Anesthetic Problems and Emergencies

• Failure to obtain an adequate history or perform an adequate physical examination on the patient

• Inadequate experience with the anesthetic machine or anesthetic agents being used

• Failure to devote sufficient time or attention to the anesthetized patient

• Errors caused by fatigue

• Failure to recognize and respond to early signs of patient difficulty

• Lack of proper patient evaluation

Failure to Obtain an Adequate History or to Perform a Physical Examination

Ideally, every patient scheduled for anesthesia should have a complete physical examination, and a thorough history should be obtained. In practice, this is not always possible. Animals are sometimes dropped off at the veterinary clinic by owners who are in a hurry and reluctant to stop and answer questions. Animals may be brought in by neighbors or friends of the owner, or by other persons unfamiliar with the animal’s history. The receptionist or other person admitting the animal to the hospital may fail to ask important questions or may not transmit the information to the anesthetist or veterinarian. The physical examination is sometimes cursory or omitted entirely. The net result is that significant information may be overlooked. For example, the anesthetist may be unaware that a patient has not been fasted or that an animal scheduled for surgery is dehydrated as a result of vomiting and diarrhea. An anesthetic protocol that is safe for a healthy patient could be inappropriate for these animals, and an anesthetic problem or even death of the patient could result.

Lack of Familiarity with the Anesthetic Machine or Anesthetic Agents

It is the responsibility of the veterinarian, and in some states or provinces a requirement of the Veterinary Association, to ensure that his or her personnel are sufficiently trained and knowledgeable to competently perform all required procedures. Although unskilled personnel working under a veterinarian’s direct supervision may assist with some aspects of an anesthetic procedure, skilled tasks, such as induction of anesthesia and monitoring of anesthetized patients, must be assigned only to personnel (veterinarians or technicians) who have sufficient training, knowledge, and experience to recognize abnormalities and danger signals and to respond appropriately.

Incorrect Administration of Drugs

Many anesthetic agents have a narrow margin of safety between therapeutic and toxic doses. The incorrect administration of drugs may have serious or even fatal consequences, and may arise from any of the following:

Personnel Who Are Preoccupied or in a Hurry

Although efficiency is desirable in any anesthetic procedure, it is not necessary or advisable that the anesthetist feel hurried. A technician who is feeling rushed is more likely to make mistakes such as injecting barbiturates perivascularly or inserting an endotracheal tube into the esophagus. Unfortunately, it is common for the technician working in a busy practice to feel pressured and distracted. The technician responsible for anesthesia may be simultaneously called on to restrain patients for examination or procedures, answer the phone, perform laboratory tests, take radiographs, discharge animals, clean soiled kennels, and carry out other similar tasks. However, when an animal is anesthetized, the technician’s top priority must be monitoring that patient, because failure to satisfactorily perform this duty may result in the death of the animal.

The technician usually does not have the luxury of being constantly by the animal’s side throughout the procedure. In most work situations, periodic absences are necessary. However, the anesthetist should return to check the patient at least once every 5 minutes, or more frequently if the patient’s status requires close monitoring. If necessary, other tasks must be temporarily set aside to allow the anesthetist to return to the patient.

Fatigue

Veterinarians and technicians often become fatigued, particularly at the end of a busy day. Anesthetic emergencies may arise when personnel are tired and less alert than normal, possibly because minor problems are not detected and corrected at an early stage. If possible, surgeries that are lengthy or difficult should be scheduled early in the day.

Inattentiveness

One of the most serious human errors in anesthesia is the failure to monitor and recognize danger signals. It is obviously better for the patient—and easier for the anesthetist—to detect and address anesthetic problems early, rather than late. For example, when anesthetic depth is excessive, an animal may show a gradually decreasing respiratory rate, from 10 to 12 breaths/min in a dog or cat to fewer than 8 breaths/min (at which point the anesthetist should consider adjusting the vaporizer to a lower setting); then to 4 breaths/min (at which point the vaporizer should be turned off and the animal bagged with oxygen); then from 4 breaths/min to 0 (at which point cardiac arrest may quickly follow if these changes are not recognized and managed).

The anesthetist’s attitude toward patient care is a key factor in the safety of anesthesia. The conscientious anesthetist will monitor the animal often to ensure that the patient is not in trouble. A brief check of the patient’s pulse rate, respiratory rate, and capillary refill time takes less than 1 minute and gives the anesthetist a good assessment of the patient’s status. The best attitude is one of low-level anxiety that is relieved only when a quick examination of the patient reveals that all vital signs and depth indicators are within acceptable limits.

Carbon Dioxide Absorbent Exhaustion

Patients on a rebreathing system rely on the carbon dioxide absorbent to remove expired CO₂ from the circuit, preventing inhalation of excessive levels of this toxic gas. If CO₂ is not removed from the circuit, the patient will experience hypercapnia (elevated blood CO₂). Signs of this disorder include tachypnea (rapid respiration), tachycardia, and cardiac arrhythmias. Examination of the CO₂ absorbent may reveal an obvious color change, if exhausted, although this does not always occur.

Empty Oxygen Tank

Failure to deliver oxygen to the patient is one of the most serious and yet one of the most easily preventable mistakes that an anesthetist can make. Before starting an anesthetic procedure, the anesthetist must ensure that the tank contains sufficient oxygen for the duration of the surgery. (For information on calculating the amount of oxygen present in a tank, refer to p. 108.) During the procedure, the oxygen tank pressure and flow meter should be checked every 5 minutes during anesthesia. The anesthetist must ensure that either oxygen or room air is continuously provided to the patient. At the end of a procedure, the patient should be disconnected from the machine before the oxygen flow meter is turned off.

It is important to be able to recognize when the machine is no longer delivering oxygen to the patient. If the oxygen flow meter reads zero, the patient is not receiving any oxygen, regardless of the oxygen tank pressure. Occasionally when the oxygen tank is nearly empty, the oxygen tank pressure gauge reads zero but the flow meter indicates some oxygen flow. Even though the tank is still delivering a small amount of oxygen, loss of oxygen pressure is imminent and the tank must be changed immediately.

The anesthetist must be aware of the proper response when oxygen delivery to the patient is stopped, whether because of machine malfunction or an empty tank. If the oxygen flow stops (i.e., the flow meter reads zero despite the efforts of the anesthetist to establish flow) and the patient is on a non-rebreathing system, the anesthetist should immediately disconnect the hose from the endotracheal tube, allowing the patient to breathe room air until oxygen delivery is reestablished. (If a circle system with a full reservoir bag is in use, the patient can remain connected for a short period of time.)

Misassembly of the Anesthetic Machine

It is essential that the person handling the anesthetic machine be familiar with every connection, hose, dial, and component of the machine. Before using an unfamiliar machine, the anesthetist should take a few minutes to examine it carefully for the location of the controls and to understand the direction and path of gas flow within the machine. Every time a connection, such as a Bain circuit, is added or removed, the anesthetist must trace the flow of gas, ensuring that the correct pattern of flow is maintained and that all connections are secure. Failure to do so can result in the patient not receiving anesthetic gases or rebreathing expired CO₂.

Endotracheal Tube Problems

Although the endotracheal tube is, strictly speaking, not a part of the anesthetic machine, it is a critical component of the anesthetic delivery system and is subject to many problems. Endotracheal tubes may become blocked during anesthesia, cutting off the flow of anesthetic gas and oxygen to the patient. Blockages may be the result of twisting or kinking of the tube; accumulation of material such as blood, mucus, or saliva within the tube; or inappropriate positioning of the tube (as may occur when the neck is flexed). The endotracheal tube should be premeasured from the incisor teeth to the mid-neck, and it should be advanced no further than the position of the carina. If the tube is accidentally advanced into a bronchus, the patient may become hypoxic and hypercapnic.

