Preanesthetic evaluation (Proceedings)

Introduction

Allen Lakein said "planning is bringing the future into the present so that you can do something about it now". In preparation for an anesthetic procedure, we must know our patients well in order to be successful. We evaluate each patient individually to determine what drugs, dosages, as well as administration routes performed. Developing a plan also helps us anticipate what techniques we need to use and what type of monitoring tools are required to be safe. Knowing the procedure performed on each patient as well as our drug protocol will keep us alerted to anticipated complications such as hypotension, hypothermia, hypoventilation, arrhythmias, etc.

Influencing Factors

Signalment

The patient's signalment, which includes the owner's information as well as species, breed, sex and age, will help influence our decisions. The client information, especially emergency phone numbers are helpful in the case of emergency contact. Various species have different metabolic requirements as well as special considerations for each individual species. A cat can develop laryngospasm during intubation. This rarely occurs in the dog. Certain breeds have influencing factors such as the sight hounds group. They may have a lower serum protein, which will reduce your dose of protein binding drugs. They also have very little to no body fat and in sight hounds; there is an alteration of liver enzymes. These things will make the recovery period longer. It is advisable to avoid highly protein bound drugs such as thiobarbiturates. The sex of the animal is particularly important especially during pregnancy. If the patient is a neonate or pediatric, they may have immature liver function as it is not well developed. If the patient is geriatric, one should consider compromised organ function even if not recognizable through neither physical exam nor diagnostic exams. We can not always see aged organs. Brachycephalics often have upper airway obstructive disease. Boxers have been seen to be sensitive to Acepromazine. People call this "boxer-ace syndrome". It has not been documented but there is an increased risk of bradycardia, which may be due to excessive vagal tone. Miniature Schnauzers have been seen developing sick sinus syndrome. They should be evaluated with an electrocardiogram prior to anesthesia. Doberman Pinschers can develop a coagulation disorder called von Willebrand's disease. It is ideal to have the von Willebrand's factor checked before surgery & or do a buccal mucosal bleeding time (normal is less than 3 minutes).

Temperament

The behavior of the animal can impact your choice of drugs dramatically. A usual attempt to sedate an animal may turn into immobilization due to the fractious nature of the patient. A proper physical exam may be near to impossible until the animal is under the influence of drugs that induce sedative and or tranquillizing effects. A stressed animal will have a catecholamine surge. There will be an increase in heart rate & blood pressure, therefore more of a work load on the heart. This will alter the effects of anesthetic dugs.

History and Client Information

If there is indication of disease prior to anesthesia, the patient may require additional diagnostics.

If a cat has a history of drinking and urinating frequently, one should suspect renal disease. It may also be behavioral but this problem should be evaluated prior to anesthesia. A dog that has exercise intolerance can indicate cardiopulmonary problems or a dog that is painful. A cat that sneezes frequently can reveal upper respiratory disease and or a foreign body. Some owner's may communicate extensively, which can benefit you in developing your anesthetic plan. Current health may reveal that an old problem is now managed such as a diabetic patient that is now under control. It is important to tap into old problems as well as new. If it's a new problem, how long has it been going on and what is the severity? What are the symptoms of the patient? All of this information factors into your plan.

Physical Exam

We must do a pre-anesthetic physical exam on every patient! Be consistent in your approach.

Either head to tail or systems approach. The systems that the anesthetists are most concerned with are cardiovascular, respiratory, renal, hepatic, nervous, gastrointestinal, integument as well as overall body condition. When evaluating the patient, know your normal vital signs with various species, breeds as well as age on each of your patients.

In evaluating the cardiovascular system, one should have a picture on how well the patient is perfusing his or her tissues. Ascultate for murmurs. Also check to see if pulses are synchronous with heart rate. One can observe rhythm via an electrocardiogram. The capillary refill time can show adequacy of perfusion but is not a real accurate method. Blood pressure monitoring is more ideal. By looking and touching the mucous membranes, one can see how well the patient is hydrated.

