance with an inhalant agent is the most commonly used method of inducing and maintaining general anesthesia in small animal patients. It has dynamic elements of both IV and inhalant administration, as illustrated in Figure 8-6, including rapid induction, good control over both increases and decreases in anesthetic depth, and a relatively rapid recovery. The sequence of events associated with this technique is summarized in Procedure 8-2, p. 258.

Intravenous Induction

Agents commonly used to induce general anesthesia in dogs and cats by IV injection include (1) a mixture of equal volumes of ketamine and diazepam or midazolam, (2) propofol, (3) neuroleptanalgesics, (4) thiopental sodium, (5) etomidate, and (6) various other combinations containing dissociatives, tranquilizers, and opioids. Protocol 8-3 shows IV induction protocols in dogs and cats.

To induce general anesthesia by the IV route, a volume of the agent to be administered is calculated based on a prescribed dose and drawn into a syringe. The agent is then injected directly into the vein, or into a winged-infusion set or indwelling catheter to effect until the patient can be intubated or until the patient is at an adequate plane of anesthesia for completion of the planned procedure.

The term “to effect” means that only the amount of injectable anesthetic necessary to produce unconsciousness is given, rather than administering the entire dose calculated on a milligram per kilogram basis. This technique is necessary because the amount of drug needed to induce or maintain anesthesia cannot be accurately predicted for a given patient, and most anesthetic agents have narrow therapeutic indices.

For example, the amount of thiopental sodium required to induce anesthesia in a quiet, older dog may be one quarter to one half of the dose required for an active, young dog of equivalent body weight. Similarly, a cat with a urinary obstruction may be deeply anesthetized after receiving a very small dose of ketamine, whereas a healthy cat may require several times more to reach an equivalent depth. The drugs used for premedication also affect the dose of general anesthetic required. For example, a patient that has not received any premedications may require two or three times as much as a patient that has been premedicated with a neuroleptanalgesic combination to reach a comparable plane of anesthesia.

For these reasons, IV drugs are given as a series of bolus injections and discontinued when the desired depth is reached—a process known as titration. A competent anesthetist monitors the patient closely and alters the amount of anesthetic given to suit the patient’s requirements, rather than relying solely on a calculated dose. (See Procedure 8-3, p. 259, for IV induction techniques in dogs and cats.)

After induction with the commonly used IV injectable agents, the duration of anesthesia varies but is usually no more than 10 to 20 minutes. If more than 20 minutes is required, anesthesia is maintained with inhalation anesthetics or administration of propofol, methohexital, or etomidate by repeat boluses or CRI. This is not recommended with other injectable agents such as thiopental sodium, because if such drugs are given this way, large amounts of anesthetic will accumulate in the body and prolong recovery.

Inhalation Agents

Inhalation agents commonly used to induce general anesthesia in dogs and cats include isoflurane and sevoflurane. These agents are administered by means of a facemask or anesthetic chamber. Protocol 8-4 shows inhalant induction protocols in dogs and cats.

Mask Induction

Mask induction involves administration of an inhalant anesthetic such as isoflurane or sevoflurane via a facemask. This method of induction is feasible only with use of inhalation anesthetics with a low blood-gas solubility coefficient, such as isoflurane or sevoflurane, because the rapid induction time associated with these agents results in passage through stage II anesthesia quickly enough to minimize excitement. In contrast, mask induction is difficult and, in some cases, not possible with agents with a higher solubility coefficient because of the prolonged length of time required to reach stage III anesthesia. For some patients in critical condition, induction by mask may be safer than induction with injectable agents because the anesthetist can decrease anesthetic depth or discontinue the agent by adjusting the vaporizer setting if problems arise.

Mask induction is challenging for several reasons. Many patients struggle, necessitating skillful restraint (enough to prevent operator and patient injury, but not so much as to restrict chest excursions or the airway). The fear associated with passage through stage I and the excitement associated with passage through stage II cause release of epinephrine and other catecholamines, which can predispose the patient to cardiac arrhythmias, hypotension, and other adverse effects. Also, mucous membrane color and refill time as well as ocular indicators of anesthetic depth are not as easily observed because the mask partially obscures the eyes and muzzle, although monitoring requirements are no different with this method of induction.

The mask should be carefully fitted before commencement of mask induction. There should be a reasonably tight seal between the muzzle and the rubber gasket without constriction of the tissues or discomfort. The mask should be long enough to accommodate the full length of the muzzle (so that the nares do not press against the end of the mask when the muzzle is fully inserted), but not too long. Be aware that relatively higher oxygen flow rates are required than when an endotracheal tube is used. The technique for mask induction is described and illustrated in Procedure 8-4, p. 259.

