Luisito S. Pablo, DVM, MS, DACVA
General anesthesia in small animals can be provided using injectable agents, inhalants or a combination of injectable and inhalant agents. Maintenance of anesthesia using inhalant agents has been a mainstay in veterinary practice especially in longer surgical procedures. However, injectable agents (IV or IM) have been utilized in shorter procedures. There are advantages and disadvantages associated with using injectable agents. Induction of anesthesia using injectable agents is more convenient than using inhalants. It is also less stressful to the patients. There is no pollution of the atmosphere associated with the use of injectable agents. Disadvantages associated with the use of injectable agents include: a) prolonged recovery especially in agents that depend primarily on metabolism for inactivation, b) prolonged effects in very young and old patients, c) possibility of hypoventilation and then hypoxemia especially in patients not supplemented with high concentration of oxygen, d) inability to reverse the depressant effects of the injectable agents especially in agents without specific reversal agents and e) more time necessary to lighten the depth of anesthesia if problem arises.
Maintenance of anesthesia using injectable agents can be performed by: a) intermittent IV boluses, b) intermittent IM injection, or c) constant rate infusion. The problems associated with intermittent IM injection, given as need arises, include delayed onset of action, inability to maintain a constant plane of anesthesia, frequent changes in cardiopulmonary status, and the possibility of using more drugs within a certain period of time compared with IV injections. Comparing intermittent IV boluses and constant rate infusion, it appears from studies that constant rate infusion is a better technique than intermittent IV boluses. Three main advantages were noted: a) fewer sudden hemodynamic changes, b) lower total amount of drug given, and c) more rapid recovery from anesthesia. More rapid recovery from anesthesia can be due to the ability to change the rate of infusion based on the anesthetic need of the patient. The major disadvantage when using an effective constant rate infusion is the cost of a precision infusion device. This device, which replaces the vaporizer in this technique, should accurately deliver the chosen dose. Presently, the most suitable agent used in this technique, propofol, is still relatively expensive. Being a new technique, there is still scant information about the doses of anesthetic drugs for various types of surgical procedures and different medical conditions.
CHOICE OF INJECTABLE AGENTS
When considering constant rate infusion technique, the properties of the anesthetic drugs to be used should be examined. An ideal drug should possess most if not all of the following properties: a) water-soluble to minimize toxicity associated with the solvent, b) stable in solution, c) does not cause sloughing of tissue if given perivascularly, d) given in concentrated solution to avoid fluid overloading, e) not absorbed by plastics, f) does not promote bacterial growth, g) rapid onset of action, h) cleared rapidly from the body resulting in rapid and predictable recovery, i) devoid of adverse side effects, j) potent and lipid-soluble, k) relatively cheap and l) chemically compatible with other drugs. There is no single agent that possesses all these properties. Propofol, a hypnotic agent, is the most commonly used agent for constant rate infusion. It has a higher elimination clearance and a shorter elimination half-life. The clearance rate of propofol is faster than the liver blood flow. The recoveries from propofol are judged to be good. The other injectable agents available commercially have major disadvantages making them less applicable for constant rate infusion. Thiopental, a commonly used IV induction agent, is not suitable for constant rate infusion because of the prolonged recovery associated with longer infusion. Following a single dose of thiopental, recovery occurs rapidly because of redistribution, however once the redistribution sites are saturated, metabolism (a slow process) takes over. Methohexital is more rapidly metabolized compared to thiopental. The main disadvantage associated with methohexital is rough recoveries. Ketamine, another commonly injectable agent in practice is not suitable for constant rate infusion. Some metabolites of ketamine are active and they will accumulate resulting in prolonged drug action. The seizure activity and hypertonus during recovery make ketamine a poor choice for this technique. Etomidate, which maintains good cardiopulmonary function, has been tried as a maintenance agent. However, hemolysis resulting in clinical signs has been reported following infusion of etomidate. A new formulation of etomidate has been touted as not hemolytic. However, this is not available yet in the United States. Etomidate is the most expensive agent compared with other injectable agents.
There are certain indications for the use of constant rate infusion as a form of general anesthesia. This technique is suitable for surgical procedures that involved the upper airway where placement of endotracheal tube will interfere with the surgery. Bronchoscopic evaluation in smaller patients also needs constant rate infusion technique. The use of IV agents as a form of anesthesia for removal of intracranial tumor has been increasing. This is brought by the fact that inhalant agents will increase blood flow to the brain while IV agents like propofol will reduce the cerebral blood flow. Other uses of constant rate infusion using propofol include patients with postoperative seizures after portosystemic shunt ligation, anesthesia for patients that require ventilators, patients undergoing magnetic resonance imaging, radiation therapy and any situation for general anesthesia where an anesthetic machine is not available.
