Anesthesia 101: designing a safe and effective anesthetic protocol (Proceedings)
Understanding how to design an anesthetic protocol is essential for the veterinary technician. Anesthesia is needed for a variety of veterinary procedures. Surgery, radiographs, CT, MRI, and endoscopy also require anesthesia and/or sedation. A thorough knowledge of patient history as well as anesthetic drugs and equipment is necessary to implement a safe anesthetic protocol
The most obvious consideration to begin with is the species must be anesthetized. Different species can have drastically different responses to anesthetic drugs. Feline patients are well known to have difficulty metabolizing opioid drugs and often become dysphoric or excited with administration. Lower doses and less frequent administration is often needed in this species (compared to canine patients). In rabbits, if an anticholinergic drug is to be given, it is often recommended that glycopyrrolate be used instead of atropine since a large percentage of domestic rabbits break atropine down very quickly due to the presence of the enzyme atropinase.
Even within a species, the breed of the patient can also determine the drug choices. Among dogs, Nordic breeds (Huskies, Malamutes) are well known to be "high-strung" and often require more sedation than some other breeds. Large breed dogs (Great Danes, Newfoundlands) frequently require less sedation based on body weight so premedication doses are often reduced to prevent excess sedation. Generally barbiturates, like thiopental, should be avoided in sighthounds (Greyhounds, Whippets) since they often have prolonged recoveries.The age of the patient must be taken into consideration as well. Neonates and pediatric patients need to maintain their heart rate in order to maintain blood pressure and cardiac output since they have less contractile function than adults and an immature sympathetic nervous system. Anticholinergics are often indicated in these patients. The respiratory system may be immature and so a higher respiratory rate is needed in order to maximize oxygenation. The liver may be immature as well so it is important to choose drugs that do not rely heavily on hepatic metabolism. Young patients are prone to hypothermia due to a lack of body fat and a high surface area to body mass ratio, so it is important to be very proactive about maintaining temperature. Hypoglycemia is common in young patients since the liver is immature and lacks the ability to efficiently store glycogen and to convert it to glucose. Blood glucose should be monitored in patients that are fasted for surgery and fluids with dextrose (2.5-5%) should be administered as necessary.
Geriatric patients should maintain a heart rate as close as possible to the patient's normal range since they can have difficulty adjusting to changes in cardiac output. As a patient gets older, the lungs become less compliant and they have a decreased functional reserve and can become hypoxic more quickly. Ventilation and oxygenation should be monitored and assisted as necessary. Geriatric patients also tend to have decreased hepatic blood flow so many drugs have an increased clearance time. Drugs that are heavily dependent on hepatic metabolism should be avoided and drug doses may need to be decreased. Some degree of renal dysfunction is often present as well, so maintaining blood pressure is especially important to renal perfusion. Fluid administration is essential and administration of inotropes and/or pressors may be needed.
The physical exam is an essential part of any pre-anesthetic work-up. Body condition should be noted as obese animals may need lower drug doses. Neurologic status is important in determining the amount of sedation that is necessary. Are they BAR? Anxious? Depressed? Doses and drug choices may need to be adjusted. Heart rate, rhythm, pulse quality and capillary refill time can tell the examiner a lot about the patient's cardiovascular system. Is the patient particularly bradycardic or tachycardic? Is there an arrhythmia or a murmur? If the pulse quality is poor or the CRT is prolonged the patient may need additional stabilization before anesthesia. Evaluation of the respiratory system can alert the anesthetist to any abnormalities that may be worsened by the administration of sedative drugs. Is the patient febrile or hypothermic? Why? Abnormalities in the patient's skin and haircoat can signify underlying medical conditions like hyperthyroidism or hyperadrenocorticism. Abdominal palpation can reveal an enlarged liver or spleen. Any of these abnormalities found on physical exam may change the anesthetic plan or may even change whether or not the procedure goes forward.