Preanesthetic Evaluation

The preanesthetic patient evaluation is critical for patient safety as it promotes identification of individual risk factors and underlying physiologic changes or pathologic compromise that will impact the anesthetic plan. Factors to be evaluated include the following:

• History: Identify risk factors such as known medical conditions and previous adverse drug responses. Clarify the use of all prescribed and over-the-counter medications (e.g., aspirin, herbal products, cannabidiol, and supplements) to avoid adverse drug interactions.⁹ Note any abnormal clinical signs, both acute and chronic, with individual questions specifically directed at the cardiovascular, respiratory, gastrointestinal, nervous, and musculoskeletal/mobility systems. Records should be evaluated for previous anesthetic events, and client communication should include specific questions regarding satisfaction with previous anesthetics and recoveries. A smooth recovery may be noted in the hospital, but the patient may go home and exhibit abnormal behaviors such as lethargy, nausea, vomiting, restlessness, and vocalization, which could indicate pain or other complications that need to be addressed.

• Physical examination: A thorough physical examination should be completed and documented within 12–24 hr previous to anesthesia and repeated just prior to anesthesia if acute clinical changes occur. Failure to record a physical exam was reported to increase the odds for death in dogs.⁴

• Age: Although age is not a disease, disease processes occur more commonly in aged patients, and physiologic systems can be immature in neonatal and pediatric patients. Advanced or very young age can increase anesthetic risk because of altered responses to drugs caused by changes or immaturity in cardiovascular, respiratory, renal, hepatic, and neurological systems.^4–6 Examples include the inability to mount a robust physiologic response to hypotension or hypothermia in these age groups. Neonatal and pediatric patients may also be impacted by hypoglycemia and geriatrics by impaired cognitive function. A fairly high percentage of health abnormalities, including those that might cause a cancellation of or change in anesthesia, have been identified during preanesthetic screening of geriatric dogs.¹⁰

• Breed/Size: Few breed-specific anesthesia “sensitivities” have been identified. Greyhounds may have prolonged recoveries after receiving some anesthetics such as barbiturates and may experience hyperkalemia associated with general anesthesia.^11,12 Breeds affected by the multiple drug resistance mutation 1 (now ABCB1 or adenosine triphosphate binding cassette subfamily B member 1) gene mutation should receive reduced dosages of acepromazine and potentially butorphanol.¹³ Conversely, breed-specific anatomy or propensity for underlying conditions commonly impact anesthesia. For instance, brachycephalic dogs and cats are more prone to upper airway obstruction, and brachycephalic breeds have been shown to have higher airway-related anesthetic complication rates compared with nonbrachycephalic breeds.¹⁴ Some breeds of dogs (e.g., Cavalier King Charles spaniel) and cats (e.g., Maine Coon) may be predisposed to cardiac disease.¹⁵ Other breed-related diseases that may impact anesthesia, such as collapsing trachea in many small-breed dogs, breed-related renal or hepatic dysfunction, low intra-erythrocyte potassium concentrations in the Shiba Inu, and drug metabolism in cats, should also be considered.¹⁶ Breed-related size can also impact anesthesia.⁶ Very small/toy-breed dogs and all cats are at increased risk for anesthetic complications because they are more prone to hypothermia and may be more difficult to intubate and monitor. These patients may experience volume overload if a means to deliver precise fluid volume (e.g., syringe pumps, buretrols, etc.) is not instituted and are more easily overdosed if high-concentration drugs, high-volume syringes, or high-volume bags of fluid are used. Giant-breed dogs can be at increased risk because they are more commonly overdosed when milligram per kilogram dosing versus body surface area dosing is used.

• Temperament: Fear, anxiety, and stress can be exhibited in many ways, including aggression, hiding, fleeing, or freezing. When any of these behaviors are exhibited, the patient may benefit from medication administered at home to provide anxiolysis and reduce fear prior to travel to the hospital. An aggressive temperament can limit the preanesthetic evaluation or make examination prior to sedation impossible. This can impair the ability to detect abnormalities and may increase anesthetic risk. Anxious patients often require high doses of sedatives or tranquilizers, which may cause respiratory and cardiovascular depression. For elective procedures, consider rescheduling with a plan to manage anxiety before admission to the clinic. Conversely, a quiet or depressed animal may require lower drug dosages for sedation or anesthesia.

• Patient Diagnostics: Risk factors and specific patient concerns provide a framework for developing individualized anesthesia plans and may indicate the need for additional diagnostic testing, stabilization before anesthesia, or adjustments in chronic medications (see textbox “Recommendations for Chronic Medications the Day of Anesthesia”). Individual patient diagnostics may include a minimum database of laboratory analysis (complete blood count, chemistry panel, urinalysis) and could include other components such as blood pressure (BP), electrocardiogram (ECG), and imaging modalities like echocardiogram or ultrasound. For example, BP should be routinely measured in patients with renal, cardiovascular, and endocrine disorders. Currently, dogs eating a grain-free diet should undergo an echocardiogram to evaluate cardiac contractility as a result of the potential link between dilated cardiomyopathy and grain-free diets.¹⁷ There is no evidence to indicate the minimum timeframe between laboratory analysis and anesthesia. A reasonable timeframe is ≤3–6 mo if values were normal and the patient is clinically healthy. If either lab values or the patient’s health is abnormal, repeat diagnostics should be performed immediately prior to anesthesia.