Preferably, an indwelling intravenous catheter should be placed. To perform an effective constant rate infusion technique, the use of a syringe pump is highly desirable. The syringe pump can accurately deliver the anesthetic dose over time and changes in delivery rate can be made more quickly. As an alternative, propofol can be infused using an IV administration set. For this technique, propofol is mixed with 5.0% dextrose in water. According to the manufacturer, propofol should only be diluted with 5.0% dextrose injection, USP and the final concentration of propofol should not be less than 2.0 mg/ml. The diluted form has 95% potency after 2 hours of running infusion in plastic. This technique is feasible, but it takes more time to prepare, will allow more chances of contamination during the preparation, and is less accurate. The drip rate may be difficult to control as changes in the height of the fluid column and occlusion of the IV line will change the drip rate.
Premedication is highly recommended before induction and maintenance of anesthesia using propofol. The quality of induction is vastly improved with the use of premedicants. The choice of premedicants will depend on the medical condition of the animal. If the animal is in pain or if pain will be involved in the procedure, an analgesic (opiates preferably) should be incorporated in the premedicants. A tranquilizer or sedative (acepromazine, benzodiazepines, or alpha-2 agonist) should be also be considered as part of premedication. Once sedation is achieved, anesthesia is induced using propofol. A calculated dose of 4.0 mg/kg is drawn up. Depending upon the degree of sedation, a typical initial dose is one-half of the calculated dose. This is given over 40-60 seconds. If the animal is still very light, the rest of propofol is given to effect. In procedures that will allow endotracheal intubation, an appropriately sized endotracheal tube is inserted and oxygen is supplied. Immediately after anesthetic induction, infusion of propofol at 0.4-0.5 mg/kg/min is started. The infusion rate should be adjusted based on the depth of anesthesia. It is important to remember that individual patients will vary in their anesthetic requirements. If the plane of anesthesia is getting deeper, characterized by lowering of blood pressure, decreased respiratory rate, and a central location eyeball without eye reflexes, the infusion rate can be reduced to as low as 0.2 mg/kg/min. In some instances, sudden application of surgical stimulus will lighten the plane of anesthesia to the point of movement of the legs and head, a bolus dose of propofol should be given to suddenly increase the blood concentration of propofol. The guideline is to give 10.0-20.0% of the induction dose, which was actually given to the patient. Following the bolus injection, the infusion rate may have to be increased by 0.1 mg/kg/min to attain a higher blood level which the patient requires. Utmost attention should be made to monitor the depth of anesthesia when the infusion rate is increased. Regular adjustments to the rate of infusion are expected for about 23 hours after the loading dose.
A modification of this technique is to add an opiate infusion to the propofol infusion. Propofol does not possess analgesic property. Fentanyl infusion has been utilized in patients that undergo painful procedures. Fentanyl infusion ranges from 0.7-2.0 ug/kg/min. If this is done, ventilatory support should be performed because of respiratory depression brought about by the two agents.
In the author's experience, the most common complication associated with constant rate infusion using propofol is respiratory depression. Hypoxemia can develop if the animal is inspiring room air. Administration of a higher concentration of oxygen, preferably 100%, is advisable. This can be administered via a facemask or endotracheal tube. Another possible complication associated with this technique is arterial hypotension. If the infusion rate is maintained at a higher level and the procedure being performed is not painful, blood pressure will decrease. If the procedure is mildly painful or not painful at all, light anesthetic depth can be maintained characterized by the presence of weak palpebral reflex.
A further modification of a constant rate infusion using propofol is a total intravenous anesthesia. This technique indicates that all components of general anesthesia are administered intravenously. Sedative-hypnotic drugs (e.g., propofol) produce unconsciousness and amnesia. Analgesic drugs (opiates) decreases or stops reflex response to surgery. Neuromuscular blocking agents (e.g., atracurium) provide the muscle relaxation. In this technique, the ventilation should be controlled because the diaphragm is paralyzed as a result of the neuromuscular blocking agent. This technique requires more equipment and expertise because of the inclusion of the muscle relaxant. If constant rate infusion gains acceptance in practice, total intravenous anesthesia may be the next step.
Constant rate infusion is another technique available to veterinary practitioners. It has its advantages and disadvantages which should be weighed when deciding for a particular case. Acceptance may not be easy at this point because of added equipment needed and the relatively high cost of propofol. Success of any technique is usually measured by its acceptance. Doubts understandably should proceed acceptance especially if other techniques seem to work. It is only by doing and trying something that someone can make the judgment about the utility of a technique.
Example of an anesthetic protocol using constant rate infusion in dogs
Acepromazine--0.05 mg/kg IM
Morphine--0.5 mg/kg IM
Propofol-- calculated dose of 4.0 mg/kg IV; initial dose given over 40-60 seconds
Propofol infusion--0.2-0.5 mg/kg/min
Oxygen supplementation is highly recommended.