Endotracheal tube blockage (if complete) results in a cessation of oxygen flow to the patient and retention of carbon dioxide. The patient may become dyspneic and may develop cardiac arrhythmias. Eventually, respiratory arrest may occur. The anesthetist usually becomes aware of the problem by observing the patient’s exaggerated breathing pattern or by noting that the reservoir bag no longer inflates and deflates with the patient’s respirations. If a problem is suspected, the anesthetist should quickly check the endotracheal tube function in two ways:

Vaporizer Problems

Each vaporizer is designed for a specific agent. A potentially disastrous problem can arise if the wrong anesthetic is put into a vaporizer. Each anesthetic liquid has its own vapor pressure (the amount of anesthetic that vaporizes at 20° C), and anesthetic vaporizers are calibrated on the basis of a particular anesthetic being used in the vaporizer with a particular vapor pressure. If an anesthetic is put into the incorrect vaporizer, it is possible that when the vaporizer is set at 1% a higher or lower concentration will be delivered because the anesthetic has a different vapor pressure than the anesthetic for which the vaporizer was designed. This leads to the patient’s anesthetic depth becoming unexpectedly deep or light. Other problems involving vaporizers are as follows:

Pop-off Valve Problems

Occasionally, an anesthetist inadvertently leaves the pop-off valve in a closed position. If the pop-off valve is closed and the oxygen flow rate is greater than the patient’s oxygen requirement, pressure within the circuit will rapidly rise. This can happen with use of a closed system in which the oxygen flow rate has inadvertently been set higher than the metabolic oxygen consumption (approximately 10 mL/kg/min). As pressure rises in the circuit, the reservoir bag will expand, as will the patient’s lungs. This prevents exhalation and also decreases the venous return to the heart. This in turn may decrease cardiac output, cause blood pressure to fall rapidly, and can lead to death within a short time unless recognized and corrected.

To detect the problem at an early stage, the anesthetist should frequently monitor the reservoir bag size and attempt to maintain it at no more than two-thirds full of gas during anesthesia. The reservoir bag size is easily adjusted by changing the oxygen flow rate or by opening and closing the pop-off valve.

• The anesthetic protocol must be chosen to reflect the special needs of the patient. For example, acepromazine is a poor preanesthetic for patients with low blood pressure because this agent may cause vasodilation, further decreasing the blood pressure. Similarly, halothane, although rarely used now, is not the preferred inhalation agent for patients with cardiac arrhythmias because it may cause arrhythmias to worsen. Isoflurane, the common replacement for halothane, may cause a significant drop in blood pressure, and therefore blood pressure monitoring should be performed on the patient. Animals that are fearful or excited may have increased levels of epinephrine circulating throughout their bodies. Drugs such as the alpha₂-agonists (e.g., xylazine) should be avoided, as they will augment the arrhythmogenic effects of epinephrine. In each case the veterinarian might choose to use an alternative agent.

• The anesthetist must be familiar with the side effects and contraindications associated with each of the preanesthetic and general anesthetic agents used in the hospital. For example, the anesthetist who administers an alpha₂-agonist should be aware of its potential to cause bradycardia, cardiac arrhythmias, vomiting, bloating, and respiratory depression.

• Multidrug use to achieve balanced anesthesia can be safer than anesthesia with a single drug, provided that the doses of the individual drugs are appropriately reduced. For example, the concentration of isoflurane needed to anesthetize an animal is significantly reduced if the animal is premedicated with acepromazine and butorphanol, as compared with an animal that is not premedicated. If the same isoflurane concentration were used in both situations, the multidrug regimen could be dangerous for the patient.

Geriatric Patients

A geriatric patient is one that has reached 75% of the average life expectancy for that species and breed. In these patients the functions of critical organs such as the heart, lungs, kidneys, and liver are reduced in comparison with the healthy, young patient. Geriatric animals have less functional reserve than do younger animals, and a relatively poor response to stress. Often they are less able to adequately maintain their state of hydration than younger patients. In addition, geriatric animals are often affected by degenerative disorders such as diabetes mellitus, cancer, congestive heart failure secondary to mitral valve insufficiency, and chronic renal disease, all of which are of concern to the anesthetist. Because of the high incidence of health problems in these animals, the importance of a thorough history and physical examination cannot be overemphasized. Preoperative tests such as a blood chemistry panel, urinalysis, chest radiographs, and an electrocardiogram (ECG) may be advisable for these patients.

Geriatric animals typically have reduced anesthetic requirements, and doses of anesthetic agents are often decreased by one half to one third compared with doses for healthy, young patients. In the case of barbiturates, dose requirements may be as little as one twentieth of the normal dose. Response to drugs is slower, and the technician should allow more time for IV injections to take effect. Recovery from anesthesia also may be prolonged in geriatric animals, partly because of decreased renal and hepatic function (and hence, decreased ability to excrete drugs). Geriatric patients also have a tendency to develop hypothermia because they have a reduced ability to regulate body temperature.

The use of IV fluids is generally advocated in geriatric patients because they have less tolerance for hypotension and often have reduced kidney function. Geriatric animals are, however, at increased risk for developing overhydration, so IV fluids should be given with care.

Pediatric Patients

A veterinary patient under 3 months of age is generally considered to be at increased risk when anesthetized, compared with a mature animal. When working with these patients, the anesthetist must be aware of special considerations during the preanesthetic, anesthetic, and recovery periods.

Preoperative fasting of the pediatric patient may not be advisable because hypoglycemia and dehydration can occur after even a short period of fasting. Oral fluids are usually allowed up to 1 hour before induction. To prevent hypoglycemia during surgery, many veterinarians use 5% dextrose in lactated Ringer’s for IV fluid therapy of anesthetized pediatric patients. (This can be formulated by adding 100 mL of 50% dextrose to 1 L of lactated Ringer’s solution.) The fluid administration rate should not exceed 5 mL/kg/hr unless shock or dehydration is present, because these animals are prone to overhydration if fluid administration is rapid. The use of a syringe driver or pediatric microdrip administration set with a delivery rate of 60 drops/mL and a burette is helpful in preventing inadvertent overinfusion of fluid.

For calculation of drug doses, an accurate weight must be obtained. For animals weighing less than 5 kg, a pediatric or gram scale gives more reliable weights than does a conventional scale. Injectable agents may require dilution because otherwise the dose may be too small to measure or administer accurately. The dose of injectable anesthetics given to pediatric animals is often one half to two thirds of the dose given to mature animals because very young animals have less plasma protein binding of drugs and lack an efficient mechanism to metabolize drugs within the liver. Injectable anesthetic agents that require liver metabolism for inactivation (e.g., thiopental and pentobarbital) can be expected to have a prolonged effect in puppies or kittens less than 8 weeks of age and should be avoided. Renal function is also inefficient compared with the adult animal, and excretion of drugs by this route may be slow.

Many veterinarians prefer to anesthetize pediatric patients with inhalant agents (particularly isoflurane) because administration and elimination of these agents is accomplished through the respiratory tract and patient recovery tends to be rapid.

Certain anesthetic procedures such as intubation and IV catheterization can be more challenging in pediatric patients because of their small body size. The larynx may be difficult to see, and use of a laryngoscope may be required. It is often necessary to cut endotracheal tubes short to avoid bronchial intubation.

Apart from obvious differences in size, the monitoring of pediatric patients is similar to that of adults. The anesthetist should be particularly watchful for bradycardia, which is associated with poor cardiac output in anesthetized animals under 4 weeks of age. Alpha₂-agonists may cause significant bradycardia and should be avoided in these patients. Premedication with atropine may not be effective because response to atropine is unpredictable in patients younger than 14 days old.

Pediatric patients are prone to hypothermia because of their lack of subcutaneous fat, their relatively large body surface area, and their reduced ability to shiver. Particular care should be taken to avoid heat loss during surgery. This is accomplished through the use of warmed IV fluids and circulating warm water heating pads or Bair Huggers. It is also essential to make sure that all air is removed from IV lines to avoid the risk of air embolism.