There are several cautions associated with mask induction with which the anesthetist must be aware:

Although a mask can be used to maintain anesthesia, the anesthetist is not able to protect the airway, prevent aspiration, provide ventilatory support, or observe respirations as readily as when a tube is present. For these reasons, most anesthetists intubate the patient immediately after induction.

Chamber Induction

Chamber induction involves placing the patient in a closed chamber infused with anesthetic gas. This technique is feasible only for patients small enough to fit comfortably into a chamber (typically weighing less than 5 to 7 kg) and is therefore primarily used for small patients that are aggressive or difficult to handle.

Most chambers are small clear boxes that resemble a 5-gallon aquarium (see Figure 4-6). The chamber should be examined before induction. A tight-fitting lid and two ports (one for entry of fresh gas and another for exit of excess gas) are required. Although there is more than one way to supply oxygen and anesthetic gases to a chamber, a common way is to remove the Y-piece from the corrugated breathing tubes of a semiclosed rebreathing system and attach the inspiratory tube to one chamber port and the expiratory tube to the other port. Place the patient in the chamber, close the lid, and proceed, following the instructions summarized in Procedure 8-5, p. 260, and illustrated in Figure 4-6.

Anesthetic chambers allow the induction of anesthesia in even the most uncooperative animal but are associated with complications from stress, trauma, vomiting, airway blockage, and other issues:

Intramuscular Induction

Agents commonly used to induce general anesthesia in dogs and cats by IM injection include neuroleptanalgesic combinations and a variety of combinations of tranquilizers, dissociatives, and opioids. IM induction is useful for animals in which IV injections are difficult, such as ferrets and very young puppies and kittens. IM induction using restraint equipment such as a squeeze cage or rabies pole is necessary to induce anesthesia in extremely aggressive domestic animals. IM induction is also standard in animals that are difficult to approach or impossible to handle or in which IV or mask induction not feasible (e.g., wild animals and captive animals in zoos). In these animals, dissociative-tranquilizer mixtures, neuroleptanalgesics, or opioids (particularly etorphine and carfentanil) are usually administered by means of a blowpipe or tranquilizing gun.

Induction by IM injection differs from IV induction in several important respects.

The differences between IM and IV administration are illustrated by the use of ketamine in cats. When ketamine is given intravenously at a dose of 5 mg/kg, induction of anesthesia occurs in less than 1 minute. Alternatively, the drug can be given intramuscularly at a dose of 15 mg/kg, inducing anesthesia in 5 to 10 minutes. Recovery from IV ketamine administration usually begins in 10 to 15 minutes, and healthy animals often appear fully recovered within 1 to 2 hours. In contrast, complete recovery from IM ketamine administration may require several hours.

Oral Administration

Oral administration of certain anesthetics (most notably ketamine) is occasionally used in circumstances in which injection is dangerous or difficult, such as induction of anesthesia in feral cats. With an open-ended tom-cat urethral catheter and syringe, the drug is forcefully squirted into the animal’s mouth from a distance when the patient opens its mouth to hiss or vocalize. When this route is used, care must be taken to avoid pulmonary aspiration or contact with the eyes. Alternatively, the agent can be mixed with a small amount of palatable food. Because ketamine is not labeled for oral use, administration by this route constitutes extra-label use.

Other routes of administration for injectable agents include the SC, rectal, and intraperitoneal routes. For dogs and cats, these routes are too slow or impractical for routine use.

• When properly maintained, an endotracheal tube helps to maintain an open airway, decreasing the likelihood of airway obstruction caused by patient position, collapse of pharyngeal tissues, foreign material, or any other cause. Because of the importance of maintaining a patent airway, it is customary to leave the tube in place throughout anesthesia and into the recovery period, until the animal regains the swallowing reflex.

• Intubation allows more efficient delivery of anesthetic gas to the animal than does a mask. Because gas flow rates can be lowered, intubation results in reduced exposure of hospital personnel to waste anesthetic gas and increased economy.

• Use of an endotracheal tube with an inflated cuff reduces the risk of aspiration of vomitus, blood, saliva, or other material that may be present in the oral cavity or breathing passages. This material may accumulate during any procedure; however, the risk of aspiration is particularly high during oral surgery or dentistry and in patients that have not been fasted.

• An endotracheal tube of the correct diameter and length will improve efficiency of gas exchange by reducing the amount of anatomic dead space. Anatomic dead space is composed of the portions of the breathing passages that contain air but in which no gas exchange can occur (i.e., the mouth, nasal passages, pharynx, trachea, and bronchi). With the anatomic dead space minimized, the endotracheal tube ensures that a larger proportion of the gas delivered to the patient reaches the exchange surface in the alveoli.