Other Plan Considerations

• Type of Procedure: In addition to patient temperament and comorbidities, consider the level of procedural invasiveness, duration of surgery, and anticipated pain level. General anesthesia with airway control is required for long, invasive, and/or potentially painful procedures (dentistry, elective ovariohysterectomy or castration, or orthopedic procedure) and for any patient with airway compromise or undergoing airway surgery. Sedation may be appropriate for shorter (<30 min) and less invasive procedures (e.g., diagnostic procedures, joint injections, suture removal, and minor wound management) in healthy patients. However, heavy sedation is not suitable for all patients and may actually increase the odds for anesthesia-related death.⁵ For instance, in older, medically compromised patients, brief general anesthesia is preferable because it is less stressful and more controlled than sedation. Some procedures may limit physical access to the patient (e.g., oral or ophthalmic procedures), necessitating individualized plans for monitoring, catheter access, etc.

• Clinical Staff Training: Trained clinical staff are essential for safe anesthesia. The number of trained staff and the level at which they are trained will also impact efficiency and scheduling. In addition, staff training can positively impact specific areas of anesthesia; for instance, staff training in local and regional anesthesia techniques will help facilitate their perioperative use.

• Time of Day: Increased anesthetic risk has been documented for procedures occurring late in the day or after normal hours.^4,6 This is because of a combination of inadequate time for stabilization, limited staff availability, and staff fatigue. The fact that many procedures are also emergency or urgent, versus scheduled or elective, is also associated with an increase in the risk of anesthetic death.^5,6 Nonemergency procedures may be best performed during the next available regular clinic day when time for preparation and planning is adequate. When possible, critical patients should be anesthetized early in the day to allow adequate time for anesthetist-supported recovery.

Step 2a

Ensure that O₂ levels in the E-tank or hospital supply tank are >200 psi or that the oxygen generator is functioning properly. Fill the vaporizer with liquid inhalant. If the patient will be breathing through an RC, change the CO₂ absorbent after 8 hr of use, which may be roughly daily or weekly, depending on the anesthesia case load.

Step 2b

Connect the breathing circuit to the machine. When choosing a breathing circuit, note that NRCs are commonly used for cats and small dogs (patients <3–5 kg [6.6–11 lbs]) because, compared with RCs, they cause less resistance to breathing and have low equipment dead space, both of which are important considerations in small patients. Excessive equipment dead space leads to rebreathing of CO₂ and subsequent hypercarbia/hypercapnia. Dead space should be kept to a minimum and should be ≤2–3 mL/kg, which is ≤20% of total tidal volume. The patient end of breathing circuits (where mixing of inspired and expired gases occurs) is a common source of dead space. Pediatric circuits, which have low dead space, are recommended for use in smaller patients. Rebreathing of CO₂ in the NRC is prevented by high O₂ flow rates, which also allows for a faster turnover in the change of anesthetic depth when adjusting the vaporizer setting. Thus, the O₂ flow should be ∼200–400 mL/kg/min when using NRCs.¹⁹ Flow adequacy should be monitored with a capnograph and adjusted to keep the inspired CO₂ to <5 mm Hg. If any occlusion occurs in the expiratory limb of the circuit, the relatively high oxygen flow rates used in NRCs will cause rapid pressurization of the entire system and can lead to airway damage and/or circulatory collapse within minutes. In addition, the oxygen flush valve should never be used in a patient breathing through an NRC because it directs a high-pressure flow directly into the patient’s airway, potentiating barotrauma. The risk of high airway pressure damage is increased in cats and small dogs because of their small lung capacity (∼400 mL per average cat). For safety, a high-pressure alarm can be inserted in the expiratory limb of the breathing circuit. This does not allow escape of gas but emits a loud noise if the pressure in the circuit rises. An example of this alarm can be found at aaha.org/anesthesia.

Rebreathing circuits (i.e., circle systems) with lightweight plastic rebreathing hoses and minimal dead space are typically used in patients >3–5 kg (6.6–11 lbs). Pediatric RCs typically have lower equipment dead space compared with hoses for adult patients and can be used for patients <3–5 kg (6.6–11 lbs) if NRCs are not available. RCs depend on functional one-way valves to ensure unidirectional gas flow and on CO₂ absorbent to prevent rebreathing of CO₂. Proper valve function can be assessed by breathing through the circuit while visualizing movement of the correct valve on inspiration and on expiration. During the maintenance phase, total O₂ flow rate should typically be 20–40 mL/kg/min, with a minimum 500 mL/min to ensure accurate vaporizer output. The benefits of lower flow rates include decreased environmental contamination and decreased consumption of O₂ and volatile anesthetic gases (i.e., inhalant anesthetic not inhaled by the patient). Lower flow rates also conserve moisture and heat. When using RCs, a minor disadvantage of lower flow rates is increased time to change anesthetic depth after changing the vaporizer setting.