Brachycephalic Dogs

Technicians are often called on to anesthetize brachycephalic dogs such as the English bulldog, pug, Boston terrier, and Pekingese. Because of their conformation, these animals may have one or more anatomic characteristics that impede air exchange, including very small nasal openings, an elongated soft palate, and a small-diameter trachea. Any anesthetic agent that depresses respiration or reduces muscle tone in the pharyngeal and laryngeal area will cause increased respiratory difficulty in these animals. In some cases this may be fatal, particularly if the animal is not intubated and an open airway cannot be maintained. These problems are most evident in animals undergoing surgery to correct conformation defects in the pharyngeal region (e.g., soft palate resection), because postoperative swelling or hemorrhage may occur, increasing the risk of respiratory difficulty.

In addition to respiratory problems, many brachycephalic animals have abnormally high parasympathetic tone, which may cause bradycardia. Use of atropine or glycopyrrolate in these patients is helpful for increasing heart rates before surgery.

The induction period is particularly difficult for brachycephalic dogs. If possible, the anesthetist should preoxygenate brachycephalic patients for 5 minutes before induction. This is done by gently restraining the animal and administering oxygen through a facemask. This procedure helps maintain adequate blood oxygen levels and gives the animal an extra margin of safety during the induction period that follows. Induction should be rapid in order to gain control over the airway, and for this reason IV induction agents are generally preferred over mask induction. Agents that are rapidly metabolized (e.g., propofol, ketamine-diazepam, and methohexital) are preferred. The dog must be adequately anesthetized to allow rapid and efficient intubation. Difficulties may be encountered because of the large amount of redundant tissue in the pharynx. This reduces visibility of the laryngeal opening, and the use of a laryngoscope is helpful in these patients. The anesthetist may find that the endotracheal tube that fits the trachea is smaller than expected, considering the size and weight of the dog.

Anesthesia usually can be safely maintained through the use of an inhalation anesthetic. With the help of an endotracheal tube, breathing during anesthesia may, in fact, be superior to that of the normal awake brachycephalic animal. Agents that allow rapid recovery (particularly isoflurane or sevoflurane) are preferred because dyspnea is common in these dogs during the early recovery period.

After surgery the patient should be observed closely until it is extubated and breathing well. Vigilance is necessary well into the recovery period because patients may develop airway obstructions even after attempting to stand. The endotracheal tube should be left in place as long as possible because the animal will maintain an open airway as long as the tube is in place. It is possible to give a low dose of morphine or hydromorphone just before turning off the vaporizer. This will allow the endotracheal tube to be left in longer, as these drugs suppress the cough reflex. Oxygen should be delivered until the patient is extubated. Once the endotracheal tube has been removed, the animal’s head and neck should be extended, and the animal should be watched closely for dyspnea and cyanosis. If dyspnea is seen, the mouth should be kept open with a mouth gag and the tongue pulled forward. Administration of oxygen by mask or even reinduction (with ketamine-diazepam, propofol, or other IV induction agent) and reintubation may occasionally be necessary. It is advisable to have supplemental oxygen and supplies for reintubation (i.e., a laryngoscope, new endotracheal tube, and the appropriate dose of an inducing agent) readily available in the recovery area in case dyspnea occurs after extubation.

Excitement and stress should be minimized as much as possible in the recovery period, especially if airway surgery was carried out. Some patients may require mild tranquilization or the use of opioid analgesics to reduce the rapid respirations that can worsen laryngeal swelling. Corticosteroids are also helpful in some patients.

Sighthounds

Several canine breeds (including the greyhound, saluki, Afghan hound, whippet, and Russian wolfhound) show increased sensitivity to anesthetic agents, particularly thiobarbiturates such as thiopental. The reason for this increased sensitivity is not entirely understood, but it may involve a lack of body fat for redistribution of the drug and inefficient hepatic metabolism of many drugs. Fortunately, many induction agents (including diazepam and ketamine, methohexital, propofol, isoflurane, and sevoflurane) can be safely used as alternatives to thiobarbiturates in these animals.

Obese Animals

Some patients presented for anesthesia have a high percentage of body fat. Because the blood supply to fat is relatively poor, anesthetics are not efficiently distributed to fat stores. Obese dogs, therefore, require lower doses of drugs on a per kilogram basis than do normal dogs. It is advisable to decrease the dose of preanesthetic and anesthetic agents so that the dose is determined according to a weight halfway between the normal breed weight and the actual weight.

Obese animals also may have some degree of respiratory difficulty, further complicating the anesthetic process. Dogs that show respiratory difficulties should receive oxygen by facemask for 5 minutes before induction. They may also require the use of induction techniques similar to those used in brachycephalic dogs and may require ventilatory support during maintenance of anesthesia (see Chapter 6).

Obese dogs and toy breeds often exhibit rapid shallow respirations during anesthesia. This breathing pattern may result in hypercapnia. The anesthetist who observes persistent rapid and shallow respirations should assume control over respiration by bagging the patient with oxygen and inhalant anesthetic, once every 5 seconds, until increased anesthetic depth and slower respirations are observed. The anesthetist can also slow down the respiratory rate by administering opioids such as hydromorphone or oxymorphone, especially if the elevated rate is a result of surgical stimulation, although occasionally opioids may cause panting.

Cesarean Section

The parturient patient faces risks that must be dealt with by the veterinary team. These include the following:

Essentially, all anesthetic drugs administered to the pregnant patient (with the exception of neuromuscular blocking agents and local anesthetics) will readily cross the placenta and affect the newborn. Although it is essential that the patient receive adequate anesthetic agent to provide immobilization and analgesia for the surgery, it is advisable to use minimal doses of those agents that depress respiration in the puppies or kittens.

Hemorrhage from the uterus is a common complication of cesarean surgery, and even nonhemorrhaging patients have an increased risk of shock. Therefore, an IV catheter should be placed and fluids should be administered intraoperatively to all cesarean patients.

It is helpful to do as much preparation of the cesarean patient preoperatively as possible, thereby reducing the anesthesia time. If possible, clipping and preparation should be initiated before induction. Whether the patient is awake or anesthetized, it is advisable that patient clipping and surgical preparation be done as much as possible with the patient gently restrained in left lateral recumbency rather than in dorsal recumbency. The latter position may cause the heavy uterus to compress the vena cava, decreasing venous return to the heart.

Various anesthetic techniques are used for cesarean surgeries, depending on the preference of the veterinarian:

In the cesarean patient the stomach empties more slowly, and the queen or bitch is more likely to eat small meals frequently, increasing the risk of food within the stomach. As time permits, the patient should be given appropriate premedication with an agent that can decrease vomiting, such as an anticholinergic. Induction should allow for quick intubation, and therefore the IV route is preferred over mask induction. During the postanesthetic period, the veterinary technician should ensure that the patient has a good swallowing reflex before extubation is performed. The technician should ensure that the patient is in a sternal position, to reduce the risk of any aspiration of vomitus. If the patient does vomit after extubation, the patient must immediately have its head lowered below the rest of the body, and suction of the oropharynx should commence immediately. Once the oropharyngeal area is clear, the patient should be placed back in sternal recumbency and may require a short period of oxygenation. The patient should be monitored for the next 24 to 48 hours for pneumonia (signs of which include pyrexia, dyspnea, hyperpnea, and depression), and the veterinarian may prescribe a course of antibiotics.

The distended uterus of the parturient patient causes lung capacity, functional residual volume, and tidal volume (V_T) to decrease. In the parturient patient there is also an increased demand for oxygen. With such patients the need for supplemental oxygen before and during induction becomes important to reduce the risk of oxygen desaturation should intubation become difficult. Induction should be done as quickly as possible; mask induction is not advisable.

Hypotension is also a potential problem for the cesarean patient. It is important that large bore catheters be used, and in some situations, such as dogs over 20 kg, it may be advantageous to have two catheters in place to ensure that rapid volume infusion is possible. Drugs chosen should help maintain blood pressure or at least not cause significant variation from normotension.

Because of physiologic anemia, the veterinary team must work with a patient that has a decreased ability to get oxygen to the necessary parts of the body. Oxygen delivery may be further compromised by an inability to control blood pressure as noted earlier. Such patients must be preoxygenated before and during the induction procedure to reduce the chance of hypoxemia and anaerobic glycolysis. Either or both of these situations can result in arrhythmias or acid-base imbalance.

