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.

Physical exam

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.

Blood work

Pre-anesthetic blood work should never be made optional. Safe anesthesia depends on knowing if essential body systems are working properly. The proper functioning of the liver and kidneys is very important during anesthesia and can be determined easily by the use of routine blood work. For young, healthy patients presenting for routine procedures a Big 4 (PCS, TS, AZO, BG) is usually adequate. Patients that are geriatric or have some kind of systemic disease, a complete blood count (CBC) and blood chemistry is recommended. Depending on the patient and the procedure, other types of blood work might be necessary such as blood type, cross match, and/or coagulation profile.

Other pre-anesthetic tests

Some patients may require additional testing if abnormities are found on the physical exam or blood work. Chest radiographs, abdominal radiographs or ultrasound should be performed as indicated. Patients with cardiac disease, hyperadrenocorticism, renal disease, or sepsis should have pre-operative blood pressure measurements. An ECG should be performed on any patient with an arrhythmia to determine what type of arrhythmia is present or those patients with known cardiac disease. An echocardiogram and cardiac workup is indicated with many of these patients as well. These tests can give the anesthetist much more information so that they can make appropriate anesthetic choices for that individual and increase the likelihood of a safe anesthesia.

Medications

A complete less of the medications that the patient is currently on should be obtained from the owner. Many commonly used medications can affect essential body systems and may cause the anesthetist to revise their anesthetic plan. NSAIDs and steroids can affect the liver, kidneys, and gastrointestinal tract. If a patient is on pain medication, preoperative doses may have to be adjusted to avoid overdose. Barbiturates and other types of anticonvulsants can cause excess sedation when combined with other pre-anesthetic drugs and can affect liver function. Medications for many type of cardiac disease, such as ACE inhibitors and beta- blockers will affect the patient's heart rate, blood pressure, and cardiac output under anesthesia so it is important to know if the patient in on these medications.

Anesthetic history

If the patient has been anesthetized before, it is helpful to review the previous anesthetic record. This record can provide future anesthetists with a wealth of pertinent information. How did the patients respond to the premedication chosen? What was used? If the patient responded poorly or had some kind of undesirable reaction, that drug or drug combination can be avoided in the future. Was it difficult to obtain an airway? What size endotracheal (ET) tube was used? Brachycephalic breeds or even seemingly normal patients can have airway abnormalities that may make intubation difficult or may require a much smaller or larger ET tube than is anticipated. Having this information ahead of time allows the anesthetist to prepare for problems. How was the anesthesia in general? Was the patient hypotensive? Were they excessively bradycardic? Were they difficult to keep anesthetized? Was recovery rough or prolonged? Knowing this information ahead of time can allow the anesthetist to create a plan that may minimize the problems that were encountered previously.

Anesthetic circuit

Next, the anesthetist must decide what type of anesthetic circuit will be used. The anesthetic circuit is important since it allows administration of oxygen and anesthetic to the patient and removes waste gas. There are two main types of anesthetic circuits. The first is the re-breathing circuit (circle system). As the name state, this type of circuit allows re-breathing of exhaled gases with the carbon dioxide removed by a chemical absorbent. The advantage of this type of circuit is that relatively low oxygen flow rates are required which minimizes heat loss and the amount of anesthetic used. Another type of anesthetic circuit is the non-rebreathing circuit. This type of circuit relies on a high oxygen flow rate to remove carbon dioxide. Higher oxygen flow rates often lead to patient hypothermia so the pros and cons of this system should be balanced before use. The main advantage of the non-rebreathing circuit is that is provides less resistance to breathing making it more suitable for very small patients.

Airway

It is important to consider how the patient's airway will be maintained while under anesthesia. It is recommended that patients be endotracheally intubated for long procedures or for procedures involving the airway, oropharynx, or nasal cavity in order to protect and maintain the airway. Patients with airway abnormalities or those with respiratory disease should be intubated as well. Any patient who has been vomiting, regurgitating, or has been fed recently should be intubated as well to prevent aspiration of gastric contents. Endotracheal intubation also offers the advantage of allowing the anesthetist to provide positive pressure ventilation and minimizes exposure of personnel to waste gases.

Maintenance of anesthesia by mask may be preferable in patients in which endotracheal intubation may be very difficult. Guinea pigs, rats, and rabbits can be difficult to intubate and trauma to the airway may occur during attempts. Airway trauma can lead to edema, hemorrhage and obstruction. In many cases, maintenance by mask may be preferable in these patients, especially for short procedures. Anesthesia with a mask is also acceptable for feline and canine patients during short procedures in patients without respiratory disease, without airway abnormalities, and those that do not require assistance with ventilation.

Fluid choice

The most common types of fluid used for anesthetic maintenance are crystalloid solutions. Isotonic crystalloids are balanced electrolyte solutions and are typically run at a surgical maintenance rate of 10 ml/kg/hr. Lactated Ringer's solution (LRS) is commonly used during anesthesia. This solution contains calcium so it cannot be administered with blood products since the calcium in this product will bind with the citrate in blood products. Plasmalyte A is another choice that is commonly used. This product may be better to administer to patients with severe hepatic dysfunction as it does not contain lactate which requires metabolism by the liver. Normal saline (0.9% NaCl) is a good choice for patients which have high potassium since it is potassium free. This crystalloid is also much more acidic (pH 5.0) than most other crytalloids so may be a good choice in patients that are alkalotic. Hypotonic crystalloids (many "maintenance" solutions), like D5W, plasmalyte 56 and 0.45% saline, should be avoided at surgical maintenance rates.