Step 2c

Leak test the machine and circuit before anesthetizing each patient and after changing any breathing circuit components. This safety measure ensures that oxygen is flowing to the patient and that there is minimal risk of inhalant leakage. For this step, the adjustable pressure limiting, also called the pop-off valve, must be closed. Occlude the end of the breathing circuit and increase the O₂ flow to raise the machine circuit pressure to 20–30 cm H₂0 on the manometer. The oxygen should be turned off, and the pressure manometer should remain steady. If no leak is detected, open the pop-off valve, and then release the breathing circuit occlusion. If a leak is detected, pull the machine from use and initiate troubleshooting procedures.

Step 2d

Ensure proper setup of the scavenging system to limit or eliminate personnel exposure to inhalant gases. The Occupational Safety and Health Administration provides advisory guidelines, although some US states have specific regulations regarding control of waste anesthetic gases. Active scavenging systems are far more effective than passive scavenging systems (e.g., activated charcoal canisters). More information on waste anesthetic gas is available through the American College of Veterinary Anesthesia and Analgesia.²⁰

Step 2e

Prepare for airway management by choosing ETTs of appropriate sizes and intubation tools (e.g., stylets, laryngoscope), along with a face mask for preoxygenation prior to induction. Veterinarians have several options for intubation, including clear cuffed polyvinylchloride, silicone, and self-sealing baffled tubes. Supraglottic airway devices, which do not require intubation, are available for airway management in cats and rabbits. See aaha.org/anesthesia for advantages and disadvantages of the various devices. ETT elbow and capnograph adapters are a source of dead space. To decrease dead space breathing without increasing airway resistance, the diameter of the capnograph and elbow adapters should always exceed the internal diameter of the ETT. A single elbow adapter may add up to 8 mL of dead space, which can be excessive in small patients. Low dead space adapters are recommended.

Step 4a. Pain Management

Effective analgesia throughout the entire anesthesia continuum is an integral component of patient health and welfare. Analgesia has numerous advantages as a component of general anesthesia. ²¹ First, analgesia improves anesthetic safety by decreasing the dose of inhalant drugs required for anesthesia maintenance, thus decreasing the likelihood of inhalant-mediated, dose-dependent adverse effects such as hypotension and hypoventilation.²² Second, provision of analgesia optimizes patient outcome with fewer pain-related adverse effects such as tachycardia, hypertension, slowed gastrointestinal motility, delayed wound healing, upregulation of pain, changes in behavior, etc.^23,24 Third, although not yet proved in animals, provision of perioperative analgesia may decrease the development of acute pain-related chronic pain.²⁵

Pain is a complex multidimensional sensation with multiple sources. Using one drug or drug class for treatment of pain is unlikely to provide adequate pain relief, at least in moderate to severe pain states. Using multiple drugs and modalities, each with activity at different sites of the pain pathway, alleviates or eliminates pain at multiple sites and from multiple sources. An example is the combination of an anti-inflammatory drug that decreases nociceptor activity and a local anesthetic that blocks pain signal transmission, plus an opioid or alpha-2 agonist to decrease receptor response in the central nervous system. This concept, known as multimodal or balanced analgesia, provides greater pain relief and promotes anesthetic safety by further decreasing required anesthetic drug dosages.²³ The use of multimodal analgesia also decreases the impact of drug unavailability, as with the recent opioid shortage. In addition, a balanced protocol includes preemptive, or preventive, analgesia and analgesia for a duration appropriate for the type/degree of pain. Preemptive administration of analgesic drugs decreases intra- and postoperative analgesic requirements.²³ Although not thoroughly researched in animals, the appropriate duration of analgesic therapy in animals can be extrapolated from the duration needed to control pain in humans suffering similar pain insults. Extrapolation is appropriate because of the similarities of the mammalian pain pathway across species.²⁶ Because animals conceal pain, treatment duration should be based on scientific knowledge of pain duration and not on presence/absence of pain signs.

Building an Analgesic Protocol: This is an overview of perioperative analgesia. For more in-depth information, see the AAHA/American Association of Feline Practitioners (AAFP) Pain Management Guidelines and World Small Animal Veterinary Association Analgesia Guidelines.^2,27 Analgesic drugs can be distributed into the four phases of anesthesia (preanesthesia, induction, maintenance, and recovery). Opioids are often the first class considered when designing protocols. Although this strategy is efficient for anesthetic plan development, when considering the sources of pain and the mechanisms of action of analgesic drugs, building an analgesic protocol is more effective if centered on anti-inflammatory and local anesthetic drugs. Inflammation is generally a major component of acute pain. Because inflammation is also the pathology that produces most acute pain syndromes, control of inflammation decreases further tissue damage and speeds healing. An anti-inflammatory drug (traditional NSAID or grapiprant) should be administered to all appropriate patients.