When evaluating the respiratory system, keep in mind that without normal function, a patient may not get adequate oxygen for the tissues and or a patient may become hypercarbic (high carbon dioxide in the circulating blood). Ascultate all lung fields as well as the upper respiratory tract. If any abnormal sound is heard, a further diagnostic work up may be necessary such as radiographs and or arterial blood gases. Look at the color of the mucous membranes (MM). Normal is pink. Pale MM can be an indication of hypotension, hypothermia, hypoxemia, vasoconstriction (from a2-agonists) and low packed cell volume. A red brick coloring can indicate venodilation, blood sludging, hypercarbia or endotoxemia.

Evaluating the renal system can be difficult on physical exam. Palpation of the kidneys should be performed for size and symmetry. Owner history and clinical findings are important as well evaluating the creatinine and blood urea nitrogen on a blood chemistry. A uremic or azotemic (retain nitrogenous wastes in blood- can indicate poor kidney function) patient must be corrected prior to anesthesia as anesthetic drugs can worsen this condition. A low urine specific gravity (USG) will tell you if your patient is concentrating urine normally. A USG less than 1.025 in the dog and 1.035 in the cat may show you early signs of kidney dysfunction prior to finding an azotemia, thus may be the first test you would do on your patient. A patient under general anesthesia must maintain adequate perfusion to support normal kidney function. This can be monitored using blood pressure measurement. If a mean arterial blood pressure of 60 millimeters of mercury is maintained, your patient is perfused adequately.

Hepatic function will indicate how well your patient can metabolize and excrete anesthetic drugs. Plasma proteins are also produced in the liver and low total plasma proteins can indicate liver disorders. The liver is also responsible for some clotting factors. Increased alkaline phosphatase (ALK Phos) alanine aminotransferase (ALT), which are liver enzymes can indicate hepatic disease. Animals with portosystemic shunts (PSSs) will often have increased bilirubin, bile acids white blood cell count and sometimes, increased clotting times. It is common with the PSSs or patients with liver disease to see decreased blood glucose, albumin, total plasma proteins and packed cell volume as well. Progressed liver disease may reveal that a patient is jaundice and or lethargic. They may even seizure.

The nervous system plays an essential role in anesthesia management as the brain and spinal cord are involved. If a patient has a disorder involving the nervous system such as head trauma or paralysis, a full neurologic exam must be performed by the veterinarian as there are a multitude of concerns one would have to mange the needs of their patient under anesthesia. The nervous system is effect by stress and pain as well and can be altered in these conditions.

When evaluation the gastrointestinal system, consider diet and how well the patient has been eating. Do they ever vomit? Do they have normal bowel movements? A patient that has had persistent diarrhea can lose valuable electrolytes such as potassium.

If skin infections or ectoparasites are noticed on the integument, this may alter your approach with skin disinfection and or local anesthetic techniques.

Look at your patient's overall body condition. What is their weight? Are they cachetic or obese? Is the obesity coming from pregnancy? This will affect your drug choice and dose. Are they hydrated? This will affect uptake, metabolism & clearance. Use skin turgor, eyeball position as well as moisture of mucous membranes. What is your patient's body temperature? This can indicate disease and or stress. Patient's young or old can be presented with disease or trauma. Age is not a disease but geriatrics should be evaluated further as they are more prone to disease. Pain should be scored prior to anesthesia all through the post-op period, therefore wheather an animal is in pain or not, they should always be evaluated for pain and discomfort.

Pre-Anesthetic Diagnostics

Pre-anesthetic laboratory tests should be performed on each patient in order to eliminate unexpected complications. If stabilizing is necessary, this will guide your therapeutic techniques.

Every hospital will have different policies depending on cost, available testing as well as environmental concerns for that particular town/state/country. Lab tests for a young patient (< 6 yrs) should include PCV, TPP, USG, & BUN. If the patient is compromised or over 6 yrs, a full CBC, chemistry profile as well as other diagnostic tests may be required.

PCV (packed cell volume)

This is a slight indicator of hydration. It will give you information on the body's oxygen carrying capacity.

Minimum PCV for pre-op is 27-30%. The minimum Intra-op is 20%. The maximum is 60% (patient's cardiac output is cut in half if blood viscosity is double what it should be).