• The anesthetist can support ventilation in intubated patients by manual or mechanical means. Manual ventilation refers to forced delivery of oxygen and anesthetic gases by squeezing the reservoir bag of the anesthetic machine. Mechanical ventilation refers to use of a mechanical ventilator to achieve the same result. Manual and mechanical ventilation are discussed in detail in Chapter 6. Because of the respiratory depression associated with anesthesia, periodic manual or mechanical ventilation is necessary in most anesthetized patients to ensure adequate gas exchange. Intermittent mandatory manual or mechanical ventilation is required in patients that have been given neuromuscular blocking agents or in which the thoracic cavity is open.

• Endotracheal intubation followed by manual or mechanical ventilation is also essential for patients in respiratory or cardiac arrest. For this reason it is advisable to have a laryngoscope and an endotracheal tube of correct size readily available for all anesthetized patients, even if endotracheal intubation is not planned.

Equipment for Endotracheal Intubation

The following equipment is required to perform endotracheal intubation (Figure 8-8):

Selecting an Endotracheal Tube

Endotracheal tubes are available in a wide variety of diameters and lengths. The tube must be of a diameter that is small enough to allow placement without causing trauma to the trachea, but large enough to produce a seal when the cuff is inflated. It must be of sufficient length to reach the thoracic inlet when fully inserted, but must not be so long as to reach the main stem bronchi or to extend beyond the end of the muzzle when inserted the correct distance.

At least three tubes of slightly different diameters should be selected so that you are prepared if your first choice does not fit. The size required by any given patient is influenced by patient species, conformation, and breed. For instance, cats require smaller-diameter tubes than dogs of comparable body weight. Most brachycephalic breeds require a tube several sizes smaller than mesocephalic or dolichocephalic breeds. An obese patient will require a smaller tube, and an emaciated patient will require a larger tube than a patient of normal conformation that weighs the same.

In view of these variances, the following guidelines can be used to estimate the proper size based on patient body weight. Most cats require a 3- to 4.5-mm tube. A dog weighing 20 kg will require a 9.5- to 10-mm tube. Increase or decrease the size approximately 1 mm for each 5 kg of body weight under or over 20 kg. For example, prepare a 7.5- to 8-mm tube for a 10-kg patient, and a 10.5- to 11-mm tube for a 25-kg patient. This guideline applies to canine patients weighing about 10 to 40 kg. (See Table 8-1 for size recommendations.)

Next determine if the tube is the appropriate length. The endotracheal tube should ideally extend from the tip of the nose to the thoracic inlet. If it is too short, it may not be long enough to reach the trachea at all. If the tube is too long, one of two problems may occur. If inserted too far, the beveled end may advance into only one main stem bronchus, thus supplying only one lung with oxygen and anesthetic. If inserted only to the thoracic inlet, the portion of the tube extending from the mouth will increase mechanical dead space (Figure 8-9). Either situation predisposes the patient to hypoventilation and hypoxemia. If a tube of the appropriate length is unavailable, the machine end of the tube can be trimmed, with care being taken to avoid cutting into the cuff inflation apparatus. Endotracheal tubes are further discussed in Chapter 4.

Preparing the Tube

Before an endotracheal tube is used, it must be checked for integrity. It should be clean, sanitized, and free of blockages, holes, deterioration, or other damage. The connector must be securely attached, and the cuff must inflate and hold pressure after the syringe is detached from the valve. A soft or narrow tube may require use of a stylette, which does not extend beyond the end of the tube, to stiffen it during placement. The tube should be lubricated with a small amount of sterile water-soluble lubricant or with the patient’s saliva immediately before placement.

Intubation Procedure

Successful endotracheal tube placement requires knowledge of the anatomy of the pharynx and larynx including the glottis, epiglottis, vocal folds, and soft palate. (See Figure 8-10 for a review of the anatomy.) Careful restraint, positioning, and lighting are necessary to maximize visibility of the larynx. Very subtle differences in these factors can make the difference between success and failure. This is especially true in cats. If you cannot easily see the larynx, insist that your assistant alter the patient position, the way the head is held, or the lighting until you can. A failure to do this frequently results in delay, frustration, and failure to successfully intubate the patient. A little time spent optimizing these factors is well worth the effort.

Endotracheal intubation is performed essentially the same way in both dogs and cats, although it is typically more challenging in cats owing to the smaller size of the larynx, decreased visibility, and the predisposition to laryngospasm typical of feline patients.