Local anesthetic drugs block sodium channels and provide complete pain relief from the nerves that are blocked. This fact led to the recommendation “…local anesthetics should be utilized, insofar as possible, with every surgical procedure.”² The task force recommends the use of local anesthesia, including the simple techniques for common procedures in Figures 3–5.

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Although opioids do not block pain at its source or stop the transmission of pain, they are potent and rapidly acting, making them excellent for acute pain relief. Full mu-opioid receptor agonists (morphine, for example) are the most potent analgesics but also the most impacted by regulatory control. Buprenorphine is moderately potent but has a longer duration (4–6 hr) than most full mu-opioid agonists, with the FDA-approved buprenorphine for cats providing 24 hr of analgesia. Butorphanol is only mildly to moderately potent and has a short duration of action (<60 min in the dog and 90 min in the cat).²⁷ See Figure 6 for opioid selection considerations.

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After an anti-inflammatory drug, a local block, and an opioid have been chosen, adjunctive drugs should be added. Dexmedetomidine and medetomidine provide both sedation and analgesia, and their analgesic effects are synergistic with those of the opioids, thus enhancing the effects of the less potent opioids.²⁸ Ketamine, administered as a subanesthetic dose infusion in a multimodal protocol, prevents or decreases the development of central sensitization, a condition that significantly amplifies pain intensity.²⁷

Adjunctive drugs with less proof of efficacy include maropitant and gabapentin. Although minimum alveolar concentration reduction does not necessarily indicate analgesia,²² maropitant decreased minimum alveolar concentration in cats (administered at label dose) and dogs (administered as infusion).^29,30 If not an analgesic, the potential for increased patient comfort secondary to decreased vomiting makes maropitant a valid addition to an anesthetic protocol. Gabapentin is used for treatment of chronic neuropathic pain but is unlikely to provide analgesia for acute inflammatory pain. However, gabapentin might be appropriate in patients with pre-existing neuropathic pain if dosed at a minimum of 10 mg/kg q 8 hr.^31,32 Tramadol appears to provide minimal acute pain relief in dogs.³³ Although it is perhaps more efficacious in cats, especially used multimodally, cats are particularly averse to the drug’s taste.^34,35 Tramadol is controlled and linked to human diversion, so dispensing for home use should be limited. Other adjunctive drugs and nonpharmacologic therapy are covered in the 2015 AAHA/AAFP Pain Management Guidelines for Dogs and Cats.²

See aaha.org/anesthesia for additional resources on designing multimodal anesthesia and analgesia protocols.

Step 4b. Preanesthetic Anxiolytics and Sedatives

In addition to preanesthetic analgesic drugs, preanesthetic sedatives and anxiolytics are an important component of the continuum of anesthesia care. Benefits include decreased stress/anxiety and dose reduction of induction and maintenance drugs, which have dose-dependent adverse effects. Reduced patient stress can reduce risk of harm to staff members who are restraining/handling patients. As stated, providing appropriate oral medication at home can alleviate anxiety and pain prior to the pet’s admission to the hospital.^36,37 In-hospital, preanesthetic medications can be administered by the intramuscular (IM) or subcutaneous route to achieve effects well in advance of anesthesia induction, or intravenously just prior to anesthesia for acute dose-sparing effects of induction drugs. Specific drugs/drug combinations should be chosen based on desired effects (e.g., reversible sedation) and individual patient needs (e.g., degree of analgesia; Figure 2). After premedication, physiologic monitoring and support (more information in the maintenance section) are conducted as indicated by the patient’s health status. Early support of body temperature should be initiated in all patients. Thermal supplementation is critical as hypothermia can cause numerous adverse events (more information in the Troubleshooting Anesthetic Complications section).

Premedicant drugs listed in Figure 2 are also useful for sedation alone, remembering from previous information that greater patient safety is often achieved with general anesthesia, even for short procedures, especially if they are painful (biopsy, laceration repair, etc.). A crucial point is that sedated patients, just as those under general anesthesia, require appropriate monitoring and supportive care. Administer supplemental oxygen to the deeply sedated patient using a facemask. The patient may require airway management, so be prepared to intubate if necessary.

Step 4c. Anesthetic Induction

Preoxygenation should be considered part of the preanesthetic/induction sequence. Delivery of 100% oxygen for only 3 min provides almost 6 min of adequate saturation of hemoglobin with oxygen.³⁸ This is especially critical in patients with airway disease (e.g., pneumonia, asthma) and breathing difficulty (e.g., upper airway dysfunction, limited thoracic movement [e.g., thoracic injury, impingement on diaphragm from dilated stomach or gravid uterus]) and in patients with expected difficult intubation (e.g., upper airway collapse or airway foreign body). All pregnant patients should be preoxygenated to ensure adequate fetal oxygen delivery.