Puppies or kittens delivered by cesarean section often show signs of reduced respiratory and cardiovascular function when first delivered. If respiration appears inadequate or if cyanosis is present, oxygen should be administered by facemask. If necessary, the newborn animal can be intubated with a 16- or 18-gauge IV catheter and gently bagged with oxygen every 5 seconds. Aspiration of fluid from the mouth and nose with an eyedropper or bulb syringe may also be useful. The use of reversal agents and doxapram (1 to 2 drops delivered under the tongue or 0.1 to 0.2 mL injected into the root of the tongue) is common. If bradycardia is present, a drop of dilute atropine (0.25 mg/mL) can be administered under the tongue or injected into the tongue. Gentle cardiac massage and oxygen may also be helpful.

The newborn should be allowed to nurse as soon as the mother appears to be recovered from anesthesia (or, with supervision, during the recovery period). The dam may be disoriented and should be closely watched to ensure the safety of the newborn puppies or kittens. Anesthetic agents excreted in the milk appear to have little effect on nursing or viability of the neonates. Postoperative analgesics such as butorphanol or buprenorphine assist the mother’s recovery.

Trauma Patients

Animals that have recently undergone trauma, such as being hit by a car, may have numerous ailments that greatly increase anesthetic risk. Respiratory difficulties are common and may be the result of pneumothorax, pulmonary contusions, hemorrhage, or diaphragmatic hernia. Any one of these decreases the V_T of the patient and therefore can cause a decrease in oxygenation. Lack of adequate oxygen exchange will lead to hypoxemia, which will lead to myocardial hypoxia and therefore arrhythmias, cell death, and acid-base imbalances. Increased CO₂ levels caused by lack of proper ventilation will also lead to acid-base imbalances and arrhythmias. Loss of blood or sequestration of fluid will result in changes in blood pressure, which must be corrected before anesthesia. Fluid sequestration can result from such situations as burns, where serum (fluid) oozes from the blood vascular system into the burn site. This causes a decrease in circulating volume and therefore a change in blood pressure.

Anemia may be present in the traumatized patient as a result of loss of blood directly or sequestration of blood into the trauma site. It has been shown in human medicine that a pelvic fracture can sequester as much as 40% of the circulating blood volume.

Details of these various issues are further described in this chapter.

Cardiovascular Disease

Common cardiovascular disorders in patients scheduled for anesthesia include anemia, shock, cardiomyopathy (primary, or secondary to hyperthyroidism), and congestive heart disease (secondary to mitral valve insufficiency). In endemic areas, heartworm disease is also common.

Hypoxia, hypercapnia, electrolyte imbalance, hypothermia, vagal stimulation, and anesthetic overdose can all lead to cardiac dysfunction. Preanesthetic evaluation and laboratory evaluation can help rule out many of these abnormalities, or at least alert the veterinary anesthetist of potential problems. Evaluation of heart rate and synchrony with the pulse is also an important aspect of cardiovascular evaluation.

The most common cardiac problem seen by the veterinary anesthetist is bradycardia. This is commonly caused by various drugs, either preanesthetics or general anesthetics, which are used as part of the anesthetic protocol. It is important for the veterinary technician to know the normal ranges for the heart rate for various species (please refer to Table 5-2). Heart rates below these values will result in decreased cardiac output and therefore decreased perfusion and blood pressure. The following equation is one of which every veterinary technician should be aware:

Anesthetic drugs may decrease not only heart rate but also force of contraction, and may decrease blood return to the heart (also known as preload). The decision to treat bradycardia will ultimately be the choice of the veterinarian, but the veterinary technician should be ready to give the appropriate dose of anticholinergic such as atropine or glycopyrrolate, although the former has a quicker onset of action (1 to 2 minutes intravenously) than the latter (up to 15 minutes). It should be remembered that anticholinergics might be selected by the attending veterinarian for treating bradycardia caused by increased vagal tone. However, many veterinary anesthesiologists no longer recommend the routine use of anticholinergics, and certainly these drugs do not work well for other types of bradyarrhythmias. Drugs that may cause an increase in parasympathetic tone include the alpha₂-agonists (medetomidine or dexmedetomidine, xylazine), the opioids, and occasionally the phenothiazines. When bradycardia is caused by anesthetic drugs that do not increase vagal tone, it is best treated with catecholamine drugs such as dopamine (1 to 5 mcg/kg/min). If the heart rate has dropped to 50% of normal value and/or blood pressure has dropped to dangerously low levels, then epinephrine becomes a drug of choice (0.01 mcg/kg).

Alternatively, reassessing the depth of anesthesia and turning the patient anesthetic level down, if appropriate, might be all that is needed. The temperature of the patient should also be assessed, as this can be another reason for bradycardia.

Arrhythmias may result from a variety of physiologic causes including the following:

Clinically it may be difficult to discern on physical examination if a patient is having arrhythmias. However, the veterinary technician can detect some arrhythmias by auscultating the heart while palpating the pulse and checking for a lack of coordination between the two, which is often called “dropped beats.” If dropped beats are discernible, an ECG should be obtained to determine what type of arrhythmia is present and what precautions may need to be taken. The only real way to determine the presence of arrhythmias is with an ECG recording.

As a veterinary paraprofessional there are certain arrhythmias that may require immediate attention by the veterinarian. It is therefore important for the veterinary technician to recognize the normal ECG so that he or she can advise the veterinarian that there may be some abnormality present. Following are some of the arrhythmias that may be encountered in trauma patients or anesthetized patients:

See Figures 5-6 to 5-15 for examples of commonly encountered arrhythmias.

The veterinarian must be informed immediately about any of the listed arrhythmias because each could be life-threatening. Various drugs such as beta blockers or lidocaine may be indicated in tachyarrhythmias, and atropine may be indicated to treat atrioventricular (AV) heart block. The veterinarian will decide what is the most appropriate drug to use.

Tachyarrhythmias may be treated either with lidocaine in dogs or with beta blockers or calcium channel blockers in cats, as lidocaine tends to be more toxic in the cat. It may be necessary to treat with a constant rate infusion (CRI) if the arrhythmia continues, as lidocaine has a very short half-life. However, a variety of different causes of and specific treatments for tachyarrhythmias exist, and discussion of them is beyond the scope of this book. On a short-term basis while drugs are being prepared, it may be necessary to slow the heart down by applying pressure to the eyeballs or massaging the carotid sinus. Either will increase parasympathetic stimulation.

Many animals with heart disease have concurrent pulmonary disease, particularly pulmonary edema, which further complicates anesthesia. Diuretics such as furosemide may be helpful in alleviating pulmonary edema before anesthesia.

As with patients that have undergone recent trauma, before initiating anesthesia it is generally advisable to stabilize the patient’s condition by treating cardiovascular and respiratory disease to alleviate the signs as much as possible. Preoxygenation using a facemask or oxygen chamber for 5 minutes immediately before induction is also extremely helpful in reducing anesthetic risk in animals with cardiovascular or respiratory difficulties.

The veterinarian and anesthetist should ensure that anesthetic agents that depress the myocardium or that exacerbate arrhythmias (e.g., alpha₂-agonists and halothane) are avoided as much as possible in these animals. Opioid agents, diazepam, and isoflurane or sevoflurane offer the advantage of relative lack of toxicity to the heart.

When anesthetizing animals with cardiovascular problems, the anesthetist should be aware of the increased risk of overhydration through excessive or rapid administration of IV fluids. Even the standard infusion rate of 10 mL/kg/hr may be dangerous for these animals. Therefore it is advisable to frequently monitor these patients for signs of pulmonary edema such as ocular or nasal discharge, increased lung sounds, and increased respiratory rate. Central venous pressure monitoring, if available, is useful for detection of overhydration.