First prepare the necessary equipment, and prepare the patient for anesthetic induction. Induce anesthesia by IV injection, mask, or chamber until the patient is in a state of readiness for intubation. This requires administration of anesthetic, combined with careful monitoring, until the patient passes through stage II. Readiness for intubation is characterized by unconsciousness, a lack of voluntary movement, an absent pedal reflex, sufficient muscle relaxation to allow the mouth to be held open, and no swallowing when the tongue is grasped. As soon as the patient reaches this point, proceed with intubation as described in Procedure 8-6, p. 260.

In most cases intubation must be performed rapidly and efficiently, because the window for successful intubation is short (typically 1 to 2 minutes with injectable protocols and often less than a minute with inhalation protocols). Immediately after intubation, place the patient in lateral recumbency, secure and cuff the tube, turn on the oxygen, attach the breathing circuit, turn on the anesthetic vaporizer, and immediately start monitoring.

With typical protocols (assuming the patient has not been overdosed), many patients are in a light plane of stage III anesthesia after intubation, although the stage and plane vary case to case. If the patient is in a light plane (as characterized by one or two forceful exhalations immediately after placement, strong jaw tone, present palpebral reflex, and in some cases spontaneous movement, swallowing, or chewing), use high oxygen flow, and keep the inhalant anesthetic agent on induction level (3% to 5% isoflurane or 4% to 6% sevoflurane), until the patient begins to enter a deeper plane. Then immediately decrease the vaporizer setting to a safe level while continuing to monitor. Adjustments are then made in response to monitoring parameters. As soon as the patient’s condition is stable, attach any mechanical monitoring devices and prepare the patient for the procedure. Details of the intubation procedure are given and illustrated in Procedure 8-6, p.

Checking for Proper Placement

The entrance to the esophagus lies just dorsal to the entrance to the trachea, and although difficult to see, easily accommodates the endotracheal tube. Inadvertent misplacement in the esophagus is common and sometimes not detected, because air may appear to move in and out of the tube and reservoir bag even if the tube is not in the trachea. This will result in an inability to keep the patient anesthetized, and possible airway blockage and hypoxemia. The following techniques may be used to confirm proper placement in the trachea:

Securing the Tube

The tube is secured in place with a 2-foot-long piece of rolled gauze or IV tubing. Tie the gauze around the tube near the connector using a surgeon’s knot, or IV tubing around the tube using a Lark’s head knot. Do not place the tie around the small tube supplying the pilot balloon. Be sure that the tie is secure enough that it does not slide up or down the tube, but not so tight that it compresses the tube. Then tie the loose ends over the nose for dolichocephalic dogs or behind the head for cats and brachycephalic dogs. When properly secured, the tube should not move in or out when manipulated (Figure 8-11).

Cuff Inflation

Immediately after successful intubation, the cuff of the endotracheal tube must be gently inflated until a seal is formed between the trachea and the cuff. “Cuffing” prevents leakage of anesthetic gases and inhalation of room air, which will result in a variety of complications including contamination of the surgery suite with waste gases and difficulty keeping the patient anesthetized. To inflate the cuff, first extend the patient’s head to straighten the airway. Attach an air-filled 12-mL syringe to the valve port. Have an assistant close the pop-off valve and gently compress the reservoir bag, watching the pressure manometer. Listen for gas leakage around the tube, which may sound like a soft hiss or gurgling. Slowly inflate the cuff until the leaking just ceases at a pressure of 20 cm H₂O but resumes at higher pressures. Avoid overinflation of the cuff, which can result in a variety of mild to serious complications. Inflation should be checked again after 15 or 30 minutes of anesthesia, because tracheal diameter may increase as a result of muscle relaxation, or a slow, undetected leak in the cuff or pilot line may cause the cuff to deflate.

Sometimes the anesthetist may see or hear recommendations that suggest use of a specific volume of air to inflate the cuff. These recommendations are not valid and in fact are potentially dangerous, because the volume necessary to seal the cuff depends on the relationship among the external diameter of the tube, the internal diameter of the trachea, and the size and type of the cuff and is thus somewhat different for every patient and anesthetic procedure.

Laryngospasm

Laryngospasm is a reflex closure of the glottis in response to contact with any object or substance. This reflex will cause the glottis to forcibly close during intubation. This complication is most commonly encountered in cats, swine, and small ruminants, especially when in a light plane of anesthesia. It is extremely difficult to place a tube in a patient experiencing laryngospasm because the glottis closes as soon as it is touched with the tube and cannot be forced open without damaging the larynx.

Laryngospasm is frustrating, makes successful intubation difficult, and in extreme cases causes hypoxemia and cyanosis. If the patient becomes cyanotic, immediately release the tongue and administer oxygen by mask. In most cases the cyanosis will quickly resolve. Laryngospasm can be prevented by using one or more of the following strategies.