A patient’s sedation level following preanesthetic drugs will influence the dose of induction drug, which should always be dosed “to effect.” Typically, appropriate premedication will result in lower doses of induction drugs. In addition, sick, debilitated, or depressed patients may require lower doses than healthy, alert patients. Anesthetic induction is most effectively and efficiently achieved by IV administration of fast-acting drugs (dosages and specific protocols in Figure 7), such as propofol, alfaxalone, etomidate, diazepam- or midazolam-ketamine, or tiletamine-zolazepam. IV induction allows rapid airway control. IM administration (Figure 8) of a combination of a sedative and either ketamine or tiletamine-zolazepam combined in the same syringe can be used to both premedicate and induce patients whose venous access is limited by size (i.e., cats and very small dogs) or temperament (i.e., fractious or aggressive). This does not allow rapid control of the airway, and patients should be observed closely as they are becoming unconscious. Supplemental oxygen administration during this period should be considered in manageable (i.e., not fractious or aggressive) patients.

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A patent airway should be secured using placement of an ETT or supraglottic device as soon after induction as possible. Tracheal intubation is an essential part of maintaining an open and protected airway. The length of the tube should be assessed prior to intubation. The proper length will allow the proximal end of the tube to be at or just external to the incisors and the distal tip of the tube to lie midway between the larynx and the thoracic inlet. The largest-diameter ETT that will easily fit through the arytenoid cartilages without trauma should be used. This will minimize resistance and the work of breathing. Correct placement can be confirmed by direct visualization of the tube between the arytenoids, movement of the rebreathing bag, condensation in the ETT during exhalation, and/or observation of a definitive end-tidal carbon dioxide (ETCO₂) tracing. A properly inflated cuff on a conventional ETT is necessary to create a seal for adequate positive pressure ventilation (PPV) and avoid inhalant leakage, being aware that overinflation may cause tracheal damage.³⁹ Applying a light coating of sterile lubricating jelly improves the cuff’s ability to seal the airway.⁴⁰ Once the patient is intubated and connected to a breathing circuit, ensure that O₂ is flowing, close the pop-off valve or push down the button on the safety pop-off valve, and administer a manual breath to 15–20 cm H₂0 while listening at the patient’s mouth for a slight hissing sound, audible if the cuff is under-deflated. If detected, slowly inflate the cuff while simultaneously administering a manual breath. Once the sound has disappeared, immediately open the pop-off valve or release the button on the safety pop-off valve and secure the ETT to the patient’s head. Self-sealing baffled ETTs may be more leak resistant, but they do not allow air to escape around them if the airway pressure is excessive. When changing the patient’s position after intubation, disconnect the ETT from the breathing tube so that the ETT does not rotate within the trachea as tube rotation may cause tracheal tears, especially if the cuff is relatively overinflated. Tracheal tears are a significant issue in anesthetized intubated cats.^39,41

Final considerations: Padding and appropriate positioning (especially for cachectic, geriatric, or large patients) should be provided. Apply corneal lubricant postinduction to protect the eyes from corneal ulceration after induction and every 2–4 hr.

Step 4d. Anesthetic Maintenance

Anesthesia is typically maintained using inhalant anesthetics delivered in O₂ and dosed “to effect.”^19,38 Maintenance can also be achieved using continuous infusions or intermittent doses of injectable agents, or a combination of injectable and inhalant drugs. Short-duration maintenance can be achieved with IM administration of sedatives plus ketamine or tiletamine/zolazepam. IM alfaxalone can be effective for short-duration, deep sedation in cats and small dogs, but the high dose required for anesthesia maintenance in healthy cats can cause excitement and hypermetria in recovery.⁴²

• Physiologic Monitoring: Regardless of the drugs used for anesthesia maintenance (i.e., inhalant or injectable), vigilant monitoring, interpretation of physiologic changes, and response to patient physiologic status by well-trained and attentive staff are critical. Monitoring decreases the odds of anesthetic death,¹⁸ whereas lack of monitoring increases the odds of anesthetic death by a factor of 5–35.⁵ Both multiparameter electronic monitors and hands-on assessment of the patient by the anesthetist should be used. Treatment decisions should be made based on information from both the electronic monitors and the anesthetist’s assessment. Monitoring respiratory function includes respiratory rate, oxygenation (percentage of hemoglobin saturated with oxygen [SpO₂]), and ventilation (ETCO₂). BP, heart rate (HR) and rhythm (ECG), capillary refill time, mucous membrane color, and pulse oximetry (SpO₂) provide the best indices of cardiovascular function. Anesthetic depth is monitored, and a surgical plane of anesthesia is typically defined as a patient with absent palpebral reflex, mild jaw tone (i.e., muscle relaxation), and lack of purposeful movement. Body temperature monitoring is critical, with heat supplementation starting early; see section on anesthetic complications.