Colloid use is indicated for patients with a total protein (TP) less than 3.5 mg/dL. When large amounts of crystalloids are administered to patients that are hypoproteinemic, fluid is not retained in the intravascular space and leaks out into the interstitium resulting in edema. Hetastarch (HES) is a synthetic colloid that can be used at up to 20 ml/kg/day. HES should be avoided in coagulopathic patients since it can interfere with platelet adhesion. Fresh frozen plasma (FFP) or frozen plasma (FP) is indicated for patients with low total protein as well as coagulation abnormalities as they can provide colloid oncotic support as well as coagulation factors.

Whole blood, packed red blood cells, or Oxyglobin ® should be administered to patients that are anemic going into surgery (less than ~20% PCV), or those that are experiencing hemorrhage. It is essential to maintain this minimum packed cell volume during anesthesia in order to maintain oxygen delivery to tissues.

Anesthetic plan

For every anesthesia, a balanced technique is recommended. The anesthetic event consists of four main parts: premedication, induction, maintenance, and recovery. Premedication accomplishes several goals. First is to decrease stress in the pre-surgical patient. Anxious patients will have high levels of circulating catecholamines which may predispose them to cardiac arrhythmias. The second goal is to provide pre-emptive analgesia and the third goal is to decrease the amount of induction drug needed to produce unconsciousness. Generally an opioid and a sedative will be used. Combining these drugs produces a synergistic effect. Combining drugs helps to decrease the amount of each drug needed and therefore decreases the incidence of side effects. An anticholinergic may be included in the premedication for some patients depending on the preference of the anesthetist. If more restraint is needed, a dissociative agent, such as ketamine can be added to help facilitate patient handling and catheter placement.

Induction to anesthesia can be accomplished in variety of ways. Box inductions, mask inductions and injectable techniques can be used. Generally, an injectable technique is preferred since the patient is less likely to struggle and the airway can be captured quickly. Also, exposure of personnel to waste gas is minimized. Box inductions can be used for fractious patients but are best avoided because of the large amount of waste gas exposure, the amount of patient struggling that can occur, and the lack of ability to capture an airway quickly. Mask inductions also present the problem of patient struggling and waste gas exposure, but can be used if an IV catheter cannot be placed, for short procedures, or patient's with severely impaired liver function.

Inhalant agents are commonly used for maintenance, especially in longer procedures. Isoflurane and sevoflurane are the most popular inhalants. Both of these have comparable cardiovascular and respiratory effects but sevoflurane provides a slightly more rapid induction and recovery. Performing a local block prior to surgery can help in the maintenance phase by significantly decreasing the amount of inhalant anesthetic needed to provide a surgical plan of anesthesia. Whenever possible, a local block should be considered as part of the anesthetic protocol. Many patients will also benefit from addition of a constant rate infusion (CRI) of an analgesic drug during their anesthesia. Besides providing additional analgesia for painful procedures, a CRI can be used to reduce inhalant requirements and minimize the cardiovascular effects of the inhalant. This can be particularly desirable in patients with cardiovascular compromise, septic shock, or other systemic disease.

Propofol can also be used as a CRI for maintenance of anesthesia. This would be indicated in patients with severe intracranial disease since propofol decreases cerebral blood flow and intracranial pressure (unlike most inhalant anesthetics). Opioid CRI's (fentanyl, remifentanil) can be used for maintenance of anesthesia without the addition of inhalant in patients who cannot tolerate the cardiovascular effects of inhalants. Total IV anesthesia with an opioid often requires a high rate of administration which often results in hypoventilation. Positive pressure ventilation is commonly needed and end tidal carbon dioxide levels should be monitored.

Recovery

The recovery phase of anesthesia involves the transition from unconsciousness back to consciousness. Priorities during this phase include ensuring that spontaneous ventilation is adequate, that the patient can maintain their airway, and that dysphoria, anxiety, and pain are minimized. Hypoventilation while under anesthesia is common. After anesthesia is discontinued, it is important to make sure that oxygen is provided and ventilation is assisted until the patient is able to ventilate themselves adequately. Extubation should occur once the patient is awake enough to maintain their airway on their own. Primarily the patient's swallowing reflex should have returned and the patient should be able to lift their head unassisted. Patients with airway abnormalities or undergoing procedures involving the nose, mouth, airway or neck should remain intubated as long as they will tolerate the ET tube since they are at higher risk of airway swelling, obstruction, or aspiration of blood.

Dysphoria and pain can be minimized post-operatively by pre-emptive administration of analgesics and sedatives. Ideally, this will start with an adequate premedication protocol and end with a plan for the recovery. A plan for post operative analgesia should be in place whether it involves an NSAID, local analgesia/anesthesia, opioid bolus dosing or a CRI. Planning ahead makes it much more likely to have a successful and safe outcome to an anesthetic event.