Complications of Intubation

A number of hazards are associated with endotracheal intubation (Box 8-4). Most are associated with tracheal irritation, trauma, or failure to protect the airway. Although the larynx and trachea of mammals are relatively resilient structures, excessive force will result in damage, perforation, rupture, or irritation of the delicate mucosa. An endotracheal tube must therefore be chosen, maintained, placed, and monitored with care.

Because of the problems associated with the use of endotracheal tubes and for reasons of convenience, not all animals undergo endotracheal intubation during anesthesia. If an endotracheal tube is not used, the inhalation agent is delivered by mask throughout the procedure. Animals lightly anesthetized with IM or IV agents for the performance of short procedures may not require the use of an endotracheal tube if the animal maintains the ability to swallow throughout the anesthesia and can breathe adequately. There should be no sounds indicating airway obstruction (such as fluid sounds in the airway), and the chest wall should move normally. With these exceptions, intubation is recommended for safety reasons if inhalation anesthetics are used or if a lengthy procedure is performed with the patient under injectable anesthesia.

Maintenance with an Inhalant Agent

To maintain general anesthesia with an inhalant agent, periodic changes in the vaporizer dial setting must be made based on information derived from physical and machine generated monitoring parameters. Frequent and effective monitoring is paramount, so that subtle changes in anesthetic depth are detected in time to make adjustments before the anesthetic depth becomes seriously too light or deep, thus risking that the patient will move or become aware or will be endangered.

In general, if the anesthetic depth is slightly too light or deep, small dial changes are necessary (about 0.5% to 1% increments with isoflurane and sevoflurane). In contrast, if anesthetic depth is significantly light or deep, large dial changes are necessary. For instance, if the patient’s anesthetic depth is significantly light (patient exhibits spontaneous movement, swallowing, active reflexes, strong muscle tone), settings of 3% to 5% isoflurane or 4% to 6% sevoflurane and a high oxygen flow rate are generally necessary to bring the patient back into surgical anesthesia. If the patient’s anesthetic depth is significantly too deep (e.g., absent reflexes; flaccid jaw tone; central, dilated pupils), then the vaporizer should be turned off, the oxygen flow increased, and the patient monitored

carefully until signs of decreased depth are evident. Be aware that these are only general guidelines, and each case must be handled differently as the circumstances warrant.

Maintenance with Repeat Boluses of Propofol or Another Ultra–Short-Acting Agent

As when maintaining anesthesia with an inhalant agent, frequent and effective monitoring is necessary so that the anesthetist has an accurate assessment of the patient’s condition and anesthetic depth. Every 3 to 5 minutes, additional boluses are administered to effect, often in volumes of approximately to ¼ of that required for induction, as needed to maintain depth at an optimal plane. If the anesthetist is distracted during this process, there is a danger that the patient will unexpectedly awake in the midst of a procedure, endangering itself and hospital staff.

Maintenance with a Constant Rate Infusion

Before induction and intubation, a calculated volume of anesthetic is drawn into a syringe large enough to accommodate the anticipated amount needed to maintain surgical anesthesia for the duration of the procedure. The syringe is placed in a syringe pump that has been programmed with an infusion rate based on a calculated dose, and attached to a winged infusion set, which in turn is placed in the injection port of an IV administration set. After induction, the pump is started, causing the agent to be delivered at a constant rate. Based on results of monitoring parameters, subtle changes are made in the infusion rate as needed to maintain surgical anesthesia. This method of administration is most suited to propofol, which does not accumulate in body fat stores, because it is rapidly metabolized. Although etomidate or methohexital can also be used to maintain anesthesia, these agents are rarely used for this purpose in small animal practice.

Maintenance with Injectable and Inhalant Agents

As an alternative, inhalant and injectable agents can be used in combination to maintain anesthesia. For example, if a patient in which anesthesia is maintained with an inhalant agent shows a response to a painful surgical stimulus or wakes prematurely, small doses of an opioid or other appropriate injectable agent can be given intravenously to rapidly deepen the anesthetic plane.

Maintenance with an Intramuscular Injection

Finally, anesthesia can be induced and maintained with a single IM injection. IM protocols generally produce general anesthesia of relatively short duration (only long enough to perform routine procedures such as spays and castrations). This technique is most commonly used in shelter medicine for elective surgeries in cats. Only tranquilizers, alpha₂-agonists, opioids, dissociatives, and anticholinergics can be given intramuscularly. In contrast, propofol, etomidate, and barbiturates should be given only by the IV route. Protocol 8-6 describes IM anesthetic protocols in cats.

• During induction the animal should be supported as it loses consciousness. Particular care should be taken to be sure that the animal does not strike its head on the table during induction or transfer to surgery.