• Physiologic Support: Regardless of the drugs used for anesthesia maintenance (i.e., inhalant or injectable), O₂ should be delivered to the patient. The O₂ flow rates depend on the breathing circuit (see section on equipment preparation). For an RC, use a relatively high flow rate (2–3 L/min) when rapid changes in anesthetic depth are needed, such as during the transition from injectables to inhalants (induction) or when discontinuing inhalants at the end of the procedure. Because of the high oxygen flow, increased flow at induction and after discontinuing inhalants is not necessary when using an NRC. Increased inspired CO₂ suggests an inadequate O₂ flow rate. Following induction and intubation, the patient may be apneic or have a low or shallow respiratory rate, requiring intermittent (1–4 breaths/min) PPV breaths delivered by the anesthetist to maintain anesthesia until the respiratory depression of the induction drugs subsides. If PPV is excessive, ETCO₂ levels will decrease below the level that stimulates ventilation and the patient may not begin spontaneously breathing. Balanced crystalloid fluids should be administered for most patients undergoing anesthesia. The basal fluid rate for healthy dogs and cats is 5 mL/kg/hr and 3 mL/kg/hr, respectively. Additional volume should be added to the basal rate for correction of hypovolemia, including dehydration, and replacement of ongoing fluid losses. Fluid volume should increase or decrease depending on the patient’s health status and fluid needs. See the 2013 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats for more information.⁴³

• Troubleshooting Anesthetic Complications: Immediate and effective response to complications during anesthesia is critical. The most common complications are hypotension, hypoventilation, hypoxemia, hypothermia, and some arrhythmias like sinus tachycardia and bradycardia. Complications in the cardiovascular and respiratory systems are generally the most acutely life-threatening. More detailed information for cats is available with the 2018 AAFP Feline Anesthesia Guidelines and online at aaha.org/anesthesia.³

  • o Cardiovascular System Complications:

    • ▪ Hypotension is a common complication during anesthesia and is defined as BP values of systolic <80–90 mm Hg, mean <60–70 mm Hg, and diastolic <40 mm Hg.⁴⁴ Evaluation of other physiologic parameters (capillary refill time, peripheral pulse palpation) can be used to aid in the diagnosis of inadequate blood flow. Balanced anesthetic techniques such as additional opioid or a local block may permit a further decrease in inhalant dose, thus improving BP. Complete avoidance of the inhalant anesthetic-mediated dose-dependent vasodilatation can be achieved by application of partial or total intravenous anesthesia techniques (Figures 7 and 8). An IV crystalloid (5–20 mL/kg, depending on patient needs) and/or colloid (1–5 mL/kg) bolus can be administered IV. If the patient is also bradycardic, administer an anticholinergic (atropine, glycopyrrolate) or sympathomimetic (e.g., ephedrine). If decreased cardiac contractility or excessive vasodilation are causing hypotension, administer a positive inotrope or vasoconstrictor, respectively (Figure 9). If hypotension continues, ensure that the patient is not hypoglycemic, hypothermic, or anemic/hypoproteinemic and that there is no electrolyte imbalance. Initiate BP support and corrective therapy if these abnormalities are present.

    • ▪ Arrhythmias commonly occurring perioperatively include sinus tachycardia, sinus bradycardia, atrioventricular block, and ventricular arrhythmias. Monitor using auscultation or ECG and/or by observing pulse–HR incongruity with Doppler ultrasound or SpO₂ waveform. The decision of whether to treat an arrhythmia should be based on the severity, the effect on other hemodynamic parameters (e.g., BP), and the likelihood of deterioration to a more significant arrhythmia. Examples of common arrhythmias and treatment considerations seen during anesthesia can be found at aaha.org/anesthesia.

    • ▪ Tachycardia, HR >180 bpm in cats³ and HR >150–190 bpm for large and small dogs, respectively,⁴⁴ during anesthesia deserves special mention, because it should prompt the anesthetist to run through a list of rule-outs and not simply assume it is a response to inadequate anesthetic depth. Tachycardia can be secondary to a noxious stimulus, hypoxemia, hypercarbia, and hypovolemia. It can also occur secondary to administration of drugs such as alfaxalone, ketamine, atropine, and dopamine.

    • ▪ Hypertension, defined as a mean arterial pressure >120–140 mm Hg or a systolic arterial pressure >160–180mm Hg,⁴⁴ is uncommon in the adequately anesthetized patient, even in patients with primary hypertension, because of the negative cardiovascular effects of inhalant anesthetics. Again, rule out a response to noxious stimulation, hypoxemia, hypercarbia, hypovolemia, and a light plane of anesthesia. The BP cuff should also be examined to ensure it has not slipped distally (e.g., around the carpus). BP can be verified with a second technique such as Doppler. Analgesics such as additional opioid should be provided to the patient who is consistently hypertensive. If hypertension persists but the patient appears to be adequately anesthetized and receiving appropriate analgesia, the increased BP may be tolerated. Increasing the vaporizer concentration in an attempt to decrease BP is not advised.