Respiratory Disease

Of all the animals to undergo anesthesia, those with respiratory problems are perhaps the most challenging for the anesthetist. Examples of these patients include animals with pleural effusion (i.e., free fluid present in the chest cavity), diaphragmatic hernia, pneumothorax, pulmonary contusions resulting from trauma, pneumonia, tracheal collapse, and pulmonary edema. Poor oxygenation is often present in these animals, and many show signs of tachypnea, dyspnea, and cyanosis. If possible, anesthesia should be delayed until respiratory function has improved. If surgery is absolutely required (e.g., to place a chest tube), local analgesia and gentle manual restraint may be preferable to general anesthesia. Administration of oxygen via previously described techniques is helpful for patients with respiratory compromise. Nitrous oxide should be avoided in patients with respiratory distress because the administration of 100% oxygen is usually necessary to maintain adequate oxygenation.

Before anesthesia, it is important to thoroughly evaluate the animal and, if possible, to find the cause of the respiratory distress. Patients that are presented for anesthesia should be assessed as to their ability to move air into and out of the lungs. Such patients as brachycephalics or those with collapsing trachea can be compromised with regard to their ability to adequately move air even when they are awake. These patients should be preoxygenated during the preanesthetic period and may also need assisted ventilation during the anesthetic procedure.

An animal under anesthesia will frequently have a reduced V_T and decreased respiratory rate. Even when standard anesthetic protocols are used, it is common for the V_T to be decreased by 25% to 40% and for the respiratory rate to drop to 8 to 12 breaths/min. During this time there can be an increase in the CO₂ levels, which can result in acid-base imbalances. Such changes may also lead to hypoxemia unless the patient is breathing 100% oxygen. The technician must remember that the inspired air is divided into two portions. Dead space ventilation (V_D) is a relatively constant portion of inspired air that fills the upper airways and bronchi. This portion of ventilation is not involved in gas exchange. Some drugs such as anticholinergics may increase dead space. Alveolar ventilation (V_A) is the inspired air that fills the alveoli, where oxygen and CO₂ are exchanged at the capillary level. Thus the total or tidal volume of air taken in (V_T) is the sum of the dead space air plus alveolar air. V_D will always be the first air that is inspired, but because V_D is a relatively constant volume and cannot be changed, any decrease in V_T will result in a decrease in alveolar gas exchange.

One can then see how levels of CO₂ may increase and levels of O₂ may decrease. These changes can come about from either decreased ventilation or apnea. Generally drugs are not used to improve the ventilatory rate or volume, because drugs such as doxapram will result in only a period of increased ventilatory rate followed by apnea. There will also be a corresponding increase in oxygen demand by the central nervous system with this drug. The technician should properly assess the depth of anesthesia and lighten it if possible. If this is not possible, the technician should provide intermittent ventilation at a volume of 12 to 15 mL/kg per breath. During the anesthetic protocol it is also important that the anesthetist remember to sigh the patient every 5 to 10 minutes to reduce these issues of hypoventilation, atelectasis, and/or apnea.

Patients that are presented for anesthesia should also be assessed as to their oxygen-carrying capabilities by evaluating the packed cell volume (PCV) and the oxygen saturation with a pulse oximeter or a blood gas analysis machine such as the i-STAT if either of these monitors is available. Patients that have a perceived or real decrease in PCV (hematocrit) or an oxygen saturation below 92% should receive oxygen supplementation before the anesthetic protocol for a minimum of 5 minutes. This will ensure saturated oxygen levels within the blood vascular system and the tissues, thus reducing the chance of arrhythmias or hypoxic events.

Hepatic Disease

Animals with liver disease are subject to increased anesthetic risk because of the central role that organ plays in drug metabolism, synthesis of blood clotting factors and other serum proteins, and carbohydrate metabolism. Some animals with liver disease are hypoproteinemic, which may lead to increased potency of barbiturate agents. Patients with chronic liver failure are also commonly dehydrated, thin, and icteric and may be anemic.

Preanesthetic medication should be given with care or omitted from the protocol because most of these agents require hepatic metabolism before they can be excreted. Acepromazine in particular may have long-lasting effects in patients with compromised hepatic function. Agents that can be reversed are preferred. Use of ketamine and diazepam should also be avoided. Induction and maintenance of anesthesia is best achieved using isoflurane or propofol, which require little or no hepatic function for elimination.

Renal Disease

The kidneys are the organs most involved in maintaining the volume and electrolyte composition of body fluids. This helps explain why animals with renal disease are often dehydrated and may have severe electrolyte and acid-base imbalances, including metabolic acidosis and hyperkalemia. General anesthesia may be particularly stressful for these patients because renal blood flow is decreased during anesthesia and renal function may be further compromised, particularly if the animal is hypotensive. Use of injectable nonsteroidal antiinflammatory agents such as ketoprofen during anesthesia may reduce renal perfusion even further.

Renal function tests such as urine specific gravity, blood urea nitrogen (BUN), and creatinine may be useful in obtaining an accurate picture of renal function. Preoperative water deprivation may not be advisable in some patients with renal disease because dehydration can occur rapidly after withdrawal of oral fluids. Water should be offered up to 1 hour before premedication. The patient with renal disease should be rehydrated as much as possible before surgery, and electrolyte problems should be identified and addressed. Administration of IV fluids is often continued throughout the anesthetic and postanesthetic periods until the animal is fully hydrated and able to drink unassisted.

Many preanesthetic and anesthetic agents and their metabolites are eliminated from the body by renal excretion. For this reason, animals with compromised renal function may show prolonged recovery after anesthesia if conventional doses are used. It is prudent to reduce doses of anesthetic drugs (including acepromazine, xylazine, diazepam, ketamine, and barbiturates) in these patients. Barbiturates, in particular, have increased potency in acidotic and uremic animals and should be used with great caution in patients with renal disease. Inhalation agents (particularly isoflurane) have some advantages over injectable agents, although halothane can produce fluoride ions, which are damaging to the kidneys.

Animals with urinary blockages (including male cats with urethral obstructions caused by struvite or calcium oxalate crystals) pose similar problems to the anesthetist. Many of these cats are depressed, dehydrated, uremic, acidotic, and hyperkalemic. Hyperkalemic animals are at particular risk for cardiac arrest. Hyperkalemia may sometimes be suspected during auscultation because bradycardia is often seen if plasma potassium levels exceed 6 mEq/L. Some blood chemistry analyzers are capable of measuring electrolytes quickly and easily. Treatment of hyperkalemia may require the use of sodium bicarbonate, 10% calcium gluconate, and/or dextrose and should be done only with close supervision and guidance from the veterinarian. Conditions stressful to the animal should be avoided as much as possible because the release of epinephrine from the adrenal glands may potentiate cardiac arrhythmias.

The administration of inhalation agents (particularly isoflurane) to cats with urinary blockages may be less hazardous than the use of injectable drugs because minimal renal excretion is required for patient recovery. Propofol or ketamine-diazepam may be used intravenously with caution and at reduced doses, provided normal kidney function is present. Cats with obstructions showing extreme depression may not require general anesthesia, particularly if a local anesthetic such as lidocaine gel is administered as part of the urethral catheterization procedure.

• Animals that will not stay anesthetized

• Animals that are too deeply anesthetized

• Pale mucous membranes

• Prolonged capillary refill time

• Cyanosis and dyspnea

• Tachypnea

• Abnormalities in cardiac rate and rhythm

• Respiratory arrest

• Cardiac arrest

Animals That Will Not Stay Anesthetized

Occasionally, the anesthetist will have difficulty in maintaining a patient at sufficient anesthetic depth. Often the veterinarian becomes aware of the problem when the patient shows signs of movement in response to surgical stimulation. If depth appears inadequate, the anesthetist should check the following:

If none of these reasons can explain the patient’s arousal, the anesthetist should consult with the veterinarian. It may be necessary to increase the vaporizer setting, administer an analgesic, or switch to a different anesthetic in order to achieve the desired anesthetic depth.

Animals That Are Too Deeply Anesthetized

An animal that is too deeply anesthetized will usually show the following signs:

The anesthetist should use judgment in interpreting the signs listed. The presence of one or two signs may not indicate excessive depth, provided the other signs are absent. Vital signs also vary depending on the preanesthetic and general anesthetics used (e.g., atropine may affect heart rate and pupil dilation). The more parameters that the anesthetist considers, the more accurate the depth assessment is likely to be.