• After IV induction, as soon as the patient is intubated, remove the needle and syringe to avoid accidental overdose in the event that the syringe plunger is accidentally pushed while the patient is being moved.

• Immediately after intubation, lay the patient in lateral recumbency, then secure and cuff the tube.

• Before preparing the patient for surgery, the anesthetist must ensure that the patient’s endotracheal tube is correctly placed (i.e., in the trachea) and that the cuff is functional and inflated. Once the surgical preparation begins, it is difficult to position the animal to allow reintubation without compromising aseptic technique.

• Check the endotracheal tube for kinks or bends. An open airway must be maintained at all times (Figure 8-12).

• Any time an intubated patient is turned over, the endotracheal tube should be temporarily disconnected from the anesthetic circuit. Rolling or twisting the animal while it is still connected to the circuit may cause the endotracheal tube to twist and collapse, resulting in an airway obstruction, or may cause the distal end of the tube to traumatize or lacerate the trachea.

• The hoses of the anesthetic machine should be supported so that there is no drag on the endotracheal tube. This could result in tracheal trauma or displacement of the tube.

• The position of the hoses and endotracheal tube should be checked during patient transfer and after repositioning. Hyperflexion of the neck should be avoided because it may lead to endotracheal tube obstruction.

• The reservoir bag should be placed so that it is clearly visible at all times.

• When positioning the animal on the surgery table, the anesthetist should ensure that the animal assumes as normal a posture as possible. In particular, hyperextension or hyperflexion of the limbs should be avoided because either may result in permanent neurologic injury.

• Heavy drapes or instruments must not compress the chest of small patients, because this may interfere with respiration.

• Do not overtighten leg restraint ropes, or circulation may be compromised.

• Place the patient on a heat-retaining surface such as a warm-water circulating blanket. Do not use an electric heating pad, which can burn the patient!

• If one lung is diseased, place the normal side up whenever possible, to maximize oxygen exchange.

• Tilting the surgery table so that the patient’s head is down gives the surgeon easier access to some abdominal organs, particularly the uterus and ovaries. However, the anesthetist should be aware that more than a 15-degree elevation of the caudal aspect of the body may cause the abdominal organs to compress the diaphragm, which may compromise heart and lung function. Head-down tilted positions should be avoided entirely in animals with breathing difficulties, especially those with diaphragmatic hernia.

• Artificial tear solution or other corneal lubricant should be instilled into the eyes of an anesthetized patient every hour, unless the patient is undergoing ocular surgery. This is particularly important if an anticholinergic is used. General anesthesia decreases tear secretion for a period of up to 24 hours after anesthesia, and some dogs may need periodic application of a corneal lubricant for up to 36 hours after anesthesia. Cats maintain some tear production throughout anesthesia; however, lubrication is advisable if an anticholinergic or ketamine is given.

• The length of the anesthetic period. As a general rule, the longer the patient is under anesthesia, the longer the expected recovery.

• The condition of the patient. Lengthy recoveries are seen in animals that have almost any debilitating disease (particularly liver and kidney disease).

• The type of anesthetic given and the route of administration. Lengthy recoveries are more common if an injectable agent is given intramuscularly rather than intravenously.

• The patient’s temperature. Hypothermic patients are slow to metabolize and excrete anesthetic drugs.

• The breed of the patient. Certain canine breeds (e.g., greyhounds, salukis, Afghan hounds, whippets, and Russian wolfhounds) are slow to recover from certain anesthetic agents, especially barbiturates.

Anesthetist’s Role in the Recovery Period

The anesthetist’s duty toward the patient does not end until the patient is awake and sternal. On completion of the procedure, the patient should be transferred to a recovery area where it can be monitored. A crash cart,

monitoring equipment, and oxygen should be readily available. The anesthetist must remain vigilant during recovery because this is often one of the most dangerous periods even for animals that have had no problems during induction or maintenance.

During anesthetic recovery, the anesthetist must fulfill each of the following responsibilities:

Procedure 8-8, p. 262, shows the sequence of events during the recovery period.

Signs of Recovery

An animal recovering from general anesthesia gradually progresses back through the anesthetic stages and planes. As the animal moves from deep to moderate to light anesthesia, vital signs and reflexes change in predictable ways. The heart rate, respiratory rate, and respiratory volume increase. After assuming a ventromedial position, the eyeballs move back to a central position. Reflex responses return, and muscle tone strengthens. The animal may shiver, swallow, chew, or attempt to lick. Shortly after swallowing reflexes return, the animal will normally show signs of consciousness, including voluntary movement of the head or limbs and possibly vocalization.