  • o Respiratory System Complications

    • ▪ Hypoventilation can be estimated by observing respiratory rate and depth (very subjective) and can be quantified using capnometry. Hypoventilation can cause hypercarbia, with subsequent respiratory acidosis, and hypoxemia. Thus, hypoventilation should be corrected. ETCO₂ is ∼35–45 mm Hg in awake patients and ∼40–50 (up to 55) mm Hg in patients in an appropriate surgical plane of anesthesia. To correct increasing CO₂, first ensure that the cause is not excessive anesthetic depth by checking the vaporizer setting and evaluating indicators of the patient’s anesthetic plane. Initiate PPV if ETCO₂ is >60 mm Hg (hypercapnia). The anesthetist can deliver breaths by manually squeezing the reservoir bag while occluding the adjustable pressure limiting valve, taking great care to not leave the valve closed except when delivering a breath. A safety pop-off relief valve will prevent this complication. A mechanical ventilator can be used if the anesthetist is knowledgeable and comfortable with ventilator use. Visit aaha.org/anesthesia for more information on mechanical ventilation. Prior to instituting PPV, the hemodynamic status of the patient should be stable, if possible, as PPV can negatively affect cardiac output through impaired venous return. Recheck BP after starting PPV. If BP declines, decrease peak airway pressure and consider a fluid bolus (5–20 mL/kg) if there is the potential that the patient is hypovolemic. PPV can also cause barotrauma, so ventilator settings should start conservatively and be adjusted based on ETCO₂. If hypercapnia persists, investigate causes of increased inspired CO₂ including excessive dead space, exhausted CO₂ absorbent or one-way valves not functioning properly in an RC, or inadequate oxygen flow in an NRC. If machine malfunction is suspected, it may be prudent to quickly replace the machine with a different machine.

    • ▪ Hypoxemia (SpO₂ < 95%, severe SpO₂ < 90%) is uncommon when a patient is intubated and breathing 100% oxygen.⁴⁴ Observation of mucous membrane color is not a sensitive indicator of hypoxemia as cyanosis will likely not occur until hypoxemia is profound.⁴⁵ Continuous assessment of oxygenation is best accomplished with pulse oximetry. With low SpO₂, the anesthetist may be tempted to troubleshoot the pulse oximeter by repositioning the probe, moistening the mucous membranes, or trying a different monitor. These measures may work if the issue is indeed the probe, but prior to troubleshooting the probe, verify that the patient is properly intubated and connected to the oxygen source and that the supply of oxygen is adequate. Hypoventilation can cause hypoxemia, so adequate ventilation should be ensured, as previously described. Insertion of the ETT past the thoracic inlet can cause one-lung intubation with decreased pulmonary surface area for gas exchange. If one-lung intubation is likely, the ETT can be pulled out slightly, with the goal to move the tip of the ETT into the trachea. Hypoxemia can be secondary to atelectasis, in patients with abdominal distention or obesity positioned in dorsal recumbency, or to primary pulmonary (e.g., pneumonia) or pleural (e.g., pleural effusion) disease. If this is expected, manual or mechanical ventilation should be instituted and a positive-end expiratory pressure (PEEP) valve (2.5–5 cm H₂O) can be added to the expiratory limb of the circuit to open collapsed airways. Decreased oxygen delivery to the tissues from perfusion issues (rather than respiratory issues) can also cause decreased SpO₂ readings. Treat indicators of poor perfusion such as slow capillary refill time, brady- or tachycardia, hypotension, and weak pulses (see section on cardiovascular complications). If no improvement occurs with these treatments, the patient should be positioned in sternal recumbency as soon as possible and recovered from anesthesia with continued oxygen support.

  • o Other Complications

    • ▪ Hypothermia, core body temperature <98°F, can result in a myriad of adverse effects, including delayed drug metabolism, cardiovascular dysfunction, impaired perfusion, respiratory compromise, cerebral depression, increased incidence of wound infection, etc., and is a very unpleasant sensation in conscious patients (as described by humans).⁴⁴ Delayed drug metabolism and cerebral depression can result in prolonged recovery.⁴⁶ The most effective heating methods are circulating warm-water blankets and warm air circulation systems. Other methods of supplemental heat that may be helpful in slowing heat loss include warm IV fluids, use of a fluid line warmer, and insulation on the patient’s feet (bubble wrap, baby socks, etc.). Do not use supplemental heat sources that are not designed specifically for anesthetized patients as they can cause severe thermal injury.⁴⁷ Because shivering significantly increases oxygen consumption, continue to provide supplemental oxygen to shivering patients, especially those with respiratory or cardiovascular compromise.

