Pre-anesthesia check on pet patients prior to surgery is widely recognized as a must-do. Anesthetics used in operation suppress the body’s normal automatic functions and even affect the cardiovascular-respiratory function of the pet patient. Thus, anesthesia procedure is associated with a certain risk. Through pre-anesthesia check, we could understand the pet’s physical condition and underlying conditions before using anesthetics. Next, prepare in advance to minimise the anesthesia-related risk. However, there are a variety of options for evaluation. The question is, which tests should veterinarians perform on patient during pre-anesthesia check?
The American Animal Hospital Association (AAHA) published a new version of the 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. It introduced the safe practice of anesthesia, a continuum of care that begins before the patient leaves home, continues at animal hospital and ends when returned home. The pre-anesthesia check includes assessing the pet’s condition at home and various test results before anesthesia. The guideline placed emphasis on pre-anaesthesia check as it is critical for animal safety. A comprehensive evaluation helps to identify individual risk factors, potential physiological changes and pathological damage that may influence the anesthesia plan. This guideline also listed the recommended evaluation procedures and test items. Let’s find out!
The following factors may affect anesthesia. Veterinarians shall communicate with pet owner to gather information for pre-anaesthesia check.
Medical history is important for pre-anaesthesia evaluation. Risk factors can be determined based on the history, including the known medical conditions and previous adverse drug reactions. It is necessary to know the prescribed and over-the-counter medications (e.g., aspirin, herbal products, cannabidiol, and supplements) to avoid harmful events attributed to the use of drugs. The AAHA guideline also stressed that veterinarians should take the drugs used in previous anesthetic events into consideration. During the communication with pet owner, veterinarians may include specific questions regarding the previous experience of pet anesthesia and recovery. A smooth recovery may be noticed during the inpatient treatment. However, it is possible that pet patients show abnormal behaviours after returning home, such as lethargy, nausea, vomiting, restlessness, and vocalization. This may indicate the patient is experiencing pain or other complications that veterinarians may not know. Hence, veterinarian-pet owner communication before anesthesia is emphasised.
Anesthetic risk is associated with pet patients regardless of their age. Some diseases are more common in the aged pet. During pre-anesthetic check, it was found that a high percentage of aged animals demonstrated health abnormalities. While for the neonatal and pediatric pet, their physiologic system such as the cardiorespiratory, renal, hepatic, and neurological systems etc. are relatively immature. This may lead to dynamic response to drugs among patients. Hence, there is an increased anesthetic risk for both aged and young pets. The AAHA guideline provided some examples, for instance these age groups have a weak response to hypotension or hypothermia. Another example is that newborns and young pets may have some congenital functions that cannot function normally due to hypoglycemia and mother-related diseases. The anesthesia plan for the patient may change or even cancel under some circumstances. As a result, age-specific risk factors should be evaluated during the pre-anesthetic check.
There is limited scientific evidence that supports the breed-specific sensitivity to anesthetic drugs – Greyhounds is one of the breeds being documented. It was noted that Greyhounds have a slower recovery than the average dog after receiving some anesthetics （e.g, barbiturates) may experience hyperkalemia. Breeds affected by the multiple drug resistance gene mutations, should receive reduced dosages of acepromazine and potentially butorphanol.
In addition to the breeds that are allergic to anesthetics, the guideline mentioned some special breeds shall be noted. Breed-specific anatomy and the underlying conditions may lead to changes of anesthesia plan. For example, brachycephalic breeds have higher chance of encountering airway-related anesthetic complications. While some dog breeds (e.g., Cavalier King Charles spaniel) and cats (e.g., Maine Coon) have a higher risk of cardiac disease. Other breed-related diseases including collapsing trachea in small-breed dogs, breed-related renal or hepatic dysfunction, low intra-erythrocyte potassium concentrations in the Shiba Inu, and drug metabolism in cats etc., may also impact the anesthesia decision.
Lastly, breed-related size should be taken into consideration too. Toy-breed dogs and cats are comparatively prone to hypothermia. They are also more difficult to intubate and monitor hence the risk associated with anesthesia increased. If the method used to deliver precise fluid volume (eg. syringe pumps, buretrols, etc.) is not appropriate, volume overload may result. Giant-breed dogs are having higher anesthesia-related risk. As “weight (ml per kg) vs body surface area (BSA, per m2)” dosage calculation is adopted, an overdose is commonly induced.
Pet characters are often being neglected. Fear, anxiety and stress will result in behaviors such as aggression, hiding, fleeing, or freezing. According to the guideline, when the pet patient exhibits those behaviors, pet owner is recommended to administer prophylactic drugs like analgesics and anxiolytics. This could effectively reduce pet’s anxiety before traveling to the hospital. As aggressive pets hinder the process of safe pre-anethesia check which increases the anesthetic risk. If the pet continues the aggressive behavior, veterinarians would use higher doses of sedatives to calm down the animal. It may cause respiratory and cardiovascular depression. Conversely, lower doses of sedatives for calm and depressed patients. For extreme cases, veterinarians may consider rescheduling the anesthesia-surgery with the pet owner. Meanwhile, it is crucial to educate the pet owner about the importance and methods of animal stress management at home.
In addition to the factors mentioned, the AAHA guideline specifically listed some risk factors that could be life-threatening. It includes anxiety, pain, hypoglycemia, hypothermia, anemia, dehydration, life-threatening cardiac arrhythmias, electrolyte and acid-base dyscrasias, cyanosis, congestive heart failure, oliguria, anuria, pneumothorax (See the details as below table) . These risk factors must be evaluated by diagnostic examination. For example, blood tests to assess hypoglycemia or anemia, and electrocardiograms (ECG) to detect arrhythmia. Therefore, the guideline suggested that the diagnostic examination should include: physical examinations, minimum database of laboratory analysis (complete blood count, biochemical examination, urinalysis), electrocardiogram (ECG), blood pressure (BP), and imaging modalities such as echocardiogram or ultrasound.
So when should veterinarians perform the diagnostic tests? Considering the timeframe between laboratory analysis and anesthesia, the patient can be identified as clinically healthy if the test findings are normal within 3 to 6 months. If abnormal values are found from the laboratory report, repeated assessment is necessary to perform prior to anesthesia.
Through a comprehensive pre-anaesthesia check and assessment, veterinarians can better understand the pet’s condition. The AAHA guidelines also emphasize the communication between veterinarians and pet owner. Pet’s conditions, potential risk factors and special circumstances shall be discussed. As a result, veterinarians are able to individualize anesthesia plans and be the pioneers of patient safety.
Reference source: 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats.
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