There are several reasons why anesthetic depth may be excessive. In most cases the vaporizer setting is too high for the patient being anesthetized or, in the case of injectable agents, too high a dose has been given. Occasionally the animal may have a preexisting problem such as shock or anemia that increases susceptibility to anesthetic overdose (Procedure 12-2, pp. 345).

Pale Mucous Membranes

Pale mucous membranes may arise from several causes. Some patients have preexisting anemia secondary to diseases such as feline leukemia, hemolytic anemia, bleeding disorders, neoplasia, or chronic renal disease. In other cases blood loss may have occurred during surgery. Some anesthetic agents (particularly inhalation agents, propofol, and acepromazine) cause vasodilation and decrease blood pressure, resulting in poor perfusion of capillary beds and pale mucous membranes in some animals. Hypothermia or pain can also reduce blood supply to the tissues and can cause pale mucous membranes.

If pale mucous membranes are observed during surgery, follow Procedure 12-3, p. 345.

Prolonged Capillary Refill Time

The observation of a capillary refill time greater than 2 seconds suggests that blood pressure is inadequate to perfuse superficial tissues. The presence of hypotension should be suspected in any animal with a slow capillary refill time. Hypotension was the most common anesthetic complication found in one study of dogs and cats (Gaynor and colleagues, 1999). Hypotension may be present before the induction of anesthesia, as in the case of animals undergoing emergency surgery after trauma. Hypotension or shock also may develop secondary to blood loss during surgery or may occur in patients that are at a very deep plane of anesthesia. Acepromazine and the inhalation agents may also cause hypotension in susceptible patients. Follow Procedure 12-4, p. 345, if a prolonged capillary refill time is observed. The treatment for shock in the anesthetized patient is similar to that in the conscious patient and should be performed under the supervision of a veterinarian (Procedure 12-5, p. 346).

Dyspnea and/or Cyanosis

Any patient showing dyspnea or cyanosis during the administration of an anesthetic should be immediately brought to the veterinarian’s attention. The presence of dyspnea indicates that the animal is unable to obtain sufficient oxygen or remove adequate CO₂ with normal respiratory movements. Cyanosis indicates that tissue oxygenation is inadequate. Dyspnea and cyanosis often are seen together and if not managed quickly may be followed by respiratory arrest, in which respiratory efforts cease and the amount of oxygen available to the tissues rapidly declines.

The most common sources of respiratory distress during anesthesia are as follows:

Respiratory problems are life-threatening and should be addressed as discussed in Procedure 12-6, p. 346.

Tachypnea

Tachypnea, or rapid respirations, must be differentiated from dyspnea, in which respiratory distress is present. Tachypnea may arise at any time during anesthesia and may be disconcerting to the anesthetist. It is particularly common during procedures using opioids. Tachypnea also may be seen if anesthetic depth is inadequate, in which case it is often accompanied by tachycardia and spontaneous movement. Paradoxically, tachypnea also may occur in deep anesthesia as a response to low blood oxygen and high blood carbon dioxide levels. Tachypnea is also seen in hyperthermic patients, including animals with malignant hyperthermia.

If tachypnea is seen, follow Procedure 12-7, p. 346.

Abnormalities in Cardiac Rate and Rhythm

Tachycardia, bradycardia, and cardiac arrhythmias are commonly seen in anesthetized patients.

Tachycardia is present if the heart rate during stage III anesthesia is greater than 140 bpm for a large dog, 160 bpm for a small dog, 200 for a cat, 60 for a horse, or 100 for a cow. It may result from the administration of drugs such as atropine, ketamine, or epinephrine. Tachycardia may also be a preexisting condition in animals with hyperthyroidism, shock, congestive heart failure, and other conditions. An elevation in heart rate is also a common response to surgical stimulation, although it does not necessarily indicate insufficient anesthetic depth unless accompanied by rapid respiration, spontaneous movement, or active reflexes.

Not all cases of tachycardia require treatment (e.g., those in patients with otherwise normal cardiac function), but the anesthetist should notify the veterinarian before assuming that tachycardia is not significant. It is also important to check the vaporizer setting and anesthetic depth and adjust them if necessary.

Bradycardia can be defined as a heart rate less than 60 to 70 bpm in a dog, <100 bpm in a cat, <25 bpm in a horse or <40 bpm in a cow. Bradycardia may be secondary to the administration of alpha₂-agonists, or opioids, particularly if anticholinergics were not administered preoperatively. Bradycardia also may result from increased activity of the vagus nerve in response to endotracheal intubation, ocular surgery, or handling of the viscera by the surgeon. Bradycardia may also occur if the animal is very deeply anesthetized, and when seen in this context it is a warning that respiratory and cardiac arrest may be imminent. Other causes of bradycardia include hypertension, hyperkalemia, hypothermia, and hypoxia.

Not all cases of bradycardia require treatment. If capillary refill, pulse oximeter readings, blood pressure, and pulse strength appear normal, tissue perfusion may be adequate and treatment may be unnecessary. The veterinarian should be consulted and should direct treatment. If anesthetic depth is excessive, the vaporizer setting should be adjusted, and bagging with 100% oxygen (empty the breathing bag into the scavenging system and refill it with pure oxygen) may be helpful. Bradycardia as a result of the administration of drugs or because of excessive vagal stimulation may be treated with anticholinergics, reversal agents, or a change in the anesthetic protocol.

The term cardiac arrhythmia (or cardiac dysrhythmia) refers to any one of a number of electrocardiographic abnormalities, including “dropped beats,” VPCs arising spontaneously from individual heart muscle cells, and sustained episodes of tachycardia. These abnormalities are most easily detected through the use of an ECG. The alert technician also may note that a pulse deficit is present in animals with some types of arrhythmias.

Cardiac arrhythmias commonly arise in animals given arrhythmogenic drugs such as barbiturates, alpha₂-agonists, and halothane. Such arrhythmias are often of short duration and well tolerated in young, healthy patients but may be a significant problem in animals with preexisting heart disease and in geriatric patients. Arrhythmias are particularly common during induction and light anesthesia. They may also be the result of respiratory depression and subsequent hypoxia. Other causes of cardiac arrhythmias include preexisting heart disease, electrolyte and acid-base disturbances, gastric volvulus, thoracic surgery, endotracheal intubation, and hypercapnia. Hypercapnia may arise from poor anesthetic technique, including exhaustion of the CO₂ absorber crystals or inadequate oxygen flow rates. Cardiac arrhythmias should be treated in consultation with the veterinarian (Procedure 12-8, p. 346).

Respiratory Arrest

Respiratory arrest is the cessation of respiratory efforts by the patient. It may lead to cardiac arrest and is therefore potentially fatal. Not all cases of respiratory arrest require immediate action by the anesthetist. Respiratory efforts may temporarily cease after the IV injection of ketamine, barbiturates, propofol, and other respiratory depressants. Minimal respiratory efforts may also be seen after a period of prolonged bagging with oxygen and in this case reflect low blood carbon dioxide and high blood oxygen levels. Whether respiratory arrest is the result of drug administration or excessive bagging with oxygen, the anesthetist must be sure that other vital signs, particularly heart rate and mucous membrane color, are normal. If the heartbeat is regular, the heart rate is greater than the minimum acceptable rate (see Table 5-2), the pulse is strong, and mucous membranes are pink, the patient does not usually require immediate treatment for respiratory arrest. Pulse oximeter readings are helpful in indicating if the patient’s oxygenation status is adequate (i.e., a reading greater than 95% usually indicates that bagging is not necessary). To be safe, occasional “breaths” of oxygen (one every 30 seconds) can be delivered to the patient during this period to prevent hypoxia. However, premature bagging with oxygen may extend the period of apnea by removing carbon dioxide from the blood, which is a stimulus for the patient to resume breathing. The anesthetist who suspects that respiratory efforts have temporarily ceased because of administration of drugs or ventilation with oxygen should closely monitor the patient’s heart rate and mucous membrane color for 1 to 2 minutes before assuming that a serious condition exists. If spontaneous respiration does not resume within this time, the veterinarian should be consulted, and it may be advisable to begin bagging the patient with oxygen.