    • ▪ GER and regurgitation can cause esophagitis and aspiration pneumonia and can lead to esophageal stricture in extreme cases. When noted, suction of the esophagus is recommended followed by lavage with saline or tap water, with concurrent endotracheal tube protection of the airway. Diluted bicarbonate can be instilled into the esophagus to increase pH.⁴⁸ Maropitant prevents vomiting, promotes more rapid return to normal feeding, and improves the quality of recovery from anesthesia but appears to have a lesser effect on the incidence of reflux or regurgitation.⁴⁹ Metoclopramide, ranitidine, and omeprazole plus maropitant also appear to have a minimal impact on regurgitation.^50,51 GER and regurgitation was minimized when cisapride 1 mg/kg was combined with omeprazole 1 mg/kg.⁵² However, as GER and regurgitation cannot be consistently prevented, the use of gastroprotectants, such as omeprazole 1 mg/kg at least twice (evening prior and morning of anesthesia), can be considered for the neutralization GER pH in at-risk patients.⁵⁰

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Step 4e. Recovery from Anesthesia

Although many complications occur throughout anesthesia, between 47 and 60% of all anesthetic-related dog and cat deaths, respectively, occur during the postoperative period of anesthesia, with most occurring within the first 3 hr.^4–6 Thus, patient care and monitoring of the recovering patient by trained personnel is critical and should be maintained with the same vigilance as during the maintenance phase of anesthesia. The anesthetist should continue monitoring specific patient physiologic parameters such as HR/RR (respiratory rate), SpO₂, BP, and body temperature. Patients should be closely observed until they are alert, normothermic, and ambulatory (unless nonambulatory preoperatively). An optimal recovery time (within 10–30 min of the end of anesthesia) for dogs and cats will depend on the patient health status, type of anesthetic technique used (i.e., inhalant versus injectable), duration of anesthesia, and body temperature.

In case sedatives, analgesics, or emergency resuscitative drugs are needed, intravenous catheters should be left in place until the patient is extubated and in sternal recumbency with physiologic parameters returned to normal. Extubate when the patient’s RR and SpO₂ are within normal limits and the patient can adequately protect its airway by vigorously swallowing. Deflate the cuff immediately before removing the ETT. With patients who have undergone a dental procedure or rhinoscopy, it is beneficial to position the nose slightly lower than the back of the head and leave the ETT cuff slightly inflated during extubation. This will help clear blood clots and debris from the trachea and deposit any fluid or debris into the pharyngeal region where it can drain from the mouth or be swallowed, thereby reducing the risk of aspiration. Respiratory depression, potentially with resultant hypercapnia and hypoxemia, often persists during early recovery from anesthesia. Severe hypercapnia can lead to cerebral impairment and potentially to respiratory arrest. The capnograph adapter can remain on the end of the ETT to assess ventilation until the patient is extubated.⁵³ In addition, a pulse oximeter should be used throughout recovery to assess the degree of oxygen saturation.

Numerous factors can impact the quality of recovery and should be addressed to aid the patient’s smooth emergence from anesthesia. Environmental stress, bright lights, excessive noise, and a cold environment can attribute to the patient’s discomfort following anesthesia. Bladder distension can be very uncomfortable. Express the bladder to minimize any discomfort, especially for those patients who may be nonambulatory and unable to urinate on their own in the immediate postoperative period.

Delayed recovery, dysphoria, and emergence delirium are common complications in the postoperative period. Delayed recovery can be caused by excessive anesthetic depth during the maintenance phase. This can not only prolong the recovery phase but also negatively impact the respiratory, cardiovascular, and thermoregulatory systems. Hypothermia (body temperature <98°F [36.7°C]) can lead to multiple physiologic complications, including delayed drug metabolism, further prolonging the patient’s recovery.⁴⁶ Patient warming devices should be used throughout the recovery phase.^46,53 Certain drugs can cause peripheral vasoconstriction (e.g., alpha-2 agonists) or vasodilation (e.g., inhalants), modifying the heat loss from the patient and influencing the effectiveness of external warming devices.^46,47 Hypoglycemia, especially in small or neonatal patients, can lead to a prolonged recovery, so monitor blood glucose frequently. Judiciously titrated drug antagonism may be considered if the patient’s recovery is concerningly prolonged. Alpha-2 agonists should be reversed only if the patient is excessively sedate or rapid recovery is needed. Opioid effects should be antagonized only if other analgesics have been administered; otherwise, the patient could experience intolerable pain.

Dysphoric recoveries and emergence delirium can often be difficult to differentiate. Emergence delirium often presents as an uncontrolled, uncoordinated thrashing of the patient, often encountered when the patient has regained partial consciousness quickly after the discontinuation from maintenance anesthetic drugs. Dysphoric recoveries can result from many issues, including uncontrolled pain, and hypoxemia due to airway obstruction. Administer an analgesic if pain is suspected. If pain management is adequate, administer an anxiolytic sedative, such as dexmedetomidine (which also provides analgesia), generally 0.001–0.005 mg/kg IV or IM (up to 0.01 mg/kg). If respiratory complications are the cause of the dysphoria/delirium, provide supplemental oxygen and assess the respiratory function of the patient. In some cases, the patient will need small doses of alfaxalone 1–2 mg/kg or propofol 1–2 mg/kg IV administered slowly until calm. Be prepared to reintubate if necessary.

Final considerations: Provide adequate padding for nonambulatory patients. Reapply eye ointment every 2–4 hr during the recovery period until an adequate blink refiex is present.