If PCV is low; consider stabilizing patient. Check hemoglobin as this is a more precise way of measurement of tissue oxygenation as well as arterial blood gas. Keep in mind that delivery of fluids will dilute PCV further. A whole blood transfusion may be necessary. You may want to consider pre-oxygenation and or intermittent positive pressure ventilation (IPPV) if improved oxygen tissue deliver is required.

TPP (total plasma protein)

The TPP will affect the patient's protein binding ability. A patient with a low TPP (hypoproteinemia) will have more drugs to bind to receptors thus increasing the potency. An increase in TPP may indicate dehydration in a patient.

CBC (complete blood count) - whole blood

This will include PCV, TPP, and Hemoglobin but also provide information about the white blood cells. An alteration may indicate stress or infection. This should also include a count of platelets which will reveal information about clotting.

Chemistry Profile- serum

There are a variety of tests that will assess organ function & disease. Listed below are some of the tests available. Each test should be considered based on each individual patient as well as disease.

Additional diagnostics are available for particular concerns of the patient such as serum electrolytes, clotting tests, arterial blood gases, urine specific gravity or full urinalysis, electrocardiogram, ultrasound, CAT scan, and magnetic resonance imaging.

ASA- American Society of Anesthesiologists

Now that you have all your pertinent information on your patient, you can develop an anesthetic plan....based on their ASA status! This is a status that the ASA developed to help the anesthetist determine what anesthetic approach is the best based on a variety of factors.

Category Class (RISK): I- minimal risk of a normal healthy patient, II- slight risk of a slight to mild systemic change, III- moderate risk & systemic change w/ some clinical alterations, IV- high risk with preexisting disease that is severe in nature and V- extreme risk where patient will probably die with or without surgery. An E, for emergency can be attached to each class if it is an emergency.

ASA Status- Examples of Various ASA

A healthy dog for castration or cruciate repair would be considered as a patient that is a class I.

An example of a class II patient would be an obese cat for a castration or a mild anemia in a dog that is being spayed. For examples of class III; a depressed dog presented for pyometra or a cat with mild cardiovascular disease presented for esophageal endoscopy. A class IV patient might be a dog with severe dehydration presented with foreign body removal or a peg tube placement on a cat that has liver failure. A class V example would be a dog with multiple organ failure leading to circulatory failure such an advanced gastric dilatation volvulus (GDV). Another example would be a trauma severe in nature such as hit by car (HBC) on shocky dog undergoing exploratory with massive abdominal hemorrhage.

After classifying your patient, consider signalment, history, client information, temperament, physical exam & diagnostics. Formulate a plan based on all this information and keep good record of your findings. Consider writing your plan on a pre-anesthetic form and your anesthesia record on another form. This helps you stay organized systematically. You will not miss key factors in helping you choose drugs, dosages and techniques. Also, legally, you have deciding factors written as to why you chose the drugs you did.

References

Greene, Stephen A: Veterinary Anesthesia and Pain Management Secrets, Philadelphia, Hanley & Belfus, Inc., 2002

Tranquilli W, Thurmon J, Grimm K: Lumb, Jones', Veterinary Anesthesia and Analgesia 4th ed. Iowa, Blackwell Publish, 2007

McKelvey D, Hollingshead KW: Veterinary Anesthesia and Analgesia, 3rd ed. St. Louis, Mosby, 2003

Meyer & Harvey: Veterinary Laboratory Medicine Interpretation & Diagnosis, 2nd ed. Philadelphia, W.B. Saunders, 1998

Muir W, Hubbell J, Bednarski R: Handbook of Veterinary Anesthesia, 4th ed. St. Louis, Mosby, 2007

Paddleford: Manual of Small Animal Anesthesia, 2nd ed. Philadelphia, W.B Saunders, 1999

Seymour C, Gleed R: Manual of Small Animal Anaesthesia and Analgesia, Shurdington, UK, BSAVA, 1999

Thrall, MA: Veterinary Hematology and Clinical Chemistry, Baltimore, Lippincoott Williams & Wilkins, 2004

Thurmon J, Tranquilli W, Benson G: Essentials of Small Animal Anesthesia & Analgesia, Philadelphia, Lippincott Williams & Wilkins, 1999