While passing through stage II, some patients may exhibit a variety of alarming signs including head-bobbing, delirium, hyperventilation, head-thrashing, and rapid limb paddling, especially if not premedicated. Occasionally a patient may attempt to stand and fall or may appear blind and bump into the sides of the cage. Some patients (particularly those recovering from ketamine anesthesia) may chew at their paws or claw their faces. Animals showing these signs of a “rough” or “stormy” recovery usually return to normal within a short time, but steps must be taken to prevent self-trauma or disruption of the surgical wound. Administration of preanesthetic medications before the procedure often prevents or moderates these signs, but additional tranquilization or administration of analgesics during the postoperative period may be necessary in these patients.

Monitoring

During recovery, the patient must be watched continuously at close range because a recovering animal may develop hypoxemia, cardiac arrhythmias, or other complications yet show no signs that are evident to the casual observer. Evaluation from across the room is not acceptable because problems such as vomiting, chewing the tube, and airway occlusion occur with some regularity and must be managed without delay to prevent serious consequences. These problems are discussed in detail in Chapter 12.

To minimize these risks, the patient should be positioned in the cage so that mucous membranes and respirations can be observed. Vital signs should be evaluated at least every 5 minutes. Abnormal vital signs or a delayed return to consciousness may indicate a variety of serious conditions that must be treated promptly by the veterinarian, such as shock, hemorrhage, hypoglycemia, or hypothermia. See Box 8-5 for ACVA monitoring guidelines for the recovery period.

Oxygen Therapy

As soon as the anesthetic depth decreases sufficiently, most recovering patients begin to shiver. Shivering is a protective response to hypothermia that raises the body temperature. During shivering, contracting muscle tissue converts oxygen and chemical fuel into heat. Consequently, muscle contractions associated with shivering increase oxygen consumption. Oxygen administration during recovery is necessary to meet these needs and to compensate for residual respiratory depression until anesthetic depth decreases.

Oxygen should be administered via the endotracheal tube at a rate of 50 to 100 mL/kg/min (0.5 to 1 L/10 kg body weight up to a maximum of 5 L/min) for 5 minutes after discontinuation of the anesthetic or until the animal swallows. This route of administration allows waste anesthetic gases to be scavenged and gives the anesthetist the ability to bag the patient during recovery to help reinflate collapsed alveoli.

If the patient is at a light depth and must be extubated, oxygen can be administered by mask. Some patients do not tolerate a mask without becoming agitated, however, and may require an alternative means of oxygen delivery. The following options are alternatives to the use of a mask during the postoperative period.

Note that a patient that is intubated and breathing oxygen from an anesthetic machine receives close to 100% oxygen. This is beneficial for the relatively short duration of most procedures, but prolonged inhalation of high levels of oxygen (e.g., greater than 50% oxygen for more than 24 hours) can be toxic.

Extubation

To prepare the patient for extubation, deflate the cuff by drawing out all the air until the pilot balloon is empty. Untie the tube so that it can be rapidly removed. At all times during recovery, keep the neck in a natural but extended position to protect the airway. Some anesthetists prefer to deflate the cuff and untie the gauze before signs of arousal are seen so that the tube can be quickly removed when swallowing occurs.

As soon as the patient shows signs of imminent arousal, the endotracheal tube must be removed using a slow, steady motion. In dogs, return of the swallowing reflex is the most appropriate time to remove the tube because this reflex will help protect the animal from pulmonary aspiration if vomiting occurs during recovery. Animals that show voluntary limb, head, or chewing movements are close to consciousness, however, and should be extubated even if swallowing has not been observed. These patients typically swallow on removal of the endotracheal tube.

A notable exception to this general rule is brachycephalic dogs. In these patients, many anesthetists prefer to delay extubation until the patient is able to lift its head unassisted, as early extubation may lead to respiratory distress from upper airway obstruction. In these patients it is wise to prepare by having a laryngoscope, an endotracheal tube (the same size as or one size smaller than used during the procedure), and the appropriate dose of an ultra–short-acting IV induction agent such as propofol nearby in case reintubation is necessary.

If a recovering patient shows signs of respiratory distress after extubation, the anesthetist must determine whether this is because of pulmonary disease (in which case oxygen therapy may be needed) or upper airway obstruction. One helpful clue is that upper airway obstruction is often associated with stridor (noisy respirations), especially on inspiration. If obstruction is present, the anesthetist should reposition the patient and gently pull the tongue forward. If the obstruction persists, it may be necessary to reinduce and reintubate the patient.

In cats the endotracheal tube may be removed when signs of impending arousal are observed. These include swallowing, an active palpebral reflex, and voluntary limb, tail, or head movements. Delaying extubation is not advisable in cats because it may predispose the patient to laryngospasm.