True respiratory arrest is much more serious and requires immediate attention. This condition may arise because of anesthetic overdose, cessation of oxygen flow, or preexisting respiratory disease such as pneumothorax or diaphragmatic hernia. Affected animals may show warning signs such as dyspnea and/or cyanosis before respiratory arrest occurs. Other vital signs, such as heart rate, capillary refill time, pulse strength, and pupil size, are often abnormal. Pulse oximetry values rapidly fall below 90%. The treatment of respiratory arrest involves the steps shown in Procedure 12-9, p. 347.

Bagging should continue until the heart rate, mucous membrane color, and pulse oximeter values have been restored to normal. Once this is achieved, the anesthetist should discontinue bagging for 15 to 30 seconds and closely observe the patient for respiratory efforts. If none are seen, bagging should resume. On occasion, the anesthetist may be faced with a patient in respiratory arrest in the absence of an anesthetic machine. It is possible to substitute an Ambu bag (Figure 12-5) or even institute mouth-to–endotracheal tube or mouth-to-muzzle resuscitation in these cases.

Cardiac Arrest

Cardiac arrest may occur at any time during anesthesia. In most cases the anesthetist receives some warning that arrest is imminent, in the form of a short period in which cyanosis, dyspnea or respiratory arrest, and prolonged capillary refill are evident, often accompanied by an arrhythmia. If cardiac arrest appears imminent, the anesthetist should immediately alert the veterinarian and the hospital staff while continuing to monitor the heart by auscultation, by palpation of the chest, or through the use of an ECG. A patient experiencing cardiac arrest rapidly develops the following signs:

Coordinated action by all hospital staff members is essential to reverse cardiac arrest by performing cardiopulmonary cerebrovascular resuscitation (CPCR). Once arrest occurs, permanent brain damage may result if oxygen delivery to the brain is not reestablished within 4 minutes, by either cardiopulmonary resuscitation (CPR) or restoration of cardiac function. Ideally, at least five staff members should participate in the resuscitative efforts as follows: one person performs chest compressions, the second person bags the animal, the third assesses the pulse during compressions and checks the pulse or ECG when compressions are temporarily suspended, the fourth draws up and administers drugs on the veterinarian’s orders, and the fifth maintains a record of patient status and resuscitative treatments. The minimum number of people required is three, with compressions performed by one person, tasks two and five by a second person, and tasks three and four by a third.

The essential steps in performing CPCR have traditionally been summarized by the mnemonic ABCDE, which stands for airway, breathing, circulation, drugs, ECG. Of these, the single most important one is circulation. Current evidence suggests that initiating steps to generate circulation of blood should be done first, particularly if the patient arrests under anesthesia and is correctly intubated, thus CABDE.

Regurgitation during Anesthesia and Postanesthesia Vomiting

Regurgitation is a passive phenomenon that may occur even during deep anesthesia. In a regurgitating animal, stomach contents exit through the cardiac sphincter, move up the esophagus, and enter the pharynx, nasopharynx, and oral cavity. Once in the pharynx, stomach contents may be aspirated into the respiratory tract. Regurgitation is most common in animals placed in a head-down position during surgery, because this causes increased pressure on the stomach, and in ruminants. Unlike vomiting, regurgitation is not accompanied by retching or other outward signs, and in fact the only sign apparent to the anesthetist may be a small amount of fluid draining from the animal’s mouth or nose. Treatment of regurgitation involves immediate intubation (if a cuffed endotracheal tube is not already present) and removal of as much regurgitated material as possible through suction.

Vomiting during or after anesthesia is a relatively common phenomenon, particularly in brachycephalic dogs. Unlike regurgitation, vomiting is an active phenomenon, often accompanied by retching. Vomiting usually occurs as the animal is losing consciousness during induction or as it is returning to consciousness during anesthetic recovery. Vomiting is potentially most dangerous if the animal is unconscious and the airway is not protected with an endotracheal tube. In this situation, the vomitus may be easily aspirated into the trachea. Aspiration of vomitus may cause immediate signs of dyspnea and cyanosis as a result of airway obstruction and bronchospasm. If the patient survives this episode, signs of aspiration pneumonia (including fever, increased respiratory rate, and increased lung sounds) may appear over the next 24 to 48 hours. It is imperative, therefore, that the anesthetist prevents the accumulation of vomitus within the oral cavity of the unconscious patient and the subsequent aspiration of the material into the air passages.

To do this, an endotracheal tube should be immediately inserted if time and level of consciousness of the patient allow. If this cannot be achieved, the animal’s head should immediately be placed at a lower level than the rest of its body (e.g., over the edge of the surgery table). This helps prevent passive flow of liquid material into the trachea. When the vomiting stops, it may be necessary to manually clean the oral cavity, using suction if available. If respiratory arrest occurs because of airway blockage, the animal should be intubated and bagged with oxygen.

Unconscious animals have a low risk of aspiration if a cuffed endotracheal tube is already in place during the vomiting episode. It is for this reason that the endotracheal tube is customarily left in place until the patient regains the swallowing reflex and is close to consciousness. If vomiting is seen in an unconscious animal that has a cuffed endotracheal tube in place, the anesthetist should ensure that the cuff of the tube is inflated and position the animal’s head lower than the rest of its body to prevent accumulation of vomitus within the oral cavity.

Occasionally, an animal may regurgitate after esophageal intubation. If this occurs the tube should be left in place to direct the contents away from the pharynx. Another endotracheal tube should be placed in the trachea, if possible, while the first tube is still in place. Fortunately, most vomiting episodes occur after the animal has regained consciousness and the ability to swallow. It is not usually necessary to intubate conscious animals during a vomiting episode; however, the anesthetist should ensure that the head is kept extended and as low as possible.

Occasionally the technician may be called on to anesthetize an animal that has not been fasted before induction. These patients may be at risk of vomiting and/or regurgitation during induction, maintenance, and recovery. The anesthetist can help avoid problems by ensuring that rapid induction and intubation techniques are used. For this reason, an injectable agent is preferred over masking in these patients. A cuffed endotracheal tube with adequate diameter is essential. If possible, head-down positions should be avoided during surgery to prevent excessive pressure on the stomach. The anesthetist should also ensure that suction is readily available in case of regurgitation or vomiting. Use of antiemetic drugs such as metoclopramide may be helpful in some cases.

Postanesthesia Seizures and Excitement

Seizures are occasionally seen in animals recovering from anesthesia. Seizures may be caused by the administration of ketamine, by diagnostic procedures such as myelography, or by patient disorders such as epilepsy or hypoglycemia.

It is important that the anesthetist differentiate between seizures and excitement during recovery. Excitement usually occurs after barbiturate anesthesia (particularly after the use of pentobarbital to treat status epilepticus) and most often appears as spontaneous paddling of the limbs and occasionally as vocalization. Geriatric animals may also vocalize and appear confused after anesthesia. Usually, treatment is unnecessary other than the calm reassurance of the patient. Sedatives can be helpful, especially if the patient did not receive a sedative in the preanesthetic period. Occasionally, excitement or dysphoria may be seen after the administration of high doses of opioids to animals that have not been tranquilized (particularly cats). Treatment with naloxone to partially reverse the opioid or a tranquilizer to reduce the dysphoric effects may be helpful in these animals. Excitement is rarely observed in animals that receive opioids for moderate or severe pain.

In contrast, seizures appear as spontaneous twitching or uncontrolled movements of the head, neck, and limbs and are often triggered by a stimulus such as sound or touch. Animals given ketamine may show stiff forelimbs, opisthotonus, and exaggerated responses to touch or noise.

Animals undergoing postoperative excitement or seizures should be brought to the veterinarian’s attention. Elimination of stimuli such as light, sound, and touch may be adequate to resolve the episode. Adequate postoperative analgesia should be provided. If seizures are present, many animals respond well to administration of IV or rectal diazepam at a rate of 0.2 to 0.4 mg/kg. If diazepam is not effective or is unavailable, the animal may be anesthetized with propofol in sufficient quantity to induce sedation and eliminate seizures.

