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 highconcentration 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.