After dental cleaning, oral surgery, or any other procedure in which blood or other fluids are present in the oral cavity, the cuff should be left partially inflated during removal to sweep out the fluid, and prevent it from entering the airways. After extubation, all animals should be placed in lateral or sternal recumbency with the neck extended. This position helps maintain a patent airway. Occasionally, fluid or mucus may accumulate in the pharynx or trachea and must be removed by suction. The anesthetist should do this before the patient is fully awake to avoid being bitten.

1. Small animal patients are restrained and anesthetized with a variety of techniques, but general anesthesia is most commonly selected for many procedures. In many practices intravenous (IV) or mask induction followed by maintenance with an inhalant agent is most commonly employed.

2. When a protocol is being selected, great care must be used in calculating drug dosages, oxygen flow rates, and fluid administration rates, and in drawing up and administering anesthetic agents and adjuncts.

3. To safely and effectively anesthetize a patient, the anesthetist must have a clear understanding of the expected sequence of events and dynamics associated with each anesthetic technique.

4. Equipment must be carefully prepared before anesthetic induction, because once induction commences, the anesthetist has little to no time to gather missing equipment.

5. The goal of anesthetic induction is to take the patient from consciousness to stage III anesthesia smoothly and rapidly. This requires finesse, watchfulness, and an ability to make decisions rapidly and act quickly based on patient monitoring parameters.

6. IV induction agents are most often given “to effect” by using a process called titration. When inducing anesthesia by IV injection, in general the entire calculated dose should not be administered at once.

7. Mask and chamber inductions involve risks of which the anesthetist must be aware to prevent serious complications. Vigilant patient monitoring is required when these methods are used.

8. There are many advantages to endotracheal intubation, which justify this technique even in patients anesthetized only with injectable drugs.

9. Successful and safe endotracheal intubation requires careful preparation, excellent technique, and watchfulness for potential complications.

10. The anesthetic period can be divided into preanesthesia, induction, maintenance, and recovery. Traditionally, anesthetic depth has been described in terms of stages and planes of anesthesia, with stage III, plane 2, being suitable for most surgical procedures.

11. Patient positioning, comfort, and safety must be considered during anesthetic maintenance to avoid problems such as traumatic injury, accidental overdose, airway blockage, hypoxemia, circulatory compromise, burns, hypostatic congestion, and corneal drying.

12. The length of the recovery period depends on many factors, including the anesthetic protocol and the patient’s condition. Return to consciousness is accompanied by increasing heart and respiratory rates, increased reflex responses, and voluntary movement.

13. During the recovery period, the anesthetist must continue to monitor the patient’s vital signs, particularly temperature, as anesthetic complications commonly occur during this period. It is often helpful to administer oxygen for several minutes after the anesthetic has been discontinued.

14. In dogs extubation should occur when the swallowing reflex returns. In cats extubation should occur when the patient shows signs of impending arousal, such as voluntary movements, swallowing, or active reflexes.

15. Other duties during recovery include stimulation of the patient, administration of oxygen, postoperative analgesia, and general nursing care.

16. Anesthetic safety is improved through the use of preanesthetic agents, use of the minimum effective dosages, selection of a protocol well suited to the patient, and close monitoring of the patient.

1. Consider the following scenario: You are performing a procedure on a canine patient that you feel is at an appropriate anesthetic depth. Which of the following settings would be most appropriate if using sevoflurane in this patient?

2. Which of the following statements about how to induce anesthesia in a young healthy patient using an IV agent such as thiopental or propofol is least accurate?

3. Which of the following statements regarding IV induction agents is least accurate?

4. Which of the following barbiturates can be used to maintain anesthesia without prolonging recovery?

5. Which of the following statements about thiopental sodium is least accurate?

6. Which of the following statements concerning endotracheal intubation is false?

7. Which of the following is an indicator that the endotracheal tube has been placed in the esophagus instead of the trachea?

8. The term atelectasis refers to:

9. After an anesthetic procedure, when is it best to extubate a dog?

10. Hypostatic congestion may be present at the end of the anesthetic protocol. This term refers to the:

11. Which of the following protocols would result in the fastest induction time and the best control over anesthetic depth?

12. Mask inductions in small animal patients are:

13. Which of the following techniques are used most often in most general small animal practices?

14. An 18-kg adult dog of normal conformation would likely require a __________ endotracheal tube.

15. An endotracheal tube is used to:

16. Problems associated with endotracheal intubation include:

17. During anesthetic maintenance:

18. The length of the anesthetic recovery period may be influenced by:

19. Which of the following is/are not a normal sign(s) of recovery?

20. Which of the following actions is appropriate during anesthetic recovery?