Animals manifesting seizures or excitement during recovery require surveillance and nursing care to prevent self-injury. In the case of cats recovering from ketamine anesthesia, it may be necessary to trim the front claws or to bandage the paws to prevent the animal from scratching its face.

All animals experiencing seizures should be monitored for hyperthermia and cyanosis. Hyperthermia can be treated by the application of cool wet towels. Cyanosis should be treated by the administration of oxygen by facemask or by endotracheal tube (if unconscious).

Dyspnea during the Recovery Period

Dyspnea resulting from upper airway obstruction is the most common cause of death in the postanesthetic period. Dyspnea in cats is usually caused by laryngospasm, whereas dyspnea in dogs is most commonly associated with breed-related (e.g., brachycephalic) obstruction of the entrance to the trachea.

Laryngospasm is a condition in which the cartilages in the laryngeal area become so tightly closed that air is unable to enter the trachea. This condition commonly arises in cats because of this species’ extremely active laryngeal reflex. In some recovering cats, the removal of the endotracheal tube initiates reflex closure of the airway. This reflex is normally useful to the cat in that it prevents the aspiration of food or water into the larynx in the conscious animal; however, in the unconscious animal it may well result in complete airway blockage. Laryngeal edema may result from repeated attempts to intubate during light anesthesia. Clinically, this resembles laryngospasm.

Cats undergoing laryngospasm or laryngeal edema may breathe with an audible stertor or wheeze. They typically show exaggerated thoracic movements, gasping, and upward movement of the head during inspiration. If conscious, the animal usually appears anxious or excited. Laryngospasm must be differentiated from growling, which is common in cats recovering from anesthesia. In the case of growling, the noises are particularly evident on expiration, whereas in laryngospasm the respiration is labored and the noise is most evident during inspiration.

If a cat shows signs of laryngospasm during recovery from anesthesia, the anesthetist should check the mucous membrane color and pulse oximeter readings (if available). If the cat’s mucous membranes appear pink and the Sao₂ is greater than 90%, the obstruction is likely partial rather than complete. In this case, the situation may resolve without treatment, although administration of oxygen by facemask may be helpful, provided it does not stress the cat. It may be helpful to extend the neck and hold the tongue rostrally. If cyanosis is present or Sao₂ readings are less than 90%, and the cat is losing consciousness—and if these signs are not alleviated by the administration of oxygen by facemask—the animal must be intubated. If intubation is impossible, the veterinarian may elect to perform a tracheotomy to reestablish airflow. The animal should be kept anesthetized for 2 hours and given furosemide and corticosteroids to reduce swelling. The cat can be extubated once the cords appear less rounded and swollen. Nasal oxygen is useful during extubation.

Laryngospasm is easier to prevent than to treat. When cats are being anesthetized, gentle intubation technique is essential to avoid unnecessary laryngeal trauma. Use of lidocaine spray and/or gel is also helpful during intubation. Early extubation is recommended in cats so that the tube is removed before the laryngeal reflex returns.

Dyspnea in brachycephalic breeds of dogs usually occurs because the airway is obstructed by the soft palate or by other redundant tissue in the pharynx. However, there are many other potential causes of obstruction, including foreign objects such as blood clots, gauze sponges, or even extracted teeth. Animals that have undergone surgery of the pharynx or larynx often undergo postoperative tissue swelling that may lead to airway obstruction.

However it arises, airway obstruction will usually not become evident until after the endotracheal tube has been removed. Strategies to prevent and treat postoperative dyspnea in brachycephalic dogs are outlined on p. 325.

A patient that requires supplemental oxygen during the recovery period can have it administered by one of four routes: facemask, nasal cannula, E-collar, or oxygen cage or tent, all as previously described in this chapter. As a general rule, the flow rate should be at least 100 mL/kg/min. If possible, oxygen being delivered to an awake animal should be humidified (e.g., by directing the flow of oxygen through a bottle of distilled water before delivery to the patient).

Prolonged Recovery from Anesthesia

Animals experiencing prolonged recovery from anesthesia should be examined by the veterinarian. There are many possible reasons why a patient may be slow to recover, including impaired renal or hepatic function, hypothermia, individual susceptibility to a particular anesthetic, breed variation (particularly sighthounds), or the presence of a disorder such as shock or hemorrhage. Excessive anesthetic depth or prolonged anesthesia may also result in delayed recovery. Use of certain agents (including intramuscular ketamine, or repeated injections of barbiturates) may be associated with prolonged recovery even in healthy animals. In the cat it is known that hypothermia in itself can cause significant delay in recovery even if all other organs are functioning properly.

Recovery may be hastened in several ways (Procedure 12-11, p. 347).

a. Instruct the receptionist to have the neighbor sign the consent form

b. Ask the neighbor to take the animal back home

c. Ask the neighbor some quick questions about the animal

d. Have the neighbor sign the consent form and ensure that the owner is called before the procedure is initiated

a. Inject a small amount of the solution and see what effect it has

b. Discard both syringes and start over

c. Ask the person who was holding the animal which syringe had saline in it

d. Discard both syringes, label some new syringes, and start over

a. Assume that the pressure gauge may be faulty and wait and see if the flow meter stops working

b. Change the oxygen tank

c. Call the repair person to have the pressure gauge checked

d. Use low-flow anesthesia techniques and ignore the pressure gauge reading

a. Disconnect the patient from the circuit, put on a new oxygen tank, and then reconnect the patient to the circuit

b. Remove the circuit from the patient to allow it to breathe room air for the remainder of the procedure

c. Resuscitate the patient with an Ambu bag

d. Switch to an injectable anesthetic

a. Stop the oxygen flow from entering the circuit

b. Convert the circuit to low-flow anesthesia

c. Cause a significant rise of pressure within the circuit

d. Cause the flutter valves to malfunction

a. The oxygen tank pressure gauge is malfunctioning and you need to recheck the flow meter

b. The oxygen pressure is adequate and the flow meter is simply not registering the flow

c. The animal is not getting oxygen, and you need to remove the animal from the circuit until a new machine is found or the problem is corrected

a. Is greater than 10 years old

b. Is greater than 15 years old

c. Has reached 50% of its life expectancy

d. Has reached 75% of its life expectancy

a. 2

b. 4

c. 6

d. 8

e. 10

a. 5:1

b. 10:1

c. 5:2

d. 10:2

a. 3 to 5 seconds

b. 5 to 20 seconds

c. 20 to 60 seconds

d. 60 to 90 seconds

True False

a. Compression of the reservoir bag does not result in the raising of the chest

b. The animal is dyspneic

c. The animal cannot be kept at an adequate plane of anesthesia

d. The reservoir bag is not moving or is moving very little

a. Reservoir bag is distended with air

b. Patient has difficulty exhaling

c. Patient wakes up

d. Flow rate starts to drop

a. Patient with cardiac disease

b. Obese patient

c. Pediatric patient

d. Brachycephalic patient

a. Physical size

b. Excess tissue around the oropharynx

c. Increased vagal tone

d. Small trachea in comparison with their physical body size

e. Increased susceptibility to barbiturates

a. Use atropine as part of the anesthetic protocol

b. Preoxygenate the animal before giving any anesthetic

c. Use an injectable anesthetic to hasten induction rather than masking

d. Ensure intubation is done quickly after induction

a. Decreased respiratory function

b. Increased chance of aspiration vomitus

c. Increased chance of hemorrhage

d. Increased workload of the heart

a. Halothane

b. Isoflurane

c. Xylazine

d. Opioids

a. More barbiturate

b. Less barbiturate

c. The same amount of barbiturate

a. A flow rate that is too low

b. An incorrect vaporizer setting

c. Incorrect placement of the endotracheal tube

d. Use of an anesthetic with a low MAC

a. Increased levels of arterial oxygen

b. Increased levels of arterial CO₂

c. The use of ketamine

d. Too light a plane of anesthesia

a. Hetastarch

b. Hypertonic saline

c. Dextran

d. Plasma

TrueFalse

TrueFalse

a. Lidocaine

b. Saline

c. Propranolol

d. Glycopyrrolate